Merge pull request #4958 from sreecha/server_try_cancel_api

Implement a TryCancel() API on ServerContext to cancel the API from the server side

4 files changed, 860 insertions, 30 deletions

diff --git a/test/cpp/end2end/async_end2end_test.cc b/test/cpp/end2end/async_end2end_test.cc
index 0616cc07ee..252bda3798 100644
--- a/test/cpp/end2end/async_end2end_test.cc
+++ b/test/cpp/end2end/async_end2end_test.cc
@@ -32,6 +32,7 @@
  */
 
 #include <memory>
+#include <thread>
 
 #include <grpc++/channel.h>
 #include <grpc++/client_context.h>
@@ -104,7 +105,10 @@ class Verifier : public PollingCheckRegion {
     expectations_[tag(i)] = expect_ok;
     return *this;
   }
-  void Verify(CompletionQueue* cq) {
+
+  void Verify(CompletionQueue* cq) { Verify(cq, false); }
+
+  void Verify(CompletionQueue* cq, bool ignore_ok) {
     GPR_ASSERT(!expectations_.empty());
     while (!expectations_.empty()) {
       bool ok;
@@ -122,7 +126,9 @@ class Verifier : public PollingCheckRegion {
       }
       auto it = expectations_.find(got_tag);
       EXPECT_TRUE(it != expectations_.end());
-      EXPECT_EQ(it->second, ok);
+      if (!ignore_ok) {
+        EXPECT_EQ(it->second, ok);
+      }
       expectations_.erase(it);
     }
   }
@@ -217,7 +223,7 @@ class AsyncEnd2endTest : public ::testing::TestWithParam<bool> {
       grpc::ServerAsyncResponseWriter<EchoResponse> response_writer(&srv_ctx);
 
       send_request.set_message("Hello");
-      std::unique_ptr<ClientAsyncResponseReader<EchoResponse> > response_reader(
+      std::unique_ptr<ClientAsyncResponseReader<EchoResponse>> response_reader(
           stub_->AsyncEcho(&cli_ctx, send_request, cq_.get()));
 
       service_.RequestEcho(&srv_ctx, &recv_request, &response_writer, cq_.get(),
@@ -270,7 +276,7 @@ TEST_P(AsyncEnd2endTest, AsyncNextRpc) {
   grpc::ServerAsyncResponseWriter<EchoResponse> response_writer(&srv_ctx);
 
   send_request.set_message("Hello");
-  std::unique_ptr<ClientAsyncResponseReader<EchoResponse> > response_reader(
+  std::unique_ptr<ClientAsyncResponseReader<EchoResponse>> response_reader(
       stub_->AsyncEcho(&cli_ctx, send_request, cq_.get()));
 
   std::chrono::system_clock::time_point time_now(
@@ -315,7 +321,7 @@ TEST_P(AsyncEnd2endTest, SimpleClientStreaming) {
   ServerAsyncReader<EchoResponse, EchoRequest> srv_stream(&srv_ctx);
 
   send_request.set_message("Hello");
-  std::unique_ptr<ClientAsyncWriter<EchoRequest> > cli_stream(
+  std::unique_ptr<ClientAsyncWriter<EchoRequest>> cli_stream(
       stub_->AsyncRequestStream(&cli_ctx, &recv_response, cq_.get(), tag(1)));
 
   service_.RequestRequestStream(&srv_ctx, &srv_stream, cq_.get(), cq_.get(),
@@ -368,7 +374,7 @@ TEST_P(AsyncEnd2endTest, SimpleServerStreaming) {
   ServerAsyncWriter<EchoResponse> srv_stream(&srv_ctx);
 
   send_request.set_message("Hello");
-  std::unique_ptr<ClientAsyncReader<EchoResponse> > cli_stream(
+  std::unique_ptr<ClientAsyncReader<EchoResponse>> cli_stream(
       stub_->AsyncResponseStream(&cli_ctx, send_request, cq_.get(), tag(1)));
 
   service_.RequestResponseStream(&srv_ctx, &recv_request, &srv_stream,
@@ -418,7 +424,7 @@ TEST_P(AsyncEnd2endTest, SimpleBidiStreaming) {
   ServerAsyncReaderWriter<EchoResponse, EchoRequest> srv_stream(&srv_ctx);
 
   send_request.set_message("Hello");
-  std::unique_ptr<ClientAsyncReaderWriter<EchoRequest, EchoResponse> >
+  std::unique_ptr<ClientAsyncReaderWriter<EchoRequest, EchoResponse>>
       cli_stream(stub_->AsyncBidiStream(&cli_ctx, cq_.get(), tag(1)));
 
   service_.RequestBidiStream(&srv_ctx, &srv_stream, cq_.get(), cq_.get(),
@@ -476,7 +482,7 @@ TEST_P(AsyncEnd2endTest, ClientInitialMetadataRpc) {
   cli_ctx.AddMetadata(meta1.first, meta1.second);
   cli_ctx.AddMetadata(meta2.first, meta2.second);
 
-  std::unique_ptr<ClientAsyncResponseReader<EchoResponse> > response_reader(
+  std::unique_ptr<ClientAsyncResponseReader<EchoResponse>> response_reader(
       stub_->AsyncEcho(&cli_ctx, send_request, cq_.get()));
 
   service_.RequestEcho(&srv_ctx, &recv_request, &response_writer, cq_.get(),
@@ -519,7 +525,7 @@ TEST_P(AsyncEnd2endTest, ServerInitialMetadataRpc) {
   std::pair<grpc::string, grpc::string> meta1("key1", "val1");
   std::pair<grpc::string, grpc::string> meta2("key2", "val2");
 
-  std::unique_ptr<ClientAsyncResponseReader<EchoResponse> > response_reader(
+  std::unique_ptr<ClientAsyncResponseReader<EchoResponse>> response_reader(
       stub_->AsyncEcho(&cli_ctx, send_request, cq_.get()));
 
   service_.RequestEcho(&srv_ctx, &recv_request, &response_writer, cq_.get(),
@@ -568,7 +574,7 @@ TEST_P(AsyncEnd2endTest, ServerTrailingMetadataRpc) {
   std::pair<grpc::string, grpc::string> meta1("key1", "val1");
   std::pair<grpc::string, grpc::string> meta2("key2", "val2");
 
