Publish messages to a topic with a schema

This document shows you how to publish messages to a topic with a schema.

Before you begin

Before configuring the publish workflow, ensure you have completed the following tasks:

Required roles

To get the permissions that you need to publish messages to a topic, ask your administrator to grant you the Pub/Sub Publisher (roles/pubsub.publisher) IAM role on the topic. For more information about granting roles, see Manage access to projects, folders, and organizations.

You might also be able to get the required permissions through custom roles or other predefined roles.

You need additional permissions to create or update topics and subscriptions.

Publish messages with schema

You can publish messages to a topic that is associated with a schema. You must encode the messages in the schema and format that you specified when you created the topic. A message matches the schema associated with the topic if it matches any of the schema's revision in the allowed range of revisions. Messages are evaluated against revisions in order from the most recent allowed revision until either a match is found or the oldest allowed revision is reached. Pub/Sub adds the following attributes to a message successfully published to a topic associated with a schema:

  • googclient_schemaname: The name of the schema used for validation.

  • googclient_schemaencoding: The encoding of the message, either JSON or BINARY.

  • googclient_schemarevisionid: The revision ID of the schema used to parse and validate the message. Each revision has a unique revision ID associated with it. The revision ID is an auto-generated eight-character UUID.

When a message does not match any of the schema revisions allowed by the topic, Pub/Sub returns an INVALID_ARGUMENT error to the publish request.

Pub/Sub only evaluates messages against schema revisions at publish time. Committing a new schema revision or changing the schema associated with a topic after publishing a message does not re-evaluate that message nor change any of the attached schema message attributes.

You can publish messages to a topic with an associated schema in a Google Cloud project using the Google Cloud console, the gcloud CLI, the Pub/Sub API, or the Cloud Client Libraries.

gcloud

  1. In the Google Cloud console, activate Cloud Shell.

    Activate Cloud Shell

    At the bottom of the Google Cloud console, a Cloud Shell session starts and displays a command-line prompt. Cloud Shell is a shell environment with the Google Cloud CLI already installed and with values already set for your current project. It can take a few seconds for the session to initialize.

  2. Publish a sample message using the gcloud pubsub topics publish command.

    gcloud pubsub topics publish TOPIC_ID \
    --message=MESSAGE

    Replace the following:

  • TOPIC_ID: Name of the topic that you already created.

  • MESSAGE: Message published to the topic. A sample message can be {"name": "Alaska", "post_abbr": "AK"}.

C++

Before trying this sample, follow the C++ setup instructions in Quickstart: Using Client Libraries. For more information, see the Pub/Sub C++ API reference documentation.

Avro 
namespace pubsub = ::google::cloud::pubsub;
using ::google::cloud::future;
using ::google::cloud::StatusOr;
[](pubsub::Publisher publisher) {
  auto constexpr kNewYork =
      R"js({ "name": "New York", "post_abbr": "NY" })js";
  auto constexpr kPennsylvania =
      R"js({ "name": "Pennsylvania", "post_abbr": "PA" })js";
  std::vector<future<void>> done;
  auto handler = [](future<StatusOr<std::string>> f) {
    auto id = f.get();
    if (!id) throw std::move(id).status();
  };
  for (auto const* data : {kNewYork, kPennsylvania}) {
    done.push_back(
        publisher.Publish(pubsub::MessageBuilder{}.SetData(data).Build())
            .then(handler));
  }
  // Block until all messages are published.
  for (auto& d : done) d.get();
}
Proto 
namespace pubsub = ::google::cloud::pubsub;
using ::google::cloud::future;
using ::google::cloud::StatusOr;
[](pubsub::Publisher publisher) {
  std::vector<std::pair<std::string, std::string>> states{
      {"New York", "NY"},
      {"Pennsylvania", "PA"},
  };
  std::vector<future<void>> done;
  auto handler = [](future<StatusOr<std::string>> f) {
    auto id = f.get();
    if (!id) throw std::move(id).status();
  };
  for (auto& data : states) {
    google::cloud::pubsub::samples::State state;
    state.set_name(data.first);
    state.set_post_abbr(data.second);
    done.push_back(publisher
                       .Publish(pubsub::MessageBuilder{}
                                    .SetData(state.SerializeAsString())
                                    .Build())
                       .then(handler));
  }
  // Block until all messages are published.
  for (auto& d : done) d.get();
}

C#

Before trying this sample, follow the C# setup instructions in Quickstart: Using Client Libraries. For more information, see the Pub/Sub C# API reference documentation.

