Docker stack - all services on same host or not?

General
1

I’ve just started using docker stack, and ran into a question to which I cannot seem to find an answer.

If I do a ‘docker stack deploy’ on a swarm that has many nodes, what is the behaviour with regard to where the services are spawned?

For example:

  • • I do a docker stack deploy of a stack that has 5 services
  • • Things are set so that only one instance of the stack is deployed
  • • But I have a swarm with 5 nodes in it

When the stack deploys, will all 5 services be deployed on the same worker node? Or is it possible for me to have one service deployed on each of the 5 nodes (but all part of the same stack)?

(You may want to answer “why would you care? they are services, and can be anywhere, right?”, but in this case I have volumes shared between the services – this would work only if they are on the same node; or if there were an easy/simple distributed volume plugin for docker (sigh)).

So, in short, does docker stack make any sort of guarantee as to where the services are placed?

Thanks in advance!

2

bump

PLEASE tell me this is not another one of those fundamental things that no-one knows the answer to? (Isn’t this implicit in the design of docker stack? It seems like such a simple thing to confirm – one way or the other?)