Propagating Error messages in Google Cloud Platform (GCP)

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I am building a near real time service. The input is a cloud storage bucket and blob path to a photo image. This horizontally-scalable service is made up of multiple components including ML models running on k8s and Google Cloud Functions, each of which has a chance of failing for a variety of reasons. The ML models are independent and run in parallel. Each component is triggered by a PubSub push message topic unique to the component. Running the entire flow for one photo may take 15 seconds.

I want to return a meaning error message back to the service requester telling which component failed if there is a failure. Essentially, I want to report which image failed and where it failed.

What is the recommended practice for returning an error back to the requester?

guillaume blaquiere

There is no built in service for this. But, because you already use PubSub for asynchronous call, I propose to use it also to push back the error.

You can do this in 2 flavors

First, create a PubSub topic for the errors, let's say 'error_topic'

1. Without message customization

In the PubSub message, the requester put which it is in the attribute (let's say 'requester' attribute name)
In the consumer service, if an error occurs, return an error code (500 for example) for push subscription or a NACK in pull subscription.
Configure the PubSub subscription to manage retry and dead letter topic (the dead letter topic is 'error_topic')
Then, create one subscription per requester on the 'error_topic' (use the filter capability for this) and consume the message in the requester services

2. With message customization

In the PubSub message, the requester put which it is in the attribute (let's say 'requester' attribute name)
The consumer service that raises the error create a new message with custom information and copies the 'requester' attribute value and then puts it in attribute of the message in the 'error_topic' (let's say 'original_requester' attribute name).
Then, create one subscription per requester on the 'error_topic' (use the filter capability for this) and consume the message in the requester services

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edited at2021-05-29

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