Merge remote-tracking branch 'upstream/master' into add_udp_server_2

33 files changed, 928 insertions, 259 deletions

diff --git a/src/objective-c/GRPCClient/GRPCCall+OAuth2.h b/src/objective-c/GRPCClient/GRPCCall+OAuth2.h
new file mode 100644
index 0000000000..2e379a7157
--- /dev/null
+++ b/src/objective-c/GRPCClient/GRPCCall+OAuth2.h
@@ -0,0 +1,49 @@
+/*
+ *
+ * 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.
+ *
+ */
+
+#import "GRPCCall.h"
+
+// Helpers for setting and reading headers compatible with OAuth2.
+@interface GRPCCall (OAuth2)
+
+// Setting this property is equivalent to setting "Bearer <passed token>" as the value of the
+// request header with key "authorization" (the authorization header). Setting it to nil removes the
+// authorization header from the request.
+// The value obtained by getting the property is the OAuth2 bearer token if the authorization header
+// of the request has the form "Bearer <token>", or nil otherwise.
+@property(atomic, copy) NSString *oauth2AccessToken;
+
+// Returns the value (if any) of the "www-authenticate" response header (the challenge header).
+@property(atomic, readonly) NSString *oauth2ChallengeHeader;
+
+@end
diff --git a/src/objective-c/GRPCClient/GRPCCall+OAuth2.m b/src/objective-c/GRPCClient/GRPCCall+OAuth2.m
new file mode 100644
index 0000000000..83b0de18e3
--- /dev/null
+++ b/src/objective-c/GRPCClient/GRPCCall+OAuth2.m
@@ -0,0 +1,63 @@
+/*
+ *
+ * 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.
+ *
+ */
+
+#import "GRPCCall+OAuth2.h"
+
+static NSString * const kAuthorizationHeader = @"authorization";
+static NSString * const kBearerPrefix = @"Bearer ";
+static NSString * const kChallengeHeader = @"www-authenticate";
+
+@implementation GRPCCall (OAuth2)
+
+- (NSString *)oauth2AccessToken {
+  NSString *headerValue = self.requestHeaders[kAuthorizationHeader];
+  if ([headerValue hasPrefix:kBearerPrefix]) {
+    return [headerValue substringFromIndex:kBearerPrefix.length];
+  } else {
+    return nil;
+  }
+}
+
+- (void)setOauth2AccessToken:(NSString *)token {
+  if (token) {
+    self.requestHeaders[kAuthorizationHeader] = [kBearerPrefix stringByAppendingString:token];
+  } else {
+    [self.requestHeaders removeObjectForKey:kAuthorizationHeader];
+  }
+}
+
+- (NSString *)oauth2ChallengeHeader {
+  return self.responseHeaders[kChallengeHeader];
+}
+
+@end
diff --git a/src/objective-c/GRPCClient/GRPCCall+Tests.h b/src/objective-c/GRPCClient/GRPCCall+Tests.h
new file mode 100644
index 0000000000..cca1614606
--- /dev/null
+++ b/src/objective-c/GRPCClient/GRPCCall+Tests.h
@@ -0,0 +1,54 @@
+/*
+ *
+ * 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.
+ *
+ */
+
+#import "GRPCCall.h"
+
+// Methods to let tune down the security of gRPC connections for specific hosts. These shouldn't be
+// used in releases, but are sometimes needed for testing.
+@interface GRPCCall (Tests)
+
+// Establish all SSL connections to the provided host using the passed SSL target name and the root
+// certificates found in the file at |certsPath|.
+//
+// Must be called before any gRPC call to that host is made. It's illegal to pass the same host to
+// more than one invocation of the methods of this category.
++ (void)useTestCertsPath:(NSString *)certsPath
+                testName:(NSString *)testName
+                 forHost:(NSString *)host;
+
+// Establish all connections to the provided host using cleartext instead of SSL.
+//
+// Must be called before any gRPC call to that host is made. It's illegal to pass the same host to
+// more than one invocation of the methods of this category.
++ (void)useInsecureConnectionsForHost:(NSString *)host;
+@end
diff --git a/src/objective-c/GRPCClient/GRPCCall+Tests.m b/src/objective-c/GRPCClient/GRPCCall+Tests.m
new file mode 100644
index 0000000000..bade0b2920
--- /dev/null
+++ b/src/objective-c/GRPCClient/GRPCCall+Tests.m
@@ -0,0 +1,53 @@
+/*
+ *
+ * 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.
+ *
+ */
+
+#import "GRPCCall+Tests.h"
+
+#import "private/GRPCHost.h"
+
+@implementation GRPCCall (Tests)
+
++ (void)useTestCertsPath:(NSString *)certsPath
+                testName:(NSString *)testName
+                 forHost:(NSString *)host {
+  GRPCHost *hostConfig = [GRPCHost hostWithAddress:host];
+  hostConfig.pathToCertificates = certsPath;
+  hostConfig.hostNameOverride = testName;
+}
+
++ (void)useInsecureConnectionsForHost:(NSString *)host {
+  GRPCHost *hostConfig = [GRPCHost hostWithAddress:host];
+  hostConfig.secure = NO;
+}
+
+@end
diff --git a/src/objective-c/GRPCClient/GRPCCall.h b/src/objective-c/GRPCClient/GRPCCall.h
index 4a8b7fff48..4eda499b1a 100644
--- a/src/objective-c/GRPCClient/GRPCCall.h
+++ b/src/objective-c/GRPCClient/GRPCCall.h
@@ -48,8 +48,10 @@
 #import <Foundation/Foundation.h>
 #import <RxLibrary/GRXWriter.h>
 
-// Key used in |NSError|'s |userInfo| dictionary to store the response metadata sent by the server.
-extern id const kGRPCStatusMetadataKey;
+// Keys used in |NSError|'s |userInfo| dictionary to store the response headers and trailers sent by
+// the server.
+extern id const kGRPCHeadersKey;
+extern id const kGRPCTrailersKey;
 
 // Represents a single gRPC remote call.
 @interface GRPCCall : GRXWriter
@@ -57,43 +59,49 @@ extern id const kGRPCStatusMetadataKey;
 // These HTTP headers will be passed to the server as part of this call. Each HTTP header is a
 // name-value pair with string names and either string or binary values.
 //
-// The passed dictionary has to use NSString keys, corresponding to the header names. The
-// value associated to each can be a NSString object or a NSData object. E.g.:
+// The passed dictionary has to use NSString keys, corresponding to the header names. The value
+// associated to each can be a NSString object or a NSData object. E.g.:
 //
-// call.requestMetadata = @{@"Authorization": @"Bearer ..."};
+// call.requestHeaders = @{@"authorization": @"Bearer ..."};
 //
-// call.requestMetadata[@"SomeBinaryHeader"] = someData;
+// call.requestHeaders[@"my-header-bin"] = someData;
 //
-// After the call is started, modifying this won't have any effect.
+// After the call is started, trying to modify this property is an error.
 //
 // For convenience, the property is initialized to an empty NSMutableDictionary, and the setter
 // accepts (and copies) both mutable and immutable dictionaries.
-- (NSMutableDictionary *)requestMetadata; // nonatomic
-- (void)setRequestMetadata:(NSDictionary *)requestMetadata; // nonatomic, copy
+- (NSMutableDictionary *)requestHeaders; // nonatomic
+- (void)setRequestHeaders:(NSDictionary *)requestHeaders; // nonatomic, copy
 
-// This dictionary is populated with the HTTP headers received from the server. When the RPC ends,
-// the HTTP trailers received are added to the dictionary too. It has the same structure as the
-// request metadata dictionary.
+// This dictionary is populated with the HTTP headers received from the server. This happens before
+// any response message is received from the server. It has the same structure as the request
+// headers dictionary: Keys are NSString header names; names ending with the suffix "-bin" have a
+// NSData value; the others have a NSString value.
 //
-// The first time this object calls |writeValue| on the writeable passed to |startWithWriteable|,
-// the |responseMetadata| dictionary already contains the response headers. When it calls
-// |writesFinishedWithError|, the dictionary contains both the response headers and trailers.
-@property(atomic, readonly) NSDictionary *responseMetadata;
+// The value of this property is nil until all response headers are received, and will change before
+// any of -writeValue: or -writesFinishedWithError: are sent to the writeable.
+@property(atomic, readonly) NSDictionary *responseHeaders;
+
+// Same as responseHeaders, but populated with the HTTP trailers received from the server before the
+// call finishes.
+//
+// The value of this property is nil until all response trailers are received, and will change
+// before -writesFinishedWithError: is sent to the writeable.
+@property(atomic, readonly) NSDictionary *responseTrailers;
 
 // The request writer has to write NSData objects into the provided Writeable. The server will
-// receive each of those separately and in order.
-// A gRPC call might not complete until the request writer finishes. On the other hand, the
-// request finishing doesn't necessarily make the call to finish, as the server might continue
-// sending messages to the response side of the call indefinitely (depending on the semantics of
-// the specific remote method called).
+// receive each of those separately and in order as distinct messages.
+// A gRPC call might not complete until the request writer finishes. On the other hand, the request
+// finishing doesn't necessarily make the call to finish, as the server might continue sending
+// messages to the response side of the call indefinitely (depending on the semantics of the
+// specific remote method called).
 // To finish a call right away, invoke cancel.
 - (instancetype)initWithHost:(NSString *)host
                         path:(NSString *)path
               requestsWriter:(GRXWriter *)requestsWriter NS_DESIGNATED_INITIALIZER;
 
