Integrating Google Cloud Pub/Sub with Spring Boot

Q: Integrating Google Cloud Pub/Sub with Spring Boot

Add the `spring-cloud-gcp-starter-pubsub` dependency, then use `PubSubTemplate` to publish messages and `@ServiceActivator` or `PubSubInboundChannelAdapter` to consume them. Spring Cloud GCP handles credential management and message serialization, letting you work with Pub/Sub using familiar Spring Integration or Spring Messaging abstractions.

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Core answer

Add the spring-cloud-gcp-starter-pubsub dependency, then use PubSubTemplate to publish messages and @ServiceActivator or PubSubInboundChannelAdapter to consume them. Spring Cloud GCP handles credential management and message serialization, letting you work with Pub/Sub using familiar Spring Integration or Spring Messaging abstractions.

Interview Answer2 min

Cloud Pub/Sub is GCP's managed messaging service. Spring Cloud GCP makes the publishing side trivial and provides annotation-based subscription handling, but understanding the delivery semantics — at-least-once, push vs pull — is what makes the integration reliable.

Publishing with PubSubTemplate

Inject PubSubTemplate and call pubSubTemplate.publish(topicName, message) — Spring Cloud GCP serializes the object to JSON by default and publishes asynchronously; the return value is a ListenableFuture for the publish confirmation
For high-throughput publishing, configure the publisher's batch settings: spring.cloud.gcp.pubsub.publisher.batching.enabled=true batches multiple messages into one API call, which reduces per-message overhead significantly

Consuming with @PubSubListener

@PubSubListener(subscriptions = "my-subscription") on a method marks it as a subscription handler — Spring Cloud GCP creates the subscription client, polls for messages, deserializes them, and calls the method; the message is automatically acknowledged if the method returns normally
If the method throws an exception, the message is not acknowledged and becomes available for redelivery after the ack deadline — at-least-once delivery; implement idempotent handlers to handle duplicate delivery safely

Push subscriptions

A push subscription has Pub/Sub call a URL when messages arrive — which means Cloud Run functions or App Engine services can receive Pub/Sub messages via HTTP POST without running a continuous polling loop
The HTTP handler receives the message as a Base64-encoded payload in a JSON envelope; @RestController + Spring's @PubSubSubscriber utility handles the deserialization

Message attributes and filtering

Pub/Sub message attributes are key-value string metadata — pubSubTemplate.publish(topic, message, Map.of("type", "OrderPlaced")) adds an attribute
Subscription filters (subscription.message_filter) allow a subscription to receive only messages matching attributes.type = "OrderPlaced", reducing traffic to consumers that only need specific event types

Interview-ready guide

Interview answer

Publishing with PubSubTemplate

Inject PubSubTemplate and call pubSubTemplate.publish(topicName, message) — Spring Cloud GCP serializes the object to JSON by default and publishes asynchronously; the return value is a ListenableFuture for the publish confirmation
For high-throughput publishing, configure the publisher's batch settings: spring.cloud.gcp.pubsub.publisher.batching.enabled=true batches multiple messages into one API call, which reduces per-message overhead significantly

Consuming with @PubSubListener

@PubSubListener(subscriptions = "my-subscription") on a method marks it as a subscription handler — Spring Cloud GCP creates the subscription client, polls for messages, deserializes them, and calls the method; the message is automatically acknowledged if the method returns normally
If the method throws an exception, the message is not acknowledged and becomes available for redelivery after the ack deadline — at-least-once delivery; implement idempotent handlers to handle duplicate delivery safely

Push subscriptions

A push subscription has Pub/Sub call a URL when messages arrive — which means Cloud Run functions or App Engine services can receive Pub/Sub messages via HTTP POST without running a continuous polling loop
The HTTP handler receives the message as a Base64-encoded payload in a JSON envelope; @RestController + Spring's @PubSubSubscriber utility handles the deserialization

Message attributes and filtering

Pub/Sub message attributes are key-value string metadata — pubSubTemplate.publish(topic, message, Map.of("type", "OrderPlaced")) adds an attribute
Subscription filters (subscription.message_filter) allow a subscription to receive only messages matching attributes.type = "OrderPlaced", reducing traffic to consumers that only need specific event types

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