-  std::unique_ptr<ClientAsyncResponseReader<EchoResponse> > response_reader(
+  std::unique_ptr<ClientAsyncResponseReader<EchoResponse>> response_reader(
       stub_->AsyncEcho(&cli_ctx, send_request, cq_.get()));
 
   service_.RequestEcho(&srv_ctx, &recv_request, &response_writer, cq_.get(),
@@ -629,7 +635,7 @@ TEST_P(AsyncEnd2endTest, MetadataRpc) {
   cli_ctx.AddMetadata(meta1.first, meta1.second);
   cli_ctx.AddMetadata(meta2.first, meta2.second);
 
-  std::unique_ptr<ClientAsyncResponseReader<EchoResponse> > response_reader(
+  std::unique_ptr<ClientAsyncResponseReader<EchoResponse>> response_reader(
       stub_->AsyncEcho(&cli_ctx, send_request, cq_.get()));
 
   service_.RequestEcho(&srv_ctx, &recv_request, &response_writer, cq_.get(),
@@ -690,7 +696,7 @@ TEST_P(AsyncEnd2endTest, ServerCheckCancellation) {
   grpc::ServerAsyncResponseWriter<EchoResponse> response_writer(&srv_ctx);
 
   send_request.set_message("Hello");
-  std::unique_ptr<ClientAsyncResponseReader<EchoResponse> > response_reader(
+  std::unique_ptr<ClientAsyncResponseReader<EchoResponse>> response_reader(
       stub_->AsyncEcho(&cli_ctx, send_request, cq_.get()));
 
   srv_ctx.AsyncNotifyWhenDone(tag(5));
@@ -725,7 +731,7 @@ TEST_P(AsyncEnd2endTest, ServerCheckDone) {
   grpc::ServerAsyncResponseWriter<EchoResponse> response_writer(&srv_ctx);
 
   send_request.set_message("Hello");
-  std::unique_ptr<ClientAsyncResponseReader<EchoResponse> > response_reader(
+  std::unique_ptr<ClientAsyncResponseReader<EchoResponse>> response_reader(
       stub_->AsyncEcho(&cli_ctx, send_request, cq_.get()));
 
   srv_ctx.AsyncNotifyWhenDone(tag(5));
@@ -759,7 +765,7 @@ TEST_P(AsyncEnd2endTest, UnimplementedRpc) {
 
   ClientContext cli_ctx;
   send_request.set_message("Hello");
-  std::unique_ptr<ClientAsyncResponseReader<EchoResponse> > response_reader(
+  std::unique_ptr<ClientAsyncResponseReader<EchoResponse>> response_reader(
       stub->AsyncUnimplemented(&cli_ctx, send_request, cq_.get()));
 
   response_reader->Finish(&recv_response, &recv_status, tag(4));
@@ -769,8 +775,384 @@ TEST_P(AsyncEnd2endTest, UnimplementedRpc) {
   EXPECT_EQ("", recv_status.error_message());
 }
 
