Hi everyone, Sorry for the delay in replying. I've finally now got some time to work on this.
Addressing Matthias's comments: 100. Good point. As Bruno mentioned, there's already AbstractReadWriteDecorator which we could leverage to provide that protection. I'll add details on this to the KIP. 101,102. It looks like these points have already been addressed by Bruno. Let me know if anything here is still unclear or you feel needs to be detailed more in the KIP. 103. I'm in favour of anything that gets the old code removed sooner, but wouldn't deprecating an API that we expect (some) users to implement cause problems? I'm thinking about implementers of custom StateStores, as they may be confused by managesOffsets() being deprecated, especially since they would have to mark their implementation as @Deprecated in order to avoid compile warnings. If deprecating an API *while it's still expected to be implemented* is something that's generally done in the project, then I'm happy to do so here. 104. I think this is technically possible, but at the cost of considerable additional code to maintain. Would we ever have a pathway to remove this downgrade code in the future? Regarding rebalance metadata: Opening all stores on start-up to read and cache their offsets is an interesting idea, especially if we can avoid re-opening the stores once the Tasks have been assigned. Scalability shouldn't be too much of a problem, because typically users have a fairly short state.cleanup.delay, so the number of on-disk Task directories should rarely exceed the number of Tasks previously assigned to that instance. An advantage of this approach is that it would also simplify StateStore implementations, as they would only need to guarantee that committed offsets are available when the store is open. I'll investigate this approach this week for feasibility and report back. I think that covers all the outstanding feedback, unless I missed anything? Regards, Nick On Mon, 6 May 2024 at 14:06, Bruno Cadonna <cado...@apache.org> wrote: > Hi Matthias, > > I see what you mean. > > To sum up: > > With this KIP the .checkpoint file is written when the store closes. > That is when: > 1. a task moves away from Kafka Streams client > 2. Kafka Streams client shuts down > > A Kafka Streams client needs the information in the .checkpoint file > 1. on startup because it does not have any open stores yet. > 2. during rebalances for non-empty state directories of tasks that are > not assigned to the Kafka Streams client. > > With hard crashes, i.e., when the Streams client is not able to close > its state stores and write the .checkpoint file, the .checkpoint file > might be quite stale. That influences the next rebalance after failover > negatively. > > > My conclusion is that Kafka Streams either needs to open the state > stores at start up or we write the checkpoint file more often. > > Writing the .checkpoint file during processing more often without > controlling the flush to disk would work. However, Kafka Streams would > checkpoint offsets that are not yet persisted on disk by the state > store. That is with a hard crash the offsets in the .checkpoint file > might be larger than the offsets checkpointed in the state store. That > might not be a problem if Kafka Streams uses the .checkpoint file only > to compute the task lag. The downside is that it makes the managing of > checkpoints more complex because now we have to maintain two > checkpoints: one for restoration and one for computing the task lag. > I think we should explore the option where Kafka Streams opens the state > stores at start up to get the offsets. > > I also checked when Kafka Streams needs the checkpointed offsets to > compute the task lag during a rebalance. Turns out Kafka Streams needs > them before sending the join request. Now, I am wondering if opening the > state stores of unassigned tasks whose state directory exists locally is > actually such a big issue due to the expected higher latency since it > happens actually before the Kafka Streams client joins the rebalance. > > Best, > Bruno > > > > > > > > On 5/4/24 12:05 AM, Matthias J. Sax wrote: > > That's good questions... I could think of a few approaches, but I admit > > it might all be a little bit tricky to code up... > > > > However if we don't solve this problem, I think this KIP does not really > > solve the core issue we are facing? In the end, if we rely on the > > `.checkpoint` file to compute a task assignment, but the `.checkpoint` > > file can be arbitrary stale after a crash because we only write it on a > > clean close, there would be still a huge gap that this KIP does not > close? > > > > For the case in which we keep the checkpoint file, this KIP would still > > help for "soft errors" in which KS can recover, and roll back the store. > > A significant win for sure. -- But hard crashes would still be an > > problem? We might assign tasks to "wrong" instance, ie, which are not > > most up to date, as the checkpoint information could be very outdated? > > Would we end up with a half-baked solution? Would this be good enough to > > justify the introduced complexity? In the, for soft failures it's still > > a win. Just want to make sure we understand the limitations and make an > > educated decision. > > > > Or do I miss something? > > > > > > -Matthias > > > > On 5/3/24 10:20 AM, Bruno Cadonna wrote: > >> Hi Matthias, > >> > >> > >> 200: > >> I like the idea in general. However, it is not clear to me how the > >> behavior should be with multiple stream threads in the same Kafka > >> Streams client. What stream thread opens which store? How can a stream > >> thread pass an open store to another stream thread that got the > >> corresponding task assigned? How does a stream thread know that a task > >> was not assigned to any of the stream threads of the Kafka Streams > >> client? I have the feeling we should just keep the .checkpoint file on > >> close for now to unblock this KIP and try to find a solution to get > >> totally rid of it later. > >> > >> > >> Best, > >> Bruno > >> > >> > >> > >> On 5/3/24 6:29 PM, Matthias J. Sax wrote: > >>> 101: Yes, but what I am saying is, that we don't need to flush the > >>> .position file to disk periodically, but only maintain it in main > >>> memory, and only write it to disk on close() to preserve it across > >>> restarts. This way, it would never be ahead, but might only lag? But > >>> with my better understanding about (102) it might be mood anyway... > >>> > >>> > >>> 102: Thanks for clarifying. Looked into the code now. Makes sense. > >>> Might be something to be worth calling out explicitly in the KIP > >>> writeup. -- Now that I realize that the position is tracked inside > >>> the store (not outside as the changelog offsets) it makes much more > >>> sense to pull position into RocksDB itself. In the end, it's actually > >>> a "store implementation" detail how it tracks the position (and kinda > >>> leaky abstraction currently, that we re-use the checkpoint file > >>> mechanism to track it and flush to disk). > >>> > >>> > >>> 200: I was thinking about this a little bit more, and maybe it's not > >>> too bad? When KS starts up, we could upon all stores we find on local > >>> disk pro-actively, and keep them all open until the first rebalance > >>> finishes: For tasks we get assigned, we hand in the already opened > >>> store (this would amortize the cost to open the store before the > >>> rebalance) and for non-assigned tasks, we know the offset information > >>> won't change and we could just cache it in-memory for later reuse > >>> (ie, next rebalance) and close the store to free up resources? -- > >>> Assuming that we would get a large percentage of opened stores > >>> assigned as tasks anyway, this could work? > >>> > >>> > >>> -Matthias > >>> > >>> On 5/3/24 1:29 AM, Bruno Cadonna wrote: > >>>> Hi Matthias, > >>>> > >>>> > >>>> 101: > >>>> Let's assume a RocksDB store, but I think the following might be > >>>> true also for other store implementations. With this KIP, if Kafka > >>>> Streams commits the offsets, the committed offsets will be stored in > >>>> an in-memory data structure (i.e. the memtable) and stay there until > >>>> RocksDB decides that it is time to persist its in-memory data > >>>> structure. If Kafka Streams writes its position to the .position > >>>> file during a commit and a crash happens before RocksDB persist the > >>>> memtable then the position in the .position file is ahead of the > >>>> persisted offset. If IQ is done between the crash and the state > >>>> store fully restored the changelog, the position might tell IQ that > >>>> the state store is more up-to-date than it actually is. > >>>> In contrast, if Kafka Streams handles persisting positions the same > >>>> as persisting offset, the position should always be consistent with > >>>> the offset, because they are persisted together. > >>>> > >>>> > >>>> 102: > >>>> I am confused about your confusion which tells me that we are > >>>> talking about two different things. > >>>> You asked > >>>> > >>>> "Do you intent to add this information [i.e. position] to the map > >>>> passed via commit(final Map<TopicPartition, Long> changelogOffsets)?" > >>>> > >>>> and with what I wrote I meant that we do not need to pass the > >>>> position into the implementation of the StateStore interface since > >>>> the position is updated within the implementation of the StateStore > >>>> interface (e.g. RocksDBStore [1]). My statement describes the > >>>> behavior now, not the change proposed in this KIP, so it does not > >>>> contradict what is stated in the KIP. > >>>> > >>>> > >>>> 200: > >>>> This is about Matthias' main concern about rebalance metadata. > >>>> As far as I understand the KIP, Kafka