Using the Momento Topics API

Momento Topics is a messaging pattern enabling real-time communication between parts of a distributed application. It enables you to publish (produce) values to a topic and subscribe (consume) from a topic. This page details the Momento API methods for interacting with Momento Topics.

TopicClient

Momento Topics API calls are made using a TopicClient object.

JavaScript

new TopicClient({
  configuration: TopicConfigurations.Default.latest(),
  credentialProvider: CredentialProvider.fromEnvironmentVariable('MOMENTO_API_KEY'),
});

Python

TopicClientAsync(
    TopicConfigurations.Default.latest(), CredentialProvider.from_environment_variable("MOMENTO_API_KEY")
)

Kotlin

TopicClient(
    CredentialProvider.fromEnvVar("MOMENTO_API_KEY"), TopicConfigurations.Laptop.latest
).use { topicClient ->
    //...
}

Go

credProvider, err := auth.NewEnvMomentoTokenProvider("MOMENTO_API_KEY")
if err != nil {
	panic(err)
}

topicClient, err = momento.NewTopicClient(
	config.TopicsDefault(),
	credProvider,
)
if err != nil {
	panic(err)
}

C#

new TopicClient(
    TopicConfigurations.Laptop.latest(),
    new EnvMomentoTokenProvider("MOMENTO_API_KEY")
);

Rust

  let _topic_client = TopicClient::builder()
      .configuration(momento::topics::configurations::Laptop::latest())
      .credential_provider(CredentialProvider::from_env_var("MOMENTO_API_KEY")?)
      .build()?;

Swift

do {
    let credentialProvider = try CredentialProvider.fromEnvironmentVariable(envVariableName: "MOMENTO_API_KEY")
    let topicClient = TopicClient(
        configuration: TopicClientConfigurations.iOS.latest(),
        credentialProvider: credentialProvider
    )
} catch {
    print("Unable to create topic client: \(error)")
    exit(1)
}

Dart

try {
  final topicClient = TopicClient(
      CredentialProvider.fromEnvironmentVariable("MOMENTO_API_KEY"),
      TopicClientConfigurations.latest());
} catch (e) {
  print("Unable to create topic client: $e");
  exit(1);
}

Topics methods

Subscribe

This method subscribes to a topic to receive subscription events with a stateful connection.

Depending on the language, you may use a callback function or an iterator to receive new subscription events, such as items, heartbeats, and discontinuities:

• Items include a string or byte message, topic sequence number, and a unique token identifier if one is present (learn more).
• Heartbeats indicate that the connection is still active.
• Discontinuities indicate that there was an interruption in the subscription and some messages may have been skipped.

Name	Type	Description
cacheName	String	Name of the cache where the topic exists.
topicName	String	Name of the topic to subscribe to.

Method response object

• Success - Returns a subscription object.
• Error

See response objects for specific information.

JavaScript

const result = await topicClient.subscribe(cacheName, 'test-topic', {
  onError: () => {
    return;
  },
  onItem: (item: TopicItem) => {
    console.log(`Received an item on subscription for 'test-topic': ${item.value().toString()}`);
    return;
  },
});
switch (result.type) {
  case TopicSubscribeResponse.Subscription:
    console.log("Successfully subscribed to topic 'test-topic'");

    console.log("Publishing a value to the topic 'test-topic'");
    // Publish a value
    await topicClient.publish(cacheName, 'test-topic', 'test-value');

    console.log('Waiting for the published value to be received.');
    await new Promise(resolve => setTimeout(resolve, 1000));

    // Need to close the stream before the example ends or else the example will hang.
    result.unsubscribe();
    break;
  case TopicSubscribeResponse.Error:
    throw new Error(
      `An error occurred while attempting to subscribe to the topic 'test-topic' in cache '${cacheName}': ${result.errorCode()}: ${result.toString()}`
    );
}

Python

response = await topic_client.subscribe("cache", "my_topic")
match response:
    case TopicSubscribe.Error() as error:
        print(f"Error subscribing to topic: {error.message}")
    case TopicSubscribe.SubscriptionAsync() as subscription:
        await topic_client.publish("cache", "my_topic", "my_value")
        async for item in subscription:
            match item:
                case TopicSubscriptionItem.Text():
                    print(f"Received message as string: {item.value}")
                    return
                case TopicSubscriptionItem.Binary():
                    print(f"Received message as bytes: {item.value!r}")
                    return
                case TopicSubscriptionItem.Error():
                    print(f"Error with received message: {item.inner_exception.message}")
                    return

