Why it matters

Pub/Sub is GCP messaging default. Understanding shapes GCP event architectures.

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The architecture

Topic: named channel. Subscription: named subscriber.

Publishers push messages; subscribers pull or receive push.

Pub/Sub structureTopicchannelSubscriptionper subscriberDeliverypull or pushOrdering keys for FIFO per key; without them, unordered
Pub/Sub components.
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How it works end to end

Delivery: at-least-once by default. Exactly-once feature available.

Ordering: per ordering key.

Dead letter topic: failed messages.

Retention: up to 7 days for undelivered.