I was just thinking that the processor would null out the record context after it finished processing the record, so I'm not sure I follow why this would not be possible? AFAIK we never call a punctuator in the middle of processing a record through the topology, and even if we did, we still know when it is about to be called and could set it to null beforehand.
I'm not trying to advocate for it here, I'm in agreement that anything you want to access within the store can and should be accessed within the calling Processor/Punctuator before reaching the store. The "we can always add it later if necessary" argument is also pretty convincing. Just trying to understand why this wouldn't be possible. FWIW, the question of "what is the current record in the context of a Punctuator" exists independently of whether we want to add this to the StateStoreContext or not. The full ProcessorContext, including the current record context, is already available within a Punctuator, so removing the current record context from the StateStoreContext does not solve the problem. Users can -- and have (see KAFKA-9584 <https://issues.apache.org/jira/browse/KAFKA-9584>) -- hit such subtle bugs without ever invoking a StateStore from their punctuator. Again, I think I do agree that we should leave the current record context off of the StateStoreContext, but I don't think the Punctuator argument against it is very convincing. It sounds to me like we need to disallow access to the current record context from within the Punctuator, independent of anything to do with state stores On Thu, Sep 10, 2020 at 7:12 AM John Roesler <vvcep...@apache.org> wrote: > Thanks for the thoughts, Sophie. > > I agree that the extra information could be useful. My only concern is > that it doesn’t seem like we can actually supply that extra information > correctly. So, then we have a situation where the system offers useful API > calls that are only correct in a narrow range of use cases. Outside of > those use cases, you get incorrect behavior. > > If it were possible to null out the context before you put a document to > which the context doesn’t apply, then the concern would be mitigated. But > it would still be pretty weird from the perspective of the store that > sometimes the context is populated and other times, it’s null. > > But that seems moot, since it doesn’t seem possible to null out the > context. Only the Processor could know whether it’s about to put a document > different from the context or not. And it would be inappropriate to offer a > public ProcessorContext api to manage the record context. > > Ultimately, it still seems like if you want to store headers, you can > store them explicitly, right? That doesn’t seem onerous to me, and it kind > of seems better than relying on undefined or asymmetrical behavior in the > store itself. > > Anyway, I’m not saying that we couldn’t solve these problems. Just that it > seems a little that we can be conservative and avoid them for now. If it > turns out we really need to solve them, we can always do it later. > > Thanks, > John > > On Wed, Sep 9, 2020, at 22:46, Sophie Blee-Goldman wrote: > > > > > > If you were to call "put" from a punctuator, or do a > > > `range()` query and then update one of those records with > > > `put()`, you'd have a very subtle bug on your hands. > > > > > > Can you elaborate on this a bit? I agree that the punctuator case is an > > obvious exemption to the assumption that store invocations always > > have a corresponding "current record", but I don't understand the > > second example. Are you envisioning a scenario where the #process > > method performs a range query and then updates records? Or were > > you just giving another example of the punctuator case? > > > > I only bring it up because I agree that the current record information > could > > still be useful within the context of the store. As a non-user my input > on > > this > > definitely has limited value, but it just isn't striking me as obvious > that > > we > > should remove access to the current record context from the state stores. > > If there is no current record, as in the punctuator case, we should just > > set > > the record context to null (or Optional.empty, etc). > > > > That said, the put() always has to come from somewhere, and that > > somewhere is always going to be either a Processor or a Punctuator, both > > of which will still have access to the full context. So additional info > > such as > > the timestamp can and should probably be supplied to the store before > > calling put(), rather than looked up by the store. But I can see some > other > > things being useful, for example the current record's headers. Maybe > if/when > > we add better (or any) support for headers in state stores this will be > > less true. > > > > Of course as John has made clear, it's pretty hard to judge without > > examples > > and more insight as to what actually goes on within a custom state store > > > > On Wed, Sep 9, 2020 at 8:07 PM John Roesler <vvcep...@apache.org> wrote: > > > > > Hi Paul, > > > > > > It's good to hear from you! > > > > > > I'm glad you're in favor of the direction. Especially when > > > it comes to public API and usability concens, I tend to > > > think that "the folks who matter" are actually the folks who > > > have to use the APIs to accomplish real tasks. It can be > > > hard for me to be sure I'm thinking clearly from that > > > perspective. > > > > > > Funny story, I also started down this road a couple of times > > > already and backed them out before the KIP because I was > > > afraid of the scope of the proposal. Unfortunately, needing > > > to make a new ProcessorContext kind of forced my hand. > > > > > > I see you've called me out about the ChangeLogging stores :) > > > In fact, I think these are the main/only reason that stores > > > might really need to invoke "forward()". My secret plan was > > > to cheat and either accomplish change-logging by a different > > > mechanism than implementing the store interface, or by just > > > breaking encapsulation to sneak the "real" ProcessorContext > > > into the ChangeLogging stores. But those are all > > > implementation details. I think the key question is whether > > > anyone else has a store implementation that needs to call > > > "forward()". It's not what you mentioned, but since you > > > spoke up, I'll just ask: if you have a use case for calling > > > "forward()" in a store, please share