Re: [DISCUSS] KIP-883: Add delete callback method to Connector API

[Chris Egerton <ch...@aiven.io.INVALID>]

Hi Hector,

Thanks for the updates!

RE 1: This doesn't have the same user-facing behavior, though. Today
failures in Connector::stop can be surfaced via the status endpoints in the
REST API. But when connectors are deleted, they won't be visible at all in
these endpoints.

RE 3: It seems like this approach would only provide guarantees on a
per-worker basis; I was wondering more about trying to wait for tasks on
all workers in the cluster to stop. One potential approach for this could
be to handle connector deletes by breaking them up into two separate
rebalances: the first would revoke all of the connector's tasks, and the
second would revoke the connector itself. But this is just an example, and
would come with its own non-negligible overhead; we can and should explore
other approaches too.

RE 4: Thanks for the clarification, makes sense 👍 Worth noting for anyone
following along that KIP-419 is similar to this KIP, but on a much smaller
scale: it only focuses on the cleanup of resources allocated by single Task
instances, whereas this KIP focuses on the cleanup of resources that are
meant to be used across the entire lifetime of the connector, which may
encompass the lifetimes of several Connector and Task instances.

Cheers,

Chris

On Wed, Nov 30, 2022 at 10:58 AM Hector Geraldino (BLOOMBERG/ 919 3RD A) <
hgeraldino@bloomberg.net> wrote:

