Merge branch 'master' of github.com:grpc/grpc into lb_resolvers

15 files changed, 2073 insertions, 0 deletions

diff --git a/src/core/lib/census/README.md b/src/core/lib/census/README.md
new file mode 100644
index 0000000000..fb615a2194
--- /dev/null
+++ b/src/core/lib/census/README.md
@@ -0,0 +1,76 @@
+<!---
+ * Copyright 2015, Google Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+-->
+
+# Census - a resource measurement and tracing system
+
+This directory contains code for Census, which will ultimately provide the
+following features for any gRPC-using system:
+* A [dapper](http://research.google.com/pubs/pub36356.html)-like tracing
+  system, enabling tracing across a distributed infrastructure.
+* RPC statistics and measurements for key metrics, such as latency, bytes
+  transferred, number of errors etc.
+* Resource measurement framework which can be used for measuring custom
+  metrics. Through the use of [tags](#Tags), these can be broken down across
+  the entire distributed stack.
+* Easy integration of the above with
+  [Google Cloud Trace](https://cloud.google.com/tools/cloud-trace) and
+  [Google Cloud Monitoring](https://cloud.google.com/monitoring/).
+
+## Concepts
+
+### Context
+
+### Operations
+
+### Tags
+
+### Metrics
+
+## API
+
+### Internal/RPC API
+
+### External/Client API
+
+### RPC API
+
+## Files in this directory
+
+Note that files and functions in this directory can be split into two
+categories:
+* Files that define core census library functions. Functions etc. in these
+  files are named census\_\*, and constitute the core census library
+  functionality. At some time in the future, these will become a standalone
+  library.
+* Files that define functions etc. that provide a convenient interface between
+  grpc and the core census functionality. These files are all named
+  grpc\_\*.{c,h}, and define function names beginning with grpc\_census\_\*.
+
diff --git a/src/core/lib/census/aggregation.h b/src/core/lib/census/aggregation.h
new file mode 100644
index 0000000000..f353368b97
--- /dev/null
+++ b/src/core/lib/census/aggregation.h
@@ -0,0 +1,66 @@
+/*
+ *
+ * Copyright 2015-2016, Google Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+#include <stddef.h>
+
+#ifndef GRPC_CORE_LIB_CENSUS_AGGREGATION_H
+#define GRPC_CORE_LIB_CENSUS_AGGREGATION_H
+
+/** Structure used to describe an aggregation type. */
+struct census_aggregation_ops {
+  /* Create a new aggregation. The pointer returned can be used in future calls
+     to clone(), free(), record(), data() and reset(). */
+  void *(*create)(const void *create_arg);
+  /* Make a copy of an aggregation created by create() */
+  void *(*clone)(const void *aggregation);
+  /* Destroy an aggregation created by create() */
+  void (*free)(void *aggregation);
+  /* Record a new value against aggregation. */
+  void (*record)(void *aggregation, double value);
+  /* Return current aggregation data. The caller must cast this object into
+     the correct type for the aggregation result. The object returned can be
+     freed by using free_data(). */
+  void *(*data)(const void *aggregation);
+  /* free data returned by data() */
+  void (*free_data)(void *data);
+  /* Reset an aggregation to default (zero) values. */
+  void (*reset)(void *aggregation);
+  /* Merge 'from' aggregation into 'to'. Both aggregations must be compatible */
+  void (*merge)(void *to, const void *from);
+  /* Fill buffer with printable string version of aggregation contents. For
+     debugging only. Returns the number of bytes added to buffer (a value == n
+     implies the buffer was of insufficient size). */
+  size_t (*print)(const void *aggregation, char *buffer, size_t n);
+};
+
+#endif /* GRPC_CORE_LIB_CENSUS_AGGREGATION_H */
diff --git a/src/core/lib/census/context.c b/src/core/lib/census/context.c
new file mode 100644
index 0000000000..5a118f46a9
--- /dev/null
+++ b/src/core/lib/census/context.c
@@ -0,0 +1,509 @@
+/*
+ *
+ * Copyright 2015-2016, Google Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+#include <grpc/census.h>
+#include <grpc/support/alloc.h>
+#include <grpc/support/log.h>
+#include <grpc/support/port_platform.h>
+#include <grpc/support/useful.h>
+#include <stdbool.h>
+#include <string.h>
+#include "src/core/lib/support/string.h"
+
+// Functions in this file support the public context API, including
+// encoding/decoding as part of context propagation across RPC's. The overall
+// requirements (in approximate priority order) for the
+// context representation:
+// 1. Efficient conversion to/from wire format
+// 2. Minimal bytes used on-wire
+// 3. Efficient context creation
+// 4. Efficient lookup of tag value for a key
+// 5. Efficient iteration over tags
+// 6. Minimal memory footprint
+//
+// Notes on tradeoffs/decisions:
+// * tag includes 1 byte length of key, as well as nil-terminating byte. These
+//   are to aid in efficient parsing and the ability to directly return key
+//   strings. This is more important than saving a single byte/tag on the wire.
+// * The wire encoding uses only single byte values. This eliminates the need
+//   to handle endian-ness conversions. It also means there is a hard upper
+//   limit of 255 for both CENSUS_MAX_TAG_KV_LEN and CENSUS_MAX_PROPAGATED_TAGS.
+// * Keep all tag information (keys/values/flags) in a single memory buffer,
+//   that can be directly copied to the wire.
+
+// min and max valid chars in tag keys and values. All printable ASCII is OK.
+#define MIN_VALID_TAG_CHAR 32   // ' '
+#define MAX_VALID_TAG_CHAR 126  // '~'
+
+// Structure representing a set of tags. Essentially a count of number of tags
+// present, and pointer to a chunk of memory that contains the per-tag details.
+struct tag_set {
+  int ntags;        // number of tags.
+  int ntags_alloc;  // ntags + number of deleted tags (total number of tags
+  // in all of kvm). This will always be == ntags, except during the process
+  // of building a new tag set.
+  size_t kvm_size;  // number of bytes allocated for key/value storage.
+  size_t kvm_used;  // number of bytes of used key/value memory
+  char *kvm;        // key/value memory. Consists of repeated entries of:
+  //   Offset  Size  Description
+  //     0      1    Key length, including trailing 0. (K)
+  //     1      1    Value length, including trailing 0 (V)
+  //     2      1    Flags
+  //     3      K    Key bytes
+  //     3 + K  V    Value bytes
+  //
+  // We refer to the first 3 entries as the 'tag header'. If extra values are
+  // introduced in the header, you will need to modify the TAG_HEADER_SIZE
+  // constant, the raw_tag structure (and everything that uses it) and the
+  // encode/decode functions appropriately.
+};
+
+// Number of bytes in tag header.
+#define TAG_HEADER_SIZE 3  // key length (1) + value length (1) + flags (1)
+// Offsets to tag header entries.
+#define KEY_LEN_OFFSET 0
+#define VALUE_LEN_OFFSET 1
+#define FLAG_OFFSET 2
+
+// raw_tag represents the raw-storage form of a tag in the kvm of a tag_set.
+struct raw_tag {
+  uint8_t key_len;
+  uint8_t value_len;
+  uint8_t flags;
+  char *key;
+  char *value;
+};
+
+// Use a reserved flag bit for indication of deleted tag.
+#define CENSUS_TAG_DELETED CENSUS_TAG_RESERVED
+#define CENSUS_TAG_IS_DELETED(flags) (flags & CENSUS_TAG_DELETED)
+
+// Primary representation of a context. Composed of 2 underlying tag_set
+// structs, one each for propagated and local (non-propagated) tags. This is
+// to efficiently support tag encoding/decoding.
+// TODO(aveitch): need to add tracing id's/structure.
+struct census_context {
+  struct tag_set tags[2];
+  census_context_status status;
+};
+
+// Indices into the tags member of census_context
+#define PROPAGATED_TAGS 0
+#define LOCAL_TAGS 1
+
+// Validate (check all characters are in range and size is less than limit) a
+// key or value string. Returns 0 if the string is invalid, or the length
+// (including terminator) if valid.
+static size_t validate_tag(const char *kv) {
+  size_t len = 1;
+  char ch;
+  while ((ch = *kv++) != 0) {
+    if (ch < MIN_VALID_TAG_CHAR || ch > MAX_VALID_TAG_CHAR) {
+      return 0;
+    }
+    len++;
+  }
+  if (len > CENSUS_MAX_TAG_KV_LEN) {
+    return 0;
+  }
+  return len;
+}
+
+// Extract a raw tag given a pointer (raw) to the tag header. Allow for some
+// extra bytes in the tag header (see encode/decode functions for usage: this
+// allows for future expansion of the tag header).