Streams will only use the > >>>> .checkpoint files to compute the task lag for unassigned tasks whose > >>>> state is locally available. For assigned tasks, it will use the > >>>> offsets managed by the open state store. > >>>> > >>>> Best, > >>>> Bruno > >>>> > >>>> [1] > >>>> > https://github.com/apache/kafka/blob/fcbfd3412eb746a0c81374eb55ad0f73de6b1e71/streams/src/main/java/org/apache/kafka/streams/state/internals/RocksDBStore.java#L397 > >>>> > >>>> On 5/1/24 3:00 AM, Matthias J. Sax wrote: > >>>>> Thanks Bruno. > >>>>> > >>>>> > >>>>> > >>>>> 101: I think I understand this better now. But just want to make > >>>>> sure I do. What do you mean by "they can diverge" and "Recovering > >>>>> after a failure might load inconsistent offsets and positions." > >>>>> > >>>>> The checkpoint is the offset from the changelog, while the position > >>>>> is the offset from the upstream source topic, right? -- In the end, > >>>>> the position is about IQ, and if we fail to update it, it only > >>>>> means that there is some gap when we might not be able to query a > >>>>> standby task, because we think it's not up-to-date enough even if > >>>>> it is, which would resolve itself soon? Ie, the position might > >>>>> "lag", but it's not "inconsistent". Do we believe that this lag > >>>>> would be highly problematic? > >>>>> > >>>>> > >>>>> > >>>>> 102: I am confused. > >>>>> > >>>>>> The position is maintained inside the state store, but is > >>>>>> persisted in the .position file when the state store closes. > >>>>> > >>>>> This contradicts the KIP: > >>>>> > >>>>>> these position offsets will be stored in RocksDB, in the same > >>>>>> column family as the changelog offsets, instead of the .position > file > >>>>> > >>>>> > >>>>> > >>>>> My main concern is currently about rebalance metadata -- opening > >>>>> RocksDB stores seems to be very expensive, but if we follow the KIP: > >>>>> > >>>>>> We will do this under EOS by updating the .checkpoint file > >>>>>> whenever a store is close()d. > >>>>> > >>>>> It seems, having the offset inside RocksDB does not help us at all? > >>>>> In the end, when we crash, we don't want to lose the state, but > >>>>> when we update the .checkpoint only on a clean close, the > >>>>> .checkpoint might be stale (ie, still contains the checkpoint when > >>>>> we opened the store when we got a task assigned). > >>>>> > >>>>> > >>>>> > >>>>> -Matthias > >>>>> > >>>>> On 4/30/24 2:40 AM, Bruno Cadonna wrote: > >>>>>> Hi all, > >>>>>> > >>>>>> 100 > >>>>>> I think we already have such a wrapper. It is called > >>>>>> AbstractReadWriteDecorator. > >>>>>> > >>>>>> > >>>>>> 101 > >>>>>> Currently, the position is checkpointed when a offset checkpoint > >>>>>> is written. If we let the state store manage the committed > >>>>>> offsets, we need to also let the state store also manage the > >>>>>> position otherwise they might diverge. State store managed offsets > >>>>>> can get flushed (i.e. checkpointed) to the disk when the state > >>>>>> store decides to flush its in-memory data structures, but the > >>>>>> position is only checkpointed at commit time. Recovering after a > >>>>>> failure might load inconsistent offsets and positions. > >>>>>> > >>>>>> > >>>>>> 102 > >>>>>> The position is maintained inside the state store, but is > >>>>>> persisted in the .position file when the state store closes. The > >>>>>> only public interface that uses the position is IQv2 in a > >>>>>> read-only mode. So the position is only updated within the state > >>>>>> store and read from IQv2. No need to add anything to the public > >>>>>> StateStore interface. > >>>>>> > >>>>>> > >>>>>> 103 > >>>>>> Deprecating managesOffsets() right away might be a good idea. > >>>>>> > >>>>>> > >>>>>> 104 > >>>>>> I agree that we should try to support downgrades without wipes. At > >>>>>> least Nick should state in the KIP why we do not support it. > >>>>>> > >>>>>> > >>>>>> Best, > >>>>>> Bruno > >>>>>> > >>>>>> > >>>>>> > >>>>>> > >>>>>> On 4/23/24 8:13 AM, Matthias J. Sax wrote: > >>>>>>> Thanks for splitting out this KIP. The discussion shows, that it > >>>>>>> is a complex beast by itself, so worth to discuss by its own. > >>>>>>> > >>>>>>> > >>>>>>> Couple of question / comment: > >>>>>>> > >>>>>>> > >>>>>>> 100 `StateStore#commit()`: The JavaDoc says "must not be called > >>>>>>> by users" -- I would propose to put a guard in place for this, by > >>>>>>> either throwing an exception (preferable) or adding a no-op > >>>>>>> implementation (at least for our own stores, by wrapping them -- > >>>>>>> we cannot enforce it for custom stores I assume), and document > >>>>>>> this contract explicitly. > >>>>>>> > >>>>>>> > >>>>>>> 101 adding `.position` to the store: Why do we actually need > >>>>>>> this? The KIP says "To ensure consistency with the committed data > >>>>>>> and changelog offsets" but I am not sure if I can follow? Can