+// This class is for testing scenarios where RPCs are cancelled on the server
+// by calling ServerContext::TryCancel()
+class AsyncEnd2endServerTryCancelTest : public AsyncEnd2endTest {
+ protected:
+  typedef enum {
+    DO_NOT_CANCEL = 0,
+    CANCEL_BEFORE_PROCESSING,
+    CANCEL_DURING_PROCESSING,
+    CANCEL_AFTER_PROCESSING
+  } ServerTryCancelRequestPhase;
+
+  void ServerTryCancel(ServerContext* context) {
+    EXPECT_FALSE(context->IsCancelled());
+    context->TryCancel();
+    gpr_log(GPR_INFO, "Server called TryCancel()");
+    EXPECT_TRUE(context->IsCancelled());
+  }
+
+  // Helper for testing client-streaming RPCs which are cancelled on the server.
+  // Depending on the value of server_try_cancel parameter, this will test one
+  // of the following three scenarios:
+  //   CANCEL_BEFORE_PROCESSING: Rpc is cancelled by the server before reading
+  //   any messages from the client
+  //
+  //   CANCEL_DURING_PROCESSING: Rpc is cancelled by the server while reading
+  //   messages from the client
+  //
+  //   CANCEL_AFTER PROCESSING: Rpc is cancelled by server after reading all
+  //   messages from the client (but before sending any status back to the
+  //   client)
+  void TestClientStreamingServerCancel(
+      ServerTryCancelRequestPhase server_try_cancel) {
+    ResetStub();
+
+    EchoRequest send_request;
+    EchoRequest recv_request;
+    EchoResponse send_response;
+    EchoResponse recv_response;
+    Status recv_status;
+
+    ClientContext cli_ctx;
+    ServerContext srv_ctx;
+    ServerAsyncReader<EchoResponse, EchoRequest> srv_stream(&srv_ctx);
+
+    // Initiate the 'RequestStream' call on client
+    std::unique_ptr<ClientAsyncWriter<EchoRequest>> cli_stream(
+        stub_->AsyncRequestStream(&cli_ctx, &recv_response, cq_.get(), tag(1)));
+    Verifier(GetParam()).Expect(1, true).Verify(cq_.get());
+
+    // On the server, request to be notified of 'RequestStream' calls
+    // and receive the 'RequestStream' call just made by the client
+    service_.RequestRequestStream(&srv_ctx, &srv_stream, cq_.get(), cq_.get(),
+                                  tag(2));
+    Verifier(GetParam()).Expect(2, true).Verify(cq_.get());
+
+    // Client sends 3 messages (tags 3, 4 and 5)
+    for (int tag_idx = 3; tag_idx <= 5; tag_idx++) {
+      send_request.set_message("Ping " + std::to_string(tag_idx));
+      cli_stream->Write(send_request, tag(tag_idx));
+      Verifier(GetParam()).Expect(tag_idx, true).Verify(cq_.get());
+    }
+    cli_stream->WritesDone(tag(6));
+    Verifier(GetParam()).Expect(6, true).Verify(cq_.get());
+
+    bool expected_server_cq_result = true;
+    bool ignore_cq_result = false;
+
+    if (server_try_cancel == CANCEL_BEFORE_PROCESSING) {
+      ServerTryCancel(&srv_ctx);
+
+      // Since cancellation is done before server reads any results, we know
+      // for sure that all cq results will return false from this point forward
+      expected_server_cq_result = false;
+    }
+
+    std::thread* server_try_cancel_thd = NULL;
+    if (server_try_cancel == CANCEL_DURING_PROCESSING) {
+      server_try_cancel_thd = new std::thread(
+          &AsyncEnd2endServerTryCancelTest::ServerTryCancel, this, &srv_ctx);
+      // Server will cancel the RPC in a parallel thread while reading the
+      // requests from the client. Since the cancellation can happen at anytime,
+      // some of the cq results (i.e those until cancellation) might be true but
+      // its non deterministic. So better to ignore the cq results
+      ignore_cq_result = true;
+    }
+
+    // Server reads 3 messages (tags 6, 7 and 8)
+    for (int tag_idx = 6; tag_idx <= 8; tag_idx++) {
+      srv_stream.Read(&recv_request, tag(tag_idx));
+      Verifier(GetParam())
+          .Expect(tag_idx, expected_server_cq_result)
+          .Verify(cq_.get(), ignore_cq_result);
+    }
+
+    if (server_try_cancel_thd != NULL) {
+      server_try_cancel_thd->join();
+      delete server_try_cancel_thd;
+    }
+
+    if (server_try_cancel == CANCEL_AFTER_PROCESSING) {
+      ServerTryCancel(&srv_ctx);
+    }
+
+    // The RPC has been cancelled at this point for sure (i.e irrespective of
+    // the value of `server_try_cancel` is). So, from this point forward, we
+    // know that cq results are supposed to return false on server.
+
+    // Server sends the final message and cancelled status (but the RPC is
+    // already cancelled at this point. So we expect the operation to fail)
+    srv_stream.Finish(send_response, Status::CANCELLED, tag(9));
+    Verifier(GetParam()).Expect(9, false).Verify(cq_.get());
+
+    // Client will see the cancellation
+    cli_stream->Finish(&recv_status, tag(10));
+    // TODO(sreek): The expectation here should be true. This is a bug (github
+    // issue #4972)
+    Verifier(GetParam()).Expect(10, false).Verify(cq_.get());
+    EXPECT_FALSE(recv_status.ok());
+    EXPECT_EQ(::grpc::StatusCode::CANCELLED, recv_status.error_code());
+  }
+
+  // Helper for testing server-streaming RPCs which are cancelled on the server.
+  // Depending on the value of server_try_cancel parameter, this will test one
+  // of the following three scenarios:
+  //   CANCEL_BEFORE_PROCESSING: Rpc is cancelled by the server before sending
+  //   any messages to the client
+  //
+  //   CANCEL_DURING_PROCESSING: Rpc is cancelled by the server while sending
+  //   messages to the client
+  //
+  //   CANCEL_AFTER PROCESSING: Rpc is cancelled by server after sending all
+  //   messages to the client (but before sending any status back to the
+  //   client)
+  void TestServerStreamingServerCancel(
+      ServerTryCancelRequestPhase server_try_cancel) {
+    ResetStub();
+
+    EchoRequest send_request;
+    EchoRequest recv_request;
+    EchoResponse send_response;
+    EchoResponse recv_response;
+    Status recv_status;
+    ClientContext cli_ctx;
+    ServerContext srv_ctx;
+    ServerAsyncWriter<EchoResponse> srv_stream(&srv_ctx);
+
+    send_request.set_message("Ping");
+    // Initiate the 'ResponseStream' call on the client
+    std::unique_ptr<ClientAsyncReader<EchoResponse>> cli_stream(
+        stub_->AsyncResponseStream(&cli_ctx, send_request, cq_.get(), tag(1)));
+    Verifier(GetParam()).Expect(1, true).Verify(cq_.get());
+    // On the server, request to be notified of 'ResponseStream' calls and
+    // receive the call just made by the client
+    service_.RequestResponseStream(&srv_ctx, &recv_request, &srv_stream,
+                                   cq_.get(), cq_.get(), tag(2));
+    Verifier(GetParam()).Expect(2, true).Verify(cq_.get());
+    EXPECT_EQ(send_request.message(), recv_request.message());
+
+    bool expected_cq_result = true;
+    bool ignore_cq_result = false;
+
+    if (server_try_cancel == CANCEL_BEFORE_PROCESSING) {
+      ServerTryCancel(&srv_ctx);
+
+      // We know for sure that all cq results will be false from this point
+      // since the server cancelled the RPC
+      expected_cq_result = false;
+    }
+
+    std::thread* server_try_cancel_thd = NULL;
+    if (server_try_cancel == CANCEL_DURING_PROCESSING) {
+      server_try_cancel_thd = new std::thread(
+          &AsyncEnd2endServerTryCancelTest::ServerTryCancel, this, &srv_ctx);
+
+      // Server will cancel the RPC in a parallel thread while writing responses
+      // to the client. Since the cancellation can happen at anytime, some of
+      // the cq results (i.e those until cancellation) might be true but it is
+      // non deterministic. So better to ignore the cq results
+      ignore_cq_result = true;
+    }
+
+    // Server sends three messages (tags 3, 4 and 5)