+static char *decode_tag(struct raw_tag *tag, char *header, int offset) {
+  tag->key_len = (uint8_t)(*header++);
+  tag->value_len = (uint8_t)(*header++);
+  tag->flags = (uint8_t)(*header++);
+  header += offset;
+  tag->key = header;
+  header += tag->key_len;
+  tag->value = header;
+  return header + tag->value_len;
+}
+
+// Make a copy (in 'to') of an existing tag_set.
+static void tag_set_copy(struct tag_set *to, const struct tag_set *from) {
+  memcpy(to, from, sizeof(struct tag_set));
+  to->kvm = gpr_malloc(to->kvm_size);
+  memcpy(to->kvm, from->kvm, from->kvm_used);
+}
+
+// Delete a tag from a tag_set, if it exists (returns true if it did).
+static bool tag_set_delete_tag(struct tag_set *tags, const char *key,
+                               size_t key_len) {
+  char *kvp = tags->kvm;
+  for (int i = 0; i < tags->ntags_alloc; i++) {
+    uint8_t *flags = (uint8_t *)(kvp + FLAG_OFFSET);
+    struct raw_tag tag;
+    kvp = decode_tag(&tag, kvp, 0);
+    if (CENSUS_TAG_IS_DELETED(tag.flags)) continue;
+    if ((key_len == tag.key_len) && (memcmp(key, tag.key, key_len) == 0)) {
+      *flags |= CENSUS_TAG_DELETED;
+      tags->ntags--;
+      return true;
+    }
+  }
+  return false;
+}
+
+// Delete a tag from a context, return true if it existed.
+static bool context_delete_tag(census_context *context, const census_tag *tag,
+                               size_t key_len) {
+  return (
+      tag_set_delete_tag(&context->tags[LOCAL_TAGS], tag->key, key_len) ||
+      tag_set_delete_tag(&context->tags[PROPAGATED_TAGS], tag->key, key_len));
+}
+
+// Add a tag to a tag_set. Return true on success, false if the tag could
+// not be added because of constraints on tag set size. This function should
+// not be called if the tag may already exist (in a non-deleted state) in
+// the tag_set, as that would result in two tags with the same key.
+static bool tag_set_add_tag(struct tag_set *tags, const census_tag *tag,
+                            size_t key_len, size_t value_len) {
+  if (tags->ntags == CENSUS_MAX_PROPAGATED_TAGS) {
+    return false;
+  }
+  const size_t tag_size = key_len + value_len + TAG_HEADER_SIZE;
+  if (tags->kvm_used + tag_size > tags->kvm_size) {
+    // allocate new memory if needed
+    tags->kvm_size += 2 * CENSUS_MAX_TAG_KV_LEN + TAG_HEADER_SIZE;
+    char *new_kvm = gpr_malloc(tags->kvm_size);
+    memcpy(new_kvm, tags->kvm, tags->kvm_used);
+    gpr_free(tags->kvm);
+    tags->kvm = new_kvm;
+  }
+  char *kvp = tags->kvm + tags->kvm_used;
+  *kvp++ = (char)key_len;
+  *kvp++ = (char)value_len;
+  // ensure reserved flags are not used.
+  *kvp++ = (char)(tag->flags & (CENSUS_TAG_PROPAGATE | CENSUS_TAG_STATS));
+  memcpy(kvp, tag->key, key_len);
+  kvp += key_len;
+  memcpy(kvp, tag->value, value_len);
+  tags->kvm_used += tag_size;
+  tags->ntags++;
+  tags->ntags_alloc++;
+  return true;
+}
+
+// Add/modify/delete a tag to/in a context. Caller must validate that tag key
+// etc. are valid.
+static void context_modify_tag(census_context *context, const census_tag *tag,
+                               size_t key_len, size_t value_len) {
+  // First delete the tag if it is already present.
+  bool deleted = context_delete_tag(context, tag, key_len);
+  bool added = false;
+  if (CENSUS_TAG_IS_PROPAGATED(tag->flags)) {
+    added = tag_set_add_tag(&context->tags[PROPAGATED_TAGS], tag, key_len,
+                            value_len);
+  } else {
+    added =
+        tag_set_add_tag(&context->tags[LOCAL_TAGS], tag, key_len, value_len);
+  }
+
+  if (deleted) {
+    context->status.n_modified_tags++;
+  } else {
+    if (added) {
+      context->status.n_added_tags++;
+    } else {
+      context->status.n_ignored_tags++;
+    }
+  }
+}
+
+// Remove memory used for deleted tags from a tag set. Basic algorithm:
+// 1) Walk through tag set to find first deleted tag. Record where it is.
+// 2) Find the next not-deleted tag. Copy all of kvm from there to the end
+//    "over" the deleted tags
+// 3) repeat #1 and #2 until we have seen all tags
+// 4) if we are still looking for a not-deleted tag, then all the end portion
+//    of the kvm is deleted. Just reduce the used amount of memory by the
+//    appropriate amount.
+static void tag_set_flatten(struct tag_set *tags) {
+  if (tags->ntags == tags->ntags_alloc) return;
+  bool found_deleted = false;  // found a deleted tag.
+  char *kvp = tags->kvm;
+  char *dbase = NULL;  // record location of deleted tag
+  for (int i = 0; i < tags->ntags_alloc; i++) {
+    struct raw_tag tag;
+    char *next_kvp = decode_tag(&tag, kvp, 0);
+    if (found_deleted) {
+      if (!CENSUS_TAG_IS_DELETED(tag.flags)) {
+        ptrdiff_t reduce = kvp - dbase;  // #bytes in deleted tags
+        GPR_ASSERT(reduce > 0);
+        ptrdiff_t copy_size = tags->kvm + tags->kvm_used - kvp;
+        GPR_ASSERT(copy_size > 0);
+        memmove(dbase, kvp, (size_t)copy_size);
+        tags->kvm_used -= (size_t)reduce;
+        next_kvp -= reduce;
+        found_deleted = false;
+      }
+    } else {
+      if (CENSUS_TAG_IS_DELETED(tag.flags)) {
+        dbase = kvp;
+        found_deleted = true;
+      }
+    }
+    kvp = next_kvp;
+  }
+  if (found_deleted) {
+    GPR_ASSERT(dbase > tags->kvm);
+    tags->kvm_used = (size_t)(dbase - tags->kvm);
+  }
+  tags->ntags_alloc = tags->ntags;
+}
+
+census_context *census_context_create(const census_context *base,
+                                      const census_tag *tags, int ntags,
+                                      census_context_status const **status) {
+  census_context *context = gpr_malloc(sizeof(census_context));
+  // If we are given a base, copy it into our new tag set. Otherwise set it
+  // to zero/NULL everything.
+  if (base == NULL) {
+    memset(context, 0, sizeof(census_context));
+  } else {
+    tag_set_copy(&context->tags[PROPAGATED_TAGS], &base->tags[PROPAGATED_TAGS]);
+    tag_set_copy(&context->tags[LOCAL_TAGS], &base->tags[LOCAL_TAGS]);
+    memset(&context->status, 0, sizeof(context->status));
+  }
+  // Walk over the additional tags and, for those that aren't invalid, modify
+  // the context to add/replace/delete as required.
+  for (int i = 0; i < ntags; i++) {
+    const census_tag *tag = &tags[i];
+    size_t key_len = validate_tag(tag->key);
+    // ignore the tag if it is invalid or too short.
+    if (key_len <= 1) {
+      context->status.n_invalid_tags++;
+    } else {
+      if (tag->value != NULL) {
+        size_t value_len = validate_tag(tag->value);
+        if (value_len != 0) {
+          context_modify_tag(context, tag, key_len, value_len);
+        } else {
+          context->status.n_invalid_tags++;
+        }
+      } else {
+        if (context_delete_tag(context, tag, key_len)) {
+          context->status.n_deleted_tags++;
+        }
+      }
+    }
+  }
+  // Remove any deleted tags, update status if needed, and return.