you > >>>>>>> elaborate why leaving the `.position` file as-is won't work? > >>>>>>> > >>>>>>>> If it's possible at all, it will need to be done by > >>>>>>>> creating temporary StateManagers and StateStores during > >>>>>>>> rebalance. I think > >>>>>>>> it is possible, and probably not too expensive, but the devil > >>>>>>>> will be in > >>>>>>>> the detail. > >>>>>>> > >>>>>>> This sounds like a significant overhead to me. We know that > >>>>>>> opening a single RocksDB takes about 500ms, and thus opening > >>>>>>> RocksDB to get this information might slow down rebalances > >>>>>>> significantly. > >>>>>>> > >>>>>>> > >>>>>>> 102: It's unclear to me, how `.position` information is added. > >>>>>>> The KIP only says: "position offsets will be stored in RocksDB, > >>>>>>> in the same column family as the changelog offsets". Do you > >>>>>>> intent to add this information to the map passed via > >>>>>>> `commit(final Map<TopicPartition, Long> changelogOffsets)`? The > >>>>>>> KIP should describe this in more detail. Also, if my assumption > >>>>>>> is correct, we might want to rename the parameter and also have a > >>>>>>> better JavaDoc description? > >>>>>>> > >>>>>>> > >>>>>>> 103: Should we make it mandatory (long-term) that all stores > >>>>>>> (including custom stores) manage their offsets internally? > >>>>>>> Maintaining both options and thus both code paths puts a burden > >>>>>>> on everyone and make the code messy. I would strongly prefer if > >>>>>>> we could have mid-term path to get rid of supporting both. -- > >>>>>>> For this case, we should deprecate the newly added > >>>>>>> `managesOffsets()` method right away, to point out that we intend > >>>>>>> to remove it. If it's mandatory to maintain offsets for stores, > >>>>>>> we won't need this method any longer. In memory stores can just > >>>>>>> return null from #committedOffset(). > >>>>>>> > >>>>>>> > >>>>>>> 104 "downgrading": I think it might be worth to add support for > >>>>>>> downgrading w/o the need to wipe stores? Leveraging > >>>>>>> `upgrade.from` parameter, we could build a two rolling bounce > >>>>>>> downgrade: (1) the new code is started with `upgrade.from` set to > >>>>>>> a lower version, telling the runtime to do the cleanup on > >>>>>>> `close()` -- (ie, ensure that all data is written into > >>>>>>> `.checkpoint` and `.position` file, and the newly added CL is > >>>>>>> deleted). In a second, rolling bounce, the old code would be able > >>>>>>> to open RocksDB. -- I understand that this implies much more > >>>>>>> work, but downgrade seems to be common enough, that it might be > >>>>>>> worth it? Even if we did not always support this in the past, we > >>>>>>> have the face the fact that KS is getting more and more adopted > >>>>>>> and as a more mature product should support this? > >>>>>>> > >>>>>>> > >>>>>>> > >>>>>>> > >>>>>>> -Matthias > >>>>>>> > >>>>>>> > >>>>>>> > >>>>>>> > >>>>>>> > >>>>>>> > >>>>>>> > >>>>>>> On 4/21/24 11:58 PM, Bruno Cadonna wrote: > >>>>>>>> Hi all, > >>>>>>>> > >>>>>>>> How should we proceed here? > >>>>>>>> > >>>>>>>> 1. with the plain .checkpoint file > >>>>>>>> 2. with a way to use the state store interface on unassigned but > >>>>>>>> locally existing task state > >>>>>>>> > >>>>>>>> While I like option 2, I think option 1 is less risky and will > >>>>>>>> give us the benefits of transactional state stores sooner. We > >>>>>>>> should consider the interface approach afterwards, though. > >>>>>>>> > >>>>>>>> > >>>>>>>> Best, > >>>>>>>> Bruno > >>>>>>>> > >>>>>>>> > >>>>>>>> > >>>>>>>> On 4/17/24 3:15 PM, Bruno Cadonna wrote: > >>>>>>>>> Hi Nick and Sophie, > >>>>>>>>> > >>>>>>>>> I think the task ID is not enough to create a state store that > >>>>>>>>> can read the offsets of non-assigned tasks for lag computation > >>>>>>>>> during rebalancing. The state store also needs the state > >>>>>>>>> directory so that it knows where to find the information that > >>>>>>>>> it needs to return from changelogOffsets(). > >>>>>>>>> > >>>>>>>>> In general, I think we should proceed with the plain > >>>>>>>>> .checkpoint file for now and iterate back to the state store > >>>>>>>>> solution later since it seems it is not that straightforward. > >>>>>>>>> Alternatively, Nick could timebox an effort to better > >>>>>>>>> understand what would be needed for the state store solution. > >>>>>>>>> Nick, let us know your decision. > >>>>>>>>> > >>>>>>>>> Regarding your question about the state store instance. I am > >>>>>>>>> not too familiar with that part of the code, but I think the > >>>>>>>>> state store is build when the processor topology is build and > >>>>>>>>> the processor topology is build per stream task. So there is > >>>>>>>>> one instance of processor topology and state store per stream > >>>>>>>>> task. Try to follow the call in [1]. > >>>>>>>>> > >>>>>>>>> Best, > >>>>>>>>> Bruno > >>>>>>>>> > >>>>>>>>> [1] > >>>>>>>>> > https://github.com/apache/kafka/blob/f52575b17225828d2ff11996030ab7304667deab/streams/src/main/java/org/apache/kafka/streams/processor/internals/ActiveTaskCreator.java#L153 > >>>>>>>>> > >>>>>>>>> > >>>>>>>>> > >>>>>>>>> On 4/16/24 8:59 PM, Nick Telford wrote: > >>>>>>>>>> That does make sense. The one thing I can't figure out is how > >>>>>>>>>> per-Task > >>>>>>>>>> StateStore instances are constructed. > >>>>>>>>>> > >>>>>>>>>> It looks like we construct one StateStore instance for the > >>>>>>>>>> whole Topology > >>>>>>>>>> (in InternalTopologyBuilder), and pass that into > >>>>>>>>>> ProcessorStateManager (via > >>>>>>>>>> StateManagerUtil) for each Task, which then initializes it. > >>>>>>>>>> > >>>>>>>>>> This can't be the case though, otherwise multiple partitions > >>>>>>>>>> of the same > >>>>>>>>>> sub-topology (aka Tasks) would share the same StateStore > >>>>>>>>>> instance, which > >>>>>>>>>> they don't. > >>>>>>>>>> > >>>>>>>>>> What am I missing? > >>>>>>>>>> > >>>>>>>>>> On Tue, 16 Apr 2024 at 16:22, Sophie Blee-Goldman > >>>>>>>>>> <sop...@responsive.dev> > >>>>>>>>>> wrote: > >>>>>>>>>> > >>>>>>>>>>> I don't think we need to *require* a constructor accept the > >>>>>>>>>>> TaskId, but we > >>>>>>>>>>> would definitely make sure that the RocksDB state store > >>>>>>>>>>> changes its > >>>>>>>>>>> constructor to one that accepts the TaskID (which we can do > >>>>>>>>>>> without > >>>>>>>>>>> deprecation since its an internal API), and custom state > >>>>>>>>>>> stores can just > >>>>>>>>>>> decide for themselves whether they want to opt-in/use the > >>>>>>>>>>> TaskId param > >>>>>>>>>>> or not. I mean custom state stores would have to opt-in > >>>>>>>>>>> anyways by > >>>>>>>>>>> implementing the new StoreSupplier#get(TaskId) API and the only > >>>>>>>>>>> reason to do that would be to have created a constructor that > >>>>>>>>>>> accepts > >>>>>>>>>>> a TaskId > >>>>>>>>>>> > >>>>>>>>>>> Just to be super clear about the proposal, this is what I had > >>>>>>>>>>> in mind. > >>>>>>>>>>> It's actually fairly simple and wouldn't add much to the > >>>>>>>>>>> scope of the > >>>>>>>>>>> KIP (I think -- if it turns out to be more complicated than > >>>>>>>>>>> I'm assuming, > >>>>>>>>>>> we should definitely do whatever has the smallest LOE to get > >>>>>>>>>>> this done > >>>>>>>>>>> > >>>>>>>>>>> Anyways, the (only) public API changes would be to add this new > >>>>>>>>>>> method to the StoreSupplier API: > >>>>>>>>>>> > >>>>>>>>>>> default T get(final TaskId taskId) { > >>>>>>>>>>> return get(); > >>>>>>>>>>> } > >>>>>>>>>>> > >>>>>>>>>>> We can decide whether or not to deprecate the old #get but > >>>>>>>>>>> it's not > >>>>>>>>>>> really necessary and might cause a lot of turmoil, so I'd > >>>>>>>>>>> personally > >>>>>>>>>>> say we just leave both APIs in place. > >>>>>>>>>>> > >>>>>>>>>>> And that's it for public API changes! Internally, we would > >>>>>>>>>>> just adapt > >>>>>>>>>>> each of the rocksdb StoreSupplier classes to implement this new > >>>>>>>>>>> API. So for example with the RocksDBKeyValueBytesStoreSupplier, > >>>>>>>>>>> we just add > >>>>>>>>>>> > >>>>>>>>>>> @Override > >>>>>>>>>>> public KeyValueStore<Bytes, byte[]> get(final TaskId taskId) { > >>>>>>>>>>> return returnTimestampedStore ? > >>>>>>>>>>> new RocksDBTimestampedStore(name, metricsScope(), > >>>>>>>>>>> taskId) : > >>>>>>>>>>> new RocksDBStore(name, metricsScope(), taskId); > >>>>>>>>>>> } > >>>>>>>>>>> > >>>>>>>>>>> And of course add the TaskId parameter to each of the actual > >>>>>>>>>>> state store constructors returned here. > >>>>>>>>>>> > >>>>>>>>>>> Does that make sense? It's entirely possible I'm missing > >>>>>>>>>>> something > >>>>>>>>>>> important here, but I think this would be a pretty small > >>>>>>>>>>> addition that > >>>>>>>>>>> would solve the problem you mentioned earlier while also being > >>>>>>>>>>> useful to anyone who uses custom state stores. > >>>>>>>>>>> > >>>>>>>>>>> On Mon, Apr 15, 2024 at 10:21 AM Nick Telford > >>>>>>>>>>> <nick.telf...@gmail.com> > >>>>>>>>>>> wrote: > >>>>>>>>>>> > >>>>>>>>>>>> Hi Sophie, > >>>>>>>>>>>> > >>>>>>>>>>>> Interesting idea! Although what would that mean for the > >>>>>>>>>>>> StateStore > >>>>>>>>>>>> interface? Obviously we can't require that the constructor > >>>>>>>>>>>> take the > >>>>>>>>>>> TaskId. > >>>>>>>>>>>> Is it enough to add the parameter to the StoreSupplier? > >>>>>>>>>>>> > >>>>>>>>>>>> Would doing this be in-scope for this KIP, or are we > >>>>>>>>>>>> over-complicating > >>>>>>>>>>> it? > >>>>>>>>>>>> > >>>>>>>>>>>> Nick > >>>>>>>>>>>> > >>>>>>>>>>>> On Fri, 12 Apr 2024 at 21:30, Sophie Blee-Goldman > >>>>>>>>>>>> <sop...