Allow limiting of the number of containers restarted in parallel for a scaled service

[ashmrtn]

This patch addresses #5124 and adds a new command line flag for docker-compose up called the rollout-factor. The flag limits the number of containers that will be restarted in parallel, allowing the service to restart with zero-downtime.

[ashmrtn]

As of now, this patch has no tests associated with it and I am not completely sure how to test it as it basically adds a command line flag and then uses pre-existing features like parallel_execute's limiter object. Do you have any suggestions as to how to test this?

[shin-]

I'm afraid that this might set up false expectations regarding the availability of the application during upgrade - there's definitely a scenario where the last container is getting restarted while the first one is still booting (i.e. not ready to receive connections). While the feature would be a nice to have, I think the implementation of it is ultimately more complex.

[ashmrtn]

That is an excellent point that I completely neglected to think about when making this patch. From what I understand, docker-compose supported something vaguely similar when it allowed people to specify healthchecks for the condition form of depends_on. The fact that compose no longer supports that makes it feel like compose technically shouldn't support something like rolling out containers as it requires similar logic to check if a container is ready to receive requests. Do you have any thoughts on how to determine if a container is ready to receive requests that could be added to the current PR?

[ijc]

Do you have any thoughts on how to determine if a container is ready to receive requests that could be added to the current PR?

Is the HEALTHCHECK stuff (builder docs, compose docs) useful/relevant here? IOW only consider any new container good once it has entered the Healthy state?

[glours]

Thanks for taking the time to create this issue/pull request!

Unfortunately, Docker Compose V1 has reached end-of-life and we are not accepting any more changes (except for security issues). Please try and reproduce your issue with Compose V2 or rewrite your pull request to be based on the v2 branch and create a new issue or PR with the relevant Compose V2 information.