+    for (int tag_idx = 3; tag_idx <= 5; tag_idx++) {
+      send_response.set_message("Pong " + std::to_string(tag_idx));
+      srv_stream.Write(send_response, tag(tag_idx));
+      Verifier(GetParam())
+          .Expect(tag_idx, expected_cq_result)
+          .Verify(cq_.get(), ignore_cq_result);
+    }
+
+    if (server_try_cancel_thd != NULL) {
+      server_try_cancel_thd->join();
+      delete server_try_cancel_thd;
+    }
+
+    if (server_try_cancel == CANCEL_AFTER_PROCESSING) {
+      ServerTryCancel(&srv_ctx);
+    }
+
+    // Client attemts to read the three messages from the server
+    for (int tag_idx = 6; tag_idx <= 8; tag_idx++) {
+      cli_stream->Read(&recv_response, tag(tag_idx));
+      Verifier(GetParam())
+          .Expect(tag_idx, expected_cq_result)
+          .Verify(cq_.get(), ignore_cq_result);
+    }
+
+    // The RPC has been cancelled at this point for sure (i.e irrespective of
+    // the value of `server_try_cancel` is). So, from this point forward, we
+    // know that cq results are supposed to return false on server.
+
+    // Server finishes the stream (but the RPC is already cancelled)
+    srv_stream.Finish(Status::CANCELLED, tag(9));
+    Verifier(GetParam()).Expect(9, false).Verify(cq_.get());
+
+    // Client will see the cancellation
+    cli_stream->Finish(&recv_status, tag(10));
+    Verifier(GetParam()).Expect(10, true).Verify(cq_.get());
+    EXPECT_FALSE(recv_status.ok());
+    EXPECT_EQ(::grpc::StatusCode::CANCELLED, recv_status.error_code());
+  }
+
+  // Helper for testing bidirectinal-streaming RPCs which are cancelled on the
+  // server.
+  //
+  // Depending on the value of server_try_cancel parameter, this will
+  // test one of the following three scenarios:
+  //   CANCEL_BEFORE_PROCESSING: Rpc is cancelled by the server before reading/
+  //   writing any messages from/to the client
+  //
+  //   CANCEL_DURING_PROCESSING: Rpc is cancelled by the server while reading
+  //   messages from the client
+  //
+  //   CANCEL_AFTER PROCESSING: Rpc is cancelled by server after reading all
+  //   messages from the client (but before sending any status back to the
+  //   client)
+  void TestBidiStreamingServerCancel(
+      ServerTryCancelRequestPhase server_try_cancel) {
+    ResetStub();
+
+    EchoRequest send_request;
+    EchoRequest recv_request;
+    EchoResponse send_response;
+    EchoResponse recv_response;
+    Status recv_status;
+    ClientContext cli_ctx;
+    ServerContext srv_ctx;
+    ServerAsyncReaderWriter<EchoResponse, EchoRequest> srv_stream(&srv_ctx);
+
+    // Initiate the call from the client side
+    std::unique_ptr<ClientAsyncReaderWriter<EchoRequest, EchoResponse>>
+        cli_stream(stub_->AsyncBidiStream(&cli_ctx, cq_.get(), tag(1)));
+    Verifier(GetParam()).Expect(1, true).Verify(cq_.get());
+
+    // On the server, request to be notified of the 'BidiStream' call and
+    // receive the call just made by the client
+    service_.RequestBidiStream(&srv_ctx, &srv_stream, cq_.get(), cq_.get(),
+                               tag(2));
+    Verifier(GetParam()).Expect(2, true).Verify(cq_.get());
+
+    // Client sends the first and the only message
+    send_request.set_message("Ping");
+    cli_stream->Write(send_request, tag(3));
+    Verifier(GetParam()).Expect(3, true).Verify(cq_.get());
+
+    bool expected_cq_result = true;
+    bool ignore_cq_result = false;
+
+    if (server_try_cancel == CANCEL_BEFORE_PROCESSING) {
+      ServerTryCancel(&srv_ctx);
+
+      // We know for sure that all cq results will be false from this point
+      // since the server cancelled the RPC
+      expected_cq_result = false;
+    }
+
+    std::thread* server_try_cancel_thd = NULL;
+    if (server_try_cancel == CANCEL_DURING_PROCESSING) {
+      server_try_cancel_thd = new std::thread(
+          &AsyncEnd2endServerTryCancelTest::ServerTryCancel, this, &srv_ctx);
+
+      // Since server is going to cancel the RPC in a parallel thread, some of
+      // the cq results (i.e those until the cancellation) might be true. Since
+      // that number is non-deterministic, it is better to ignore the cq results
+      ignore_cq_result = true;
+    }
+
+    srv_stream.Read(&recv_request, tag(4));
+    Verifier(GetParam())
+        .Expect(4, expected_cq_result)
+        .Verify(cq_.get(), ignore_cq_result);
+
+    send_response.set_message("Pong");
+    srv_stream.Write(send_response, tag(5));
+    Verifier(GetParam())
+        .Expect(5, expected_cq_result)
+        .Verify(cq_.get(), ignore_cq_result);
+
+    cli_stream->Read(&recv_response, tag(6));
+    Verifier(GetParam())
+        .Expect(6, expected_cq_result)
+        .Verify(cq_.get(), ignore_cq_result);
+
+    // This is expected to succeed in all cases
+    cli_stream->WritesDone(tag(7));
+    Verifier(GetParam()).Expect(7, true).Verify(cq_.get());
+
+    // This is expected to fail in all cases i.e for all values of
+    // server_try_cancel. This is becasue at this point, either there are no
+    // more msgs from the client (because client called WritesDone) or the RPC
+    // is cancelled on the server
+    srv_stream.Read(&recv_request, tag(8));
+    Verifier(GetParam()).Expect(8, false).Verify(cq_.get());
+
+    if (server_try_cancel_thd != NULL) {
+      server_try_cancel_thd->join();
+      delete server_try_cancel_thd;
+    }
+
+    if (server_try_cancel == CANCEL_AFTER_PROCESSING) {
+      ServerTryCancel(&srv_ctx);
+    }
+
+    // The RPC has been cancelled at this point for sure (i.e irrespective of
+    // the value of `server_try_cancel` is). So, from this point forward, we
+    // know that cq results are supposed to return false on server.
+
+    srv_stream.Finish(Status::CANCELLED, tag(9));
+    Verifier(GetParam()).Expect(9, false).Verify(cq_.get());
+
+    cli_stream->Finish(&recv_status, tag(10));
+    Verifier(GetParam()).Expect(10, true).Verify(cq_.get());
+    EXPECT_FALSE(recv_status.ok());
+    EXPECT_EQ(grpc::StatusCode::CANCELLED, recv_status.error_code());
+  }
+};
+
+TEST_P(AsyncEnd2endServerTryCancelTest, ClientStreamingServerTryCancelBefore) {
+  TestClientStreamingServerCancel(CANCEL_BEFORE_PROCESSING);
+}
+
+TEST_P(AsyncEnd2endServerTryCancelTest, ClientStreamingServerTryCancelDuring) {
+  TestClientStreamingServerCancel(CANCEL_DURING_PROCESSING);
+}
+
+TEST_P(AsyncEnd2endServerTryCancelTest, ClientStreamingServerTryCancelAfter) {
+  TestClientStreamingServerCancel(CANCEL_AFTER_PROCESSING);
+}
+
+TEST_P(AsyncEnd2endServerTryCancelTest, ServerStreamingServerTryCancelBefore) {
+  TestServerStreamingServerCancel(CANCEL_BEFORE_PROCESSING);
+}
+
+TEST_P(AsyncEnd2endServerTryCancelTest, ServerStreamingServerTryCancelDuring) {
+  TestServerStreamingServerCancel(CANCEL_DURING_PROCESSING);
+}
+
+TEST_P(AsyncEnd2endServerTryCancelTest, ServerStreamingServerTryCancelAfter) {
+  TestServerStreamingServerCancel(CANCEL_AFTER_PROCESSING);
+}
+
+TEST_P(AsyncEnd2endServerTryCancelTest, ServerBidiStreamingTryCancelBefore) {
+  TestBidiStreamingServerCancel(CANCEL_BEFORE_PROCESSING);
+}
+
+TEST_P(AsyncEnd2endServerTryCancelTest, ServerBidiStreamingTryCancelDuring) {
+  TestBidiStreamingServerCancel(CANCEL_DURING_PROCESSING);
+}
+
+TEST_P(AsyncEnd2endServerTryCancelTest, ServerBidiStreamingTryCancelAfter) {
+  TestBidiStreamingServerCancel(CANCEL_AFTER_PROCESSING);
+}
+
 INSTANTIATE_TEST_CASE_P(AsyncEnd2end, AsyncEnd2endTest,
                         ::testing::Values(false, true));
+INSTANTIATE_TEST_CASE_P(AsyncEnd2endServerTryCancel,
+                        AsyncEnd2endServerTryCancelTest,
+                        ::testing::Values(false));
 
