Replay Mechanisms and Dead Letter Topics in Cloud Pub/Sub

Replay Mechanisms in Cloud Pub/Sub

Cloud Pub/Sub is primarily designed as a streaming messaging service, where messages are delivered to subscribers in real-time. However, there are scenarios when you may want to replay past messages, such as:


Recovering from a bug in a subscriber application.


Reprocessing data with updated logic.


Testing or debugging with real data.


How Replays Work in Pub/Sub

Pub/Sub does not natively support full message replay like some systems (e.g., Kafka). However, there are workarounds and best practices to enable replay-like functionality:


Retain Acknowledged Messages:


By default, Pub/Sub retains unacknowledged messages for 7 days.


For message replay beyond 7 days, you must store messages externally (e.g., in Cloud Storage or BigQuery).


Using a Message Archival Pattern:


Use Cloud Dataflow or Pub/Sub subscriptions to stream and archive messages to a durable storage system.


Replay is then performed by reading from that archive and re-publishing to the topic.


Seek Operation:


Cloud Pub/Sub supports seek operations for subscriptions:


Seek to a specific timestamp.


Seek to a specific snapshot (a point-in-time view of a subscription).


This allows limited replay within the 7-day window of message retention.


Dead Letter Topics (DLTs) in Cloud Pub/Sub

A Dead Letter Topic is a secondary topic where Pub/Sub forwards messages that cannot be successfully processed after a certain number of delivery attempts.


Why Use Dead Letter Topics?

Prevents endless retries for unprocessable messages.


Allows isolation of problematic messages for inspection and debugging.


Improves the reliability and observability of your system.


How Dead Letter Topics Work:

Configuration:


Set a dead letter topic on a subscription.


Define a maximum delivery attempt count (e.g., 5 attempts).


Behavior:


If a subscriber fails to acknowledge a message after the max attempts, the message is forwarded to the DLT.


The message is not delivered again to the original subscription.


Use Cases:


Handling poison messages (e.g., malformed data).


Logging and alerting on recurring errors.


Reprocessing later with manual or automated review.


Summary Table

Feature Description

Replay Mechanisms Reprocessing past messages via seek, snapshots, or external archives.

Dead Letter Topics Redirects unprocessable messages after max attempts to a secondary topic.


Best Practices

Use Snapshots regularly if you expect the need to replay recent messages.


Archive messages if long-term replay is needed (beyond 7 days).


Monitor dead letter topics for operational issues or bugs.


Ensure IAM permissions are properly configured for publishing and subscribing to DLTs.

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