> Thanks for your feedback Chris,
>
> 1. I think the behavior should remain the same as it is today. The worker
> stops the connector when its configuration is updated, and if the update is
> a deletion, it won't start the connector again. If an error happens during
> stop() today, the statusListener will update the backing store with a
> FAILED state. The only thing that changes on this path is that the
> Connector#stop() method will include an additional boolean parameter, so
> the connector knows that the reason it is being stopped is because of a
> deletion, and can perform additional actions if necessary.
>
> 2. I agree; at first I thought it made sense, but after reading KIP-875
> and finding out that connectors can use custom offsets topics to store
> offsets, I think this idea needs more refinement. There's probably a way to
> reuse the work proposed by this KIP with the "Automatically delete offsets
> with connectors" feature mentioned on the "Future work" section of KIP-875,
> and am happy to explore it more.
>
> 3. I didn't consider that. There is some asymmetry here on how the
> StandaloneHerder handles this (tasks are stopped before the connector is)
> and the DistributedHerder. One option would be not to handle this on the
> #processConnectorConfigUpdates(...) method, but instead wait for the
> RebalanceListener#onRevoked(...) callback, which already stops the revoked
> connectors and tasks synchronously. The idea would be to enhance this to
> check the configState store and, if the configuration of the revoked
> connector(s) is gone, then we can let the connector know about that fact
> when stopping it (by the aforementioned mechanism). I'll update the KIP and
> PR if you think it is worth it.
>
> 4. That's correct. As the KIP motivates, we have connectors that need to
> do some provisioning/setup when they are deployed (we run connectors for
> internal clients), and when tenants delete a connector, we don't have a
> clear signal that allows us to cleanup those resources. The goal is
> probably similar to
> https://cwiki.apache.org/confluence/display/KAFKA/KIP-419%3A+Safely+notify+Kafka+Connect+SourceTask+is+stopped,
> just took a different approach.
>
>
> From: dev@kafka.apache.org At: 11/29/22 15:31:31 UTC-5:00To:
> dev@kafka.apache.org
> Subject: Re: [DISCUSS] KIP-883: Add delete callback method to Connector API
>
> Hi Hector,
>
> Thanks for the KIP! Here are my initial thoughts:
>
> 1. I like the simplicity of an overloaded stop method, but there is some
> asymmetry between stopping a connector and deleting one. If a connector is
> stopped (for rebalance, to be reconfigured, etc.) and a failure occurs
> then, the failure will be clearly visible in the REST API via, e.g., the
> GET /connectors/{connector}/status endpoint. If a connector is deleted and
> a failure occurs, with the current proposal, users won't have the same
> level of visibility. How can we clearly surface failures caused during the
> "destroy" phase of a connector's lifecycle to users?
>
> 2. I don't think that this new feature should be used to control (delete)
> offsets for connectors. We're addressing that separately in KIP-875, and it
> could be a source of headaches for users if they discover that some
> connectors' offsets persist across deletion/recreation while others do not.
> If anything, we should explicitly recommend against this kind of logic in
> the Javadocs for the newly-introduced method.
>
> 3. Is it worth trying to give all of the connector's tasks a chance to shut
> down before invoking "stop(true)" on the Connector? If so, any thoughts on
> how we can accomplish that?
>
> 4. Just to make sure we're on the same page--this feature is not being
> proposed so that connectors can try to delete the data that they've
> produced (i.e., that sink connectors have written to the external system,
> or that source connectors have written to Kafka), right?
>
> Cheers,
>
> Chris
>
> On Thu, Nov 17, 2022 at 5:31 PM Hector Geraldino (BLOOMBERG/ 919 3RD A) <
> hgeraldino@bloomberg.net> wrote:
>
> > Hi,
> >
> > I've updated the KIP with the new #stop(boolean isDeleted) overloaded
> > method, and have also amended the PR and JIRA tickets. I also added a
> > couple entries to the "Rejected alternatives" section with the reasons
> why
> > I pivoted from introducing new callback methods to retrofit the existing
> > one.
> >
> > Please let me know what your thoughts are.
> >
> > Cheers,
> > Hector
> >
> > From: Hector Geraldino (BLOOMBERG/ 919 3RD A) At: 11/16/22 17:38:59
> > UTC-5:00To:  dev@kafka.apache.org
> > Subject: Re: [DISCUSS] KIP-883: Add delete callback method to Connector
> API
> >
> > Hi Mickael,
> >
> > I agree that the new STOPPED state proposed in KIP-875 will improve the
> > connector lifecycle. The changes proposed in this KIP aim to cover the
> gap
> > where connectors need to actually be deleted, but because the API doesn't
> > provide any hooks, external assets are left lingering where they
> shouldn't.
> >
> > I agree that this proposal is similar to KIP-419, maybe the main
> > difference is their focus on Tasks whereas KIP-833 proposes changes to
