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https://issues.apache.org/jira/browse/FLINK-9592?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16647600#comment-16647600
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Kostas Kloudas commented on FLINK-9592:
---------------------------------------
Hi [~kent2171],
I had a look at the PR.
In general, as I said earlier, I like the idea of having Callbacks to notify
when a file changes state.
As far as the design/implementation of the current PR is concerned, the
following are my comments:
1) The FileStateChangedCallback seems to be pretty limitting, and probably
designed with a specific usecase
in mind. It assumes that the user would like to do sth with the underlying file
system when the file changes
state (e.g. write a special file). But other usecases may need to do a REST
call, or update a DB, or in
general communicate with another system.
Given the above, I would suggest that the function should have an open() and a
close() method which are called
once and are responsible for allocating and freeing resources. The open()
should potentially take the flink config
as argument and initialize any long-living resources, e.g. connections to
databases, a connection to the filesystem,
etc, and the close should be responsible for freeing them. This will allow the
sink to accommodate a broader variety
of usecases. Now for the methods themselves, I do not yet have a
definite answer on what should be included as argument, but I would also
include a Context as an argument. This
will allow for future-proofing the method, as we will be able to add stuff in
the Context if we want to expose
more stuff in the future, rather than deprecating the already existing API and
creating a new one.
2) IMPORTANT CONSIDERATIONS to keep in mind: all this is a "best-effort"
reporting of state changes, as, for example, if a
failure happens after transitioning a file to its "final" state, but before
calling the hook, then you will
never get the notification. This behavior is alligned with Flink's metric
system, where metrics are not checkpointed.
In our case though, the scenario described above is more tricky to accomodate
as we are talking about integration
with external systems.
Let me know what you think about the above!
> Notify on moving file into pending/ final state
> -----------------------------------------------
>
> Key: FLINK-9592
> URL: https://issues.apache.org/jira/browse/FLINK-9592
> Project: Flink
> Issue Type: New Feature
> Components: filesystem-connector
> Reporter: Rinat Sharipov
> Assignee: Kostas Kloudas
> Priority: Major
> Labels: pull-request-available
>
> Hi mates, I got a proposal about functionality of BucketingSink.
>
> During implementation of one of our tasks we got the following need - create
> a meta-file, with the path and additional information about the file, created
> by BucketingSink, when it’s been moved into final place.
> Unfortunately such behaviour is currently not available for us.
>
> We’ve implemented our own Sink, that provides an opportunity to register
> notifiers, that will be called, when file state is changing, but current API
> doesn’t allow us to add such behaviour using inheritance ...
>
> It seems, that such functionality could be useful, and could be a part of
> BucketingSink API
> What do you sink, should I make a PR ?
> Sincerely yours,
> *Rinat Sharipov*
> Software Engineer at 1DMP CORE Team
>
> email: [[email protected]|mailto:[email protected]]
> mobile: +7 (925) 416-37-26
> Clever{color:#4f8f00}DATA{color}
> make your data clever
>
> ------------------------------------------------------------------------------------------------------------------------
>
> Hi,
> I see that could be a useful feature. What exactly now is preventing you from
> inheriting from BucketingSink? Maybe it would be just enough to make the
> BucketingSink easier extendable.
> One thing now that could collide with such feature is that Kostas is now
> working on larger BucketingSink rework/refactor.
> Piotrek
> ________________________________________________________________________
>
> Hi guys, thx for your reply.
