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cuppy_path: reference/messaging/errors.md
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cuppy_updated: 2026-10-23T16:16:52.575Z
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---


# Messaging error catalogue

## 1. The consumer sees the same message twice

| | |
|---|---|
| **cause** | **not a bug.** Standard Pub/Sub subscriptions are **at-least-once** |
| **check** | is exactly-once delivery enabled on this subscription? By default it is not |
| **fix** | make the consumer **idempotent** — the correct answer. Or enable **exactly-once delivery**, knowing you have changed the delivery semantics |

This is the first row in the catalogue because it is the first thing a queue consumer ported from elsewhere
gets wrong, and because "fix the duplicates" is the wrong framing.

## 2. A push subscription stops delivering

| | |
|---|---|
| **cause** | push is an **HTTP POST** to your endpoint. If the endpoint does not return success, Pub/Sub retries |
| **check** | what is the endpoint returning? On Cloud Run look for **429** "The request was aborted because there was no available instance." and **503** "The request failed because the instance could not start successfully." |
| **fix** | a **429** is capacity: raise maximum instances, or switch to a **pull** subscription so the consumer sets its own pace. A **503** is configuration — usually a secret the runtime identity cannot read |

**A push subscription couples your delivery rate to your service's scaling.** That is the trade: pull gives the
consumer flow control, push gives you no consumer to run.

## 3. Messages disappeared

| | |
|---|---|
| **cause** | no **dead-letter topic**, and the message exceeded the subscription's **retention** |
| **check** | does the subscription have a dead-letter topic? What is its retention set to? |
| **fix** | configure a dead-letter topic on every subscription whose consumer can fail. Without one, a permanently-failing message is retried until retention expires and is then gone with no record |

*Recorded as an absence:* the Pub/Sub overview states **no default retention value**, so none is quoted here.
Read it from the subscription before relying on a replay window.

## 4. Throughput collapsed after enabling ordering

| | |
|---|---|
| **cause** | **ordering keys** serialise delivery per key |
| **check** | how many distinct keys are in use? If the answer is "one", every message is in one queue |
| **fix** | choose a key with high cardinality, or drop ordering where the consumer does not need it. Pub/Sub's native model is **per-message parallelism** "unlike partition-based systems" — ordering is the thing that gives that up |

This is the row a Kafka migration meets: a partition count became an ordering key, and a low-cardinality key is
a one-partition topic.

## 5. Tasks dispatch too slowly, or all at once

| | |
|---|---|
| **cause** | a Cloud Tasks **queue** carries the rate limits and the retry parameters — all tasks in it share them |
| **check** | which queue is this task in, and what are that queue's rate limits? |
| **fix** | separate queues for separate policies. A queue is a policy, not a container |

## 6. An Eventarc trigger never fires

| | |
|---|---|
| **cause** | usually the **source**, not the destination: the event you expect is not published directly and must come through **Cloud Audit Logs** — which means the relevant audit log type has to be enabled |
| **check** | is this source a direct publisher or an audit-log-derived event? Is that audit log type on for the resource? |
| **fix** | enable the audit log type, or re-target the trigger at a source that publishes directly |

And one shape error: **a trigger that needs the message transformed** is an Eventarc **Advanced** design (bus
plus pipelines), not a Standard trigger with extra configuration.

Next: `guides/messaging/deliver-pubsub-to-cloud-run.md`, `guides/parity/messaging.md`.


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