 }  // namespace
 }  // namespace testing
diff --git a/test/cpp/end2end/end2end_test.cc b/test/cpp/end2end/end2end_test.cc
index cdc123d124..65da71b391 100644
--- a/test/cpp/end2end/end2end_test.cc
+++ b/test/cpp/end2end/end2end_test.cc
@@ -306,6 +306,301 @@ static void SendRpc(grpc::testing::EchoTestService::Stub* stub, int num_rpcs,
   }
 }
 
+// This class is for testing scenarios where RPCs are cancelled on the server
+// by calling ServerContext::TryCancel()
+class End2endServerTryCancelTest : public End2endTest {
+ protected:
+  // Helper for testing client-streaming RPCs which are cancelled on the server.
+  // Depending on the value of server_try_cancel parameter, this will test one
+  // of the following three scenarios:
+  //   CANCEL_BEFORE_PROCESSING: Rpc is cancelled by the server before reading
+  //   any messages from the client
+  //
+  //   CANCEL_DURING_PROCESSING: Rpc is cancelled by the server while reading
+  //   messages from the client
+  //
+  //   CANCEL_AFTER PROCESSING: Rpc is cancelled by server after reading all
+  //   the messages from the client
+  //
+  // NOTE: Do not call this function with server_try_cancel == DO_NOT_CANCEL.
+  void TestRequestStreamServerCancel(
+      ServerTryCancelRequestPhase server_try_cancel, int num_msgs_to_send) {
+    ResetStub();
+    EchoRequest request;
+    EchoResponse response;
+    ClientContext context;
+
+    // Send server_try_cancel value in the client metadata
+    context.AddMetadata(kServerTryCancelRequest,
+                        std::to_string(server_try_cancel));
+
+    auto stream = stub_->RequestStream(&context, &response);
+
+    int num_msgs_sent = 0;
+    while (num_msgs_sent < num_msgs_to_send) {
+      request.set_message("hello");
+      if (!stream->Write(request)) {
+        break;
+      }
+      num_msgs_sent++;
+    }
+    gpr_log(GPR_INFO, "Sent %d messages", num_msgs_sent);
+
+    stream->WritesDone();
+    Status s = stream->Finish();
+
+    // At this point, we know for sure that RPC was cancelled by the server
+    // since we passed server_try_cancel value in the metadata. Depending on the
+    // value of server_try_cancel, the RPC might have been cancelled by the
+    // server at different stages. The following validates our expectations of
+    // number of messages sent in various cancellation scenarios:
+
+    switch (server_try_cancel) {
+      case CANCEL_BEFORE_PROCESSING:
+      case CANCEL_DURING_PROCESSING:
+        // If the RPC is cancelled by server before / during messages from the
+        // client, it means that the client most likely did not get a chance to
+        // send all the messages it wanted to send. i.e num_msgs_sent <=
+        // num_msgs_to_send
+        EXPECT_LE(num_msgs_sent, num_msgs_to_send);
+        break;
+
+      case CANCEL_AFTER_PROCESSING:
+        // If the RPC was cancelled after all messages were read by the server,
+        // the client did get a chance to send all its messages
+        EXPECT_EQ(num_msgs_sent, num_msgs_to_send);
+        break;
+
+      default:
+        gpr_log(GPR_ERROR, "Invalid server_try_cancel value: %d",
+                server_try_cancel);
+        EXPECT_TRUE(server_try_cancel > DO_NOT_CANCEL &&
+                    server_try_cancel <= CANCEL_AFTER_PROCESSING);
+        break;
+    }
+
+    EXPECT_FALSE(s.ok());
+    EXPECT_EQ(grpc::StatusCode::CANCELLED, s.error_code());
+  }
+
+  // Helper for testing server-streaming RPCs which are cancelled on the server.
+  // Depending on the value of server_try_cancel parameter, this will test one
+  // of the following three scenarios:
+  //   CANCEL_BEFORE_PROCESSING: Rpc is cancelled by the server before writing
+  //   any messages to the client
+  //
+  //   CANCEL_DURING_PROCESSING: Rpc is cancelled by the server while writing
+  //   messages to the client
+  //
+  //   CANCEL_AFTER PROCESSING: Rpc is cancelled by server after writing all
+  //   the messages to the client
+  //
+  // NOTE: Do not call this function with server_try_cancel == DO_NOT_CANCEL.
+  void TestResponseStreamServerCancel(
+      ServerTryCancelRequestPhase server_try_cancel) {
+    ResetStub();
+    EchoRequest request;
+    EchoResponse response;
+    ClientContext context;
+
+    // Send server_try_cancel in the client metadata
+    context.AddMetadata(kServerTryCancelRequest,
+                        std::to_string(server_try_cancel));
+
+    request.set_message("hello");
+    auto stream = stub_->ResponseStream(&context, request);
+
+    int num_msgs_read = 0;
+    while (num_msgs_read < kNumResponseStreamsMsgs) {
+      if (!stream->Read(&response)) {
+        break;
+      }
+      EXPECT_EQ(response.message(),
+                request.message() + std::to_string(num_msgs_read));
+      num_msgs_read++;
+    }
+    gpr_log(GPR_INFO, "Read %d messages", num_msgs_read);
+
+    Status s = stream->Finish();
+
+    // Depending on the value of server_try_cancel, the RPC might have been
+    // cancelled by the server at different stages. The following validates our
+    // expectations of number of messages read in various cancellation
+    // scenarios:
+    switch (server_try_cancel) {
+      case CANCEL_BEFORE_PROCESSING:
+        // Server cancelled before sending any messages. Which means the client
+        // wouldn't have read any
+        EXPECT_EQ(num_msgs_read, 0);
+        break;
+
+      case CANCEL_DURING_PROCESSING:
+        // Server cancelled while writing messages. Client must have read less
+        // than or equal to the expected number of messages
+        EXPECT_LE(num_msgs_read, kNumResponseStreamsMsgs);
+        break;
+
+      case CANCEL_AFTER_PROCESSING:
+        // Server cancelled after writing all messages. Client must have read
+        // all messages
+        EXPECT_EQ(num_msgs_read, kNumResponseStreamsMsgs);
+        break;
+
+      default: {