Kotlin

when (val response = topicClient.subscribe("test-cache", "test-topic")) {
    is TopicSubscribeResponse.Subscription -> coroutineScope {
        launch {
            withTimeoutOrNull(2000) {
                response.collect { item ->
                    when (item) {
                        is TopicMessage.Text -> println("Received text message: ${item.value}")
                        is TopicMessage.Binary -> println("Received binary message: ${item.value}")
                        is TopicMessage.Error -> throw RuntimeException(
                            "An error occurred reading messages from topic 'test-topic': ${item.errorCode}", item
                        )
                    }
                }
            }
        }
    }

    is TopicSubscribeResponse.Error -> throw RuntimeException(
        "An error occurred while attempting to subscribe to topic 'test-topic': ${response.errorCode}", response
    )
}

Go

// Instantiate subscriber
sub, subErr := topicClient.Subscribe(ctx, &momento.TopicSubscribeRequest{
	CacheName: cacheName,
	TopicName: "test-topic",
})
if subErr != nil {
	panic(subErr)
}

time.Sleep(time.Second)
_, pubErr := topicClient.Publish(ctx, &momento.TopicPublishRequest{
	CacheName: cacheName,
	TopicName: "test-topic",
	Value:     momento.String("test-message"),
})
if pubErr != nil {
	panic(pubErr)
}
time.Sleep(time.Second)

// Receive only subscription items with messages
item, err := sub.Item(ctx)
if err != nil {
	panic(err)
}
switch msg := item.(type) {
case momento.String:
	fmt.Printf("received message as string: '%v'\n", msg)
case momento.Bytes:
	fmt.Printf("received message as bytes: '%v'\n", msg)
}

// Receive all subscription events (messages, discontinuities, heartbeats)
event, err := sub.Event(ctx)
if err != nil {
	panic(err)
}
switch e := event.(type) {
case momento.TopicHeartbeat:
	fmt.Printf("received heartbeat\n")
case momento.TopicDiscontinuity:
	fmt.Printf("received discontinuity\n")
case momento.TopicItem:
	fmt.Printf(
		"received message with sequence number %d and publisher id %s: %v \n",
		e.GetTopicSequenceNumber(),
		e.GetPublisherId(),
		e.GetValue(),
	)
}

C#

var produceCancellation = new CancellationTokenSource();
produceCancellation.CancelAfter(2000);

var subscribeResponse = await topicClient.SubscribeAsync("test-cache", "test-topic");
switch (subscribeResponse)
{
    case TopicSubscribeResponse.Subscription subscription:
        var cancellableSubscription = subscription.WithCancellation(produceCancellation.Token);

        await Task.Delay(1_000);
        await topicClient.PublishAsync("test-cache", "test-topic", "test-topic-value");
        await Task.Delay(1_000);

        await foreach (var message in cancellableSubscription)
        {
            switch (message)
            {
                case TopicMessage.Binary:
                    Console.WriteLine("Received unexpected binary message from topic.");
                    break;
                case TopicMessage.Text text:
                    Console.WriteLine($"Received string message from topic: {text.Value}");
                    break;
                case TopicMessage.Error error:
                    throw new Exception($"An error occurred while receiving topic message: {error.ErrorCode}: {error}");
                default:
                    throw new Exception("Bad message received");
            }
        }
        subscription.Dispose();
        break;
    case TopicSubscribeResponse.Error error:
        throw new Exception($"An error occurred subscribing to a topic: {error.ErrorCode}: {error}");
}

Rust

  // Make a subscription
  let mut subscription = topic_client
      .subscribe(cache_name, topic_name)
      .await
      .expect("subscribe rpc failed");

  // Consume the subscription
  while let Some(item) = subscription.next().await {
      println!("Received subscription item: {item:?}")
  }

Swift

let subscribeResponse = await topicClient.subscribe(cacheName: cacheName, topicName: "topic")

#if swift(>=5.9)
let subscription = switch subscribeResponse {
    case .error(let err): fatalError("Error subscribing to topic: \(err)")
    case .subscription(let sub): sub
}
#else 
let subscription: TopicSubscription
switch subscribeResponse {
    case .error(let err):
        fatalError("Error subscribing to topic: \(err)")
    case .subscription(let sub):
        subscription = sub
}
#endif