-// Finishes the request side of this call, notifies the server that the RPC
-// should be cancelled, and finishes the response side of the call with an error
-// of code CANCELED.
+// Finishes the request side of this call, notifies the server that the RPC should be cancelled, and
+// finishes the response side of the call with an error of code CANCELED.
 - (void)cancel;
 
 // TODO(jcanizales): Let specify a deadline. As a category of GRXWriter?
diff --git a/src/objective-c/GRPCClient/GRPCCall.m b/src/objective-c/GRPCClient/GRPCCall.m
index 9d9648ae28..ff5d1c5aaf 100644
--- a/src/objective-c/GRPCClient/GRPCCall.m
+++ b/src/objective-c/GRPCClient/GRPCCall.m
@@ -37,15 +37,18 @@
 #include <grpc/support/time.h>
 #import <RxLibrary/GRXConcurrentWriteable.h>
 
-#import "private/GRPCChannel.h"
 #import "private/GRPCWrappedCall.h"
 #import "private/NSData+GRPC.h"
 #import "private/NSDictionary+GRPC.h"
 #import "private/NSError+GRPC.h"
 
-NSString * const kGRPCStatusMetadataKey = @"io.grpc.StatusMetadataKey";
+NSString * const kGRPCHeadersKey = @"io.grpc.HeadersKey";
+NSString * const kGRPCTrailersKey = @"io.grpc.TrailersKey";
 
 @interface GRPCCall () <GRXWriteable>
+// Make them read-write.
+@property(atomic, strong) NSDictionary *responseHeaders;
+@property(atomic, strong) NSDictionary *responseTrailers;
 @end
 
 // The following methods of a C gRPC call object aren't reentrant, and thus
@@ -70,21 +73,27 @@ NSString * const kGRPCStatusMetadataKey = @"io.grpc.StatusMetadataKey";
   GRPCWrappedCall *_wrappedCall;
   dispatch_once_t _callAlreadyInvoked;
 
-  GRPCChannel *_channel;
-
   // The C gRPC library has less guarantees on the ordering of events than we
   // do. Particularly, in the face of errors, there's no ordering guarantee at
   // all. This wrapper over our actual writeable ensures thread-safety and
   // correct ordering.
   GRXConcurrentWriteable *_responseWriteable;
+
+  // The network thread wants the requestWriter to resume (when the server is ready for more input),
+  // or to stop (on errors), concurrently with user threads that want to start it, pause it or stop
+  // it. Because a writer isn't thread-safe, we'll synchronize those operations on it.
+  // We don't use a dispatch queue for that purpose, because the writer can call writeValue: or
+  // writesFinishedWithError: on this GRPCCall as part of those operations. We want to be able to
+  // pause the writer immediately on writeValue:, so we need our locking to be recursive.
   GRXWriter *_requestWriter;
 
   // To create a retain cycle when a call is started, up until it finishes. See
-  // |startWithWriteable:| and |finishWithError:|.
-  GRPCCall *_self;
+  // |startWithWriteable:| and |finishWithError:|. This saves users from having to retain a
+  // reference to the call object if all they're interested in is the handler being executed when
+  // the response arrives.
+  GRPCCall *_retainSelf;
 
-  NSMutableDictionary *_requestMetadata;
-  NSMutableDictionary *_responseMetadata;
+  NSMutableDictionary *_requestHeaders;
 }
 
 @synthesize state = _state;
@@ -105,46 +114,41 @@ NSString * const kGRPCStatusMetadataKey = @"io.grpc.StatusMetadataKey";
                 format:@"The requests writer can't be already started."];
   }
   if ((self = [super init])) {
-    _channel = [GRPCChannel channelToHost:host];
-
-    _wrappedCall = [[GRPCWrappedCall alloc] initWithChannel:_channel
-                                                       path:path
-                                                       host:host];
+    _wrappedCall = [[GRPCWrappedCall alloc] initWithHost:host path:path];
+    if (!_wrappedCall) {
+      return nil;
+    }
 
     // Serial queue to invoke the non-reentrant methods of the grpc_call object.
     _callQueue = dispatch_queue_create("org.grpc.call", NULL);
 
     _requestWriter = requestWriter;
 
-    _requestMetadata = [NSMutableDictionary dictionary];
-    _responseMetadata = [NSMutableDictionary dictionary];
+    _requestHeaders = [NSMutableDictionary dictionary];
   }
   return self;
 }
 
 #pragma mark Metadata
 
-- (NSMutableDictionary *)requestMetadata {
-  return _requestMetadata;
-}
-
-- (void)setRequestMetadata:(NSDictionary *)requestMetadata {
-  _requestMetadata = [NSMutableDictionary dictionaryWithDictionary:requestMetadata];
+- (NSMutableDictionary *)requestHeaders {
+  return _requestHeaders;
 }
 
-- (NSDictionary *)responseMetadata {
-  return _responseMetadata;
+- (void)setRequestHeaders:(NSDictionary *)requestHeaders {
+  _requestHeaders = [NSMutableDictionary dictionaryWithDictionary:requestHeaders];
 }
 
 #pragma mark Finish
 
 - (void)finishWithError:(NSError *)errorOrNil {
   // If the call isn't retained anywhere else, it can be deallocated now.
-  _self = nil;
+  _retainSelf = nil;
 
   // If there were still request messages coming, stop them.
-  _requestWriter.state = GRXWriterStateFinished;
-  _requestWriter = nil;
+  @synchronized(_requestWriter) {
+    _requestWriter.state = GRXWriterStateFinished;
+  }
 
   if (errorOrNil) {
     [_responseWriteable cancelWithError:errorOrNil];
@@ -226,11 +230,10 @@ NSString * const kGRPCStatusMetadataKey = @"io.grpc.StatusMetadataKey";
 
 #pragma mark Send headers
 
-// TODO(jcanizales): Rename to commitHeaders.
-- (void)sendHeaders:(NSDictionary *)metadata {
+- (void)sendHeaders:(NSDictionary *)headers {
   // TODO(jcanizales): Add error handlers for async failures
   [_wrappedCall startBatchWithOperations:@[[[GRPCOpSendMetadata alloc]
-                                            initWithMetadata:metadata ?: @{} handler:nil]]];
+                                            initWithMetadata:headers ?: @{} handler:nil]]];
 }
 
 #pragma mark GRXWriteable implementation
@@ -244,12 +247,14 @@ NSString * const kGRPCStatusMetadataKey = @"io.grpc.StatusMetadataKey";
     // Resume the request writer.
     GRPCCall *strongSelf = weakSelf;
     if (strongSelf) {
-      strongSelf->_requestWriter.state = GRXWriterStateStarted;
+      @synchronized(strongSelf->_requestWriter) {
+        strongSelf->_requestWriter.state = GRXWriterStateStarted;
+      }
     }
   };
-  [_wrappedCall startBatchWithOperations:@[[[GRPCOpSendMessage alloc]
-                                            initWithMessage:message
-                                            handler:resumingHandler]] errorHandler:errorHandler];
+  [_wrappedCall startBatchWithOperations:@[[[GRPCOpSendMessage alloc] initWithMessage:message
+                                                                              handler:resumingHandler]]
+                            errorHandler:errorHandler];
 }
 
 - (void)writeValue:(id)value {
@@ -257,7 +262,9 @@ NSString * const kGRPCStatusMetadataKey = @"io.grpc.StatusMetadataKey";
 
   // Pause the input and only resume it when the C layer notifies us that writes
   // can proceed.
-  _requestWriter.state = GRXWriterStatePaused;
+  @synchronized(_requestWriter) {
+    _requestWriter.state = GRXWriterStatePaused;
+  }
 
   __weak GRPCCall *weakSelf = self;
   dispatch_async(_callQueue, ^{
@@ -277,7 +284,6 @@ NSString * const kGRPCStatusMetadataKey = @"io.grpc.StatusMetadataKey";
 }
 
 - (void)writesFinishedWithError:(NSError *)errorOrNil {
-  _requestWriter = nil;
   if (errorOrNil) {
     [self cancel];
   } else {
@@ -296,42 +302,54 @@ NSString * const kGRPCStatusMetadataKey = @"io.grpc.StatusMetadataKey";
 
 // Both handlers will eventually be called, from the network queue. Writes can start immediately
 // after this.
-// The first one (metadataHandler), when the response headers are received.
+// The first one (headersHandler), when the response headers are received.
 // The second one (completionHandler), whenever the RPC finishes for any reason.
-- (void)invokeCallWithMetadataHandler:(void(^)(NSDictionary *))metadataHandler
+- (void)invokeCallWithHeadersHandler:(void(^)(NSDictionary *))headersHandler
                     completionHandler:(void(^)(NSError *, NSDictionary *))completionHandler {
   // TODO(jcanizales): Add error handlers for async failures
   [_wrappedCall startBatchWithOperations:@[[[GRPCOpRecvMetadata alloc]
-                                            initWithHandler:metadataHandler]]];
+                                            initWithHandler:headersHandler]]];
   [_wrappedCall startBatchWithOperations:@[[[GRPCOpRecvStatus alloc]
                                             initWithHandler:completionHandler]]];
 }
 
