WithTopic

WithTopic<T>()

Creates an SNS topic for publishing messages of type T. Multiple subscribers can receive messages published to this topic, enabling fan-out scenarios where different services react to the same event.

• An SNS topic will be created for messages of type T.
• The topic name will be determined using the supplied (or default if not) ITopicNamingConvention, applied to the message type T.
  • This convention can be overridden using WithTopicName in the topic configuration.

Example:

config.Publications(x =>
{
    x.WithTopic<OrderPlacedEvent>();
});

This describes the following infrastructure:

• An SNS topic of name orderplacedevent

Further configuration options can be defined by passing a configuration lambda to the WithTopic method.

Example with configuration:

config.Publications(x =>
{
    x.WithTopic<OrderReadyEvent>(cfg =>
    {
        cfg.WithTag("IsOrderEvent")
           .WithTag("Publisher", "KitchenConsole");
    });
});

WithTopicArn<T>(string topicArn)

Publishes messages of type T to an existing SNS topic specified by its ARN (Amazon Resource Name). Use this when the topic already exists and you don't want JustSaying to create or manage it.

Example:

config.Publications(x =>
{
    x.WithTopicArn<OrderPlacedEvent>("arn:aws:sns:us-east-1:123456789012:existing-topic");
});

This configuration allows publishing to a topic that may be owned by another AWS account or managed separately from your JustSaying application.

Configuration Options

When using the configuration lambda, the following options are available:

WithTopicName(string name)

Override the naming convention to use a specific topic name.

x.WithTopic<OrderPlacedEvent>(cfg =>
{
    cfg.WithTopicName("custom-order-topic");
});

WithTopicName(Func<Message, string> topicNameFunc)

Dynamically determine the topic name based on the message content. Useful for multi-tenant scenarios.

x.WithTopic<OrderPlacedEvent>(cfg =>
{
    cfg.WithTopicName(msg =>
    {
        var order = (OrderPlacedEvent)msg;
        return $"tenant-{order.TenantId}-orders";
    });
});

See Dynamic Topics for more details.

WithTag(string key)

Add a tag to the SNS topic resource (without a value).

WithTag(string key, string value)

Add a tag with a key and value to the SNS topic resource. Tags are useful for cost allocation and resource management in AWS.

x.WithTopic<OrderPlacedEvent>(cfg =>
{
    cfg.WithTag("Environment", "Production")
       .WithTag("Team", "Orders");
});

WithWriteConfiguration(Action<SnsWriteConfigurationBuilder> configure)

Configure advanced publishing options such as encryption, compression, and error handling. See Write Configuration for details.

x.WithTopic<OrderPlacedEvent>(cfg =>
{
    cfg.WithWriteConfiguration(w =>
    {
        w.Encryption = new ServerSideEncryption
        {
            KmsMasterKeyId = "your-kms-key-id"
        };
    });
});

When to Use Topics

Use topics when:

• Multiple services need to react to the same event (fan-out pattern)
• You're publishing events that represent something that has happened
• You need to decouple publishers from subscribers
• Subscribers can be added or removed without changing the publisher

For point-to-point messaging where only one consumer should process the message, use WithQueue instead.