+        gpr_log(GPR_ERROR, "Invalid server_try_cancel value: %d",
+                server_try_cancel);
+        EXPECT_TRUE(server_try_cancel > DO_NOT_CANCEL &&
+                    server_try_cancel <= CANCEL_AFTER_PROCESSING);
+        break;
+      }
+    }
+
+    EXPECT_FALSE(s.ok());
+    EXPECT_EQ(grpc::StatusCode::CANCELLED, s.error_code());
+  }
+
+  // Helper for testing bidirectional-streaming RPCs which are cancelled on the
+  // server. Depending on the value of server_try_cancel parameter, this will
+  // test one of the following three scenarios:
+  //   CANCEL_BEFORE_PROCESSING: Rpc is cancelled by the server before reading/
+  //   writing any messages from/to the client
+  //
+  //   CANCEL_DURING_PROCESSING: Rpc is cancelled by the server while reading/
+  //   writing messages from/to the client
+  //
+  //   CANCEL_AFTER PROCESSING: Rpc is cancelled by server after reading/writing
+  //   all the messages from/to the client
+  //
+  // NOTE: Do not call this function with server_try_cancel == DO_NOT_CANCEL.
+  void TestBidiStreamServerCancel(ServerTryCancelRequestPhase server_try_cancel,
+                                  int num_messages) {
+    ResetStub();
+    EchoRequest request;
+    EchoResponse response;
+    ClientContext context;
+
+    // Send server_try_cancel in the client metadata
+    context.AddMetadata(kServerTryCancelRequest,
+                        std::to_string(server_try_cancel));
+
+    auto stream = stub_->BidiStream(&context);
+
+    int num_msgs_read = 0;
+    int num_msgs_sent = 0;
+    while (num_msgs_sent < num_messages) {
+      request.set_message("hello " + std::to_string(num_msgs_sent));
+      if (!stream->Write(request)) {
+        break;
+      }
+      num_msgs_sent++;
+
+      if (!stream->Read(&response)) {
+        break;
+      }
+      num_msgs_read++;
+
+      EXPECT_EQ(response.message(), request.message());
+    }
+    gpr_log(GPR_INFO, "Sent %d messages", num_msgs_sent);
+    gpr_log(GPR_INFO, "Read %d messages", num_msgs_read);
+
+    stream->WritesDone();
+    Status s = stream->Finish();
+
+    // Depending on the value of server_try_cancel, the RPC might have been
+    // cancelled by the server at different stages. The following validates our
+    // expectations of number of messages read in various cancellation
+    // scenarios:
+    switch (server_try_cancel) {
+      case CANCEL_BEFORE_PROCESSING:
+        EXPECT_EQ(num_msgs_read, 0);
+        break;
+
+      case CANCEL_DURING_PROCESSING:
+        EXPECT_LE(num_msgs_sent, num_messages);
+        EXPECT_LE(num_msgs_read, num_msgs_sent);
+        break;
+
+      case CANCEL_AFTER_PROCESSING:
+        EXPECT_EQ(num_msgs_sent, num_messages);
+        EXPECT_EQ(num_msgs_read, num_msgs_sent);
+        break;
+
+      default:
+        gpr_log(GPR_ERROR, "Invalid server_try_cancel value: %d",
+                server_try_cancel);
+        EXPECT_TRUE(server_try_cancel > DO_NOT_CANCEL &&
+                    server_try_cancel <= CANCEL_AFTER_PROCESSING);
+        break;
+    }
+
+    EXPECT_FALSE(s.ok());
+    EXPECT_EQ(grpc::StatusCode::CANCELLED, s.error_code());
+  }
+};
+
+TEST_P(End2endServerTryCancelTest, RequestEchoServerCancel) {
+  ResetStub();
+  EchoRequest request;
+  EchoResponse response;
+  ClientContext context;
+
+  context.AddMetadata(kServerTryCancelRequest,
+                      std::to_string(CANCEL_BEFORE_PROCESSING));
+  Status s = stub_->Echo(&context, request, &response);
+  EXPECT_FALSE(s.ok());
+  EXPECT_EQ(grpc::StatusCode::CANCELLED, s.error_code());
+}
+
+// Server to cancel before doing reading the request
+TEST_P(End2endServerTryCancelTest, RequestStreamServerCancelBeforeReads) {
+  TestRequestStreamServerCancel(CANCEL_BEFORE_PROCESSING, 1);
+}
+
+// Server to cancel while reading a request from the stream in parallel
+TEST_P(End2endServerTryCancelTest, RequestStreamServerCancelDuringRead) {
+  TestRequestStreamServerCancel(CANCEL_DURING_PROCESSING, 10);
+}
+
+// Server to cancel after reading all the requests but before returning to the
+// client
+TEST_P(End2endServerTryCancelTest, RequestStreamServerCancelAfterReads) {
+  TestRequestStreamServerCancel(CANCEL_AFTER_PROCESSING, 4);
+}
+
+// Server to cancel before sending any response messages
+TEST_P(End2endServerTryCancelTest, ResponseStreamServerCancelBefore) {
+  TestResponseStreamServerCancel(CANCEL_BEFORE_PROCESSING);
+}
+
+// Server to cancel while writing a response to the stream in parallel
+TEST_P(End2endServerTryCancelTest, ResponseStreamServerCancelDuring) {
+  TestResponseStreamServerCancel(CANCEL_DURING_PROCESSING);
+}
+
+// Server to cancel after writing all the respones to the stream but before
+// returning to the client
+TEST_P(End2endServerTryCancelTest, ResponseStreamServerCancelAfter) {
+  TestResponseStreamServerCancel(CANCEL_AFTER_PROCESSING);
+}
+
+// Server to cancel before reading/writing any requests/responses on the stream
+TEST_P(End2endServerTryCancelTest, BidiStreamServerCancelBefore) {
+  TestBidiStreamServerCancel(CANCEL_BEFORE_PROCESSING, 2);
+}
+
+// Server to cancel while reading/writing requests/responses on the stream in
+// parallel
+TEST_P(End2endServerTryCancelTest, BidiStreamServerCancelDuring) {
+  TestBidiStreamServerCancel(CANCEL_DURING_PROCESSING, 10);
+}
+
+// Server to cancel after reading/writing all requests/responses on the stream
+// but before returning to the client
+TEST_P(End2endServerTryCancelTest, BidiStreamServerCancelAfter) {
+  TestBidiStreamServerCancel(CANCEL_AFTER_PROCESSING, 5);
+}
+
 TEST_P(End2endTest, MultipleRpcsWithVariedBinaryMetadataValue) {
   ResetStub();
   std::vector<std::thread*> threads;
@@ -1059,6 +1354,9 @@ INSTANTIATE_TEST_CASE_P(End2end, End2endTest,
                         ::testing::Values(TestScenario(false, false),
                                           TestScenario(false, true)));
 