 - (void)invokeCall {
   __weak GRPCCall *weakSelf = self;
-  [self invokeCallWithMetadataHandler:^(NSDictionary *headers) {
+  [self invokeCallWithHeadersHandler:^(NSDictionary *headers) {
     // Response headers received.
     GRPCCall *strongSelf = weakSelf;
     if (strongSelf) {
-      [strongSelf->_responseMetadata addEntriesFromDictionary:headers];
+      strongSelf.responseHeaders = headers;
       [strongSelf startNextRead];
     }
   } completionHandler:^(NSError *error, NSDictionary *trailers) {
     GRPCCall *strongSelf = weakSelf;
     if (strongSelf) {
-      [strongSelf->_responseMetadata addEntriesFromDictionary:trailers];
+      strongSelf.responseTrailers = trailers;
 
       if (error) {
-        NSMutableDictionary *userInfo =
-            [NSMutableDictionary dictionaryWithDictionary:error.userInfo];
-        userInfo[kGRPCStatusMetadataKey] = strongSelf->_responseMetadata;
+        NSMutableDictionary *userInfo = [NSMutableDictionary dictionary];
+        if (error.userInfo) {
+          [userInfo addEntriesFromDictionary:error.userInfo];
+        }
+        userInfo[kGRPCTrailersKey] = strongSelf.responseTrailers;
+        // TODO(jcanizales): The C gRPC library doesn't guarantee that the headers block will be
+        // called before this one, so an error might end up with trailers but no headers. We
+        // shouldn't call finishWithError until ater both blocks are called. It is also when this is
+        // done that we can provide a merged view of response headers and trailers in a thread-safe
+        // way.
+        if (strongSelf.responseHeaders) {
+          userInfo[kGRPCHeadersKey] = strongSelf.responseHeaders;
+        }
         error = [NSError errorWithDomain:error.domain code:error.code userInfo:userInfo];
       }
       [strongSelf finishWithError:error];
     }
   }];
   // Now that the RPC has been initiated, request writes can start.
-  [_requestWriter startWithWriteable:self];
+  @synchronized(_requestWriter) {
+    [_requestWriter startWithWriteable:self];
+  }
 }
 
 #pragma mark GRXWriter implementation
@@ -342,10 +360,10 @@ NSString * const kGRPCStatusMetadataKey = @"io.grpc.StatusMetadataKey";
   // before being autoreleased).
   // Care is taken not to retain self strongly in any of the blocks used in this implementation, so
   // that the life of the instance is determined by this retain cycle.
-  _self = self;
+  _retainSelf = self;
 
   _responseWriteable = [[GRXConcurrentWriteable alloc] initWithWriteable:writeable];
-  [self sendHeaders:_requestMetadata];
+  [self sendHeaders:_requestHeaders];
   [self invokeCall];
 }
 
diff --git a/src/objective-c/GRPCClient/private/GRPCChannel.h b/src/objective-c/GRPCClient/private/GRPCChannel.h
index bc6a47d469..2a7b701576 100644
--- a/src/objective-c/GRPCClient/private/GRPCChannel.h
+++ b/src/objective-c/GRPCClient/private/GRPCChannel.h
@@ -35,17 +35,12 @@
 
 struct grpc_channel;
 
-// Each separate instance of this class represents at least one TCP
-// connection to the provided host. To create a grpc_call, pass the
-// value of the unmanagedChannel property to grpc_channel_create_call.
-// Release this object when the call is finished.
+// Each separate instance of this class represents at least one TCP connection to the provided host.
+// Create them using one of the subclasses |GRPCSecureChannel| and |GRPCUnsecuredChannel|.
 @interface GRPCChannel : NSObject
 @property(nonatomic, readonly) struct grpc_channel *unmanagedChannel;
 
-// Convenience constructor to allow for reuse of connections.
-+ (instancetype)channelToHost:(NSString *)host;
-
-- (instancetype)initWithHost:(NSString *)host NS_DESIGNATED_INITIALIZER;
-
+// This initializer takes ownership of the passed channel, and will destroy it when this object is
+// deallocated. It's illegal to pass the same grpc_channel to two different GRPCChannel objects.
 - (instancetype)initWithChannel:(struct grpc_channel *)unmanagedChannel NS_DESIGNATED_INITIALIZER;
 @end
diff --git a/src/objective-c/GRPCClient/private/GRPCChannel.m b/src/objective-c/GRPCClient/private/GRPCChannel.m
index af4326332f..4366e63320 100644
--- a/src/objective-c/GRPCClient/private/GRPCChannel.m
+++ b/src/objective-c/GRPCClient/private/GRPCChannel.m
@@ -35,53 +35,17 @@
 
 #include <grpc/grpc.h>
 
-#import "GRPCSecureChannel.h"
-#import "GRPCUnsecuredChannel.h"
-
 @implementation GRPCChannel
 
-+ (instancetype)channelToHost:(NSString *)host {
-  // TODO(mlumish): Investigate whether a cache with strong links is a good idea
-  static NSMutableDictionary *channelCache;
-  static dispatch_once_t cacheInitialization;
-  dispatch_once(&cacheInitialization, ^{
-    channelCache = [NSMutableDictionary dictionary];
-  });
-  GRPCChannel *channel = channelCache[host];
-  if (!channel) {
-    channel = [[self alloc] initWithHost:host];
-    channelCache[host] = channel;
-  }
-  return channel;
-}
-
 - (instancetype)init {
-  return [self initWithHost:nil];
+  return [self initWithChannel:NULL];
 }
 
-- (instancetype)initWithHost:(NSString *)host {
-  if (![host rangeOfString:@"://"].length) {
-    // No scheme provided; assume https.
-    host = [@"https://" stringByAppendingString:host];
-  }
-  NSURL *hostURL = [NSURL URLWithString:host];
-  if (!hostURL) {
-    [NSException raise:NSInvalidArgumentException format:@"Invalid URL: %@", host];
+// Designated initializer
+- (instancetype)initWithChannel:(grpc_channel *)unmanagedChannel {
+  if (!unmanagedChannel) {
+    return nil;
   }
-  if ([hostURL.scheme isEqualToString:@"https"]) {
-    host = [@[hostURL.host, hostURL.port ?: @443] componentsJoinedByString:@":"];
-    return [[GRPCSecureChannel alloc] initWithHost:host];
-  }
-  if ([hostURL.scheme isEqualToString:@"http"]) {
-    host = [@[hostURL.host, hostURL.port ?: @80] componentsJoinedByString:@":"];
-    return [[GRPCUnsecuredChannel alloc] initWithHost:host];
-  }
-  [NSException raise:NSInvalidArgumentException
-              format:@"URL scheme %@ isn't supported.", hostURL.scheme];
-  return nil; // silence warning.
-}
-
-- (instancetype)initWithChannel:(struct grpc_channel *)unmanagedChannel {
   if ((self = [super init])) {
     _unmanagedChannel = unmanagedChannel;
   }
@@ -89,12 +53,8 @@
 }
 
 - (void)dealloc {
-  // _unmanagedChannel is NULL when deallocating an object of the base class (because the
-  // initializer returns a different object).
-  if (_unmanagedChannel) {
-    // TODO(jcanizales): Be sure to add a test with a server that closes the connection prematurely,
-    // as in the past that made this call to crash.
-    grpc_channel_destroy(_unmanagedChannel);
-  }
+  // TODO(jcanizales): Be sure to add a test with a server that closes the connection prematurely,
+  // as in the past that made this call to crash.
+  grpc_channel_destroy(_unmanagedChannel);
 }
 @end
diff --git a/src/objective-c/GRPCClient/private/GRPCCompletionQueue.m b/src/objective-c/GRPCClient/private/GRPCCompletionQueue.m
index 12535c9616..696069c200 100644
--- a/src/objective-c/GRPCClient/private/GRPCCompletionQueue.m
+++ b/src/objective-c/GRPCClient/private/GRPCCompletionQueue.m
@@ -38,8 +38,6 @@
 @implementation GRPCCompletionQueue
 
 + (instancetype)completionQueue {
-  // TODO(jcanizales): Reuse completion queues to consume only one thread,
-  // instead of one per call.
   return [[self alloc] init];
 }
 