+  tag_set_flatten(&context->tags[PROPAGATED_TAGS]);
+  tag_set_flatten(&context->tags[LOCAL_TAGS]);
+  context->status.n_propagated_tags = context->tags[PROPAGATED_TAGS].ntags;
+  context->status.n_local_tags = context->tags[LOCAL_TAGS].ntags;
+  if (status) {
+    *status = &context->status;
+  }
+  return context;
+}
+
+const census_context_status *census_context_get_status(
+    const census_context *context) {
+  return &context->status;
+}
+
+void census_context_destroy(census_context *context) {
+  gpr_free(context->tags[PROPAGATED_TAGS].kvm);
+  gpr_free(context->tags[LOCAL_TAGS].kvm);
+  gpr_free(context);
+}
+
+void census_context_initialize_iterator(const census_context *context,
+                                        census_context_iterator *iterator) {
+  iterator->context = context;
+  iterator->index = 0;
+  if (context->tags[PROPAGATED_TAGS].ntags != 0) {
+    iterator->base = PROPAGATED_TAGS;
+    iterator->kvm = context->tags[PROPAGATED_TAGS].kvm;
+  } else if (context->tags[LOCAL_TAGS].ntags != 0) {
+    iterator->base = LOCAL_TAGS;
+    iterator->kvm = context->tags[LOCAL_TAGS].kvm;
+  } else {
+    iterator->base = -1;
+  }
+}
+
+int census_context_next_tag(census_context_iterator *iterator,
+                            census_tag *tag) {
+  if (iterator->base < 0) {
+    return 0;
+  }
+  struct raw_tag raw;
+  iterator->kvm = decode_tag(&raw, iterator->kvm, 0);
+  tag->key = raw.key;
+  tag->value = raw.value;
+  tag->flags = raw.flags;
+  if (++iterator->index == iterator->context->tags[iterator->base].ntags) {
+    do {
+      if (iterator->base == LOCAL_TAGS) {
+        iterator->base = -1;
+        return 1;
+      }
+    } while (iterator->context->tags[++iterator->base].ntags == 0);
+    iterator->index = 0;
+    iterator->kvm = iterator->context->tags[iterator->base].kvm;
+  }
+  return 1;
+}
+
+// Find a tag in a tag_set by key. Return true if found, false otherwise.
+static bool tag_set_get_tag(const struct tag_set *tags, const char *key,
+                            size_t key_len, census_tag *tag) {
+  char *kvp = tags->kvm;
+  for (int i = 0; i < tags->ntags; i++) {
+    struct raw_tag raw;
+    kvp = decode_tag(&raw, kvp, 0);
+    if (key_len == raw.key_len && memcmp(raw.key, key, key_len) == 0) {
+      tag->key = raw.key;
+      tag->value = raw.value;
+      tag->flags = raw.flags;
+      return true;
+    }
+  }
+  return false;
+}
+
+int census_context_get_tag(const census_context *context, const char *key,
+                           census_tag *tag) {
+  size_t key_len = strlen(key) + 1;
+  if (key_len == 1) {
+    return 0;
+  }
+  if (tag_set_get_tag(&context->tags[PROPAGATED_TAGS], key, key_len, tag) ||
+      tag_set_get_tag(&context->tags[LOCAL_TAGS], key, key_len, tag)) {
+    return 1;
+  }
+  return 0;
+}
+
+// Context encoding and decoding functions.
+//
+// Wire format for tag_set's on the wire:
+//
+// First, a tag set header:
+//
+// offset   bytes  description
+//   0        1    version number
+//   1        1    number of bytes in this header. This allows for future
+//                 expansion.
+//   2        1    number of bytes in each tag header.
+//   3        1    ntags value from tag set.
+//
+//   This is followed by the key/value memory from struct tag_set.
+
+#define ENCODED_VERSION 0      // Version number
+#define ENCODED_HEADER_SIZE 4  // size of tag set header
+
+// Encode a tag set. Returns 0 if buffer is too small.
+static size_t tag_set_encode(const struct tag_set *tags, char *buffer,
+                             size_t buf_size) {
+  if (buf_size < ENCODED_HEADER_SIZE + tags->kvm_used) {
+    return 0;
+  }
+  buf_size -= ENCODED_HEADER_SIZE;
+  *buffer++ = (char)ENCODED_VERSION;
+  *buffer++ = (char)ENCODED_HEADER_SIZE;
+  *buffer++ = (char)TAG_HEADER_SIZE;
+  *buffer++ = (char)tags->ntags;
+  if (tags->ntags == 0) {
+    return ENCODED_HEADER_SIZE;
+  }
+  memcpy(buffer, tags->kvm, tags->kvm_used);
+  return ENCODED_HEADER_SIZE + tags->kvm_used;
+}
+
+size_t census_context_encode(const census_context *context, char *buffer,
+                             size_t buf_size) {
+  return tag_set_encode(&context->tags[PROPAGATED_TAGS], buffer, buf_size);
+}
+
+// Decode a tag set.
+static void tag_set_decode(struct tag_set *tags, const char *buffer,
+                           size_t size) {
+  uint8_t version = (uint8_t)(*buffer++);
+  uint8_t header_size = (uint8_t)(*buffer++);
+  uint8_t tag_header_size = (uint8_t)(*buffer++);
+  tags->ntags = tags->ntags_alloc = (int)(*buffer++);
+  if (tags->ntags == 0) {
+    tags->ntags_alloc = 0;
+    tags->kvm_size = 0;
+    tags->kvm_used = 0;
+    tags->kvm = NULL;
+    return;
+  }
+  if (header_size != ENCODED_HEADER_SIZE) {
+    GPR_ASSERT(version != ENCODED_VERSION);
+    GPR_ASSERT(ENCODED_HEADER_SIZE < header_size);
+    buffer += (header_size - ENCODED_HEADER_SIZE);
+  }
+  tags->kvm_used = size - header_size;
+  tags->kvm_size = tags->kvm_used + CENSUS_MAX_TAG_KV_LEN;
+  tags->kvm = gpr_malloc(tags->kvm_size);
+  if (tag_header_size != TAG_HEADER_SIZE) {
+    // something new in the tag information. I don't understand it, so
+    // don't copy it over.
+    GPR_ASSERT(version != ENCODED_VERSION);
+    GPR_ASSERT(tag_header_size > TAG_HEADER_SIZE);
+    char *kvp = tags->kvm;
+    for (int i = 0; i < tags->ntags; i++) {
+      memcpy(kvp, buffer, TAG_HEADER_SIZE);
+      kvp += header_size;
+      struct raw_tag raw;
+      buffer =
+          decode_tag(&raw, (char *)buffer, tag_header_size - TAG_HEADER_SIZE);
+      memcpy(kvp, raw.key, (size_t)raw.key_len + raw.value_len);
+      kvp += raw.key_len + raw.value_len;
+    }
+  } else {
+    memcpy(tags->kvm, buffer, tags->kvm_used);
+  }
+}
+
+census_context *census_context_decode(const char *buffer, size_t size) {
+  census_context *context = gpr_malloc(sizeof(census_context));
+  memset(&context->tags[LOCAL_TAGS], 0, sizeof(struct tag_set));
+  if (buffer == NULL) {
+    memset(&context->tags[PROPAGATED_TAGS], 0, sizeof(struct tag_set));
+  } else {
+    tag_set_decode(&context->tags[PROPAGATED_TAGS], buffer, size);
+  }
+  memset(&context->status, 0, sizeof(context->status));
+  context->status.n_propagated_tags = context->tags[PROPAGATED_TAGS].ntags;
+  return context;
+}
diff --git a/src/core/lib/census/grpc_context.c b/src/core/lib/census/grpc_context.c
new file mode 100644
index 0000000000..457c176355
--- /dev/null
+++ b/src/core/lib/census/grpc_context.c
@@ -0,0 +1,53 @@
+/*
+ *
+ * Copyright 2015-2016, Google Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+#include <grpc/census.h>
+#include <grpc/grpc.h>
+#include "src/core/lib/surface/api_trace.h"
+#include "src/core/lib/surface/call.h"
+
+void grpc_census_call_set_context(grpc_call *call, census_context *context) {
+  GRPC_API_TRACE("grpc_census_call_set_context(call=%p, census_context=%p)", 2,
+                 (call, context));
+  if (census_enabled() == CENSUS_FEATURE_NONE) {
+    return;
+  }
+  if (context != NULL) {
+    grpc_call_context_set(call, GRPC_CONTEXT_TRACING, context, NULL);
+  }
+}
+
+census_context *grpc_census_call_get_context(grpc_call *call) {
+  GRPC_API_TRACE("grpc_census_call_get_context(call=%p)", 1, (call));
+  return (census_context *)grpc_call_context_get(call, GRPC_CONTEXT_TRACING);
+}
diff --git a/src/core/lib/census/grpc_filter.c b/src/core/lib/census/grpc_filter.c
new file mode 100644
index 0000000000..d27d789aa1
--- /dev/null
+++ b/src/core/lib/census/grpc_filter.c
@@ -0,0 +1,198 @@
+/*
+ *
+ * Copyright 2015-2016, Google Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+#include "src/core/lib/census/grpc_filter.h"
+
+#include <stdio.h>
+#include <string.h>
+
+#include <grpc/census.h>
+#include <grpc/support/alloc.h>
+#include <grpc/support/log.h>
+#include <grpc/support/slice.h>
+#include <grpc/support/time.h>
