Document ability to control task termination order when shutting down a swarm service

[mdlinville]

See

moby/moby#33392
moby/swarmkit#2264
moby/swarmkit#2265

[gbarr01]

@MistyHacks , what do you think about updating this info on these pages:

https://docs.docker.com/engine/swarm/swarm-tutorial/scale-service/
https://docs.docker.com/engine/swarm/how-swarm-mode-works/services/

[gbarr01]

@MistyHacks , the title of this PR implies that the user can control task termination. But looking at the code, it seems that tasks are shutdown in reverse slot order. The user cannot control this with syntax.

@thaJeztah , can you confirm?

[gbarr01]

This is my understanding:

Tasks are shut down gracefully by balancing remaining tasks across nodes. Unassigned tasks are shutdown first and running tasks are shutdown by Slot number in reverse order.

[gbarr01]

@thaJeztah , how does the user actually control termination order?

[thaJeztah]

I don't think the changes that were merged allow you to influence the order; the change that was implemented is that the order in which tasks are shutdown is more deterministic (i.e., when scaling down, the last task is shutdown first). Order in which tasks are shutdown is still not guaranteed though (at least, to my understanding), just that SwarmKit now takes it into account

[gbarr01]

Okay, thanks, @thaJeztah. That makes sense. I'm going to close this issue unless you think it needs documentation.

[kamalhakim]

Hi, Is this explained/documented anywhere? Can we rely on that tasks are terminated by reverse slot order in a swarm? Thanks!