diff --git a/src/objective-c/GRPCClient/private/GRPCHost.h b/src/objective-c/GRPCClient/private/GRPCHost.h
new file mode 100644
index 0000000000..f0bbd53023
--- /dev/null
+++ b/src/objective-c/GRPCClient/private/GRPCHost.h
@@ -0,0 +1,58 @@
+/*
+ *
+ * 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.
+ *
+ */
+
+#import <Foundation/Foundation.h>
+
+@class GRPCCompletionQueue;
+struct grpc_call;
+
+@interface GRPCHost : NSObject
+
+@property(nonatomic, readonly) NSString *address;
+
+// The following properties should only be modified for testing:
+
+@property(nonatomic, getter=isSecure) BOOL secure;
+
+@property(nonatomic, copy) NSString *pathToCertificates;
+@property(nonatomic, copy) NSString *hostNameOverride;
+
+// Host objects initialized with the same address are the same.
++ (instancetype)hostWithAddress:(NSString *)address;
+- (instancetype)initWithAddress:(NSString *)address NS_DESIGNATED_INITIALIZER;
+
+// Create a grpc_call object to the provided path on this host.
+- (struct grpc_call *)unmanagedCallWithPath:(NSString *)path
+                            completionQueue:(GRPCCompletionQueue *)queue;
+
+@end
diff --git a/src/objective-c/GRPCClient/private/GRPCHost.m b/src/objective-c/GRPCClient/private/GRPCHost.m
new file mode 100644
index 0000000000..d902f95b51
--- /dev/null
+++ b/src/objective-c/GRPCClient/private/GRPCHost.m
@@ -0,0 +1,126 @@
+/*
+ *
+ * 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.
+ *
+ */
+
+#import "GRPCHost.h"
+
+#include <grpc/grpc.h>
+
+#import "GRPCChannel.h"
+#import "GRPCCompletionQueue.h"
+#import "GRPCSecureChannel.h"
+#import "GRPCUnsecuredChannel.h"
+
+@interface GRPCHost ()
+// TODO(mlumish): Investigate whether caching channels with strong links is a good idea.
+@property(nonatomic, strong) GRPCChannel *channel;
+@end
+
+@implementation GRPCHost
+
++ (instancetype)hostWithAddress:(NSString *)address {
+  return [[self alloc] initWithAddress:address];
+}
+
+- (instancetype)init {
+  return [self initWithAddress:nil];
+}
+
+// Default initializer.
+- (instancetype)initWithAddress:(NSString *)address {
+
+  // To provide a default port, we try to interpret the address. If it's just a host name without
+  // scheme and without port, we'll use port 443. If it has a scheme, we pass it untouched to the C
+  // gRPC library.
+  // TODO(jcanizales): Add unit tests for the types of addresses we want to let pass untouched.
+  NSURL *hostURL = [NSURL URLWithString:[@"https://" stringByAppendingString:address]];
+  if (hostURL && !hostURL.port) {
+    address = [hostURL.host stringByAppendingString:@":443"];
+  }
+
+  // Look up the GRPCHost in the cache.
+  static NSMutableDictionary *hostCache;
+  static dispatch_once_t cacheInitialization;
+  dispatch_once(&cacheInitialization, ^{
+    hostCache = [NSMutableDictionary dictionary];
+  });
+  @synchronized(hostCache) {
+    GRPCHost *cachedHost = hostCache[address];
+    if (cachedHost) {
+      return cachedHost;
+    }
+
+    if ((self = [super init])) {
+      _address = address;
+      _secure = YES;
+      hostCache[address] = self;
+    }
+  }
+  return self;
+}
+
+- (grpc_call *)unmanagedCallWithPath:(NSString *)path completionQueue:(GRPCCompletionQueue *)queue {
+  if (!queue || !path || !self.channel) {
+    return NULL;
+  }
+  return grpc_channel_create_call(self.channel.unmanagedChannel,
+                                  NULL, GRPC_PROPAGATE_DEFAULTS,
+                                  queue.unmanagedQueue,
+                                  path.UTF8String,
+                                  self.hostName.UTF8String,
+                                  gpr_inf_future(GPR_CLOCK_REALTIME));
+}
+
+- (GRPCChannel *)channel {
+  // Create it lazily, because we don't want to open a connection just because someone is
+  // configuring a host.
+  if (!_channel) {
+    if (_secure) {
+      _channel = [[GRPCSecureChannel alloc] initWithHost:_address
+                                      pathToCertificates:_pathToCertificates
+                                        hostNameOverride:_hostNameOverride];
+    } else {
+      _channel = [[GRPCUnsecuredChannel alloc] initWithHost:_address];
+    }
+  }
+  return _channel;
+}
+
+- (NSString *)hostName {
+  // TODO(jcanizales): Default to nil instead of _address when Issue #2635 is clarified.
+  return _hostNameOverride ?: _address;
+}
+
+// TODO(jcanizales): Don't let set |secure| to |NO| if |pathToCertificates| or |hostNameOverride|
+// have been set. Don't let set either of the latter if |secure| has been set to |NO|.
+
+@end
diff --git a/src/objective-c/GRPCClient/private/GRPCSecureChannel.h b/src/objective-c/GRPCClient/private/GRPCSecureChannel.h
index d34ceaea0c..74257eb058 100644
--- a/src/objective-c/GRPCClient/private/GRPCSecureChannel.h
+++ b/src/objective-c/GRPCClient/private/GRPCSecureChannel.h
@@ -31,8 +31,23 @@
  *
  */
 
+#include <grpc/grpc.h>
+
 #import "GRPCChannel.h"
 
+struct grpc_credentials;
+
 @interface GRPCSecureChannel : GRPCChannel
+- (instancetype)initWithHost:(NSString *)host;
+
+// Only in tests shouldn't pathToCertificates or hostNameOverride be nil. Passing nil for
+// pathToCertificates results in using the default root certificates distributed with the library.
+- (instancetype)initWithHost:(NSString *)host
+          pathToCertificates:(NSString *)path
+            hostNameOverride:(NSString *)hostNameOverride;
 
+// The passed arguments aren't required to be valid beyond the invocation of this initializer.
+- (instancetype)initWithHost:(NSString *)host
+                 credentials:(struct grpc_credentials *)credentials
+                        args:(grpc_channel_args *)args NS_DESIGNATED_INITIALIZER;
 @end
diff --git a/src/objective-c/GRPCClient/private/GRPCSecureChannel.m b/src/objective-c/GRPCClient/private/GRPCSecureChannel.m
index 43a8bd539e..0a54804bb2 100644
--- a/src/objective-c/GRPCClient/private/GRPCSecureChannel.m
+++ b/src/objective-c/GRPCClient/private/GRPCSecureChannel.m
@@ -33,28 +33,83 @@
 
 #import "GRPCSecureChannel.h"
 
-#import <grpc/grpc_security.h>
+#include <grpc/grpc_security.h>
+
+// Returns NULL if the file at path couldn't be read. In that case, if errorPtr isn't NULL,
+// *errorPtr will be an object describing what went wrong.
+static grpc_credentials *CertificatesAtPath(NSString *path, NSError **errorPtr) {
+  // Files in PEM format can have non-ASCII characters in their comments (e.g. for the name of the
+  // issuer). Load them as UTF8 and produce an ASCII equivalent.
+  NSString *contentInUTF8 = [NSString stringWithContentsOfFile:path
+                                                     encoding:NSUTF8StringEncoding
+                                                        error:errorPtr];
+  NSData *contentInASCII = [contentInUTF8 dataUsingEncoding:NSASCIIStringEncoding
+                                       allowLossyConversion:YES];
+  if (!contentInASCII.bytes) {
+    // Passing NULL to grpc_ssl_credentials_create produces behavior we don't want, so return.
+    return NULL;
+  }
+  return grpc_ssl_credentials_create(contentInASCII.bytes, NULL);
+}
 
 @implementation GRPCSecureChannel
 
 - (instancetype)initWithHost:(NSString *)host {
-  static grpc_credentials *kCredentials;
+  return [self initWithHost:host pathToCertificates:nil hostNameOverride:nil];
+}
+
+- (instancetype)initWithHost:(NSString *)host
+          pathToCertificates:(NSString *)path
+            hostNameOverride:(NSString *)hostNameOverride {
+  // Load default SSL certificates once.
+  static grpc_credentials *kDefaultCertificates;
   static dispatch_once_t loading;
   dispatch_once(&loading, ^{
+    NSString *defaultPath = @"gRPCCertificates.bundle/roots"; // .pem
     // Do not use NSBundle.mainBundle, as it's nil for tests of library projects.
     NSBundle *bundle = [NSBundle bundleForClass:self.class];
-    NSString *certsPath = [bundle pathForResource:@"gRPCCertificates.bundle/roots" ofType:@"pem"];
-    NSAssert(certsPath.length,
-             @"gRPCCertificates.bundle/roots.pem not found under %@. This file, with the root "
-             "certificates, is needed to establish TLS (HTTPS) connections.", bundle.bundlePath);
-    NSData *certsData = [NSData dataWithContentsOfFile:certsPath];
-    NSAssert(certsData.length, @"No data read from %@", certsPath);
-    NSString *certsString = [[NSString alloc] initWithData:certsData encoding:NSUTF8StringEncoding];
-    kCredentials = grpc_ssl_credentials_create(certsString.UTF8String, NULL);
+    NSString *path = [bundle pathForResource:defaultPath ofType:@"pem"];
+    NSError *error;
+    kDefaultCertificates = CertificatesAtPath(path, &error);
+    NSAssert(kDefaultCertificates, @"Could not read %@/%@.pem. This file, with the root "
+             "certificates, is needed to establish secure (TLS) connections. Because the file is "
+             "distributed with the gRPC library, this error is usually a sign that the library "
+             "wasn't configured correctly for your project. Error: %@",
+             bundle.bundlePath, defaultPath, error);
   });
-  return (self = [super initWithChannel:grpc_secure_channel_create(kCredentials,
-                                                                   host.UTF8String,
-                                                                   NULL)]);
+
+  //TODO(jcanizales): Add NSError** parameter to the initializer.
+  grpc_credentials *certificates = path ? CertificatesAtPath(path, NULL) : kDefaultCertificates;
+  if (!certificates) {
+    return nil;
+  }
+
+  // Ritual to pass the SSL host name override to the C library.
+  grpc_channel_args channelArgs;
+  grpc_arg nameOverrideArg;
+  channelArgs.num_args = 1;
+  channelArgs.args = &nameOverrideArg;
+  nameOverrideArg.type = GRPC_ARG_STRING;
+  nameOverrideArg.key = GRPC_SSL_TARGET_NAME_OVERRIDE_ARG;
+  // Cast const away. Hope C gRPC doesn't modify it!
+  nameOverrideArg.value.string = (char *) hostNameOverride.UTF8String;
+  grpc_channel_args *args = hostNameOverride ? &channelArgs : NULL;
+
+  return [self initWithHost:host credentials:certificates args:args];
+}
+
+- (instancetype)initWithHost:(NSString *)host
+                 credentials:(grpc_credentials *)credentials
+                        args:(grpc_channel_args *)args {
+  return (self =
+          [super initWithChannel:grpc_secure_channel_create(credentials, host.UTF8String, args)]);
+}
+
+// TODO(jcanizales): GRPCSecureChannel and GRPCUnsecuredChannel are just convenience initializers
+// for GRPCChannel. Move them into GRPCChannel, which will make the following unnecessary.
+- (instancetype)initWithChannel:(grpc_channel *)unmanagedChannel {
+  [NSException raise:NSInternalInconsistencyException format:@"use another initializer"];
+  return [self initWithHost:nil]; // silence warnings
 }
 