> the
> > Connector. My goal is to figure out the correct semantics for notifying
> > connectors that they're being stopped because the connector has been
> > deleted.
> >
> > Now, computing the "deleted" state in both the Standalone and Distributed
> > herders is not hard, so the question is: when shall the connector be
> > notified? The "easiest" option would be to do it by calling an overloaded
> > Connector#stop(deleted) method, but there are other - more expressive -
> > ways, like providing an 'onDelete()' or 'destroy()' method.
> >
> > I'm leaning towards adding an overload method (less complexity, known
> > corner cases), and will amend the KIP with the reasoning behind that
> > decision soon.
> >
> > Thanks for your feedback!
> >
> > From: dev@kafka.apache.org At: 11/16/22 11:13:17 UTC-5:00To:
> > dev@kafka.apache.org
> > Subject: Re: [DISCUSS] KIP-883: Add delete callback method to Connector
> API
> >
> > Hi Hector,
> >
> > Thanks for the KIP.
> >
> > One tricky aspect is that currently there's no real way to stop a
> > connector so to do so people often just delete them temporarily.
> > KIP-875 proposes adding a mechanism to properly stop connectors which
> > should reduce the need to deleting them and avoid doing potentially
> > expensive cleanup operations repetitively.
> >
> > This KIP also reminds me of KIP-419:
> >
> >
>
> https://cwiki.apache.org/confluence/display/KAFKA/KIP-419%3A+Safely+notify+Kafka
> > +Connect+SourceTask+is+stopped.
> > Is it guaranteed the new delete callback will be the last method
> > called?
> >
> > Thanks,
> > Mickael
> >
> >
> > On Tue, Nov 15, 2022 at 5:40 PM Sagar <sa...@gmail.com> wrote:
> > >
> > > Hey Hector,
> > >
> > > Thanks for the KIP. I have a minor suggestion in terms of naming. Since
> > > this is a callback method, would it make sense to call it onDelete()?
> > >
> > > Also, the failure scenarios discussed by Greg would need handling.
> Among
> > > other things, I like the idea of having a timeout for graceful shutdown
> > or
> > > else try a force shutdown. What do you think about that approach?
> > >
> > > Thanks!
> > > Sagar.
> > >
> > > On Sat, Nov 12, 2022 at 1:53 AM Hector Geraldino (BLOOMBERG/ 919 3RD
> A) <
> > > hgeraldino@bloomberg.net> wrote:
> > >
> > > > Thanks Greg for taking your time to review not just the KIP but also
> > the
> > > > PR.
> > > >
> > > > 1. You made very valid points regarding the behavior of the destroy()
> > > > callback for connectors that don't follow the happy path. After
> > thinking
> > > > about it, I decided to tweak the implementation a bit and have the
> > > > destroy() method be called during the worker shutdown: this means it
> > will
> > > > share the same guarantees the connector#stop() method has. An
> > alternative
> > > > implementation can be to have an overloaded connector#stop(boolean
> > deleted)
> > > > method that signals a connector that it is being stopped due to
> > deletion,
> > > > but I think that having a separate destroy() method provides clearer
> > > > semantics.
> > > >
> > > > I'll make sure to ammend the KIP with these details.
> > > >
> > > > 3. Without going too deep on the types of operations that can be
> > performed
> > > > by a connector when it's being deleted, I can imagine the
> > > > org.apache.kafka.connect.source.SourceConnector base class having a
> > default
> > > > implementation that deletes the connector's offsets automatically
> > > > (controlled by a property); this is in the context of KIP-875
> > (first-class
> > > > offsets support in Kafka Connect). Similar behaviors can be
> introduced
> > for
> > > > the SinkConnector, however I'm not sure if this KIP is the right
> place
> > to
> > > > discuss all the possibilities, or if we shoold keeping it more
> > > > narrow-focused on  providing a callback mechanism for when connectors
> > are
> > > > deleted, and what the expectations are around this newly introduced
> > method.
> > > > What do you think?
> > > >
> > > >
> > > > From: dev@kafka.apache.org At: 11/09/22 16:55:04 UTC-5:00To:
> > > > dev@kafka.apache.org
> > > > Subject: Re: [DISCUSS] KIP-883: Add delete callback method to
> > Connector API
> > > >
> > > > Hi Hector,
> > > >
> > > > Thanks for the KIP!
> > > >
> > > > This is certainly missing functionality from the native Connect
> > framework,
> > > > and we should try to make it possible to inform connectors about this
> > part
> > > > of their lifecycle.
> > > > However, as with most functionality that was left out of the initial
> > > > implementation of the framework, the details are more challenging to
> > work
> > > > out.
> > > >
> > > > 1. What happens when the destroy call throws an error, how does the
> > > > framework respond?
> > > >
> > > > This is unspecified in the KIP, and it appears that your proposed
> > changes
> > > > could cause the herder to fail.
> > > > From the perspective of operators & connector developers, what is a
> > > > reasonable expectation to have for failure of a destroy?
> > > > I could see operators wanting both a graceful-delete to make use of
> > this
