Handling a Cloud Run container shutdown

Question

When writing a Cloud Run service, we develop a container that listens on the PORT environment variable to handle an incoming HTTP request. An instance of the container springs to life and handles the request and then lives a while longer after concluding the original request in case there are further requests to arrive. If there are no further requests, GCP shuts down the container. It is in this area that I have a question.

Is there a hook, signal or other indication that the container is being shut down within the container?

In my example, my container wants to cleanly end. Perhaps it wants to close connections or perform some quick flush of cache.

Answer 1

An FAQ of great collected Q&A on Cloud Run has been found here. Within that FAQ there is an item which reads:

What is the termination signal for Cloud Run services?

Currently, Cloud Run terminates containers while scaling to zero with unix signal 9 (SIGKILL). SIGKILL is not trappable (capturable) by applications. Therefore, your applications should be okay to be killed abruptly.

A related and important entry also reads:

When will my service scale to zero?

Cloud Run does not provide any guarantees on how long it will keep a service "warm". It depends on factors like capacity and Google’s implementation details.

Some users see their services staying warm up to an hour, or longer.

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Opinion

I find it interesting that the story appears to be an immediate SIGKILL. If we take Docker as a basis for a container environment, we can read about docker stop which appears to be the way to cleanly stop a container. In its own description it says:

The main process inside the container will receive SIGTERM, and after a grace period, SIGKILL.

This would appear to indicate that for a normal Docker container stop, the process running the container will receive a signal.

Answer 2

SIGTERM (unix signal 15) is now sent to the Cloud Run service and you are given 10 seconds to handle closing any resources etc, if you handle the signal in your code.

See here.

Container instances can be shut down at any time. When a container instance needs to be shut down, new incoming requests are routed to other instances and requests currently being processed are given time to complete. The container instance then receives a SIGTERM signal indicating the start of a 10 second period before being shut down (with a SIGKILL signal). During this period, the container instance is allocated CPU and billed. If the container instance does not catch the SIGTERM signal, it is immediately shut down.

(image of code taken from here)