+
+#include "src/core/lib/channel/channel_stack.h"
+#include "src/core/lib/statistics/census_interface.h"
+#include "src/core/lib/statistics/census_rpc_stats.h"
+#include "src/core/lib/transport/static_metadata.h"
+
+typedef struct call_data {
+  census_op_id op_id;
+  census_context *ctxt;
+  gpr_timespec start_ts;
+  int error;
+
+  /* recv callback */
+  grpc_metadata_batch *recv_initial_metadata;
+  grpc_closure *on_done_recv;
+  grpc_closure finish_recv;
+} call_data;
+
+typedef struct channel_data { uint8_t unused; } channel_data;
+
+static void extract_and_annotate_method_tag(grpc_metadata_batch *md,
+                                            call_data *calld,
+                                            channel_data *chand) {
+  grpc_linked_mdelem *m;
+  for (m = md->list.head; m != NULL; m = m->next) {
+    if (m->md->key == GRPC_MDSTR_PATH) {
+      gpr_log(GPR_DEBUG, "%s",
+              (const char *)GPR_SLICE_START_PTR(m->md->value->slice));
+      /* Add method tag here */
+    }
+  }
+}
+
+static void client_mutate_op(grpc_call_element *elem,
+                             grpc_transport_stream_op *op) {
+  call_data *calld = elem->call_data;
+  channel_data *chand = elem->channel_data;
+  if (op->send_initial_metadata) {
+    extract_and_annotate_method_tag(op->send_initial_metadata, calld, chand);
+  }
+}
+
+static void client_start_transport_op(grpc_exec_ctx *exec_ctx,
+                                      grpc_call_element *elem,
+                                      grpc_transport_stream_op *op) {
+  client_mutate_op(elem, op);
+  grpc_call_next_op(exec_ctx, elem, op);
+}
+
+static void server_on_done_recv(grpc_exec_ctx *exec_ctx, void *ptr,
+                                bool success) {
+  grpc_call_element *elem = ptr;
+  call_data *calld = elem->call_data;
+  channel_data *chand = elem->channel_data;
+  if (success) {
+    extract_and_annotate_method_tag(calld->recv_initial_metadata, calld, chand);
+  }
+  calld->on_done_recv->cb(exec_ctx, calld->on_done_recv->cb_arg, success);
+}
+
+static void server_mutate_op(grpc_call_element *elem,
+                             grpc_transport_stream_op *op) {
+  call_data *calld = elem->call_data;
+  if (op->recv_initial_metadata) {
+    /* substitute our callback for the op callback */
+    calld->recv_initial_metadata = op->recv_initial_metadata;
+    calld->on_done_recv = op->recv_initial_metadata_ready;
+    op->recv_initial_metadata_ready = &calld->finish_recv;
+  }
+}
+
+static void server_start_transport_op(grpc_exec_ctx *exec_ctx,
+                                      grpc_call_element *elem,
+                                      grpc_transport_stream_op *op) {
+  /* TODO(ctiller): this code fails. I don't know why. I expect it's
+                    incomplete, and someone should look at it soon.
+
+  call_data *calld = elem->call_data;
+  GPR_ASSERT((calld->op_id.upper != 0) || (calld->op_id.lower != 0)); */
+  server_mutate_op(elem, op);
+  grpc_call_next_op(exec_ctx, elem, op);
+}
+
+static void client_init_call_elem(grpc_exec_ctx *exec_ctx,
+                                  grpc_call_element *elem,
+                                  grpc_call_element_args *args) {
+  call_data *d = elem->call_data;
+  GPR_ASSERT(d != NULL);
+  memset(d, 0, sizeof(*d));
+  d->start_ts = gpr_now(GPR_CLOCK_REALTIME);
+}
+
+static void client_destroy_call_elem(grpc_exec_ctx *exec_ctx,
+                                     grpc_call_element *elem) {
+  call_data *d = elem->call_data;
+  GPR_ASSERT(d != NULL);
+  /* TODO(hongyu): record rpc client stats and census_rpc_end_op here */
+}
+
+static void server_init_call_elem(grpc_exec_ctx *exec_ctx,
+                                  grpc_call_element *elem,
+                                  grpc_call_element_args *args) {
+  call_data *d = elem->call_data;
+  GPR_ASSERT(d != NULL);
+  memset(d, 0, sizeof(*d));
+  d->start_ts = gpr_now(GPR_CLOCK_REALTIME);
+  /* TODO(hongyu): call census_tracing_start_op here. */
+  grpc_closure_init(&d->finish_recv, server_on_done_recv, elem);
+}
+
+static void server_destroy_call_elem(grpc_exec_ctx *exec_ctx,
+                                     grpc_call_element *elem) {
+  call_data *d = elem->call_data;
+  GPR_ASSERT(d != NULL);
+  /* TODO(hongyu): record rpc server stats and census_tracing_end_op here */
+}
+
+static void init_channel_elem(grpc_exec_ctx *exec_ctx,
+                              grpc_channel_element *elem,
+                              grpc_channel_element_args *args) {
+  channel_data *chand = elem->channel_data;
+  GPR_ASSERT(chand != NULL);
+}
+
+static void destroy_channel_elem(grpc_exec_ctx *exec_ctx,
+                                 grpc_channel_element *elem) {
+  channel_data *chand = elem->channel_data;
+  GPR_ASSERT(chand != NULL);
+}
+
+const grpc_channel_filter grpc_client_census_filter = {
+    client_start_transport_op,
+    grpc_channel_next_op,
+    sizeof(call_data),
+    client_init_call_elem,
+    grpc_call_stack_ignore_set_pollset,
+    client_destroy_call_elem,
+    sizeof(channel_data),
+    init_channel_elem,
+    destroy_channel_elem,
+    grpc_call_next_get_peer,
+    "census-client"};
+
+const grpc_channel_filter grpc_server_census_filter = {
+    server_start_transport_op,
+    grpc_channel_next_op,
+    sizeof(call_data),
+    server_init_call_elem,
+    grpc_call_stack_ignore_set_pollset,
+    server_destroy_call_elem,
+    sizeof(channel_data),
+    init_channel_elem,
+    destroy_channel_elem,
+    grpc_call_next_get_peer,
+    "census-server"};
diff --git a/src/core/lib/census/grpc_filter.h b/src/core/lib/census/grpc_filter.h
new file mode 100644
index 0000000000..7ceafe56e4
--- /dev/null
+++ b/src/core/lib/census/grpc_filter.h
@@ -0,0 +1,44 @@
+/*
+ *
+ * Copyright 2015-2016, Google Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+#ifndef GRPC_CORE_LIB_CENSUS_GRPC_FILTER_H
+#define GRPC_CORE_LIB_CENSUS_GRPC_FILTER_H
+
+#include "src/core/lib/channel/channel_stack.h"
+
+/* Census filters: provides tracing and stats collection functionalities. It
+   needs to reside right below the surface filter in the channel stack. */
+extern const grpc_channel_filter grpc_client_census_filter;
+extern const grpc_channel_filter grpc_server_census_filter;
+
+#endif /* GRPC_CORE_LIB_CENSUS_GRPC_FILTER_H */
diff --git a/src/core/lib/census/grpc_plugin.c b/src/core/lib/census/grpc_plugin.c
new file mode 100644
index 0000000000..12aca76745
--- /dev/null
+++ b/src/core/lib/census/grpc_plugin.c
@@ -0,0 +1,70 @@
+/*
+ *
+ * Copyright 2015-2016, Google Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+#include "src/core/lib/census/grpc_plugin.h"
+
+#include <limits.h>
+
+#include <grpc/census.h>
+
+#include "src/core/lib/census/grpc_filter.h"
+#include "src/core/lib/channel/channel_stack_builder.h"
+#include "src/core/lib/surface/channel_init.h"
+
+static bool maybe_add_census_filter(grpc_channel_stack_builder *builder,
+                                    void *arg_must_be_null) {
+  const grpc_channel_args *args =
+      grpc_channel_stack_builder_get_channel_arguments(builder);
+  if (grpc_channel_args_is_census_enabled(args)) {
+    return grpc_channel_stack_builder_prepend_filter(
+        builder, &grpc_client_census_filter, NULL, NULL);
+  }
+  return true;
+}
+
+void census_grpc_plugin_init(void) {
+  /* Only initialize census if no one else has and some features are
+   * available. */
+  if (census_enabled() == CENSUS_FEATURE_NONE &&
+      census_supported() != CENSUS_FEATURE_NONE) {
+    if (census_initialize(census_supported())) { /* enable all features. */
+      gpr_log(GPR_ERROR, "Could not initialize census.");
+    }
+  }
+  grpc_channel_init_register_stage(GRPC_CLIENT_CHANNEL, INT_MAX,
+                                   maybe_add_census_filter, NULL);