 @end
diff --git a/src/objective-c/GRPCClient/private/GRPCUnsecuredChannel.h b/src/objective-c/GRPCClient/private/GRPCUnsecuredChannel.h
index 9d89cfb541..8528be44c0 100644
--- a/src/objective-c/GRPCClient/private/GRPCUnsecuredChannel.h
+++ b/src/objective-c/GRPCClient/private/GRPCUnsecuredChannel.h
@@ -34,5 +34,5 @@
 #import "GRPCChannel.h"
 
 @interface GRPCUnsecuredChannel : GRPCChannel
-
+- (instancetype)initWithHost:(NSString *)host NS_DESIGNATED_INITIALIZER;
 @end
diff --git a/src/objective-c/GRPCClient/private/GRPCUnsecuredChannel.m b/src/objective-c/GRPCClient/private/GRPCUnsecuredChannel.m
index 8518f78c5b..070a529629 100644
--- a/src/objective-c/GRPCClient/private/GRPCUnsecuredChannel.m
+++ b/src/objective-c/GRPCClient/private/GRPCUnsecuredChannel.m
@@ -41,4 +41,10 @@
   return (self = [super initWithChannel:grpc_insecure_channel_create(host.UTF8String, NULL)]);
 }
 
+// TODO(jcanizales): GRPCSecureChannel and GRPCUnsecuredChannel are just convenience initializers
+// for GRPCChannel. Move them into GRPCChannel, which will make the following unnecessary.
+- (instancetype)initWithChannel:(grpc_channel *)unmanagedChannel {
+  [NSException raise:NSInternalInconsistencyException format:@"use the other initializer"];
+  return [self initWithHost:nil]; // silence warnings
+}
 @end
diff --git a/src/objective-c/GRPCClient/private/GRPCWrappedCall.h b/src/objective-c/GRPCClient/private/GRPCWrappedCall.h
index 18f8bb5531..da11cbb761 100644
--- a/src/objective-c/GRPCClient/private/GRPCWrappedCall.h
+++ b/src/objective-c/GRPCClient/private/GRPCWrappedCall.h
@@ -81,11 +81,12 @@
 
 @end
 
+#pragma mark GRPCWrappedCall
+
 @interface GRPCWrappedCall : NSObject
 
-- (instancetype)initWithChannel:(GRPCChannel *)channel
-                           path:(NSString *)path
-                           host:(NSString *)host NS_DESIGNATED_INITIALIZER;
+- (instancetype)initWithHost:(NSString *)host
+                        path:(NSString *)path NS_DESIGNATED_INITIALIZER;
 
 - (void)startBatchWithOperations:(NSArray *)ops errorHandler:(void(^)())errorHandler;
 
diff --git a/src/objective-c/GRPCClient/private/GRPCWrappedCall.m b/src/objective-c/GRPCClient/private/GRPCWrappedCall.m
index ecc512edca..951c051036 100644
--- a/src/objective-c/GRPCClient/private/GRPCWrappedCall.m
+++ b/src/objective-c/GRPCClient/private/GRPCWrappedCall.m
@@ -32,11 +32,14 @@
  */
 
 #import "GRPCWrappedCall.h"
+
 #import <Foundation/Foundation.h>
 #include <grpc/grpc.h>
 #include <grpc/byte_buffer.h>
 #include <grpc/support/alloc.h>
+
 #import "GRPCCompletionQueue.h"
+#import "GRPCHost.h"
 #import "NSDictionary+GRPC.h"
 #import "NSData+GRPC.h"
 #import "NSError+GRPC.h"
@@ -219,37 +222,36 @@
 
 @end
 
-@implementation GRPCWrappedCall{
-  grpc_call *_call;
+#pragma mark GRPCWrappedCall
+
+@implementation GRPCWrappedCall {
   GRPCCompletionQueue *_queue;
+  grpc_call *_call;
 }
 
 - (instancetype)init {
-  return [self initWithChannel:nil path:nil host:nil];
+  return [self initWithHost:nil path:nil];
 }
 
-- (instancetype)initWithChannel:(GRPCChannel *)channel
-                           path:(NSString *)path
-                           host:(NSString *)host {
-  if (!channel || !path || !host) {
+- (instancetype)initWithHost:(NSString *)host
+                        path:(NSString *)path {
+  if (!path || !host) {
     [NSException raise:NSInvalidArgumentException
-                format:@"channel, method, and host cannot be nil."];
+                format:@"path and host cannot be nil."];
   }
-  
+
   if (self = [super init]) {
     static dispatch_once_t initialization;
     dispatch_once(&initialization, ^{
       grpc_init();
     });
-    
+
+    // Each completion queue consumes one thread. There's a trade to be made between creating and
+    // consuming too many threads and having contention of multiple calls in a single completion
+    // queue. Currently we favor latency and use one per call.
     _queue = [GRPCCompletionQueue completionQueue];
-    if (!_queue) {
-      return nil;
-    }
-    _call = grpc_channel_create_call(
-        channel.unmanagedChannel, NULL, GRPC_PROPAGATE_DEFAULTS,
-        _queue.unmanagedQueue, path.UTF8String, host.UTF8String,
-        gpr_inf_future(GPR_CLOCK_REALTIME));
+
+    _call = [[GRPCHost hostWithAddress:host] unmanagedCallWithPath:path completionQueue:_queue];
     if (_call == NULL) {
       return nil;
     }
diff --git a/src/objective-c/RxLibrary/GRXBufferedPipe.h b/src/objective-c/RxLibrary/GRXBufferedPipe.h
index b6296e1ed7..ca94ce275f 100644
--- a/src/objective-c/RxLibrary/GRXBufferedPipe.h
+++ b/src/objective-c/RxLibrary/GRXBufferedPipe.h
@@ -36,13 +36,11 @@
 #import "GRXWriteable.h"
 #import "GRXWriter.h"
 
-// A buffered pipe is a Writeable that also acts as a Writer (to whichever other writeable is passed
-// to -startWithWriteable:).
+// A buffered pipe is a Writer that also acts as a Writeable.
 // Once it is started, whatever values are written into it (via -writeValue:) will be propagated
 // immediately, unless flow control prevents it.
 // If it is throttled and keeps receiving values, as well as if it receives values before being
-// started, it will buffer them and propagate them in order as soon as its state becomes
-// GRXWriterStateStarted.
+// started, it will buffer them and propagate them in order as soon as its state becomes Started.
 // If it receives an error (via -writesFinishedWithError:), it will drop any buffered values and
 // propagate the error immediately.
 //
@@ -51,6 +49,9 @@
 // pipe will keep buffering all data written to it, your application could run out of memory and
 // crash. If you want to react to flow control signals to prevent that, instead of using this class
 // you can implement an object that conforms to GRXWriter.
+//
+// Thread-safety:
+// The methods of an object of this class should not be called concurrently from different threads.
 @interface GRXBufferedPipe : GRXWriter<GRXWriteable>
 
 // Convenience constructor.
diff --git a/src/objective-c/RxLibrary/GRXForwardingWriter.h b/src/objective-c/RxLibrary/GRXForwardingWriter.h
index d004333d2b..f310832284 100644
--- a/src/objective-c/RxLibrary/GRXForwardingWriter.h
+++ b/src/objective-c/RxLibrary/GRXForwardingWriter.h
@@ -33,11 +33,17 @@
 
 #import "GRXWriter.h"
 
-// A "proxy" class that simply forwards values, completion, and errors from its
-// input writer to its writeable.
+// A "proxy" class that simply forwards values, completion, and errors from its input writer to its
+// writeable.
 // It is useful as a superclass for pipes that act as a transformation of their
 // input writer, and for classes that represent objects with input and
 // output sequences of values, like an RPC.
+//
+// Thread-safety:
+// All messages sent to this object need to be serialized. When it is started, the writer it wraps
+// is started in the same thread. Manual state changes are propagated to the wrapped writer in the
+// same thread too. Importantly, all messages the wrapped writer sends to its writeable need to be
+// serialized with any message sent to this object.
 @interface GRXForwardingWriter : GRXWriter
 - (instancetype)initWithWriter:(GRXWriter *)writer NS_DESIGNATED_INITIALIZER;
 @end
diff --git a/src/objective-c/RxLibrary/GRXForwardingWriter.m b/src/objective-c/RxLibrary/GRXForwardingWriter.m
index 2342f51ab3..a72be9ace2 100644
--- a/src/objective-c/RxLibrary/GRXForwardingWriter.m
+++ b/src/objective-c/RxLibrary/GRXForwardingWriter.m
@@ -48,7 +48,11 @@
 // Designated initializer
 - (instancetype)initWithWriter:(GRXWriter *)writer {
   if (!writer) {
-    [NSException raise:NSInvalidArgumentException format:@"writer can't be nil."];
+    return nil;
+  }
+  if (writer.state != GRXWriterStateNotStarted) {
+    [NSException raise:NSInvalidArgumentException
+                format:@"The writer argument must not have already started."];
   }
   if ((self = [super init])) {
     _writer = writer;
diff --git a/src/objective-c/RxLibrary/GRXImmediateWriter.h b/src/objective-c/RxLibrary/GRXImmediateWriter.h
index b171f0c760..3fcc259434 100644
--- a/src/objective-c/RxLibrary/GRXImmediateWriter.h
+++ b/src/objective-c/RxLibrary/GRXImmediateWriter.h
@@ -36,10 +36,17 @@
 #import "GRXWriter.h"
 
