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https://issues.apache.org/jira/browse/HBASE-6992?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13475788#comment-13475788
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ramkrishna.s.vasudevan commented on HBASE-6992:
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[~mbertozzi]
As you noticed the reason for introducing the pre/postOperationHandlers as part
of HBASE-5584 is first of all to have control on the async operations.
In our use case the create/delete coprocessors where used to create/delete
another table with some modified HTD.
Generally any async operations we need to wait for the successful operation and
that too when we have coprocessors we need to wait for the operation to be done
on the coprocessor hook tii.
When we tried to see the exisiting impl the reason for having pre/Post
operation hook for async operations and not waiting for the operation to get
completed is to not to block the RPC threads that does the operation.
So we thought of having other hooks that is sync with the handlers so that the
RPC threads need not wait.
The similar case is applicable for enable/disable table also. The new hooks
now gives us the advantage of being sure that the postOpreationHandler hook
will be exeucted only on success of the main operation.
bq.but in case of failure of async operations like deleteTable() we've removed
rights that we still need.
But the above problem should be a problem even without the new hooks right?
bq.for example: modifyTable() is just a helper to avoid multiple
addColumn()/deleteColumn() calls
but the problem here is that modifyTable() has its own pre/post operation()
This is again a general problem right?
> Coprocessors semantic issues: post async operations, helper methods, ...
> ------------------------------------------------------------------------
>
> Key: HBASE-6992
> URL: https://issues.apache.org/jira/browse/HBASE-6992
> Project: HBase
> Issue Type: Brainstorming
> Components: Coprocessors
> Affects Versions: 0.92.2, 0.94.2, 0.96.0
> Reporter: Matteo Bertozzi
> Assignee: Matteo Bertozzi
>
> Discussion ticket around coprocessor pre/post semantic.
> For each rpc in HMaster we have a pre/post operation that allows a coprocessor
> to execute some code before and after the operation
> * preOperation()
> * my operation
> * postOperation()
> This is used for example by the AccessController to verify if the user can
> execute or not the operation.
> Everything is fine, unless the master operation is asynchronous (like
> create/delete table)
> * preOperation()
> * executor.submit(new OperationHandler())
> * postOperation()
> The pre operation is still fine, since is executed before the operation and
> need to throw exceptions to the client in case of failures...
> The post operation, instead, is no longer post... is just post submit. And if
> someone subscribe to postCreateTable() the notification can arrive before the
> table creation.
> To "solve" this problem, HBASE-5584 added pre/post handlers and now the
> situation looks like this:
> {code}
> client request client response
> | |
> +------+-- submit op --+----+--- (HMaster)
> pre op post op
> (executor) +---- handler ----+
> pre handler post handler
> {code}
> Now, we've two types of pre/post operation and the semantical correct are
> preOperation() and postOperationHandler()
> since the preOperation() needs to reply to the client (e.g AccessController
> NotAllowException) and the postOperatioHandler() is really post operation.
> postOperation() is not post... and preOperationHandler() can't communicate
> with the client.
> The AccessController coprocessor uses the postOperation() that is fine for
> the sync operation like addColumn(), deleteColumn()... but in case of failure
> of async operations like deleteTable() we've removed rights that we still
> need.
> I think that we should get back just to the single pre/post operation but
> with the right semantic...
> Other then the "when is executed" problem, we've also functions that can be
> described with other simpler functions
> for example: modifyTable() is just a helper to avoid multiple
> addColumn()/deleteColumn() calls
> but the problem here is that modifyTable() has its own pre/post operation()
> and if I've implemented the pre/post addColumn I don't get notified when I
> call modifyTable(). This is another problem in the access controller
> coprocessor
> In this case I'm not sure what the best solution can be... but in this way,
> adding new helper methods means breaking the coprocessors, because they don't
> get notified even if something is changed...
> Any idea, thoughts, ...?
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