Compression Cookbook

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+# gRPC (Core) Compression Cookbook
+
+## Introduction
+
+This document describes compression as implemented by the gRPC C core. See [the
+full compression specification](compression.md) for details.
+
+### Intended Audience
+
+Wrapped languages developers, for the purposes of supporting compression by
+interacting with the C core.
+
+## Criteria for GA readiness
+
+1. Be able to set compression at [channel](#per-channel-settings),
+   [call](#per-call-settings) and [message](#per-message-settings) level.
+   In principle this API should be based on _compression levels_ as opposed to
+   algorithms. See the discussion [below](#level-vs-algorithms).
+1. Have unit tests covering [the cases from the
+   spec](https://github.com/grpc/grpc/blob/master/doc/compression.md#test-cases).
+1. Interop tests implemented and passing on Jenkins. The two relevant interop
+   test cases are
+   [large_compressed_unary](https://github.com/grpc/grpc/blob/master/doc/interop-test-descriptions.md#large_compressed_unary)
+   and
+   [server_compressed_streaming](https://github.com/grpc/grpc/blob/master/doc/interop-test-descriptions.md#server_compressed_streaming).
+
+## Summary Flowcharts
+
+The following flowcharts depict the evolution of a message, both _incoming_ and
+_outgoing_, irrespective of the client/server character of the call. Aspects
+still not symmetric between clients and servers (e.g. the [use of compression
+levels](https://github.com/grpc/grpc/blob/master/doc/compression.md#compression-levels-and-algorithms))
+are explicitly marked. The in-detail textual description for the different
+scenarios is described in subsequent sections.
+
+## Incoming Messages
+
+![image](images/compression_cookbook_incoming.png)
+
+## Outgoing Messages
+
+![image](images/compression_cookbook_outgoing.png)
+
+## Levels vs Algorithms
+
+As mentioned in [the relevant discussion on the spec
+document](https://github.com/grpc/grpc/blob/master/doc/compression.md#compression-levels-and-algorithms),
+compression _levels_ are the primary mechanism for compression selection _at the
+server side_. In the future, it'll also be at the client side. The use of levels
+abstracts away the intricacies of selecting a concrete algorithm supported by a
+peer, on top of removing the burden of choice from the developer.
+As of this writing (Q2 2016), clients can only specify compression _algorithms_.
+Clients will support levels as soon as an automatic retry/negotiation mechanism
+is in place.
+
+## Per Channel Settings
+
+Compression may be configured at channel creation. This is a convenience to
+avoid having to repeatedly configure compression for every call. Note that any
+compression setting on individual [calls](#per-call-settings) or
+[messages](#per-message-settings) overrides channel settings.
+
+The following aspects can be configured at channel-creation time via channel arguments:
+
+#### Disable Compression _Algorithms_
+
+Use the channel argument key
+`GRPC_COMPRESSION_CHANNEL_ENABLED_ALGORITHMS_BITSET` (from
+[`grpc/impl/codegen/compression_types.h`](https://github.com/grpc/grpc/blob/master/include/grpc/impl/codegen/compression_types.h)),
+takes a 32 bit bitset value. A set bit means the algorithm with that enum value
+according to `grpc_compression_algorithm` is _enabled_.
+For example, `GRPC_COMPRESS_GZIP` currently has a numeric value of 2. To
+enable/disable GZIP for a channel, one would set/clear the 3rd LSB (eg, 0b100 =
+0x4). Note that setting/clearing 0th position, that corresponding to
+`GRPC_COMPRESS_NONE`, has no effect, as no-compression (a.k.a. _identity_) is
+always supported.
+Incoming messages compressed (ie, encoded) with a disabled algorithm will result
+in the call being closed with `GRPC_STATUS_UNIMPLEMENTED`.
+
+#### Default Compression _Level_
+
+**(currently, Q2 2016, only applicable for server side channels. It's ignored
+for clients.)**
+Use the channel argument key `GRPC_COMPRESSION_CHANNEL_DEFAULT_LEVEL` (from
+[`grpc/impl/codegen/compression_types.h`](https://github.com/grpc/grpc/blob/master/include/grpc/impl/codegen/compression_types.h)),
+valued by an integer corresponding to a value from the `grpc_compression_level`
+enum.
+
+#### Default Compression _Algorithm_
+
+Use the channel argument key `GRPC_COMPRESSION_CHANNEL_DEFAULT_ALGORITHM` (from
+[`grpc/impl/codegen/compression_types.h`](https://github.com/grpc/grpc/blob/master/include/grpc/impl/codegen/compression_types.h)),
+valued by an integer corresponding to a value from the `grpc_compression_level`
+enum.
+
+## Per Call Settings
+
+### Compression **Level** in Call Responses
+
+The server requests a compression level via initial metadata. The
+`send_initial_metadata` `grpc_op` contains a `maybe_compression_level` field
+with two fields, `is_set` and `compression_level`. The former must be set when
+actively choosing a level to disambiguate the default value of zero (no
+compression) from the proactive selection of no compression.
+
+The core will receive the request for the compression level and automatically
+choose a compression algorithm based on its knowledge about the peer
+(communicated by the client via the `grpc-accept-encoding` header. Note that the
+absence of this header means no compression is supported by the client/peer).
+
+### Compression **Algorithm** in Call Responses
+
+**Server should avoid setting the compression algorithm directly**. Prefer
+setting compression levels unless there's a _very_ compelling reason to choose
+specific algorithms (benchmarking, testing).
+
+Selection of concrete compression algorithms is performed by adding a
+`(GRPC_COMPRESS_REQUEST_ALGORITHM_KEY, <algorithm-name>)` key-value pair to the
+initial metadata, where `GRPC_COMPRESS_REQUEST_ALGORITHM_KEY` is defined in
+[`grpc/impl/codegen/compression_types.h`](https://github.com/grpc/grpc/blob/master/include/grpc/impl/codegen/compression_types.h)),
+and `<algorithm-name>` is the human readable name of the algorithm as given in
+[the HTTP2 spec](https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-HTTP2.md)
+for `Message-Encoding` (e.g. gzip, identity, etc.). See
+[`grpc_compression_algorithm_name`](https://github.com/grpc/grpc/blob/master/src/core/lib/compression/compression.c)
+for the mapping between the `grpc_compression_algorithm` enum values and their
+textual representation.
+
+## Per Message Settings
+
+To disable compression for a specific message, the `flags` field of `grpc_op`
+instances of type `GRPC_OP_SEND_MESSAGE` must have its `GRPC_WRITE_NO_COMPRESS`
+bit set. Refer to
+[`grpc/impl/codegen/compression_types.h`](https://github.com/grpc/grpc/blob/master/include/grpc/impl/codegen/compression_types.h)),