 // Utility to construct GRXWriter instances from values that are immediately available when
-// required. The returned writers all support pausing and early termination.
+// required.
 //
-// Unless the writeable callback pauses them or stops them early, these writers will do all their
-// interactions with the writeable before the start method returns.
+// Thread-safety:
+//
+// An object of this class shouldn't be messaged concurrently by more than one thread. It will start
+// messaging the writeable before |startWithWriteable:| returns, in the same thread. That is the
+// only place where the writer can be paused or stopped prematurely.
+//
+// If a paused writer of this class is resumed, it will start messaging the writeable, in the same
+// thread, before |setState:| returns. Because the object can't be legally accessed concurrently,
+// that's the only place where it can be paused again (or stopped).
 @interface GRXImmediateWriter : GRXWriter
 
 // Returns a writer that pulls values from the passed NSEnumerator instance and pushes them to
diff --git a/src/objective-c/RxLibrary/GRXWriteable.h b/src/objective-c/RxLibrary/GRXWriteable.h
index 216de30735..45613d6dd0 100644
--- a/src/objective-c/RxLibrary/GRXWriteable.h
+++ b/src/objective-c/RxLibrary/GRXWriteable.h
@@ -48,15 +48,15 @@
 
 typedef void (^GRXValueHandler)(id value);
 typedef void (^GRXCompletionHandler)(NSError *errorOrNil);
-typedef void (^GRXSingleValueHandler)(id value, NSError *errorOrNil);
-typedef void (^GRXStreamHandler)(BOOL done, id value, NSError *error);
+typedef void (^GRXSingleHandler)(id value, NSError *errorOrNil);
+typedef void (^GRXEventHandler)(BOOL done, id value, NSError *error);
 
 // Utility to create objects that conform to the GRXWriteable protocol, from
 // blocks that handle each of the two methods of the protocol.
 @interface GRXWriteable : NSObject<GRXWriteable>
 
-+ (instancetype)writeableWithSingleValueHandler:(GRXSingleValueHandler)handler;
-+ (instancetype)writeableWithStreamHandler:(GRXStreamHandler)handler;
++ (instancetype)writeableWithSingleHandler:(GRXSingleHandler)handler;
++ (instancetype)writeableWithEventHandler:(GRXEventHandler)handler;
 
 - (instancetype)initWithValueHandler:(GRXValueHandler)valueHandler
                    completionHandler:(GRXCompletionHandler)completionHandler
diff --git a/src/objective-c/RxLibrary/GRXWriteable.m b/src/objective-c/RxLibrary/GRXWriteable.m
index 63f7c3e7f3..2729d62b72 100644
--- a/src/objective-c/RxLibrary/GRXWriteable.m
+++ b/src/objective-c/RxLibrary/GRXWriteable.m
@@ -38,7 +38,7 @@
   GRXCompletionHandler _completionHandler;
 }
 
-+ (instancetype)writeableWithSingleValueHandler:(GRXSingleValueHandler)handler {
++ (instancetype)writeableWithSingleHandler:(GRXSingleHandler)handler {
   if (!handler) {
     return [[self alloc] init];
   }
@@ -51,7 +51,7 @@
   }];
 }
 
-+ (instancetype)writeableWithStreamHandler:(GRXStreamHandler)handler {
++ (instancetype)writeableWithEventHandler:(GRXEventHandler)handler {
   if (!handler) {
     return [[self alloc] init];
   }
diff --git a/src/objective-c/RxLibrary/GRXWriter.h b/src/objective-c/RxLibrary/GRXWriter.h
index 5d6e1a472a..b1c994aa38 100644
--- a/src/objective-c/RxLibrary/GRXWriter.h
+++ b/src/objective-c/RxLibrary/GRXWriter.h
@@ -35,84 +35,73 @@
 
 #import "GRXWriteable.h"
 
+// States of a writer.
 typedef NS_ENUM(NSInteger, GRXWriterState) {
 
-  // The writer has not yet been given a writeable to which it can push its
-  // values. To have an writer transition to the Started state, send it a
-  // startWithWriteable: message.
+  // The writer has not yet been given a writeable to which it can push its values. To have a writer
+  // transition to the Started state, send it a startWithWriteable: message.
   //
-  // An writer's state cannot be manually set to this value.
+  // A writer's state cannot be manually set to this value.
   GRXWriterStateNotStarted,
 
   // The writer might push values to the writeable at any moment.
   GRXWriterStateStarted,
 
-  // The writer is temporarily paused, and won't send any more values to the
-  // writeable unless its state is set back to Started. The writer might still
-  // transition to the Finished state at any moment, and is allowed to send
-  // writesFinishedWithError: to its writeable.
-  //
-  // Not all implementations of writer have to support pausing, and thus
-  // trying to set an writer's state to this value might have no effect.
+  // The writer is temporarily paused, and won't send any more values to the writeable unless its
+  // state is set back to Started. The writer might still transition to the Finished state at any
+  // moment, and is allowed to send writesFinishedWithError: to its writeable.
   GRXWriterStatePaused,
 
   // The writer has released its writeable and won't interact with it anymore.
   //
-  // One seldomly wants to set an writer's state to this value, as its
-  // writeable isn't notified with a writesFinishedWithError: message. Instead, sending
-  // finishWithError: to the writer will make it notify the writeable and then
-  // transition to this state.
+  // One seldomly wants to set a writer's state to this value, as its writeable isn't notified with
+  // a writesFinishedWithError: message. Instead, sending finishWithError: to the writer will make
+  // it notify the writeable and then transition to this state.
   GRXWriterStateFinished
 };
 
-// An object that conforms to this protocol can produce, on demand, a sequence
-// of values. The sequence may be produced asynchronously, and it may consist of
-// any number of elements, including none or an infinite number.
+// An GRXWriter object can produce, on demand, a sequence of values. The sequence may be produced
+// asynchronously, and it may consist of any number of elements, including none or an infinite
+// number.
+//
+// GRXWriter is the active dual of NSEnumerator. The difference between them is thus whether the
+// object plays an active or passive role during usage: A user of NSEnumerator pulls values off it,
+// and passes the values to a writeable. A user of GRXWriter, though, just gives it a writeable, and
+// the GRXWriter instance pushes values to the writeable. This makes this protocol suitable to
+// represent a sequence of future values, as well as collections with internal iteration.
 //
-// GRXWriter is the active dual of NSEnumerator. The difference between them
-// is thus whether the object plays an active or passive role during usage: A
-// user of NSEnumerator pulls values off it, and passes the values to a writeable.
-// A user of GRXWriter, though, just gives it a writeable, and the
-// GRXWriter instance pushes values to the writeable. This makes this protocol
-// suitable to represent a sequence of future values, as well as collections
-// with internal iteration.
+// An instance of GRXWriter can start producing values after a writeable is passed to it. It can
+// also be commanded to finish the sequence immediately (with an optional error). Finally, it can be
+// asked to pause, and resumed later. All GRXWriter objects support pausing and early termination.
 //
-// An instance of GRXWriter can start producing values after a writeable is
-// passed to it. It can also be commanded to finish the sequence immediately
-// (with an optional error). Finally, it can be asked to pause, but the
-// conforming instance is not required to oblige.
+// Thread-safety:
 //
-// Unless otherwise indicated by a conforming class, no messages should be sent
-// concurrently to a GRXWriter. I.e., conforming classes aren't required to
-// be thread-safe.
+// State transitions take immediate effect if the object is used from a single thread. Subclasses
+// might offer stronger guarantees.
+//
+// Unless otherwise indicated by a conforming subclass, no messages should be sent concurrently to a
+// GRXWriter. I.e., conforming classes aren't required to be thread-safe.
 @interface GRXWriter : NSObject
 
-// This property can be used to query the current state of the writer, which
-// determines how it might currently use its writeable. Some state transitions can
-// be triggered by setting this property to the corresponding value, and that's
-// useful for advanced use cases like pausing an writer. For more details,
-// see the documentation of the enum.
+// This property can be used to query the current state of the writer, which determines how it might
+// currently use its writeable. Some state transitions can be triggered by setting this property to
+// the corresponding value, and that's useful for advanced use cases like pausing an writer. For
+// more details, see the documentation of the enum further down.
 @property(nonatomic) GRXWriterState state;
 
-// Start sending messages to the writeable. Messages may be sent before the method
-// returns, or they may be sent later in the future. See GRXWriteable.h for the
-// different messages a writeable can receive.
+// Transition to the Started state, and start sending messages to the writeable (a reference to it
+// is retained). Messages to the writeable may be sent before the method returns, or they may be
+// sent later in the future. See GRXWriteable.h for the different messages a writeable can receive.
 //
-// If this writer draws its values from an external source (e.g. from the
-// filesystem or from a server), calling this method will commonly trigger side
-// effects (like network connections).
+// If this writer draws its values from an external source (e.g. from the filesystem or from a
+// server), calling this method will commonly trigger side effects (like network connections).
 //
 // This method might only be called on writers in the NotStarted state.
 - (void)startWithWriteable:(id<GRXWriteable>)writeable;
 
-// Send writesFinishedWithError:errorOrNil immediately to the writeable, and don't send
-// any more messages to it.
-//
-// This method might only be called on writers in the Started or Paused
-// state.
+// Send writesFinishedWithError:errorOrNil to the writeable. Then release the reference to it and
+// transition to the Finished state.
 //
-// TODO(jcanizales): Consider adding some guarantee about the immediacy of that
-// stopping. I know I've relied on it in part of the code that uses this, but
-// can't remember the details in the presence of concurrency.
+// This method might only be called on writers in the Started or Paused state.
 - (void)finishWithError:(NSError *)errorOrNil;
 @end
diff --git a/src/objective-c/tests/GRPCClientTests.m b/src/objective-c/tests/GRPCClientTests.m
index 103e5ca3d4..06581e7599 100644
--- a/src/objective-c/tests/GRPCClientTests.m
+++ b/src/objective-c/tests/GRPCClientTests.m
@@ -35,6 +35,8 @@
 #import <XCTest/XCTest.h>
 
 #import <GRPCClient/GRPCCall.h>
+#import <GRPCClient/GRPCCall+OAuth2.h>
+#import <GRPCClient/GRPCCall+Tests.h>
 #import <ProtoRPC/ProtoMethod.h>
 #import <RemoteTest/Messages.pbobjc.h>
 #import <RxLibrary/GRXWriteable.h>
@@ -43,8 +45,7 @@
 // These are a few tests similar to InteropTests, but which use the generic gRPC client (GRPCCall)
 // rather than a generated proto library on top of it.
 