+INSTANTIATE_TEST_CASE_P(End2endServerTryCancel, End2endServerTryCancelTest,
+                        ::testing::Values(TestScenario(false, false)));
+
 INSTANTIATE_TEST_CASE_P(ProxyEnd2end, ProxyEnd2endTest,
                         ::testing::Values(TestScenario(false, false),
                                           TestScenario(false, true),
diff --git a/test/cpp/end2end/test_service_impl.cc b/test/cpp/end2end/test_service_impl.cc
index 0eebe64148..66d11d0dfc 100644
--- a/test/cpp/end2end/test_service_impl.cc
+++ b/test/cpp/end2end/test_service_impl.cc
@@ -33,6 +33,8 @@
 
 #include "test/cpp/end2end/test_service_impl.h"
 
+#include <thread>
+
 #include <grpc++/security/credentials.h>
 #include <grpc++/server_context.h>
 #include <grpc/grpc.h>
@@ -82,6 +84,17 @@ void CheckServerAuthContext(const ServerContext* context,
 
 Status TestServiceImpl::Echo(ServerContext* context, const EchoRequest* request,
                              EchoResponse* response) {
+  int server_try_cancel = GetIntValueFromMetadata(
+      kServerTryCancelRequest, context->client_metadata(), DO_NOT_CANCEL);
+  if (server_try_cancel > DO_NOT_CANCEL) {
+    // Since this is a unary RPC, by the time this server handler is called,
+    // the 'request' message is already read from the client. So the scenarios
+    // in server_try_cancel don't make much sense. Just cancel the RPC as long
+    // as server_try_cancel is not DO_NOT_CANCEL
+    ServerTryCancel(context);
+    return Status::CANCELLED;
+  }
+
   response->set_message(request->message());
   MaybeEchoDeadline(context, request, response);
   if (host_) {
@@ -143,18 +156,39 @@ Status TestServiceImpl::Echo(ServerContext* context, const EchoRequest* request,
 Status TestServiceImpl::RequestStream(ServerContext* context,
                                       ServerReader<EchoRequest>* reader,
                                       EchoResponse* response) {
+  // If 'server_try_cancel' is set in the metadata, the RPC is cancelled by
+  // the server by calling ServerContext::TryCancel() depending on the value:
+  //   CANCEL_BEFORE_PROCESSING: The RPC is cancelled before the server reads
+  //   any message from the client
+  //   CANCEL_DURING_PROCESSING: The RPC is cancelled while the server is
+  //   reading messages from the client
+  //   CANCEL_AFTER_PROCESSING: The RPC is cancelled after the server reads
+  //   all the messages from the client
+  int server_try_cancel = GetIntValueFromMetadata(
+      kServerTryCancelRequest, context->client_metadata(), DO_NOT_CANCEL);
+
+  // If 'cancel_after_reads' is set in the metadata AND non-zero, the server
+  // will cancel the RPC (by just returning Status::CANCELLED - doesn't call
+  // ServerContext::TryCancel()) after reading the number of records specified
+  // by the 'cancel_after_reads' value set in the metadata.
+  int cancel_after_reads = GetIntValueFromMetadata(
+      kServerCancelAfterReads, context->client_metadata(), 0);
+
   EchoRequest request;
   response->set_message("");
-  int cancel_after_reads = 0;
-  const std::multimap<grpc::string_ref, grpc::string_ref>&
-      client_initial_metadata = context->client_metadata();
-  if (client_initial_metadata.find(kServerCancelAfterReads) !=
-      client_initial_metadata.end()) {
-    std::istringstream iss(ToString(
-        client_initial_metadata.find(kServerCancelAfterReads)->second));
-    iss >> cancel_after_reads;
-    gpr_log(GPR_INFO, "cancel_after_reads %d", cancel_after_reads);
+
+  if (server_try_cancel == CANCEL_BEFORE_PROCESSING) {
+    ServerTryCancel(context);
+    return Status::CANCELLED;
+  }
+
+  std::thread* server_try_cancel_thd = NULL;
+  if (server_try_cancel == CANCEL_DURING_PROCESSING) {
+    server_try_cancel_thd =
+        new std::thread(&TestServiceImpl::ServerTryCancel, this, context);
   }
+
+  int num_msgs_read = 0;
   while (reader->Read(&request)) {
     if (cancel_after_reads == 1) {
       gpr_log(GPR_INFO, "return cancel status");
@@ -164,21 +198,65 @@ Status TestServiceImpl::RequestStream(ServerContext* context,
     }
     response->mutable_message()->append(request.message());
   }
+  gpr_log(GPR_INFO, "Read: %d messages", num_msgs_read);
+
+  if (server_try_cancel_thd != NULL) {
+    server_try_cancel_thd->join();
+    delete server_try_cancel_thd;
+    return Status::CANCELLED;
+  }
+
+  if (server_try_cancel == CANCEL_AFTER_PROCESSING) {
+    ServerTryCancel(context);
+    return Status::CANCELLED;
+  }
+
   return Status::OK;
 }
 