> The following code info is actual for *release-1.5.0 tag,
> org.apache.flink.streaming.connectors.fs.bucketing.BucketingSink class*
>
> For now, BucketingSink has the following lifecycle of files
>
> When moving files from opened to pending state:
> # on each item (*method* *invoke:434* *line*), we check that suitable bucket
> exist, and contain opened file, in case, when opened file doesn’t exist, we
> create one, and write item to it
> # on each item (*method* *invoke:434* *line*), we check that suitable opened
> file doesn’t exceed the limits, and if limits are exceeded, we close it and
> move into pending state using *closeCurrentPartFile:568 line - private method*
> # on each timer request (*onProcessingTime:482 line*), we check, if items
> haven't been added to the opened file longer, than specified period of time,
> we close it, using the same private method *closeCurrentPartFile:588 line*
>
> So, the only way, that we have, is to call our hook from
> *closeCurrentPartFile*, that is private, so we copy-pasted the current impl
> and injected our logic there
>
>
> Files are moving from pending state into final, during checkpointing
> lifecycle, in *notifyCheckpointComplete:657 line*, that is public, and
> contains a lot of logic, including discovery of files in pending states,
> synchronization of state access and it’s modification, etc …
>
> So we couldn’t override it, or call super method and add some logic, because
> when current impl changes the state of files, it removes them from state, and
> we don’t have any opportunity to know,
> for which files state have been changed.
>
> To solve such problem, we've created the following interface
>
> /**
> * The \{@code FileStateChangeCallback}is used to perform any additional
> operations, when
> {@link BucketingSink}
> * moves file from one state to another. For more information about state
> management of \{@code BucketingSink}, look
> * through it's official documentation.
> */
> public interface FileStateChangeCallback extends Serializable \{ /** * Used
> to perform any additional operations, related with moving of file into next
> state. * * @param fs provides access for working with file system * @param
> path path to the file, moved into next state * * @throws IOException if
> something went wrong, while performing any operations with file system */
> void call(FileSystem fs, Path path) throws IOException; }
> And have added an ability to register this callbacks in BucketingSink impl in
> the following manner
>
> public BucketingSink<T>
> registerOnFinalStateChangeCallback(FileStateChangeCallback… callbacks) \{...}
> public BucketingSink<T>
> registerOnPendingStateChangeCallback(FileStateChangeCallback... callbacks)
> \{...}
>
> I’m ready to discuss the best ways, how such hooks could be implemented in
> the core impl or any other improvements, that will help us to add such
> functionality into our extension, using public api, instead of copy-pasting
> the source code.
>
> Thx for your help, mates =)
> [*See More* from Piotr Nowojski|x-redundant-cluster-toggle://0]
>
> Sincerely yours,
> *Rinat Sharipov*
> Software Engineer at 1DMP CORE Team
>
> email: [[email protected]|mailto:[email protected]]
> mobile: +7 (925) 416-37-26
> Clever{color:#4f8f00}DATA{color}
> make your data clever
>
> ________________________________________________________________________
>
> Hi,
>
> Couple of things:
>
> 1. Please create a Jira ticket with this proposal, so we can move discussion
> from user mailing list.
>
> I haven’t thought it through, so take my comments with a grain of salt,
> however:
>
> 2. If we were to go with such callback, I would prefer to have one
> BucketStateChangeCallback, with methods like `onInProgressToPending(…)`,
> `onPendingToFinal`, `onPendingToCancelled(…)`, etc, in oppose to having one
> interface passed three times/four times for different purposes.
>
> 3. Other thing that I had in mind is that BucketingSink could be rewritten to
> extend TwoPhaseCommitSinkFunction. In that case, with
>
> public class BucketingSink2 extends TwoPhaseCommitSinkFunction<???>
>
> user could add his own hooks by overriding following methods
>
> BucketingSink2#beginTransaction, BucketingSink2#preCommit,
> BucketingSink2#commit, BucketingSink2#abort. For example:
>
> public class MyBucketingSink extends BucketingSink2 {
> @Override
> protected void commit(??? txn)
> { super.commit(txn); // My hook on moving file from pending to commit
> state }
> ;
> }
>
> Alternatively, we could implement before mentioned callbacks support in
> TwoPhaseCommitSinkFunction and provide such feature to
> Kafka/Pravega/BucketingSink at once.
>
> Piotrek
>
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