Steve Loughran commented on YARN-913:

w.r.t [~aw]'s comments:

bq. If a client needs to talk to more than one ZK, it sounds like they are 
basically screwed.

If you are grabbing binding/configs via the CLI, it's not a worry, nor if you 
are talking to >1 ZK quorum with the same auth policy. Its when you start 
tuning SASL auth and some various timeouts that this arises. This is not an 
issue with the registry, it's the ZK client here.

bq. I was mainly looking at the hostname pattern:
+      "([a-z0-9]|[a-z0-9][a-z0-9\\-]*[a-z0-9])";
bq. It doesn't appear to support periods/dots.

That's just the pattern for entries in the registry path itself; you can't give 
a service a name like "-#foo" as DNS won't like it. Stick whatever you want in 
the fields themselves.

I'll javadoc that field

> Add a way to register long-lived services in a YARN cluster
> -----------------------------------------------------------
>                 Key: YARN-913
>                 URL: https://issues.apache.org/jira/browse/YARN-913
>             Project: Hadoop YARN
>          Issue Type: New Feature
>          Components: api, resourcemanager
>    Affects Versions: 2.5.0, 2.4.1
>            Reporter: Steve Loughran
>            Assignee: Steve Loughran
>         Attachments: 2014-09-03_Proposed_YARN_Service_Registry.pdf, 
> 2014-09-08_YARN_Service_Registry.pdf, RegistrationServiceDetails.txt, 
> YARN-913-001.patch, YARN-913-002.patch, YARN-913-003.patch, 
> YARN-913-003.patch, YARN-913-004.patch, YARN-913-006.patch, 
> YARN-913-007.patch, YARN-913-008.patch, YARN-913-009.patch, 
> YARN-913-010.patch, YARN-913-011.patch, YARN-913-012.patch, 
> YARN-913-013.patch, yarnregistry.pdf, yarnregistry.tla
> In a YARN cluster you can't predict where services will come up -or on what 
> ports. The services need to work those things out as they come up and then 
> publish them somewhere.
> Applications need to be able to find the service instance they are to bond to 
> -and not any others in the cluster.
> Some kind of service registry -in the RM, in ZK, could do this. If the RM 
> held the write access to the ZK nodes, it would be more secure than having 
> apps register with ZK themselves.

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