+  grpc_channel_init_register_stage(GRPC_SERVER_CHANNEL, INT_MAX,
+                                   maybe_add_census_filter, NULL);
+}
+
+void census_grpc_plugin_destroy(void) { census_shutdown(); }
diff --git a/src/core/lib/census/grpc_plugin.h b/src/core/lib/census/grpc_plugin.h
new file mode 100644
index 0000000000..33e5f0b701
--- /dev/null
+++ b/src/core/lib/census/grpc_plugin.h
@@ -0,0 +1,40 @@
+/*
+ *
+ * Copyright 2015-2016, Google Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+#ifndef GRPC_CORE_LIB_CENSUS_GRPC_PLUGIN_H
+#define GRPC_CORE_LIB_CENSUS_GRPC_PLUGIN_H
+
+void census_grpc_plugin_init(void);
+void census_grpc_plugin_destroy(void);
+
+#endif /* GRPC_CORE_LIB_CENSUS_GRPC_PLUGIN_H */
diff --git a/src/core/lib/census/initialize.c b/src/core/lib/census/initialize.c
new file mode 100644
index 0000000000..ce7ec09b89
--- /dev/null
+++ b/src/core/lib/census/initialize.c
@@ -0,0 +1,54 @@
+/*
+ *
+ * Copyright 2015-2016, Google Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+#include <grpc/census.h>
+
+static int features_enabled = CENSUS_FEATURE_NONE;
+
+int census_initialize(int features) {
+  if (features_enabled != CENSUS_FEATURE_NONE) {
+    // Must have been a previous call to census_initialize; return error
+    return 1;
+  }
+  features_enabled = features;
+  return 0;
+}
+
+void census_shutdown(void) { features_enabled = CENSUS_FEATURE_NONE; }
+
+int census_supported(void) {
+  /* TODO(aveitch): improve this as we implement features... */
+  return CENSUS_FEATURE_NONE;
+}
+
+int census_enabled(void) { return features_enabled; }
diff --git a/src/core/lib/census/mlog.c b/src/core/lib/census/mlog.c
new file mode 100644
index 0000000000..9d47e80297
--- /dev/null
+++ b/src/core/lib/census/mlog.c
@@ -0,0 +1,600 @@
+/*
+ *
+ * Copyright 2015-2016, Google Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+// Implements an efficient in-memory log, optimized for multiple writers and
+// a single reader. Available log space is divided up in blocks of
+// CENSUS_LOG_2_MAX_RECORD_SIZE bytes. A block can be in one of the following
+// three data structures:
+// - Free blocks (free_block_list)
+// - Blocks with unread data (dirty_block_list)
+// - Blocks currently attached to cores (core_local_blocks[])
+//
+// census_log_start_write() moves a block from core_local_blocks[] to the end of
+// dirty_block_list when block:
+// - is out-of-space OR
+// - has an incomplete record (an incomplete record occurs when a thread calls
+//   census_log_start_write() and is context-switched before calling
+//   census_log_end_write()
+// So, blocks in dirty_block_list are ordered, from oldest to newest, by the
+// time when block is detached from the core.
+//
+// census_log_read_next() first iterates over dirty_block_list and then
+// core_local_blocks[]. It moves completely read blocks from dirty_block_list
+// to free_block_list. Blocks in core_local_blocks[] are not freed, even when
+// completely read.
+//
+// If the log is configured to discard old records and free_block_list is empty,
+// census_log_start_write() iterates over dirty_block_list to allocate a
+// new block. It moves the oldest available block (no pending read/write) to
+// core_local_blocks[].
+//
+// core_local_block_struct is used to implement a map from core id to the block
+// associated with that core. This mapping is advisory. It is possible that the
+// block returned by this mapping is no longer associated with that core. This
+// mapping is updated, lazily, by census_log_start_write().
+//
+// Locking in block struct:
+//
+// Exclusive g_log.lock must be held before calling any functions operating on
+// block structs except census_log_start_write() and census_log_end_write().
+//
+// Writes to a block are serialized via writer_lock. census_log_start_write()
+// acquires this lock and census_log_end_write() releases it. On failure to
+// acquire the lock, writer allocates a new block for the current core and
+// updates core_local_block accordingly.
+//
+// Simultaneous read and write access is allowed. Readers can safely read up to
+// committed bytes (bytes_committed).
+//
+// reader_lock protects the block, currently being read, from getting recycled.
+// start_read() acquires reader_lock and end_read() releases the lock.
+//
+// Read/write access to a block is disabled via try_disable_access(). It returns
+// with both writer_lock and reader_lock held. These locks are subsequently
+// released by enable_access() to enable access to the block.
+//
+// A note on naming: Most function/struct names are prepended by cl_
+// (shorthand for census_log). Further, functions that manipulate structures
+// include the name of the structure, which will be passed as the first
+// argument. E.g. cl_block_initialize() will initialize a cl_block.
+
+#include "src/core/lib/census/mlog.h"
+#include <grpc/support/alloc.h>
+#include <grpc/support/atm.h>
+#include <grpc/support/cpu.h>
+#include <grpc/support/log.h>
+#include <grpc/support/sync.h>
+#include <grpc/support/useful.h>
+#include <stdbool.h>
+#include <string.h>
+
+// End of platform specific code
+
+typedef struct census_log_block_list_struct {
+  struct census_log_block_list_struct* next;
+  struct census_log_block_list_struct* prev;
+  struct census_log_block* block;
+} cl_block_list_struct;
+
+typedef struct census_log_block {
+  // Pointer to underlying buffer.
+  char* buffer;
+  gpr_atm writer_lock;
+  gpr_atm reader_lock;
+  // Keeps completely written bytes. Declared atomic because accessed
+  // simultaneously by reader and writer.
+  gpr_atm bytes_committed;
+  // Bytes already read.
+  size_t bytes_read;
+  // Links for list.
+  cl_block_list_struct link;
+// We want this structure to be cacheline aligned. We assume the following
+// sizes for the various parts on 32/64bit systems:
+// type                 32b size    64b size
+// char*                   4           8
+// 3x gpr_atm             12          24
+// size_t                  4           8
+// cl_block_list_struct   12          24
+// TOTAL                  32          64
+//
+// Depending on the size of our cacheline and the architecture, we
+// selectively add char buffering to this structure. The size is checked
+// via assert in census_log_initialize().
+#if defined(GPR_ARCH_64)
+#define CL_BLOCK_PAD_SIZE (GPR_CACHELINE_SIZE - 64)
+#else
+#if defined(GPR_ARCH_32)
+#define CL_BLOCK_PAD_SIZE (GPR_CACHELINE_SIZE - 32)
+#else
+#error "Unknown architecture"
+#endif
+#endif
+#if CL_BLOCK_PAD_SIZE > 0
+  char padding[CL_BLOCK_PAD_SIZE];
+#endif
+} cl_block;
+
+// A list of cl_blocks, doubly-linked through cl_block::link.
+typedef struct census_log_block_list {
+  int32_t count;            // Number of items in list.
+  cl_block_list_struct ht;  // head/tail of linked list.
+} cl_block_list;
+
+// Cacheline aligned block pointers to avoid false sharing. Block pointer must
+// be initialized via set_block(), before calling other functions
+typedef struct census_log_core_local_block {
+  gpr_atm block;
+// Ensure cachline alignment: we assume sizeof(gpr_atm) == 4 or 8
+#if defined(GPR_ARCH_64)
+#define CL_CORE_LOCAL_BLOCK_PAD_SIZE (GPR_CACHELINE_SIZE - 8)
+#else
+#if defined(GPR_ARCH_32)
+#define CL_CORE_LOCAL_BLOCK_PAD_SIZE (GPR_CACHELINE_SIZE - 4)
+#else
+#error "Unknown architecture"
+#endif
+#endif
+#if CL_CORE_LOCAL_BLOCK_PAD_SIZE > 0
+  char padding[CL_CORE_LOCAL_BLOCK_PAD_SIZE];
+#endif
+} cl_core_local_block;
+
+struct census_log {
+  int discard_old_records;
+  // Number of cores (aka hardware-contexts)
+  unsigned num_cores;
+  // number of CENSUS_LOG_2_MAX_RECORD_SIZE blocks in log
+  uint32_t num_blocks;
+  cl_block* blocks;                        // Block metadata.
+  cl_core_local_block* core_local_blocks;  // Keeps core to block mappings.
+  gpr_mu lock;
+  int initialized;  // has log been initialized?