Avro 

using Avro.IO;
using Avro.Specific;
using Google.Cloud.PubSub.V1;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;

public class PublishAvroMessagesAsyncSample
{
    public async Task<int> PublishAvroMessagesAsync(string projectId, string topicId, IEnumerable<AvroUtilities.State> messageStates)
    {
        TopicName topicName = TopicName.FromProjectTopic(projectId, topicId);
        PublisherClient publisher = await PublisherClient.CreateAsync(topicName);

        PublisherServiceApiClient publishApi = PublisherServiceApiClient.Create();
        var topic = publishApi.GetTopic(topicName);

        int publishedMessageCount = 0;
        var publishTasks = messageStates.Select(async state =>
        {

            try
            {
                string messageId = null;
                switch (topic.SchemaSettings.Encoding)
                {
                    case Encoding.Binary:
                        using (var ms = new MemoryStream())
                        {
                            var encoder = new BinaryEncoder(ms);
                            var writer = new SpecificDefaultWriter(state.Schema);
                            writer.Write(state, encoder);
                            messageId = await publisher.PublishAsync(ms.ToArray());
                        }
                        break;
                    case Encoding.Json:
                        var jsonMessage = AvroUtilities.StateUtils.StateToJsonString(state);
                        messageId = await publisher.PublishAsync(jsonMessage);
                        break;
                }
                Console.WriteLine($"Published message {messageId}");
                Interlocked.Increment(ref publishedMessageCount);
            }
            catch (Exception exception)
            {
                Console.WriteLine($"An error occurred when publishing message {state}: {exception.Message}");
            }
        });
        await Task.WhenAll(publishTasks);
        // PublisherClient instance should be shutdown after use.
        // The TimeSpan specifies for how long to attempt to publish locally queued messages.
        await publisher.ShutdownAsync(TimeSpan.FromSeconds(15));
        return publishedMessageCount;
    }
}
Proto 

using Google.Cloud.PubSub.V1;
using Google.Protobuf;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;

public class PublishProtoMessagesAsyncSample
{
    public async Task<int> PublishProtoMessagesAsync(string projectId, string topicId, IEnumerable<Utilities.State> messageStates)
    {
        TopicName topicName = TopicName.FromProjectTopic(projectId, topicId);
        PublisherClient publisher = await PublisherClient.CreateAsync(topicName);

        PublisherServiceApiClient publishApi = PublisherServiceApiClient.Create();
        var topic = publishApi.GetTopic(topicName);

        int publishedMessageCount = 0;
        var publishTasks = messageStates.Select(async state =>
        {
            try
            {
                string messageId = null;
                switch (topic.SchemaSettings.Encoding)
                {
                    case Encoding.Binary:
                        var binaryMessage = state.ToByteString();
                        messageId = await publisher.PublishAsync(binaryMessage);
                        break;
                    case Encoding.Json:
                        var jsonMessage = JsonFormatter.Default.Format(state);
                        messageId = await publisher.PublishAsync(jsonMessage);
                        break;
                }
                Console.WriteLine($"Published message {messageId}");
                Interlocked.Increment(ref publishedMessageCount);
            }
            catch (Exception exception)
            {
                Console.WriteLine($"An error occurred when publishing message {state}: {exception.Message}");
            }
        });
        await Task.WhenAll(publishTasks);
        // PublisherClient instance should be shutdown after use.
        // The TimeSpan specifies for how long to attempt to publish locally queued messages.
        await publisher.ShutdownAsync(TimeSpan.FromSeconds(15));
        return publishedMessageCount;
    }
}

Go

The following sample uses the major version of the Go Pub/Sub client library (v2). If you are still using the v1 library, see the migration guide to v2. To see a list of v1 code samples, see the deprecated code samples.

Before trying this sample, follow the Go setup instructions in Quickstart: Using Client Libraries. For more information, see the Pub/Sub Go API reference documentation.

Avro 
import (
	"context"
	"fmt"
	"io"
	"os"

	"cloud.google.com/go/pubsub/v2"
	"cloud.google.com/go/pubsub/v2/apiv1/pubsubpb"
	"github.com/linkedin/goavro/v2"
)

func publishAvroRecords(w io.Writer, projectID, topicID, avscFile string) error {
	// projectID := "my-project-id"
	// topicID := "my-topic"
	// avscFile = "path/to/an/avro/schema/file(.avsc)/formatted/in/json"
	ctx := context.Background()
	client, err := pubsub.NewClient(ctx, projectID)
	if err != nil {
		return fmt.Errorf("pubsub.NewClient: %w", err)
	}

	avroSource, err := os.ReadFile(avscFile)
	if err != nil {
		return