I have a requirement to track the undelivered messages in PubSub. But when a subscriber to a PubSub Pull subscription is unavailable after the retention period the message will be lost forever from the subscription. It is not been captured by the dead letter topic created for the subscription.
It seems the PubSub only sends a message to a dead letter topic if the number of retries exceeds and the acknowledgement not been received by the subscriber.
Is there a way to push a message to dead letter topic before the message get lost for forever?
There is no way to send messages to a dead letter topic before the message is deleted due to the retention period expiring, no. The goal of the dead letter topic is to capture messages that are causing issues for subscribers and potentially preventing the processing of other messages, e.g., if the subscribers are crashing due to an unexpected message. The way this state is detected is via the retry count.
Related
I found this docs: https://cloud.google.com/nodejs/docs/reference/pubsub/0.19.x/v1.SubscriberClient#acknowledge
If a message is acknowledged failed, will it be put back to the message queue and wait for redelivering later? Or this message is lost?
Acknowledgements in Google Cloud Pub/Sub are best effort and the service as a whole has at-least-once delivery of messages. What this means is that if an acknowledgement fails (and even in rare cases, if you get back that an ack succeeded), the messages will be redelivered to a subscriber. A message is only deleted from Pub/Sub if the service successfully receives and processes an ack for the messages messageRetentionDuration passes, which defaults to seven days.
We recently integrated google pubsub into our app, and some of our long running tasks are now under problem, as they take more than 1 minute sometimes. We have configured our subscriber's ack deadline to 600 seconds, yet, anything that is taking more than 600ms, is being retried by pubsub.
this is our config:
gcloud pubsub subscriptions describe name
ackDeadlineSeconds: 600
expirationPolicy: {}
messageRetentionDuration: 604800s
Not sure what is the issue. Most of our tasks will get repeated because of this
Pub/Sub has a built in At-least-once delivery system which will retry messages that were not acknowledged. In this case, after 600s have passed, the message you first sent becomes unacknowledged, thus Pub/Sub retries the message. It will keep retrying it for 600s until it reaches the messageRetentionDuration or you acknowledge it.
Keep in mind that it's specified in the documentation that your subscriber should be idempotent. So, making your code be able to handle multiple messages should be the best approach to this issue.
You could also decrease the messageRetentionDuration to 600s(it's minimum) so anything that passes the 10 min mark will not be retried.
Also, it is stated in the FAQs that:
Why are there too many duplicate messages?
Cloud Pub/Sub guarantees at-least-once message delivery, which means
that occasional duplicates are to be expected. However, a high rate of
duplicates may indicate that the client is not acknowledging messages
within the configured ack_deadline_seconds, and Cloud Pub/Sub is
retrying the message delivery. This can be observed in the monitoring
metrics.
pubsub.googleapis.com/subscription/pull_ack_message_operation_count
for pull subscriptions, and
pubsub.googleapis.com/subscription/push_request_count for push
subscriptions. Look for elevated expired or webhook_timeout values in
the /response_code. This is particularly likely if there are many
small messages, since Cloud Pub/Sub may batch messages internally and
a partially acknowledged batch will be fully redelivered.
Another possibility is that the subscriber is not acknowledging some
messages because the code path processing those specific messages
fails, and the Acknowledge call is never made; or the push endpoint
never responds or responds with an error.
In a Pub/Sub 'push' model the docs say this:
If the push endpoint returns an error code, messages are retried for up to 7 days with an exponential backoff policy (capped at 10 seconds).
Is there a way to decide what to do with the message after the retry period ? i.e. send it to some error queue etc ?
The seven-day retry period represents the maximum amount of time unacknowledged messages are retrained in Cloud Pub/Sub to be delivered to subscribers. After the seven days pass, a message is automatically deleted from Cloud Pub/Sub and no longer delivered. The system does not currently support performing any actions on these deleted messages such as sending them to an error queue.
My question is related to this description of the pub/sub message flow from The Basics of a Publish/Subscribe Service:
The description apears to suggest that it's possible for a subscriber to only receive some of the messages hitting a subscription point: Subscriber 1 appears to be getting just the B message and Subscriber 2 getting just the A message, despite the fact that both A and B messages are coming from Subscription 1.
Nowhere else in the docs I encountered such concept, message receiving appears to be done based on particular subscription and a subscription appears to be done for a particular topic, but not for a particular publisher.
Am I misinterpreting the above description or is it really possible for a subscriber to select only some of the messages it receives (based on the publisher)?
The subscribers themselves do not choose which messages they get. When there are multiple subscribers for a single subscription, they can both pull from the same subscription and receive an arbitrary subset of the messages. This can be used to load balance across multiple subscribers and process more messages in parallel by increasing the number of subscribers.
I need this ability to send push notifications for an action in a mobile app but wait for the user to undo the action until say 10 seconds.
Is it possible to delay the processing of a message published in a topic by 10 seconds ? And then (sometimes, if user does undo) delete the message before 10 seconds, if it doesn't need to be processed ?
Depends on if you write the subscribers as well or not:
You have control over your subscriber's code:
In your PubSub messages add a timestamp for when you want that message to be processed.
In your clients (subscribers), have logic to acknowledge the message only if the timestamp to process the message is reached.
PubSub will retry delivering the message until it's acknowledged (or 10 days)
If you don't have control over your subscriber you can have a my-topic and my-delayed-topic. Folks can publish to the former topic and that topic will have only one subscriber which you will implement:
Publish message as before to my-topic.
You will have a subscriber for that topic that can do the same throttling as shown above.
If the time for that message has reached your handler will publish/relay that message to my-delayed-topic.
You can also implement the logic above with task-queue+pubsub-topic instead of pubsub-topic+pubsub-topic.
If architecturally possible at all, you could use Cloud Tasks. This API has the following features that might suit your usecase:
You can schedule the delivery of the message (task)
You can delete the tasks from the queue (before they are executed)
Assuming that your client has a storage for some task Ids:
Create a task with schedule_time set to 10s in the future.
Store the task name in memory (you can either assign a name to the task at creation time, or use the automatically generated ID returned from the create response).
If user undid the job, then call DeleteTask.
Just wanted to share that I noticed Pub/Sub supports retry policies 1 that are GA as of 2020-06-16 2.
If the acknowledgement deadline expires or a subscriber responds with a negative acknowledgement, Pub/Sub can send the message again using exponential backoff.
If the retry policy isn't set, Pub/Sub resends the message as soon as the acknowledgement deadline expires or a subscriber responds with a negative acknowledgement.
If the maximum backoff duration is set, the default minimum backoff duration is 10 seconds. If the minimum backoff duration is set, the default maximum backoff duration is 600 seconds.
The longest backoff duration that you can specify is 600 seconds.