@responsive.dev > >>>>>>>>>>>> > >>>>>>>>>>>> wrote: > >>>>>>>>>>>> > >>>>>>>>>>>>> Somewhat minor point overall, but it actually drives me > >>>>>>>>>>>>> crazy that you > >>>>>>>>>>>>> can't get access to the taskId of a StateStore until #init > >>>>>>>>>>>>> is called. > >>>>>>>>>>>> This > >>>>>>>>>>>>> has caused me a huge headache personally (since the same is > >>>>>>>>>>>>> true for > >>>>>>>>>>>>> processors and I was trying to do something that's probably > >>>>>>>>>>>>> too hacky > >>>>>>>>>>> to > >>>>>>>>>>>>> actually complain about here lol) > >>>>>>>>>>>>> > >>>>>>>>>>>>> Can we just change the StateStoreSupplier to receive and > >>>>>>>>>>>>> pass along the > >>>>>>>>>>>>> taskId when creating a new store? Presumably by adding a > >>>>>>>>>>>>> new version of > >>>>>>>>>>>> the > >>>>>>>>>>>>> #get method that takes in a taskId parameter? We can have > >>>>>>>>>>>>> it default to > >>>>>>>>>>>>> invoking the old one for compatibility reasons and it > >>>>>>>>>>>>> should be > >>>>>>>>>>>> completely > >>>>>>>>>>>>> safe to tack on. > >>>>>>>>>>>>> > >>>>>>>>>>>>> Would also prefer the same for a ProcessorSupplier, but > that's > >>>>>>>>>>> definitely > >>>>>>>>>>>>> outside the scope of this KIP > >>>>>>>>>>>>> > >>>>>>>>>>>>> On Fri, Apr 12, 2024 at 3:31 AM Nick Telford > >>>>>>>>>>>>> <nick.telf...@gmail.com> > >>>>>>>>>>>>> wrote: > >>>>>>>>>>>>> > >>>>>>>>>>>>>> On further thought, it's clear that this can't work for > >>>>>>>>>>>>>> one simple > >>>>>>>>>>>>> reason: > >>>>>>>>>>>>>> StateStores don't know their associated TaskId (and hence, > >>>>>>>>>>>>>> their > >>>>>>>>>>>>>> StateDirectory) until the init() call. Therefore, > >>>>>>>>>>>>>> committedOffset() > >>>>>>>>>>>> can't > >>>>>>>>>>>>>> be called before init(), unless we also added a > >>>>>>>>>>>>>> StateStoreContext > >>>>>>>>>>>>> argument > >>>>>>>>>>>>>> to committedOffset(), which I think might be trying to > >>>>>>>>>>>>>> shoehorn too > >>>>>>>>>>>> much > >>>>>>>>>>>>>> into committedOffset(). > >>>>>>>>>>>>>> > >>>>>>>>>>>>>> I still don't like the idea of the Streams engine > >>>>>>>>>>>>>> maintaining the > >>>>>>>>>>> cache > >>>>>>>>>>>>> of > >>>>>>>>>>>>>> changelog offsets independently of stores, mostly because > >>>>>>>>>>>>>> of the > >>>>>>>>>>>>>> maintenance burden of the code duplication, but it looks > >>>>>>>>>>>>>> like we'll > >>>>>>>>>>>> have > >>>>>>>>>>>>> to > >>>>>>>>>>>>>> live with it. > >>>>>>>>>>>>>> > >>>>>>>>>>>>>> Unless you have any better ideas? > >>>>>>>>>>>>>> > >>>>>>>>>>>>>> Regards, > >>>>>>>>>>>>>> Nick > >>>>>>>>>>>>>> > >>>>>>>>>>>>>> On Wed, 10 Apr 2024 at 14:12, Nick Telford > >>>>>>>>>>>>>> <nick.telf...@gmail.com> > >>>>>>>>>>>>> wrote: > >>>>>>>>>>>>>> > >>>>>>>>>>>>>>> Hi Bruno, > >>>>>>>>>>>>>>> > >>>>>>>>>>>>>>> Immediately after I sent my response, I looked at the > >>>>>>>>>>>>>>> codebase and > >>>>>>>>>>>> came > >>>>>>>>>>>>>> to > >>>>>>>>>>>>>>> the same conclusion. If it's possible at all, it will > >>>>>>>>>>>>>>> need to be > >>>>>>>>>>> done > >>>>>>>>>>>>> by > >>>>>>>>>>>>>>> creating temporary StateManagers and StateStores during > >>>>>>>>>>>>>>> rebalance. > >>>>>>>>>>> I > >>>>>>>>>>>>>> think > >>>>>>>>>>>>>>> it is possible, and probably not too expensive, but the > >>>>>>>>>>>>>>> devil will > >>>>>>>>>>> be > >>>>>>>>>>>>> in > >>>>>>>>>>>>>>> the detail. > >>>>>>>>>>>>>>> > >>>>>>>>>>>>>>> I'll try to find some time to explore the idea to see if > >>>>>>>>>>>>>>> it's > >>>>>>>>>>>> possible > >>>>>>>>>>>>>> and > >>>>>>>>>>>>>>> report back, because we'll need to determine this before > >>>>>>>>>>>>>>> we can > >>>>>>>>>>> vote > >>>>>>>>>>>> on > >>>>>>>>>>>>>> the > >>>>>>>>>>>>>>> KIP. > >>>>>>>>>>>>>>> > >>>>>>>>>>>>>>> Regards, > >>>>>>>>>>>>>>> Nick > >>>>>>>>>>>>>>> > >>>>>>>>>>>>>>> On Wed, 10 Apr 2024 at 11:36, Bruno Cadonna > >>>>>>>>>>>>>>> <cado...@apache.org> > >>>>>>>>>>>>> wrote: > >>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>> Hi Nick, > >>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>> Thanks for reacting on my comments so quickly! > >>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>> 2. > >>>>>>>>>>>>>>>> Some thoughts on your proposal. > >>>>>>>>>>>>>>>> State managers (and state stores) are parts of tasks. If > >>>>>>>>>>>>>>>> the task > >>>>>>>>>>> is > >>>>>>>>>>>>> not > >>>>>>>>>>>>>>>> assigned locally, we do not create those tasks. To get > >>>>>>>>>>>>>>>> the offsets > >>>>>>>>>>>>> with > >>>>>>>>>>>>>>>> your approach, we would need to either create kind of > >>>>>>>>>>>>>>>> inactive > >>>>>>>>>>> tasks > >>>>>>>>>>>>>>>> besides active and standby tasks or store and manage state > >>>>>>>>>>> managers > >>>>>>>>>>>> of > >>>>>>>>>>>>>>>> non-assigned tasks differently than the state managers > >>>>>>>>>>>>>>>> of assigned > >>>>>>>>>>>>>>>> tasks. Additionally, the cleanup thread that removes > >>>>>>>>>>>>>>>> unassigned > >>>>>>>>>>> task > >>>>>>>>>>>>>>>> directories needs to concurrently delete those inactive > >>>>>>>>>>>>>>>> tasks or > >>>>>>>>>>>>>>>> task-less