+  // Keeps the state of the reader iterator. A value of 0 indicates that
+  // iterator has reached the end. census_log_init_reader() resets the value
+  // to num_core to restart iteration.
+  uint32_t read_iterator_state;
+  // Points to the block being read. If non-NULL, the block is locked for
+  // reading(block_being_read_->reader_lock is held).
+  cl_block* block_being_read;
+  char* buffer;
+  cl_block_list free_block_list;
+  cl_block_list dirty_block_list;
+  gpr_atm out_of_space_count;
+};
+
+// Single internal log.
+static struct census_log g_log;
+
+// Functions that operate on an atomic memory location used as a lock.
+
+// Returns non-zero if lock is acquired.
+static int cl_try_lock(gpr_atm* lock) { return gpr_atm_acq_cas(lock, 0, 1); }
+
+static void cl_unlock(gpr_atm* lock) { gpr_atm_rel_store(lock, 0); }
+
+// Functions that operate on cl_core_local_block's.
+
+static void cl_core_local_block_set_block(cl_core_local_block* clb,
+                                          cl_block* block) {
+  gpr_atm_rel_store(&clb->block, (gpr_atm)block);
+}
+
+static cl_block* cl_core_local_block_get_block(cl_core_local_block* clb) {
+  return (cl_block*)gpr_atm_acq_load(&clb->block);
+}
+
+// Functions that operate on cl_block_list_struct's.
+
+static void cl_block_list_struct_initialize(cl_block_list_struct* bls,
+                                            cl_block* block) {
+  bls->next = bls->prev = bls;
+  bls->block = block;
+}
+
+// Functions that operate on cl_block_list's.
+
+static void cl_block_list_initialize(cl_block_list* list) {
+  list->count = 0;
+  cl_block_list_struct_initialize(&list->ht, NULL);
+}
+
+// Returns head of *this, or NULL if empty.
+static cl_block* cl_block_list_head(cl_block_list* list) {
+  return list->ht.next->block;
+}
+
+// Insert element *e after *pos.
+static void cl_block_list_insert(cl_block_list* list, cl_block_list_struct* pos,
+                                 cl_block_list_struct* e) {
+  list->count++;
+  e->next = pos->next;
+  e->prev = pos;
+  e->next->prev = e;
+  e->prev->next = e;
+}
+
+// Insert block at the head of the list
+static void cl_block_list_insert_at_head(cl_block_list* list, cl_block* block) {
+  cl_block_list_insert(list, &list->ht, &block->link);
+}
+
+// Insert block at the tail of the list.
+static void cl_block_list_insert_at_tail(cl_block_list* list, cl_block* block) {
+  cl_block_list_insert(list, list->ht.prev, &block->link);
+}
+
+// Removes block *b. Requires *b be in the list.
+static void cl_block_list_remove(cl_block_list* list, cl_block* b) {
+  list->count--;
+  b->link.next->prev = b->link.prev;
+  b->link.prev->next = b->link.next;
+}
+
+// Functions that operate on cl_block's
+
+static void cl_block_initialize(cl_block* block, char* buffer) {
+  block->buffer = buffer;
+  gpr_atm_rel_store(&block->writer_lock, 0);
+  gpr_atm_rel_store(&block->reader_lock, 0);
+  gpr_atm_rel_store(&block->bytes_committed, 0);
+  block->bytes_read = 0;
+  cl_block_list_struct_initialize(&block->link, block);
+}
+
+// Guards against exposing partially written buffer to the reader.
+static void cl_block_set_bytes_committed(cl_block* block,
+                                         size_t bytes_committed) {
+  gpr_atm_rel_store(&block->bytes_committed, (gpr_atm)bytes_committed);
+}
+
+static size_t cl_block_get_bytes_committed(cl_block* block) {
+  return (size_t)gpr_atm_acq_load(&block->bytes_committed);
+}
+
+// Tries to disable future read/write access to this block. Succeeds if:
+// - no in-progress write AND
+// - no in-progress read AND
+// - 'discard_data' set to true OR no unread data
+// On success, clears the block state and returns with writer_lock_ and
+// reader_lock_ held. These locks are released by a subsequent
+// cl_block_access_enable() call.
+static bool cl_block_try_disable_access(cl_block* block, int discard_data) {
+  if (!cl_try_lock(&block->writer_lock)) {
+    return false;
+  }
+  if (!cl_try_lock(&block->reader_lock)) {
+    cl_unlock(&block->writer_lock);
+    return false;
+  }
+  if (!discard_data &&
+      (block->bytes_read != cl_block_get_bytes_committed(block))) {
+    cl_unlock(&block->reader_lock);
+    cl_unlock(&block->writer_lock);
+    return false;
+  }
+  cl_block_set_bytes_committed(block, 0);
+  block->bytes_read = 0;
+  return true;
+}
+
+static void cl_block_enable_access(cl_block* block) {
+  cl_unlock(&block->reader_lock);
+  cl_unlock(&block->writer_lock);
+}
+
+// Returns with writer_lock held.
+static void* cl_block_start_write(cl_block* block, size_t size) {
+  if (!cl_try_lock(&block->writer_lock)) {
+    return NULL;
+  }
+  size_t bytes_committed = cl_block_get_bytes_committed(block);
+  if (bytes_committed + size > CENSUS_LOG_MAX_RECORD_SIZE) {
+    cl_unlock(&block->writer_lock);
+    return NULL;
+  }
+  return block->buffer + bytes_committed;
+}
+
+// Releases writer_lock and increments committed bytes by 'bytes_written'.
+// 'bytes_written' must be <= 'size' specified in the corresponding
+// StartWrite() call. This function is thread-safe.
+static void cl_block_end_write(cl_block* block, size_t bytes_written) {
+  cl_block_set_bytes_committed(
+      block, cl_block_get_bytes_committed(block) + bytes_written);
+  cl_unlock(&block->writer_lock);
+}
+
+// Returns a pointer to the first unread byte in buffer. The number of bytes
+// available are returned in 'bytes_available'. Acquires reader lock that is
+// released by a subsequent cl_block_end_read() call. Returns NULL if:
+// - read in progress
+// - no data available
+static void* cl_block_start_read(cl_block* block, size_t* bytes_available) {
+  if (!cl_try_lock(&block->reader_lock)) {
+    return NULL;
+  }
+  // bytes_committed may change from under us. Use bytes_available to update
+  // bytes_read below.
+  size_t bytes_committed = cl_block_get_bytes_committed(block);
+  GPR_ASSERT(bytes_committed >= block->bytes_read);
+  *bytes_available = bytes_committed - block->bytes_read;
+  if (*bytes_available == 0) {
+    cl_unlock(&block->reader_lock);
+    return NULL;
+  }
+  void* record = block->buffer + block->bytes_read;
+  block->bytes_read += *bytes_available;
+  return record;
+}
+
+static void cl_block_end_read(cl_block* block) {
+  cl_unlock(&block->reader_lock);
+}
+
+// Internal functions operating on g_log
+
+// Allocates a new free block (or recycles an available dirty block if log is
+// configured to discard old records). Returns NULL if out-of-space.
+static cl_block* cl_allocate_block(void) {
+  cl_block* block = cl_block_list_head(&g_log.free_block_list);
+  if (block != NULL) {
+    cl_block_list_remove(&g_log.free_block_list, block);
+    return block;
+  }
+  if (!g_log.discard_old_records) {
+    // No free block and log is configured to keep old records.
+    return NULL;
+  }
+  // Recycle dirty block. Start from the oldest.
+  for (block = cl_block_list_head(&g_log.dirty_block_list); block != NULL;
+       block = block->link.next->block) {
+    if (cl_block_try_disable_access(block, 1 /* discard data */)) {
+      cl_block_list_remove(&g_log.dirty_block_list, block);
+      return block;
+    }
+  }
+  return NULL;
+}
+
+// Allocates a new block and updates core id => block mapping. 'old_block'
+// points to the block that the caller thinks is attached to
+// 'core_id'. 'old_block' may be NULL. Returns true if:
+// - allocated a new block OR
+// - 'core_id' => 'old_block' mapping changed (another thread allocated a
+//   block before lock was acquired).
+static bool cl_allocate_core_local_block(uint32_t core_id,
+                                         cl_block* old_block) {
+  // Now that we have the lock, check if core-local mapping has changed.
