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https://issues.apache.org/jira/browse/HADOOP-5640?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12698516#action_12698516
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Todd Lipcon commented on HADOOP-5640:
-------------------------------------
That's somewhat feasible, but lacking deep-copy support in a lot of the hadoop
classes, it's a bit hard to ensure.
I guess we're probably best off just leaving this with a warning... the two
warning options are:
a) Your plugin hooks are called in the same thread as the service you are
plugging into. Ensure that you do not ever block in a plugin hook.
b) Your plugin hooks are called in a separate thread from the service you are
plugging into. Ensure that you do are aware of any synchronization issues that
might arise as a result.
Personally I prefer (a) still, since it's trivial to understand whether your
code blocks, whereas it is somewhat more difficult to know whether you have a
race. But, if others still prefer (b), that's fine.
> Allow ServicePlugins to hook callbacks into key service events
> --------------------------------------------------------------
>
> Key: HADOOP-5640
> URL: https://issues.apache.org/jira/browse/HADOOP-5640
> Project: Hadoop Core
> Issue Type: Improvement
> Components: util
> Reporter: Todd Lipcon
>
> HADOOP-5257 added the ability for NameNode and DataNode to start and stop
> ServicePlugin implementations at NN/DN start/stop. However, this is
> insufficient integration for some common use cases.
> We should add some functionality for Plugins to subscribe to events generated
> by the service they're plugging into. Some potential hook points are:
> NameNode:
> - new datanode registered
> - datanode has died
> - exception caught
> - etc?
> DataNode:
> - startup
> - initial registration with NN complete (this is important for HADOOP-4707
> to sync up datanode.dnRegistration.name with the NN-side registration)
> - namenode reconnect
> - some block transfer hooks?
> - exception caught
> I see two potential routes for implementation:
> 1) We make an enum for the types of hookpoints and have a general function in
> the ServicePlugin interface. Something like:
> {code:java}
> enum HookPoint {
> DN_STARTUP,
> DN_RECEIVED_NEW_BLOCK,
> DN_CAUGHT_EXCEPTION,
> ...
> }
> void runHook(HookPoint hp, Object value);
> {code}
> 2) We make classes specific to each "pluggable" as was originally suggested
> in HADDOP-5257. Something like:
> {code:java}
> class DataNodePlugin {
> void datanodeStarted() {}
> void receivedNewBlock(block info, etc) {}
> void caughtException(Exception e) {}
> ...
> }
> {code}
> I personally prefer option (2) since we can ensure plugin API compatibility
> at compile-time, and we avoid an ugly switch statement in a runHook()
> function.
> Interested to hear what people's thoughts are here.
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