-// grpc-test.sandbox.google.com
-static NSString * const kHostAddress = @"http://localhost:5050";
+static NSString * const kHostAddress = @"localhost:5050";
 static NSString * const kPackage = @"grpc.testing";
 static NSString * const kService = @"TestService";
 
@@ -58,6 +59,9 @@ static ProtoMethod *kUnaryCallMethod;
 @implementation GRPCClientTests
 
 - (void)setUp {
+  // Register test server as non-SSL.
+  [GRPCCall useInsecureConnectionsForHost:kHostAddress];
+
   // This method isn't implemented by the remote server.
   kInexistentMethod = [[ProtoMethod alloc] initWithPackage:kPackage
                                                    service:kService
@@ -110,7 +114,7 @@ static ProtoMethod *kUnaryCallMethod;
 
   [call startWithWriteable:responsesWriteable];
 
-  [self waitForExpectationsWithTimeout:4 handler:nil];
+  [self waitForExpectationsWithTimeout:8 handler:nil];
 }
 
 - (void)testSimpleProtoRPC {
@@ -142,7 +146,7 @@ static ProtoMethod *kUnaryCallMethod;
 
   [call startWithWriteable:responsesWriteable];
 
-  [self waitForExpectationsWithTimeout:4 handler:nil];
+  [self waitForExpectationsWithTimeout:8 handler:nil];
 }
 
 - (void)testMetadata {
@@ -157,18 +161,20 @@ static ProtoMethod *kUnaryCallMethod;
                                              path:kUnaryCallMethod.HTTPPath
                                    requestsWriter:requestsWriter];
 
-  call.requestMetadata[@"Authorization"] = @"Bearer bogusToken";
+  call.oauth2AccessToken = @"bogusToken";
 
   id<GRXWriteable> responsesWriteable = [[GRXWriteable alloc] initWithValueHandler:^(NSData *value) {
     XCTFail(@"Received unexpected response: %@", value);
   } completionHandler:^(NSError *errorOrNil) {
     XCTAssertNotNil(errorOrNil, @"Finished without error!");
     XCTAssertEqual(errorOrNil.code, 16, @"Finished with unexpected error: %@", errorOrNil);
-    XCTAssertEqualObjects(call.responseMetadata, errorOrNil.userInfo[kGRPCStatusMetadataKey],
-                          @"Metadata in the NSError object and call object differ.");
-    NSString *challengeHeader = call.responseMetadata[@"www-authenticate"];
+    XCTAssertEqualObjects(call.responseHeaders, errorOrNil.userInfo[kGRPCHeadersKey],
+                          @"Headers in the NSError object and call object differ.");
+    XCTAssertEqualObjects(call.responseTrailers, errorOrNil.userInfo[kGRPCTrailersKey],
+                          @"Trailers in the NSError object and call object differ.");
+    NSString *challengeHeader = call.oauth2ChallengeHeader;
     XCTAssertGreaterThan(challengeHeader.length, 0,
-                         @"No challenge in response headers %@", call.responseMetadata);
+                         @"No challenge in response headers %@", call.responseHeaders);
     [expectation fulfill];
   }];
 
diff --git a/src/objective-c/tests/InteropTests.h b/src/objective-c/tests/InteropTests.h
new file mode 100644
index 0000000000..1045c3d124
--- /dev/null
+++ b/src/objective-c/tests/InteropTests.h
@@ -0,0 +1,43 @@
+/*
+ *
+ * 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.
+ *
+ */
+
+#import <XCTest/XCTest.h>
+
+// Implements tests as described here:
+// https://github.com/grpc/grpc/blob/master/doc/interop-test-descriptions.md
+
+@interface InteropTests : XCTestCase
+// Returns @"grpc-test.sandbox.google.com".
+// Override in a subclass to perform the same tests against a different address.
++ (NSString *)host;
+@end
diff --git a/src/objective-c/tests/InteropTests.m b/src/objective-c/tests/InteropTests.m
index b473d73422..1b63fe2059 100644
--- a/src/objective-c/tests/InteropTests.m
+++ b/src/objective-c/tests/InteropTests.m
@@ -31,11 +31,11 @@
  *
  */
 
-#include <grpc/status.h>
+#import "InteropTests.h"
 
-#import <UIKit/UIKit.h>
-#import <XCTest/XCTest.h>
+#include <grpc/status.h>
 
+#import <GRPCClient/GRPCCall+Tests.h>
 #import <ProtoRPC/ProtoRPC.h>
 #import <RemoteTest/Empty.pbobjc.h>
 #import <RemoteTest/Messages.pbobjc.h>
@@ -76,21 +76,22 @@
 }
 @end
 
-@interface InteropTests : XCTestCase
-@end
+#pragma mark Tests
+
+static NSString * const kRemoteSSLHost = @"grpc-test.sandbox.google.com";
 
 @implementation InteropTests {
   RMTTestService *_service;
 }
 
-// grpc-test.sandbox.google.com
++ (NSString *)host {
+  return kRemoteSSLHost;
+}
 
 - (void)setUp {
-  _service = [[RMTTestService alloc] initWithHost:@"http://localhost:5050"];
+  _service = [[RMTTestService alloc] initWithHost:self.class.host];
 }
 
-// Tests as described here: https://github.com/grpc/grpc/blob/master/doc/interop-test-descriptions.md
-
 - (void)testEmptyUnaryRPC {
   __weak XCTestExpectation *expectation = [self expectationWithDescription:@"EmptyUnary"];
 
@@ -127,7 +128,7 @@
     [expectation fulfill];
   }];
 
-  [self waitForExpectationsWithTimeout:8 handler:nil];
+  [self waitForExpectationsWithTimeout:16 handler:nil];
 }
 
 - (void)testClientStreamingRPC {
diff --git a/src/objective-c/tests/InteropTestsLocalCleartext.m b/src/objective-c/tests/InteropTestsLocalCleartext.m
new file mode 100644
index 0000000000..2d7d3c4b2c
--- /dev/null
+++ b/src/objective-c/tests/InteropTestsLocalCleartext.m
@@ -0,0 +1,59 @@
+/*
+ *
+ * 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.
+ *
+ */
+
+// Repeat of the tests in InteropTests.m, but sending the RPCs to a local cleartext server instead
+// of the remote SSL one.
+
+#import <GRPCClient/GRPCCall+Tests.h>
+
+#import "InteropTests.h"
+
+static NSString * const kLocalCleartextHost = @"localhost:5050";
+
+@interface InteropTestsLocalCleartext : InteropTests
+@end
+
+@implementation InteropTestsLocalCleartext
+
++ (NSString *)host {
+  return kLocalCleartextHost;
+}
+
+- (void)setUp {
+  // Register test server as non-SSL.
+  [GRPCCall useInsecureConnectionsForHost:kLocalCleartextHost];
+
+  [super setUp];
+}
+
+@end
diff --git a/src/objective-c/tests/InteropTestsLocalSSL.m b/src/objective-c/tests/InteropTestsLocalSSL.m
new file mode 100644
index 0000000000..f69f806dcf
--- /dev/null
+++ b/src/objective-c/tests/InteropTestsLocalSSL.m
@@ -0,0 +1,62 @@
+/*
+ *
+ * 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.
+ *
+ */
+
+// Repeat of the tests in InteropTests.m, but sending the RPCs to a local SSL server instead of the
+// remote one.
+
+#import <GRPCClient/GRPCCall+Tests.h>
+
+#import "InteropTests.h"
+
+static NSString * const kLocalSSLHost = @"localhost:5051";
+
+@interface InteropTestsLocalSSL : InteropTests
+@end
+
+@implementation InteropTestsLocalSSL
+
++ (NSString *)host {
+  return kLocalSSLHost;
+}
+
+- (void)setUp {
+  // Register test server certificates and name.
+  NSBundle *bundle = [NSBundle bundleForClass:self.class];
+  NSString *certsPath = [bundle pathForResource:@"TestCertificates.bundle/test-certificates"
+                                         ofType:@"pem"];
+  [GRPCCall useTestCertsPath:certsPath testName:@"foo.test.google.fr" forHost:kLocalSSLHost];
+
+  [super setUp];
+}
+
+@end
diff --git a/src/objective-c/tests/RxLibraryUnitTests.m b/src/objective-c/tests/RxLibraryUnitTests.m
index 5e3162875a..a67a4c6cd9 100644
--- a/src/objective-c/tests/RxLibraryUnitTests.m
+++ b/src/objective-c/tests/RxLibraryUnitTests.m
@@ -55,7 +55,7 @@
   return [[self alloc] init];
 }
 