+  cl_core_local_block* core_local_block = &g_log.core_local_blocks[core_id];
+  cl_block* block = cl_core_local_block_get_block(core_local_block);
+  if ((block != NULL) && (block != old_block)) {
+    return true;
+  }
+  if (block != NULL) {
+    cl_core_local_block_set_block(core_local_block, NULL);
+    cl_block_list_insert_at_tail(&g_log.dirty_block_list, block);
+  }
+  block = cl_allocate_block();
+  if (block == NULL) {
+    return false;
+  }
+  cl_core_local_block_set_block(core_local_block, block);
+  cl_block_enable_access(block);
+  return true;
+}
+
+static cl_block* cl_get_block(void* record) {
+  uintptr_t p = (uintptr_t)((char*)record - g_log.buffer);
+  uintptr_t index = p >> CENSUS_LOG_2_MAX_RECORD_SIZE;
+  return &g_log.blocks[index];
+}
+
+// Gets the next block to read and tries to free 'prev' block (if not NULL).
+// Returns NULL if reached the end.
+static cl_block* cl_next_block_to_read(cl_block* prev) {
+  cl_block* block = NULL;
+  if (g_log.read_iterator_state == g_log.num_cores) {
+    // We are traversing dirty list; find the next dirty block.
+    if (prev != NULL) {
+      // Try to free the previous block if there is no unread data. This
+      // block
+      // may have unread data if previously incomplete record completed
+      // between
+      // read_next() calls.
+      block = prev->link.next->block;
+      if (cl_block_try_disable_access(prev, 0 /* do not discard data */)) {
+        cl_block_list_remove(&g_log.dirty_block_list, prev);
+        cl_block_list_insert_at_head(&g_log.free_block_list, prev);
+      }
+    } else {
+      block = cl_block_list_head(&g_log.dirty_block_list);
+    }
+    if (block != NULL) {
+      return block;
+    }
+    // We are done with the dirty list; moving on to core-local blocks.
+  }
+  while (g_log.read_iterator_state > 0) {
+    g_log.read_iterator_state--;
+    block = cl_core_local_block_get_block(
+        &g_log.core_local_blocks[g_log.read_iterator_state]);
+    if (block != NULL) {
+      return block;
+    }
+  }
+  return NULL;
+}
+
+#define CL_LOG_2_MB 20  // 2^20 = 1MB
+
+// External functions: primary stats_log interface
+void census_log_initialize(size_t size_in_mb, int discard_old_records) {
+  // Check cacheline alignment.
+  GPR_ASSERT(sizeof(cl_block) % GPR_CACHELINE_SIZE == 0);
+  GPR_ASSERT(sizeof(cl_core_local_block) % GPR_CACHELINE_SIZE == 0);
+  GPR_ASSERT(!g_log.initialized);
+  g_log.discard_old_records = discard_old_records;
+  g_log.num_cores = gpr_cpu_num_cores();
+  // Ensure that we will not get any overflow in calaculating num_blocks
+  GPR_ASSERT(CL_LOG_2_MB >= CENSUS_LOG_2_MAX_RECORD_SIZE);
+  GPR_ASSERT(size_in_mb < 1000);
+  // Ensure at least 2x as many blocks as there are cores.
+  g_log.num_blocks =
+      (uint32_t)GPR_MAX(2 * g_log.num_cores, (size_in_mb << CL_LOG_2_MB) >>
+                                                 CENSUS_LOG_2_MAX_RECORD_SIZE);
+  gpr_mu_init(&g_log.lock);
+  g_log.read_iterator_state = 0;
+  g_log.block_being_read = NULL;
+  g_log.core_local_blocks = (cl_core_local_block*)gpr_malloc_aligned(
+      g_log.num_cores * sizeof(cl_core_local_block), GPR_CACHELINE_SIZE_LOG);
+  memset(g_log.core_local_blocks, 0,
+         g_log.num_cores * sizeof(cl_core_local_block));
+  g_log.blocks = (cl_block*)gpr_malloc_aligned(
+      g_log.num_blocks * sizeof(cl_block), GPR_CACHELINE_SIZE_LOG);
+  memset(g_log.blocks, 0, g_log.num_blocks * sizeof(cl_block));
+  g_log.buffer = gpr_malloc(g_log.num_blocks * CENSUS_LOG_MAX_RECORD_SIZE);
+  memset(g_log.buffer, 0, g_log.num_blocks * CENSUS_LOG_MAX_RECORD_SIZE);
+  cl_block_list_initialize(&g_log.free_block_list);
+  cl_block_list_initialize(&g_log.dirty_block_list);
+  for (uint32_t i = 0; i < g_log.num_blocks; ++i) {
+    cl_block* block = g_log.blocks + i;
+    cl_block_initialize(block, g_log.buffer + (CENSUS_LOG_MAX_RECORD_SIZE * i));
+    cl_block_try_disable_access(block, 1 /* discard data */);
+    cl_block_list_insert_at_tail(&g_log.free_block_list, block);
+  }
+  gpr_atm_rel_store(&g_log.out_of_space_count, 0);
+  g_log.initialized = 1;
+}
+
+void census_log_shutdown(void) {
+  GPR_ASSERT(g_log.initialized);
+  gpr_mu_destroy(&g_log.lock);
+  gpr_free_aligned(g_log.core_local_blocks);
+  g_log.core_local_blocks = NULL;
+  gpr_free_aligned(g_log.blocks);
+  g_log.blocks = NULL;
+  gpr_free(g_log.buffer);
+  g_log.buffer = NULL;
+  g_log.initialized = 0;
+}
+
+void* census_log_start_write(size_t size) {
+  // Used to bound number of times block allocation is attempted.
+  GPR_ASSERT(size > 0);
+  GPR_ASSERT(g_log.initialized);
+  if (size > CENSUS_LOG_MAX_RECORD_SIZE) {
+    return NULL;
+  }
+  uint32_t attempts_remaining = g_log.num_blocks;
+  uint32_t core_id = gpr_cpu_current_cpu();
+  do {
+    void* record = NULL;
+    cl_block* block =
+        cl_core_local_block_get_block(&g_log.core_local_blocks[core_id]);
+    if (block && (record = cl_block_start_write(block, size))) {
+      return record;
+    }
+    // Need to allocate a new block. We are here if:
+    // - No block associated with the core OR
+    // - Write in-progress on the block OR
+    // - block is out of space
+    gpr_mu_lock(&g_log.lock);
+    bool allocated = cl_allocate_core_local_block(core_id, block);
+    gpr_mu_unlock(&g_log.lock);
+    if (!allocated) {
+      gpr_atm_no_barrier_fetch_add(&g_log.out_of_space_count, 1);
+      return NULL;
+    }
+  } while (attempts_remaining--);
+  // Give up.
+  gpr_atm_no_barrier_fetch_add(&g_log.out_of_space_count, 1);
+  return NULL;
+}
+
+void census_log_end_write(void* record, size_t bytes_written) {
+  GPR_ASSERT(g_log.initialized);
+  cl_block_end_write(cl_get_block(record), bytes_written);
+}
+
+void census_log_init_reader(void) {
+  GPR_ASSERT(g_log.initialized);
+  gpr_mu_lock(&g_log.lock);
+  // If a block is locked for reading unlock it.
+  if (g_log.block_being_read != NULL) {
+    cl_block_end_read(g_log.block_being_read);
+    g_log.block_being_read = NULL;
+  }
+  g_log.read_iterator_state = g_log.num_cores;
+  gpr_mu_unlock(&g_log.lock);
+}
+
+const void* census_log_read_next(size_t* bytes_available) {
+  GPR_ASSERT(g_log.initialized);
+  gpr_mu_lock(&g_log.lock);
+  if (g_log.block_being_read != NULL) {
+    cl_block_end_read(g_log.block_being_read);
+  }
+  do {
+    g_log.block_being_read = cl_next_block_to_read(g_log.block_being_read);
+    if (g_log.block_being_read != NULL) {
+      void* record =
+          cl_block_start_read(g_log.block_being_read, bytes_available);
+      if (record != NULL) {
+        gpr_mu_unlock(&g_log.lock);
+        return record;
+      }
+    }
+  } while (g_log.block_being_read != NULL);
+  gpr_mu_unlock(&g_log.lock);
+  return NULL;
+}
+
+size_t census_log_remaining_space(void) {
+  GPR_ASSERT(g_log.initialized);
+  size_t space = 0;
+  gpr_mu_lock(&g_log.lock);
+  if (g_log.discard_old_records) {
+    // Remaining space is not meaningful; just return the entire log space.