> > > > new feature, and a force-delete for when the graceful-delete fails.
> > > > A connector developer could choose to swallow all errors encountered,
> > or
> > > > fail-fast to indicate to the operator that there is an issue with the
> > > > graceful-delete flow.
> > > > If the alternative is crashing the herder, connector developers may
> > choose
> > > > to hide serious errors, which is undesirable.
> > > >
> > > > 2. What happens when the destroy() call takes a long time to
> complete,
> > or
> > > > is interrupted?
> > > >
> > > > It appears that your implementation serially destroy()s each
> > appropriate
> > > > connector, and may prevent the herder thread from making progress
> > while the
> > > > operation is ongoing.
> > > > We have previously had to patch Connect to perform all connector and
> > task
> > > > operations on a background thread, because some connector method
> > > > implementations can stall indefinitely.
> > > > Connect also has the notion of "cancelling" a connector/task if a
> > graceful
> > > > shutdown timeout operation takes too long. Perhaps some of that
> design
> > or
> > > > machinery may be useful to protect this method call as well.
> > > >
> > > > More specific to the destroy() call itself, what happens when a
> > connector
> > > > completes part of a destroy operation and then cannot complete the
> > > > remainder, either due to timing out or a worker crashing?
> > > > What is the contract with the connector developer about this method?
> > Is the
> > > > destroy() only started exactly once during the lifetime of the
> > connector,
> > > > or may it be retried?
> > > >
> > > > 3. What should be considered a reasonable custom implementation of
> the
> > > > destroy() call? What resources should it clean up by default?
> > > >
> > > > I think we can broadly categorize the state a connector mutates among
> > the
> > > > following
> > > > * Framework-managed state (e.g. source offsets, consumer offsets)
> > > > * Implementation detail state (e.g. debezium db history topic, audit
> > > > tables, temporary accounts)
> > > > * Third party system data (e.g. the actual data being written by a
> sink
> > > > connector)
> > > > * Third party system metadata (e.g. tables in a database, delivery
> > > > receipts, permissions)
> > > >
> > > > I think it's apparent that the framework-managed state cannot/should
> > not be
> > > > interacted with by the destroy() call. However, the framework could
> be
> > > > changed to clean up these resources at the same time that destroy()
> is
> > > > called. Is that out-of-scope of this proposal, and better handled by
> > manual
> > > > intervention?
> > > > From the text of the KIP, I think it explicitly includes the
> > Implementation
> > > > detail state, which should not be depended on externally and should
> be
> > safe
> > > > to clean up during a destroy(). I think this is completely
> reasonable.
> > > > Are the third-party data and metadata out-of-scope for this proposal?
> > Can
> > > > we officially recommend against it, or should we accommodate users
> and
> > > > connector developers that wish to clean up data/metadata during
> > destroy()?
> > > >
> > > > 4. How should connector implementations of destroy handle backwards
> > > > compatibility?
> > > >
> > > > In terms of backward-compatibility for the framework vs connector
> > versions,
> > > > I think the default-noop method is very reasonable.
> > > > However, what happens when someone upgrades from a version of a
> > connector
> > > > without a destroy() implementation to one with an implementation, and
> > > > maintain backwards compatibility?
> > > > To replicate the same behavior, the connector might include something
> > like
> > > > an `enable.cleanup` config which allows users to opt-in to the new
> > > > behavior. This could mean the proliferation of many different
> > > > configurations to handle this behavior.
> > > > Maybe we can provide some recommendations to developers, or some
> > mechanism
> > > > to standardize this opt-in behavior.
> > > >
> > > > I'm interested to hear if you have any experience with the above, if
> > you've
> > > > experimented with this feature in your fork.
> > > >
> > > > Thanks,
> > > > Greg
> > > >
> > > >
> > > > On Thu, Nov 3, 2022 at 11:55 AM Hector Geraldino (BLOOMBERG/ 919 3RD
> > A) <
> > > > hgeraldino@bloomberg.net> wrote:
> > > >
> > > > > Hi everyone,
> > > > >
> > > > > I've submitted KIP-883, which introduces a callback to the public
> > > > > Connector API called when deleting a connector:
> > > > >
> > > > >
> > > > >
> > > >
> > > >
> >
> >
>
> https://cwiki.apache.org/confluence/display/KAFKA/KIP-883%3A+Add+delete+callback
> > > > +method+to+Connector+API
> > > > >
> > > > > It adds a new `deleted()` method (open to better naming
> suggestions)
> > to
> > > > > the org.apache.kafka.connect.connector.Connector abstract class,
> > which
> > > > will
> > > > > be invoked by connect Workers when a connector is being deleted.
> > > > >
> > > > > Feedback and comments are welcome.
> > > > >
> > > > > Thank you!
> > > > > Hector
> > > > >
> > > > >
> > > >
> > > >
> > > >
> >
> >
> >
>
>
>