-- (GRXSingleValueHandler)block {
+- (GRXSingleHandler)block {
   return ^(id value, NSError *errorOrNil) {
     ++_timesCalled;
     _value = value;
@@ -71,13 +71,13 @@
 
 #pragma mark Writeable
 
-- (void)testWriteableSingleValueHandlerIsCalledForValue {
+- (void)testWriteableSingleHandlerIsCalledForValue {
   // Given:
   CapturingSingleValueHandler *handler = [CapturingSingleValueHandler handler];
   id anyValue = @7;
 
   // If:
-  id<GRXWriteable> writeable = [GRXWriteable writeableWithSingleValueHandler:handler.block];
+  id<GRXWriteable> writeable = [GRXWriteable writeableWithSingleHandler:handler.block];
   [writeable writeValue:anyValue];
 
   // Then:
@@ -86,13 +86,13 @@
   XCTAssertEqualObjects(handler.errorOrNil, nil);
 }
 
-- (void)testWriteableSingleValueHandlerIsCalledForError {
+- (void)testWriteableSingleHandlerIsCalledForError {
   // Given:
   CapturingSingleValueHandler *handler = [CapturingSingleValueHandler handler];
   NSError *anyError = [NSError errorWithDomain:@"domain" code:7 userInfo:nil];
 
   // If:
-  id<GRXWriteable> writeable = [GRXWriteable writeableWithSingleValueHandler:handler.block];
+  id<GRXWriteable> writeable = [GRXWriteable writeableWithSingleHandler:handler.block];
   [writeable writesFinishedWithError:anyError];
 
   // Then:
@@ -106,7 +106,7 @@
 - (void)testBufferedPipePropagatesValue {
   // Given:
   CapturingSingleValueHandler *handler = [CapturingSingleValueHandler handler];
-  id<GRXWriteable> writeable = [GRXWriteable writeableWithSingleValueHandler:handler.block];
+  id<GRXWriteable> writeable = [GRXWriteable writeableWithSingleHandler:handler.block];
   id anyValue = @7;
 
   // If:
@@ -123,7 +123,7 @@
 - (void)testBufferedPipePropagatesError {
   // Given:
   CapturingSingleValueHandler *handler = [CapturingSingleValueHandler handler];
-  id<GRXWriteable> writeable = [GRXWriteable writeableWithSingleValueHandler:handler.block];
+  id<GRXWriteable> writeable = [GRXWriteable writeableWithSingleHandler:handler.block];
   NSError *anyError = [NSError errorWithDomain:@"domain" code:7 userInfo:nil];
 
   // If:
diff --git a/src/objective-c/tests/TestCertificates.bundle/test-certificates.pem b/src/objective-c/tests/TestCertificates.bundle/test-certificates.pem
new file mode 100644
index 0000000000..6c8511a73c
--- /dev/null
+++ b/src/objective-c/tests/TestCertificates.bundle/test-certificates.pem
@@ -0,0 +1,15 @@
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----
diff --git a/src/objective-c/tests/Tests.xcodeproj/project.pbxproj b/src/objective-c/tests/Tests.xcodeproj/project.pbxproj
index f13fb8288b..3a1c3d940a 100644
--- a/src/objective-c/tests/Tests.xcodeproj/project.pbxproj
+++ b/src/objective-c/tests/Tests.xcodeproj/project.pbxproj
@@ -13,6 +13,9 @@
 		63423F511B151B77006CF63C /* RxLibraryUnitTests.m in Sources */ = {isa = PBXBuildFile; fileRef = 63423F501B151B77006CF63C /* RxLibraryUnitTests.m */; };
 		635697CD1B14FC11007A7283 /* Tests.m in Sources */ = {isa = PBXBuildFile; fileRef = 635697CC1B14FC11007A7283 /* Tests.m */; };
 		635ED2EC1B1A3BC400FDE5C3 /* InteropTests.m in Sources */ = {isa = PBXBuildFile; fileRef = 635ED2EB1B1A3BC400FDE5C3 /* InteropTests.m */; };
+		63715F561B780C020029CB0B /* InteropTestsLocalCleartext.m in Sources */ = {isa = PBXBuildFile; fileRef = 63715F551B780C020029CB0B /* InteropTestsLocalCleartext.m */; };
+		63E240CE1B6C4E2B005F3B0E /* InteropTestsLocalSSL.m in Sources */ = {isa = PBXBuildFile; fileRef = 63E240CD1B6C4E2B005F3B0E /* InteropTestsLocalSSL.m */; };
+		63E240D01B6C63DC005F3B0E /* TestCertificates.bundle in Resources */ = {isa = PBXBuildFile; fileRef = 63E240CF1B6C63DC005F3B0E /* TestCertificates.bundle */; };
 		7D8A186224D39101F90230F6 /* libPods.a in Frameworks */ = {isa = PBXBuildFile; fileRef = 35F2B6BF3BAE8F0DC4AFD76E /* libPods.a */; };
 /* End PBXBuildFile section */
 
@@ -49,6 +52,10 @@
 		635697CC1B14FC11007A7283 /* Tests.m */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.objc; path = Tests.m; sourceTree = "<group>"; };
 		635697D81B14FC11007A7283 /* Info.plist */ = {isa = PBXFileReference; lastKnownFileType = text.plist.xml; path = Info.plist; sourceTree = "<group>"; };
 		635ED2EB1B1A3BC400FDE5C3 /* InteropTests.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = InteropTests.m; sourceTree = "<group>"; };
+		63715F551B780C020029CB0B /* InteropTestsLocalCleartext.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = InteropTestsLocalCleartext.m; sourceTree = "<group>"; };
+		63E240CC1B6C4D3A005F3B0E /* InteropTests.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = InteropTests.h; sourceTree = "<group>"; };
+		63E240CD1B6C4E2B005F3B0E /* InteropTestsLocalSSL.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = InteropTestsLocalSSL.m; sourceTree = "<group>"; };
+		63E240CF1B6C63DC005F3B0E /* TestCertificates.bundle */ = {isa = PBXFileReference; lastKnownFileType = "wrapper.plug-in"; path = TestCertificates.bundle; sourceTree = "<group>"; };
 		FF7B5489BCFE40111D768DD0 /* Pods.debug.xcconfig */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = text.xcconfig; name = Pods.debug.xcconfig; path = "Pods/Target Support Files/Pods/Pods.debug.xcconfig"; sourceTree = "<group>"; };
 /* End PBXFileReference section */
 
@@ -93,6 +100,7 @@
 			isa = PBXGroup;
 			children = (
 				635697C91B14FC11007A7283 /* Tests */,
+				63E240CF1B6C63DC005F3B0E /* TestCertificates.bundle */,
 				635697C81B14FC11007A7283 /* Products */,
 				51E4650F34F854F41FF053B3 /* Pods */,
 				136D535E19727099B941D7B1 /* Frameworks */,
@@ -112,9 +120,12 @@
 			isa = PBXGroup;
 			children = (
 				6312AE4D1B1BF49B00341DEE /* GRPCClientTests.m */,
-				63175DFE1B1B9FAF00027841 /* LocalClearTextTests.m */,
+				63E240CC1B6C4D3A005F3B0E /* InteropTests.h */,
 				635ED2EB1B1A3BC400FDE5C3 /* InteropTests.m */,
+				63E240CD1B6C4E2B005F3B0E /* InteropTestsLocalSSL.m */,
+				63715F551B780C020029CB0B /* InteropTestsLocalCleartext.m */,
 				63423F501B151B77006CF63C /* RxLibraryUnitTests.m */,
+				63175DFE1B1B9FAF00027841 /* LocalClearTextTests.m */,
 				635697CC1B14FC11007A7283 /* Tests.m */,
 				635697D71B14FC11007A7283 /* Supporting Files */,
 			);
@@ -209,6 +220,7 @@
 			isa = PBXResourcesBuildPhase;
 			buildActionMask = 2147483647;
 			files = (
+				63E240D01B6C63DC005F3B0E /* TestCertificates.bundle in Resources */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;
 		};
@@ -252,8 +264,10 @@
 			isa = PBXSourcesBuildPhase;
 			buildActionMask = 2147483647;
 			files = (
+				63715F561B780C020029CB0B /* InteropTestsLocalCleartext.m in Sources */,
 				63175DFF1B1B9FAF00027841 /* LocalClearTextTests.m in Sources */,
 				63423F511B151B77006CF63C /* RxLibraryUnitTests.m in Sources */,
+				63E240CE1B6C4E2B005F3B0E /* InteropTestsLocalSSL.m in Sources */,
 				6312AE4E1B1BF49B00341DEE /* GRPCClientTests.m in Sources */,
 				635ED2EC1B1A3BC400FDE5C3 /* InteropTests.m in Sources */,
 			);
diff --git a/src/objective-c/tests/run_tests.sh b/src/objective-c/tests/run_tests.sh
index b13c0f0633..7b133c1782 100755
--- a/src/objective-c/tests/run_tests.sh
+++ b/src/objective-c/tests/run_tests.sh
@@ -37,7 +37,8 @@ cd $(dirname $0)
 
 # Run the tests server.
 ../../../bins/$CONFIG/interop_server --port=5050 &
-# Kill it when this script exits.
+../../../bins/$CONFIG/interop_server --port=5051 --enable_ssl &
+# Kill them when this script exits.
 trap 'kill -9 `jobs -p`' EXIT
 
 # xcodebuild is very verbose. We filter its output and tell Bash to fail if any