+    space = g_log.num_blocks << CENSUS_LOG_2_MAX_RECORD_SIZE;
+  } else {
+    GPR_ASSERT(g_log.free_block_list.count >= 0);
+    space = (size_t)g_log.free_block_list.count * CENSUS_LOG_MAX_RECORD_SIZE;
+  }
+  gpr_mu_unlock(&g_log.lock);
+  return space;
+}
+
+int64_t census_log_out_of_space_count(void) {
+  GPR_ASSERT(g_log.initialized);
+  return gpr_atm_acq_load(&g_log.out_of_space_count);
+}
diff --git a/src/core/lib/census/mlog.h b/src/core/lib/census/mlog.h
new file mode 100644
index 0000000000..7fbdeda986
--- /dev/null
+++ b/src/core/lib/census/mlog.h
@@ -0,0 +1,95 @@
+/*
+ *
+ * Copyright 2015-2016, Google Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+/* A very fast in-memory log, optimized for multiple writers. */
+
+#ifndef GRPC_CORE_LIB_CENSUS_MLOG_H
+#define GRPC_CORE_LIB_CENSUS_MLOG_H
+
+#include <grpc/support/port_platform.h>
+#include <stddef.h>
+
+/* Maximum record size, in bytes. */
+#define CENSUS_LOG_2_MAX_RECORD_SIZE 14 /* 2^14 = 16KB */
+#define CENSUS_LOG_MAX_RECORD_SIZE (1 << CENSUS_LOG_2_MAX_RECORD_SIZE)
+
+/* Initialize the statistics logging subsystem with the given log size. A log
+   size of 0 will result in the smallest possible log for the platform
+   (approximately CENSUS_LOG_MAX_RECORD_SIZE * gpr_cpu_num_cores()). If
+   discard_old_records is non-zero, then new records will displace older ones
+   when the log is full. This function must be called before any other
+   census_log functions.
+*/
+void census_log_initialize(size_t size_in_mb, int discard_old_records);
+
+/* Shutdown the logging subsystem. Caller must ensure that:
+   - no in progress or future call to any census_log functions
+   - no incomplete records
+*/
+void census_log_shutdown(void);
+
+/* Allocates and returns a 'size' bytes record and marks it in use. A
+   subsequent census_log_end_write() marks the record complete. The
+   'bytes_written' census_log_end_write() argument must be <=
+   'size'. Returns NULL if out-of-space AND:
+       - log is configured to keep old records OR
+       - all blocks are pinned by incomplete records.
+*/
+void* census_log_start_write(size_t size);
+
+void census_log_end_write(void* record, size_t bytes_written);
+
+void census_log_init_reader(void);
+
+/* census_log_read_next() iterates over blocks with data and for each block
+   returns a pointer to the first unread byte. The number of bytes that can be
+   read are returned in 'bytes_available'. Reader is expected to read all
+   available data. Reading the data consumes it i.e. it cannot be read again.
+   census_log_read_next() returns NULL if the end is reached i.e last block
+   is read. census_log_init_reader() starts the iteration or aborts the
+   current iteration.
+*/
+const void* census_log_read_next(size_t* bytes_available);
+
+/* Returns estimated remaining space across all blocks, in bytes. If log is
+   configured to discard old records, returns total log space. Otherwise,
+   returns space available in empty blocks (partially filled blocks are
+   treated as full).
+*/
+size_t census_log_remaining_space(void);
+
+/* Returns the number of times gprc_stats_log_start_write() failed due to
+   out-of-space. */
+int64_t census_log_out_of_space_count(void);
+
+#endif /* GRPC_CORE_LIB_CENSUS_MLOG_H */
diff --git a/src/core/lib/census/operation.c b/src/core/lib/census/operation.c
new file mode 100644
index 0000000000..315f9c3534
--- /dev/null
+++ b/src/core/lib/census/operation.c
@@ -0,0 +1,63 @@
+/*
+ * Copyright 2015-2016, Google Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+#include <grpc/census.h>
+
+/* TODO(aveitch): These are all placeholder implementations. */
+
+census_timestamp census_start_rpc_op_timestamp(void) {
+  census_timestamp ct;
+  /* TODO(aveitch): assumes gpr_timespec implementation of census_timestamp. */
+  ct.ts = gpr_now(GPR_CLOCK_MONOTONIC);
+  return ct;
+}
+
+census_context *census_start_client_rpc_op(
+    const census_context *context, int64_t rpc_name_id,
+    const census_rpc_name_info *rpc_name_info, const char *peer, int trace_mask,
+    const census_timestamp *start_time) {
+  return NULL;
+}
+
+census_context *census_start_server_rpc_op(
+    const char *buffer, int64_t rpc_name_id,
+    const census_rpc_name_info *rpc_name_info, const char *peer, int trace_mask,
+    census_timestamp *start_time) {
+  return NULL;
+}
+
+census_context *census_start_op(census_context *context, const char *family,
+                                const char *name, int trace_mask) {
+  return NULL;
+}
+
+void census_end_op(census_context *context, int status) {}
diff --git a/src/core/lib/census/placeholders.c b/src/core/lib/census/placeholders.c
new file mode 100644
index 0000000000..fe23d13971
--- /dev/null
+++ b/src/core/lib/census/placeholders.c
@@ -0,0 +1,109 @@
+/*
+ *
+ * Copyright 2016, Google Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+#include <grpc/census.h>
+
+#include <grpc/support/log.h>
+
+/* Placeholders for the pending APIs */
+
+int census_get_trace_record(census_trace_record *trace_record) {
+  (void)trace_record;
+  abort();
+}
+
+void census_record_values(census_context *context, census_value *values,
+                          size_t nvalues) {
+  (void)context;
+  (void)values;
+  (void)nvalues;
+  abort();
+}
+
+void census_set_rpc_client_peer(census_context *context, const char *peer) {
+  (void)context;
+  (void)peer;
+  abort();
+}
+
+void census_trace_scan_end() { abort(); }
+
+int census_trace_scan_start(int consume) {
+  (void)consume;
+  abort();
+}
+
+const census_aggregation *census_view_aggregrations(const census_view *view) {
+  (void)view;
+  abort();
+}
+
+census_view *census_view_create(uint32_t metric_id, const census_context *tags,
+                                const census_aggregation *aggregations,
+                                size_t naggregations) {
+  (void)metric_id;
+  (void)tags;
+  (void)aggregations;
+  (void)naggregations;
+  abort();
+}
+
+const census_context *census_view_tags(const census_view *view) {
+  (void)view;
+  abort();
+}
+
+void census_view_delete(census_view *view) {
+  (void)view;
+  abort();
+}
+
+const census_view_data *census_view_get_data(const census_view *view) {
+  (void)view;
+  abort();
+}
+
+size_t census_view_metric(const census_view *view) {
+  (void)view;
+  abort();
+}
+
+size_t census_view_naggregations(const census_view *view) {
+  (void)view;
+  abort();
+}
+
+void census_view_reset(census_view *view) {
+  (void)view;
+  abort();
+}
diff --git a/src/core/lib/census/rpc_metric_id.h b/src/core/lib/census/rpc_metric_id.h
new file mode 100644
index 0000000000..aad0588fb3
--- /dev/null
+++ b/src/core/lib/census/rpc_metric_id.h
@@ -0,0 +1,51 @@
+/*
+ *
+ * Copyright 2015-2016, Google Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+#ifndef GRPC_CORE_LIB_CENSUS_RPC_METRIC_ID_H
+#define GRPC_CORE_LIB_CENSUS_RPC_METRIC_ID_H
+
+/* Metric ID's used for RPC measurements. */
+/* Count of client requests sent. */
+#define CENSUS_METRIC_RPC_CLIENT_REQUESTS ((uint32_t)0)
+/* Count of server requests sent. */
+#define CENSUS_METRIC_RPC_SERVER_REQUESTS ((uint32_t)1)
+/* Client error counts. */
+#define CENSUS_METRIC_RPC_CLIENT_ERRORS ((uint32_t)2)
+/* Server error counts. */
+#define CENSUS_METRIC_RPC_SERVER_ERRORS ((uint32_t)3)
+/* Client side request latency. */
+#define CENSUS_METRIC_RPC_CLIENT_LATENCY ((uint32_t)4)
+/* Server side request latency. */
+#define CENSUS_METRIC_RPC_SERVER_LATENCY ((uint32_t)5)
+
+#endif /* GRPC_CORE_LIB_CENSUS_RPC_METRIC_ID_H */
diff --git a/src/core/lib/census/tracing.c b/src/core/lib/census/tracing.c
new file mode 100644
index 0000000000..e508996af3
--- /dev/null
+++ b/src/core/lib/census/tracing.c
@@ -0,0 +1,45 @@
+/*
+ *
+ * Copyright 2015-2016, Google Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+#include <grpc/census.h>
+
+/* TODO(aveitch): These are all placeholder implementations. */
+
+int census_trace_mask(const census_context *context) {
+  return CENSUS_TRACE_MASK_NONE;
+}
+
+void census_set_trace_mask(int trace_mask) {}
+
+void census_trace_print(census_context *context, uint32_t type,
+                        const char *buffer, size_t n) {}