state managers of unassigned tasks. This seems > >>>>>>>>>>>>>>>> all quite > >>>>>>>>>>>>> messy > >>>>>>>>>>>>>>>> to me. > >>>>>>>>>>>>>>>> Could we create those state managers (or state stores) > >>>>>>>>>>>>>>>> for locally > >>>>>>>>>>>>>>>> existing but unassigned tasks on demand when > >>>>>>>>>>>>>>>> TaskManager#getTaskOffsetSums() is executed? Or have a > >>>>>>>>>>>>>>>> different > >>>>>>>>>>>>>>>> encapsulation for the unused task directories? > >>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>> Best, > >>>>>>>>>>>>>>>> Bruno > >>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>> On 4/10/24 11:31 AM, Nick Telford wrote: > >>>>>>>>>>>>>>>>> Hi Bruno, > >>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>> Thanks for the review! > >>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>> 1, 4, 5. > >>>>>>>>>>>>>>>>> Done > >>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>> 3. > >>>>>>>>>>>>>>>>> You're right. I've removed the offending paragraph. I had > >>>>>>>>>>>> originally > >>>>>>>>>>>>>>>>> adapted this from the guarantees outlined in KIP-892. > >>>>>>>>>>>>>>>>> But it's > >>>>>>>>>>>>>>>> difficult to > >>>>>>>>>>>>>>>>> provide these guarantees without the KIP-892 transaction > >>>>>>>>>>> buffers. > >>>>>>>>>>>>>>>> Instead, > >>>>>>>>>>>>>>>>> we'll add the guarantees back into the JavaDoc when > >>>>>>>>>>>>>>>>> KIP-892 > >>>>>>>>>>> lands. > >>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>> 2. > >>>>>>>>>>>>>>>>> Good point! This is the only part of the KIP that was > >>>>>>>>>>>>> (significantly) > >>>>>>>>>>>>>>>>> changed when I extracted it from KIP-892. My prototype > >>>>>>>>>>>>>>>>> currently > >>>>>>>>>>>>>>>> maintains > >>>>>>>>>>>>>>>>> this "cache" of changelog offsets in .checkpoint, but > >>>>>>>>>>>>>>>>> doing so > >>>>>>>>>>>>> becomes > >>>>>>>>>>>>>>>> very > >>>>>>>>>>>>>>>>> messy. My intent with this change was to try to better > >>>>>>>>>>> encapsulate > >>>>>>>>>>>>>> this > >>>>>>>>>>>>>>>>> offset "caching", especially for StateStores that can > >>>>>>>>>>>>>>>>> cheaply > >>>>>>>>>>>>> provide > >>>>>>>>>>>>>>>> the > >>>>>>>>>>>>>>>>> offsets stored directly in them without needing to > >>>>>>>>>>>>>>>>> duplicate > >>>>>>>>>>> them > >>>>>>>>>>>> in > >>>>>>>>>>>>>>>> this > >>>>>>>>>>>>>>>>> cache. > >>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>> It's clear some more work is needed here to better > >>>>>>>>>>>>>>>>> encapsulate > >>>>>>>>>>>> this. > >>>>>>>>>>>>>> My > >>>>>>>>>>>>>>>>> immediate thought is: what if we construct *but don't > >>>>>>>>>>> initialize* > >>>>>>>>>>>>> the > >>>>>>>>>>>>>>>>> StateManager and StateStores for every Task directory > >>>>>>>>>>>>>>>>> on-disk? > >>>>>>>>>>>> That > >>>>>>>>>>>>>>>> should > >>>>>>>>>>>>>>>>> still be quite cheap to do, and would enable us to > >>>>>>>>>>>>>>>>> query the > >>>>>>>>>>>> offsets > >>>>>>>>>>>>>> for > >>>>>>>>>>>>>>>>> all on-disk stores, even if they're not open. If the > >>>>>>>>>>> StateManager > >>>>>>>>>>>>>> (aka. > >>>>>>>>>>>>>>>>> ProcessorStateManager/GlobalStateManager) proves too > >>>>>>>>>>>>>>>>> expensive > >>>>>>>>>>> to > >>>>>>>>>>>>> hold > >>>>>>>>>>>>>>>> open > >>>>>>>>>>>>>>>>> for closed stores, we could always have a > >>>>>>>>>>>>>>>>> "StubStateManager" in > >>>>>>>>>>>> its > >>>>>>>>>>>>>>>> place, > >>>>>>>>>>>>>>>>> that enables the querying of offsets, but nothing else? > >>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>> IDK, what do you think? > >>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>> Regards, > >>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>> Nick > >>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>> On Tue, 9 Apr 2024 at 15:00, Bruno Cadonna > >>>>>>>>>>>>>>>>> <cado...@apache.org> > >>>>>>>>>>>>>> wrote: > >>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>>> Hi Nick, > >>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>>> Thanks for breaking out the KIP from KIP-892! > >>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>>> Here a couple of comments/questions: > >>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>>> 1. > >>>>>>>>>>>>>>>>>> In Kafka Streams, we have a design guideline which > >>>>>>>>>>>>>>>>>> says to not > >>>>>>>>>>>> use > >>>>>>>>>>>>>> the > >>>>>>>>>>>>>>>>>> "get"-prefix for getters on the public API. Could you > >>>>>>>>>>>>>>>>>> please > >>>>>>>>>>>> change > >>>>>>>>>>>>>>>>>> getCommittedOffsets() to committedOffsets()? > >>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>>> 2. > >>>>>>>>>>>>>>>>>> It is not clear to me how > TaskManager#getTaskOffsetSums() > >>>>>>>>>>> should > >>>>>>>>>>>>> read > >>>>>>>>>>>>>>>>>> offsets of tasks the stream thread does not own but > >>>>>>>>>>>>>>>>>> that have a > >>>>>>>>>>>>> state > >>>>>>>>>>>>>>>>>> directory on the Streams client by calling > >>>>>>>>>>>>>>>>>> StateStore#getCommittedOffsets(). If the thread does > >>>>>>>>>>>>>>>>>> not own a > >>>>>>>>>>>> task > >>>>>>>>>>>>>> it > >>>>>>>>>>>>>>>>>> does also not create any state stores for the task, > >>>>>>>>>>>>>>>>>> which means > >>>>>>>>>>>>> there > >>>>>>>>>>>>>>>> is > >>>>>>>>>>>>>>>>>> no state store on which to call getCommittedOffsets(). > >>>>>>>>>>>>>>>>>> I would have rather expected that a checkpoint file is > >>>>>>>>>>>>>>>>>> written > >>>>>>>>>>>> for > >>>>>>>>>>>>>> all > >>>>>>>>>>>>>>>>>> state stores on close -- not only for the RocksDBStore > >>>>>>>>>>>>>>>>>> -- and > >>>>>>>>>>>> that > >>>>>>>>>>>>>> this > >>>>>>>>>>>>>>>>>> checkpoint file is read in > >>>>>>>>>>>>>>>>>> TaskManager#getTaskOffsetSums() for > >>>>>>>>>>>> the > >>>>>>>>>>>>>>>> tasks > >>>>>>>>>>>>>>>>>> that have a state directory on the client but are not > >>>>>>>>>>>>>>>>>> currently > >>>>>>>>>>>>>>>> assigned > >>>>>>>>>>>>>>>>>> to any stream thread of the Streams client. > >>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>>> 3. > >>>>>>>>>>>>>>>>>> In the javadocs for commit() you write > >>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>>> "... all writes since the last commit(Map), or since > >>>>>>>>>>>>> init(StateStore) > >>>>>>>>>>>>>>>>>> *MUST* be available to readers, even after a restart." > >>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>>> This is only true for a clean close before the > >>>>>>>>>>>>>>>>>> restart, isn't > >>>>>>>>>>> it? > >>>>>>>>>>>>>>>>>> If the task fails with a dirty close, Kafka Streams > >>>>>>>>>>>>>>>>>> cannot > >>>>>>>>>>>>> guarantee > >>>>>>>>>>>>>>>>>> that the in-memory structures of the state store (e.g. > >>>>>>>>>>>>>>>>>> memtable > >>>>>>>>>>>> in > >>>>>>>>>>>>>> the > >>>>>>>>>>>>>>>>>> case of RocksDB) are flushed so that the records and the > >>>>>>>>>>>> committed > >>>>>>>>>>>>>>>>>> offsets are persisted. > >>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>>> 4. > >>>>>>>>>>>>>>>>>> The wrapper that provides the legacy checkpointing > >>>>>>>>>>>>>>>>>> behavior is > >>>>>>>>>>>>>> actually > >>>>>>>>>>>>>>>>>> an implementation detail. I would remove it from the > >>>>>>>>>>>>>>>>>> KIP, but > >>>>>>>>>>>> still > >>>>>>>>>>>>>>>>>> state that the legacy checkpointing behavior will be > >>>>>>>>>>>>>>>>>> supported > >>>>>>>>>>>> when > >>>>>>>>>>>>>> the > >>>>>>>>>>>>>>>>>> state store does not manage the checkpoints. > >>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>>> 5. > >>>>>>>>>>>>>>>>>> Regarding the metrics, could you please add the tags, > >>>>>>>>>>>>>>>>>> and the > >>>>>>>>>>>>>> recording > >>>>>>>>>>>>>>>>>> level (DEBUG or INFO) as done in KIP-607 or KIP-444. > >>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>>> Best, > >>>>>>>>>>>>>>>>>> Bruno > >>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>>> On 4/7/24 5:35 PM, Nick Telford wrote: > >>>>>>>>>>>>>>>>>>> Hi everyone, > >>>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>>>> Based on some offline discussion, I've split out the > >>>>>>>>>>>>>>>>>>> "Atomic > >>>>>>>>>>>>>>>>>> Checkpointing" > >>>>>>>>>>>>>>>>>>> section from KIP-892: Transactional Semantics for > >>>>>>>>>>>>>>>>>>> StateStores, > >>>>>>>>>>>>> into > >>>>>>>>>>>>>>>> its > >>>>>>>>>>>>>>>>>> own > >>>>>>>>>>>>>>>>>>> KIP > >>>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>>>> KIP-1035: StateStore managed changelog offsets > >>>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>> > >>>>>>>>>>>>>> > >>>>>>>>>>>>> > >>>>>>>>>>>> > >>>>>>>>>>> > https://cwiki.apache.org/confluence/display/KAFKA/KIP-1035%3A+StateStore+managed+changelog+offsets > >>>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>>>> While KIP-892 was adopted *with* the changes outlined > in > >>>>>>>>>>>> KIP-1035, > >>>>>>>>>>>>>>>> these > >>>>>>>>>>>>>>>>>>> changes were always the most contentious part, and > >>>>>>>>>>>>>>>>>>> continued > >>>>>>>>>>> to > >>>>>>>>>>>>> spur > >>>>>>>>>>>>>>>>>>> discussion even after KIP-892 was adopted. > >>>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>>>> All the changes introduced in KIP-1035 have been > >>>>>>>>>>>>>>>>>>> removed from > >>>>>>>>>>>>>> KIP-892, > >>>>>>>>>>>>>>>>>> and > >>>>>>>>>>>>>>>>>>> a hard dependency on KIP-1035 has been added to > >>>>>>>>>>>>>>>>>>> KIP-892 in > >>>>>>>>>>> their > >>>>>>>>>>>>>>>> place. > >>>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>>>> I'm hopeful that with some more focus on this set of > >>>>>>>>>>>>>>>>>>> changes, > >>>>>>>>>>> we > >>>>>>>>>>>>> can > >>>>>>>>>>>>>>>>>>> deliver something that we're all happy with. > >>>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>>>> Regards, > >>>>>>>>>>>>>>>>>>> Nick > >>>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>>> > >>>>>>>>>>>>>>> > >>>>>>>>>>>>>> > >>>>>>>>>>>>> > >>>>>>>>>>>> > >>>>>>>>>>> > >>>>>>>>>> >
