Add zenPings(ImmutableList) again

[dadoonet]

Commit 95c2d84 removed zenPings(ImmutableList<? extends ZenPing> pings) which actually used by discovery plugins.
For example here: https://github.com/elasticsearch/elasticsearch-cloud-aws/blob/master/src/main/java/org/elasticsearch/discovery/ec2/Ec2Discovery.java#L68

This PR add it back.

[javanna]

can we explain why it may be useful for discovery plugins? This missing bit is exactly why it got removed in the first place :)

I also wonder if we can find a different way to achieve the same result, avoiding to expose a public method that is used by plugins only. That said I don't know this part of es that much...

[dadoonet]

Well, we are adding a HostsProvider to unicastZenPing. This hosts provider makes AWS/Azure/GCE API calls to get back the list of potential elasticsearch nodes.

See https://github.com/elasticsearch/elasticsearch-cloud-aws/blob/master/src/main/java/org/elasticsearch/discovery/ec2/Ec2Discovery.java#L64-71

[javanna]

By "can we explain" I meant that I would like to see this info in the comment, anything else is going to get lost. Even if I understand it now and I look at the link you provided, this is not going to be useful the next time we wonder why this method is there.

[dadoonet]

Yeah. I understood your comment as a suggestion to enhance my Java comment. I agree.

I was answering actually to the last part of your comment on how we could do something different in core. :)

[dadoonet]

@javanna PR updated. Let me know.

[imotov]

@dadoonet, wouldn't it be easier to register this unicast provider in Ec2DiscoveryModule constructor by calling addUnicastHostProvider method of ZenDiscoveryModule instead of fishing for UnicastZenPing? It seems a bit cleaner and simpler to me. What do you think?

[dadoonet]

@imotov Agreed. Something like this? https://github.com/dadoonet/elasticsearch-cloud-aws/compare/pr/ec2-host-provider

I think this should work fine.

[imotov]

@dadoonet I think adding it in bindDiscovery is a bit too late because by this time all unicast providers are already added to multi-binder. I think Ec2DiscoveryModule constructor might be a better place.

[dadoonet]

@imotov Closing this one in favor of elastic/elasticsearch-cloud-aws#160

[javanna]

Happy to see we found another way to do the same, without requiring to add this method back to core. Thanks @imotov and @dadoonet !