Cloud Pub/Sub C++ Client Library

Table of Contents

• Quickstart
• Setting up your repo
• Environment Variables
• Error Handling
• Override the default endpoint
• Override the authentication configuration
• Next Steps

The Cloud Pub/Sub C++ Client library offers types and functions to use Cloud Pub/Sub from C++ applications.

This library requires a C++14 compiler. It is supported (and tested) on multiple Linux distributions, macOS, and Windows.

Quickstart

The following instructions show you how to perform basic tasks in Cloud Pub/Sub using the C++ client library.

Before you begin

1. Select or create a Google Cloud Platform (GCP) project using the manage resource page. Make a note of the project id as you will need to use it later.
2. Make sure that billing is enabled for your project.
3. Learn about key terms and concepts for Cloud Pub/Sub.
4. Setup the authentication for the examples:
   • Configure a service account,
   • or login with your personal account

Setting up your repo

In order to use the Cloud Pub/Sub C++ client library from your own code, you'll need to configure your build system how to fetch and compile the Cloud C++ client library. The Cloud Pub/Sub C++ client library natively supports the Bazel and CMake build systems. We've created a minimal, "Hello world", quickstart repo that includes detailed instructions on how to compile the library for use in your application. You can fetch the source from GitHub as normal:

git clone https://github.com/googleapis/google-cloud-cpp.git
cd google-cloud-cpp/google/cloud/pubsub/quickstart

Example: Hello World

The following shows the code that you'll run in the google/cloud/pubsub/quickstart/ directory, which should give you a taste of the Cloud Pub/Sub C++ client library API.

// Copyright 2020 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
 
#include "google/cloud/pubsub/publisher.h"
#include <iostream>
 
int main(int argc, char* argv[]) try {
  if (argc != 3) {
    std::cerr << "Usage: " << argv[0] << " <project-id> <topic-id>\n";
    return 1;
  }
 
  std::string const project_id = argv[1];
  std::string const topic_id = argv[2];
 
  // Create a namespace alias to make the code easier to read.
  namespace pubsub = ::google::cloud::pubsub;
  auto publisher = pubsub::Publisher(
      pubsub::MakePublisherConnection(pubsub::Topic(project_id, topic_id)));
  auto id =
      publisher
          .Publish(pubsub::MessageBuilder{}.SetData("Hello World!").Build())
          .get();
  if (!id) throw std::move(id).status();
  std::cout << "Hello World published with id=" << *id << "\n";
 
  return 0;
} catch (google::cloud::Status const& status) {
  std::cerr << "google::cloud::Status thrown: " << status << "\n";
  return 1;
}

Environment Variables

There are several environment variables that can be set to configure certain behaviors in the library.

• GOOGLE_CLOUD_CPP_ENABLE_TRACING=rpc turns on tracing for most gRPC calls. The library injects an additional Stub decorator that prints each gRPC request and response. Unless you have configured you own logging backend, you should also set GOOGLE_CLOUD_CPP_ENABLE_CLOG to produce any output on the program's console.
• GOOGLE_CLOUD_CPP_TRACING_OPTIONS=... modifies the behavior of gRPC tracing, including whether messages will be output on multiple lines, or whether string/bytes fields will be truncated.
• GOOGLE_CLOUD_PROJECT=... is used in examples and integration tests to configure the GCP project.
• PUBSUB_EMULATOR_HOST=host:port tells the library to connect to the specified emulator rather than the default endpoint.
• GOOGLE_CLOUD_CPP_ENABLE_CLOG=yes turns on logging in the library, basically the library always "logs" but the logging infrastructure has no backend to actually print anything until the application sets a backend or they set this environment variable.

Error Handling

This library never throws exceptions to signal error. In general, the library returns a StatusOr<T> if an error is possible. Some functions return objects that are not wrapped in a StatusOr<> but will themselves return a StatusOr<T> to signal an error. For example, wrappers for asynchronous operations return future<StatusOr<T>>.

Applications should check if the StatusOr<T> contains a value before using it, much like how you might check that a pointer is not null before dereferencing it. Indeed, a StatusOr<T> object can be used like a smart-pointer to T, with the main difference being that when it does not hold a T it will instead hold a Status object with extra information about the error.

You can check that a StatusOr<T> contains a value by calling the .ok() method, or by using operator bool() (like with other smart pointers). If there is no value, you can access the contained Status object using the .status() member. If there is a value, you may access it by dereferencing with operator*() or operator->(). As with all smart pointers, callers must first check that the StatusOr<T> contains a value before dereferencing and accessing the contained value. Alternatively, callers may instead use the .value() member function which is defined to throw a RuntimeStatusError if there is no value.

Note

If you're compiling with exceptions disabled, calling .value() on a StatusOr<T> that does not contain a value will terminate the program instead of throwing.

Example

  namespace pubsub = ::google::cloud::pubsub;
  [](pubsub::TopicAdminClient client, std::string const& project_id) {
    // The actual type of `topic` is
    // google::cloud::StatusOr<google::pubsub::v1::Topic>, but
    // we expect it'll most often be declared with auto like this.
    for (auto const& topic : client.ListTopics(project_id)) {
      // Use `topic` like a smart pointer; check it before de-referencing
      if (!topic) {
        // `topic` doesn't contain a value, so `.status()` will contain error
        // info
        std::cerr << topic.status() << "\n";
        break;
      }
      std::cout << topic->DebugString() << "\n";
    }
  }

Override the default endpoint

In some cases, you may need to override the default endpoint used by the client library. Use the google::cloud::EndpointOption when initializing the client library to change this default. For example, this will override the default endpoint for google::cloud::pubsub::Publisher:

  namespace pubsub = ::google::cloud::pubsub;
  using ::google::cloud::Options;
  [](std::string project_id, std::string topic_id) {
    auto topic = pubsub::Topic(std::move(project_id), std::move(topic_id));
    return pubsub::Publisher(pubsub::MakePublisherConnection(
        std::move(topic), Options{}.set<google::cloud::EndpointOption>(
                              "private.googleapis.com")));
  }

Follow these links for additional examples Subscriber BlockingPublisher TopicAdminClient SubscriptionAdminClient SchemaAdminClient Publisher

Override the authentication configuration

Some applications cannot use the default authentication mechanism (known as Application Default Credentials). You can override this default using google::cloud::UnifiedCredentialsOption. The following example shows how to explicitly load a service account key file.

  namespace pubsub = ::google::cloud::pubsub;
  using ::google::cloud::Options;
  [](std::string project_id, std::string topic_id, std::string const& keyfile) {
    auto is = std::ifstream(keyfile);
    is.exceptions(std::ios::badbit);
    auto contents = std::string(std::istreambuf_iterator<char>(is.rdbuf()), {});
    auto topic = pubsub::Topic(std::move(project_id), std::move(topic_id));
    return pubsub::Publisher(pubsub::MakePublisherConnection(
        std::move(topic),
        Options{}.set<google::cloud::UnifiedCredentialsOption>(
            google::cloud::MakeServiceAccountCredentials(contents))));
  }

Keep in mind that we chose this as an example because it is relatively easy to understand. Consult the Best practices for managing service account keys guide for more details.

See also

Authentication Components - for more information on the factory functions to create google::cloud::Credentials objects.

Follow these links for additional examples Subscriber BlockingPublisher TopicAdminClient SubscriptionAdminClient SchemaAdminClient Publisher

Next Steps

• Testing your Cloud Pub/Sub publisher application with googlemock
• Testing your Cloud Pub/Sub subscriber application with googlemock