// unsubscribe in 5 seconds
Task {
    try await Task.sleep(nanoseconds: 5_000_000_000)
    subscription.unsubscribe()
}

// loop over messages as they are received
for try await item in subscription.stream {
    var value: String = ""
    switch item {
    case .itemText(let textItem):
        value = textItem.value
        print("Subscriber recieved text message: \(value)")
    case .itemBinary(let binaryItem):
        value = String(decoding: binaryItem.value, as: UTF8.self)
        print("Subscriber recieved binary message: \(value)")
    case .error(let err):
        print("Subscriber received error: \(err)")
    }
}

Dart

final subscription = await topicClient.subscribe("test-cache", "test-topic");
final messageStream = switch (subscription) {
  TopicSubscription() => subscription.stream,
  TopicSubscribeError() => throw Exception(
      "Subscribe error: ${subscription.errorCode} ${subscription.message}"),
};

// cancel subscription 5 seconds from now
Timer(const Duration(seconds: 5), () {
  print("Cancelling subscription!");
  subscription.unsubscribe();
});

try {
  await for (final msg in messageStream) {
    switch (msg) {
      case TopicSubscriptionItemBinary():
        print("Binary value: ${msg.value}");
      case TopicSubscriptionItemText():
        print("String value: ${msg.value}");
    }
  }
} catch (e) {
  print("Runtime type: ${e.runtimeType}");
  print("Error with await for loop: $e");
}

Publish

Publishes a message to a topic.

Name	Type	Description
cacheName	String	Name of the cache where the topic exists.
topicName	String	Name of the topic to publish the value to.
value	String / bytes	Value to publish to the topic.

Method response object

• Success
• Error

See response objects for specific information.

JavaScript

const result = await topicClient.publish(cacheName, 'test-topic', 'test-topic-value');
switch (result.type) {
  case TopicPublishResponse.Success:
    console.log("Value published to topic 'test-topic'");
    break;
  case TopicPublishResponse.Error:
    throw new Error(
      `An error occurred while attempting to publish to the topic 'test-topic' in cache '${cacheName}': ${result.errorCode()}: ${result.toString()}`
    );
}

Python

response = await topic_client.publish("cache", "my_topic", "my_value")
match response:
    case TopicPublish.Success():
        print("Successfully published a message")
    case TopicPublish.Error() as error:
        print(f"Error publishing a message: {error.message}")

Kotlin

when (val response = topicClient.publish("test-cache", "test-topic", "test-message")) {
    is TopicPublishResponse.Success -> println("Message published successfully")
    is TopicPublishResponse.Error -> throw RuntimeException(
        "An error occurred while attempting to publish message to topic 'test-topic': ${response.errorCode}",
        response
    )
}

Go

_, err := topicClient.Publish(ctx, &momento.TopicPublishRequest{
	CacheName: cacheName,
	TopicName: "test-topic",
	Value:     momento.String("test-message"),
})
if err != nil {
	panic(err)
}

C#

var publishResponse =
     await topicClient.PublishAsync("test-cache", "test-topic", "test-topic-value");
switch (publishResponse)
{
    case TopicPublishResponse.Success:
        Console.WriteLine("Successfully published message to 'test-topic'");
        break;
    case TopicPublishResponse.Error error:
        throw new Exception($"An error occurred while publishing topic message: {error.ErrorCode}: {error}");
}

Rust

  topic_client
      .publish(cache_name, topic_name, "Hello, Momento!")
      .await?;
  println!("Published message");

Swift

let result = await topicClient.publish(
    cacheName: cacheName,
    topicName: "topic",
    value: "value"
)
switch result {
case .success(_):
    print("Successfully published message!")
case .error(let err):
    print("Unable to publish message: \(err)")
    exit(1)
}

Dart

final result = await topicClient.publish("cache", "topic", "hello message!");
switch (result) {
  case TopicPublishSuccess():
    print("Successful publish!");
  case TopicPublishError():
    print("Publish error: ${result.errorCode} ${result.message}");
}

Example apps using Momento Topics APIs

A growing list of example apps using the Momento Topics.

• A serverless item publishing microservice This microservice is written in TypeScript and runs on AWS using API Gateway, a Lambda function, and Momento Topics. It can be used by any of your other services (with the correct security on API Gateway) to publish messages to various topics that are then subscribed to by other applications. You pass into this API a topicName and topicValue and this service publishes the value to that topic.