-// Return 3 messages.
+// Return 'kNumResponseStreamMsgs' messages.
 // TODO(yangg) make it generic by adding a parameter into EchoRequest
 Status TestServiceImpl::ResponseStream(ServerContext* context,
                                        const EchoRequest* request,
                                        ServerWriter<EchoResponse>* writer) {
+  // If server_try_cancel is set in the metadata, the RPC is cancelled by the
+  // server by calling ServerContext::TryCancel() depending on the value:
+  //   CANCEL_BEFORE_PROCESSING: The RPC is cancelled before the server writes
+  //   any messages to the client
+  //   CANCEL_DURING_PROCESSING: The RPC is cancelled while the server is
+  //   writing messages to the client
+  //   CANCEL_AFTER_PROCESSING: The RPC is cancelled after the server writes
+  //   all the messages to the client
+  int server_try_cancel = GetIntValueFromMetadata(
+      kServerTryCancelRequest, context->client_metadata(), DO_NOT_CANCEL);
+
+  if (server_try_cancel == CANCEL_BEFORE_PROCESSING) {
+    ServerTryCancel(context);
+    return Status::CANCELLED;
+  }
+
   EchoResponse response;
-  response.set_message(request->message() + "0");
-  writer->Write(response);
-  response.set_message(request->message() + "1");
-  writer->Write(response);
-  response.set_message(request->message() + "2");
-  writer->Write(response);
+  std::thread* server_try_cancel_thd = NULL;
+  if (server_try_cancel == CANCEL_DURING_PROCESSING) {
+    server_try_cancel_thd =
+        new std::thread(&TestServiceImpl::ServerTryCancel, this, context);
+  }
+
+  for (int i = 0; i < kNumResponseStreamsMsgs; i++) {
+    response.set_message(request->message() + std::to_string(i));
+    writer->Write(response);
+  }
+
+  if (server_try_cancel_thd != NULL) {
+    server_try_cancel_thd->join();
+    delete server_try_cancel_thd;
+    return Status::CANCELLED;
+  }
+
+  if (server_try_cancel == CANCEL_AFTER_PROCESSING) {
+    ServerTryCancel(context);
+    return Status::CANCELLED;
+  }
 
   return Status::OK;
 }
@@ -186,15 +264,70 @@ Status TestServiceImpl::ResponseStream(ServerContext* context,
 Status TestServiceImpl::BidiStream(
     ServerContext* context,
     ServerReaderWriter<EchoResponse, EchoRequest>* stream) {
+  // If server_try_cancel is set in the metadata, the RPC is cancelled by the
+  // server by calling ServerContext::TryCancel() depending on the value:
+  //   CANCEL_BEFORE_PROCESSING: The RPC is cancelled before the server reads/
+  //   writes any messages from/to the client
+  //   CANCEL_DURING_PROCESSING: The RPC is cancelled while the server is
+  //   reading/writing messages from/to the client
+  //   CANCEL_AFTER_PROCESSING: The RPC is cancelled after the server
+  //   reads/writes all messages from/to the client
+  int server_try_cancel = GetIntValueFromMetadata(
+      kServerTryCancelRequest, context->client_metadata(), DO_NOT_CANCEL);
+
   EchoRequest request;
   EchoResponse response;
+
+  if (server_try_cancel == CANCEL_BEFORE_PROCESSING) {
+    ServerTryCancel(context);
+    return Status::CANCELLED;
+  }
+
+  std::thread* server_try_cancel_thd = NULL;
+  if (server_try_cancel == CANCEL_DURING_PROCESSING) {
+    server_try_cancel_thd =
+        new std::thread(&TestServiceImpl::ServerTryCancel, this, context);
+  }
+
   while (stream->Read(&request)) {
     gpr_log(GPR_INFO, "recv msg %s", request.message().c_str());
     response.set_message(request.message());
     stream->Write(response);
   }
+
+  if (server_try_cancel_thd != NULL) {
+    server_try_cancel_thd->join();
+    delete server_try_cancel_thd;
+    return Status::CANCELLED;
+  }
+
+  if (server_try_cancel == CANCEL_AFTER_PROCESSING) {
+    ServerTryCancel(context);
+    return Status::CANCELLED;
+  }
+
   return Status::OK;
 }
 
+int TestServiceImpl::GetIntValueFromMetadata(
+    const char* key,
+    const std::multimap<grpc::string_ref, grpc::string_ref>& metadata,
+    int default_value) {
+  if (metadata.find(key) != metadata.end()) {
+    std::istringstream iss(ToString(metadata.find(key)->second));
+    iss >> default_value;
+    gpr_log(GPR_INFO, "%s : %d", key, default_value);
+  }
+
+  return default_value;
+}
+
+void TestServiceImpl::ServerTryCancel(ServerContext* context) {
+  EXPECT_FALSE(context->IsCancelled());
+  context->TryCancel();
+  gpr_log(GPR_INFO, "Server called TryCancel() to cancel the request");
+  EXPECT_TRUE(context->IsCancelled());
+}
+
 }  // namespace testing
 }  // namespace grpc
diff --git a/test/cpp/end2end/test_service_impl.h b/test/cpp/end2end/test_service_impl.h
index 2c35b5614c..1ab6ced9e0 100644
--- a/test/cpp/end2end/test_service_impl.h
+++ b/test/cpp/end2end/test_service_impl.h
@@ -44,7 +44,16 @@
 namespace grpc {
 namespace testing {
 
+const int kNumResponseStreamsMsgs = 3;
 const char* const kServerCancelAfterReads = "cancel_after_reads";
+const char* const kServerTryCancelRequest = "server_try_cancel";
+
+typedef enum {
+  DO_NOT_CANCEL = 0,
+  CANCEL_BEFORE_PROCESSING,
+  CANCEL_DURING_PROCESSING,
+  CANCEL_AFTER_PROCESSING
+} ServerTryCancelRequestPhase;
 
 class TestServiceImpl : public ::grpc::testing::EchoTestService::Service {
  public:
@@ -74,6 +83,14 @@ class TestServiceImpl : public ::grpc::testing::EchoTestService::Service {
   }
 
  private:
+  int GetIntValueFromMetadata(
+      const char* key,
+      const std::multimap<grpc::string_ref, grpc::string_ref>& metadata,
+      int default_value);
+
+  void ServerTryCancel(ServerContext* context);
+
+ private:
   bool signal_client_;
   std::mutex mu_;
   std::unique_ptr<grpc::string> host_;