Add --scheduler option in manage command to prepare pluggable schedulers

[francisbouvier]

Hi,

I've made some changes in order to add pluggable schedulers in the future.
Scheduler is an interface and SwarmScheduler its default implementation.

My idea is to add other scheduler, e.g. an ApiScheduler to connect to external services.

[vieux]

Hi @francisbouvier thanks for the contribution!

First tests are failing: https://travis-ci.org/docker/swarm/builds/44228278
Second, I have an issue here about the --strategy flag (and the future --filter) because if you don't use --scheduler swarm, theses flags are useless right ?

I would love to see your ApiScheduler or any other scheduler before merging this, to make sure the interface is correct.

[francisbouvier]

Thanks for your quick reply.

I've updated the pull request, tests are passing now: https://travis-ci.org/docker/swarm/builds/44246565.
Sorry about that.

Yes the --strategy and --filter flags are only used with the default swarm scheduler.
May be we can have a --scheduler-opts common flag (as pluggable scheduler may have their own options too) ?

[vieux]

@francisbouvier why not, that's why I'd love to see your scheduler

[vieux]

😈

[francisbouvier]

@vieux I'm working on it :)

[francisbouvier]

@vieux

Hi,

I've commited the ApiScheduler.

For the option I've created a --scheduler-option.
It's a cli.StringSliceFlag wich content is parsed as <option_name>:<option_content> and passed to the Initialize method of the Scheduler.
So the default options (for the SwarmScheduler) are --scheduler-option=strategy:binpacking:0.05 and --scheduler-option=filters:health,label,port.
Option for the ApiScheduler is --scheduler-option=url<ip>:<port>.

I've created a basic implementation of the corresponding API swarm-scheduler-api. It's a very basic API with random strategy, for tests purposes.

[vieux]

@francisbouvier thanks, I'll get back to this tomorrow once I have tried it out.

[vieux]

@francisbouvier sorry for the delay. If I understand correctly your api scheduler creates the container on the node ? I think it should only return the right node and let swarm create the container.

[francisbouvier]

@vieux
Yes, the external API creates the container. If you want to plug to an external scheduler like Mesos-Marathon or Kubernetes I think you have to let him creates the container (if I understand well how they works).
Using only the node selection can be usefull too, but it's more a pluggable strategy.

[vieux]

@francisbouvier thank for this, but it's a bit too abstract for us to merge. Be sure we are going to discuss the scheduler API publicly and I'm going to include you in the discussions.

[vieux]

See #213 for reference