it. > > > > > > Regarding the other record-specific context methods, I think > > > you have a good point, but I also can't quite wrap my head > > > around how we can actually guarantee it to work in general. > > > For example, the case you cited, where the implementation of > > > `KeyValueStore#put(key, value)` uses the context to augment > > > the record with timestamp information. This relies on the > > > assumption that you would only call "put()" from inside a > > > `Processor#process(key, value)` call in which the record > > > being processed is the same record that you're trying to put > > > into the store. > > > > > > If you were to call "put" from a punctuator, or do a > > > `range()` query and then update one of those records with > > > `put()`, you'd have a very subtle bug on your hands. Right > > > now, the Streams component that actually calls the Processor > > > takes care to set the right record context before invoking > > > the method, and in the case of caching, etc., it also takes > > > care to swap out the old context and keep it somewhere safe. > > > But when it comes to public API Processors calling methods > > > on StateStores, there's no opportunity for any component to > > > make sure the context is always correct. > > > > > > In the face of that situation, it seemed better to just move > > > in the direction of a "normal" data store. I.e., when you > > > use a HashMap or RocksDB or other "state stores", you don't > > > expect them to automatically know extra stuff about the > > > record you're storing. If you need them to know something, > > > you just put it in the value. > > > > > > All of that said, I'm just reasoning from first principles > > > here. To really know if this is a mistake or not, I need to > > > be in your place. So please push back if you think what I > > > said is nonsense. My personal plan was to keep an eye out > > > during the period where the old API was still present, but > > > deprecated, to see if people were struggling to use the new > > > API. If so, then we'd have a chance to address it before > > > dropping the old API. But it's even better if you can help > > > think it through now. > > > > > > It did also cross my mind to _not_ add the > > > StateStoreContext, but just to continue to punt on the > > > question by just dropping in the new ProcessorContext to the > > > new init method. If StateStoreContext seems too bold, we can > > > go that direction. But if we actually add some methods to > > > StateStoreContext, I'd like to be able to ensure they would > > > be well defined. I think the current situation was more of > > > an oversight than a choice. > > > > > > Thanks again for your reply, > > > -John > > > > > > > > > On Wed, 2020-09-09 at 21:23 -0500, Paul Whalen wrote: > > > > John, > > > > > > > > It's exciting to see this KIP head in this direction! In the last > year > > > or > > > > so I've tried to sketch out some usability improvements for custom > state > > > > stores, and I also ended up splitting out the StateStoreContext from > the > > > > ProcessorContext in an attempt to facilitate what I was doing. I > sort of > > > > abandoned it when I realized how large the ideal change might have > to be, > > > > but it's great to see that there is other interest in moving in this > > > > direction (from the folks that matter :) ). > > > > > > > > Having taken a stab at it myself, I have a comment/question on this > > > bullet > > > > about StateStoreContext: > > > > > > > > It does *not* include anything processor- or record- specific, like > > > > > `forward()` or any information about the "current" record, which is > > > only a > > > > > well-defined in the context of the Processor. Processors process > one > > > record > > > > > at a time, but state stores may be used to store and fetch many > > > records, so > > > > > there is no "current record". > > > > > > > > > > > > > I totally agree that record-specific or processor-specific context > in a > > > > state store is often not well-defined and it would be good to > separate > > > that > > > > out, but sometimes it (at least record-specific context) is actually > > > > useful, for example, passing the record's timestamp through to the > > > > underlying storage (or changelog topic): > > > > > > > > https://github.com/apache/kafka/blob/trunk/streams/src/main/java/org/apache/kafka/streams/state/internals/ChangeLoggingKeyValueBytesStore.java#L121 > > > > > > > > You could have the writer client of the state store pass this > through, > > > but > > > > it would be nice to be able to write state stores where the client > did > > > not > > > > have this responsibility. I'm not sure if the solution is to add > some > > > > things back to StateStoreContext, or make yet another context that > > > > represents record-specific context while inside a state store. > > > > > > > > Best, > > > > Paul > > > > > > > > On Wed, Sep 9, 2020 at 5:43 PM John Roesler <j...@vvcephei.org> > wrote: > > > > > > > > > Hello all, > > > > > > > > > > I've been slowly pushing KIP-478 forward over the last year, > > > > > and I'm happy to say that we're making good progress now. > > > > > However, several issues with the original design have come > > > > > to light. > > > > > > > > > > The major changes: > > > > > > > > > > We discovered that the original plan of just adding generic > > > > > parameters to ProcessorContext was too disruptive, so we are > > > > > now adding a new api.ProcessorContext. > > > > > > > > > > That choice forces us to add a new StateStore.init method > > > > > for the new context, but ProcessorContext really isn't ideal > > > > > for state stores to begin with, so I'm proposing a new > > > > > StateStoreContext for this purpose. In a nutshell, there are > > > > > quite a few methods in ProcessorContext that actually should > > > > > never be called from inside a StateStore. > > > > > > > > > > Also, since there is a new ProcessorContext interface, we > > > > > need a new MockProcessorContext implementation in the test- > > > > > utils module. > > > > > > > > > > > > > > > > > > > > The changeset for the KIP document is here: > > > > > > > > > > > > > > https://cwiki.apache.org/confluence/pages/diffpagesbyversion.action?pageId=118172121&selectedPageVersions=14&selectedPageVersions=10 > > > > > > > > > > And the KIP itself is here: > > > > > > > > > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP-478+-+Strongly+typed+Processor+API > > > > > > > > > > > > > > > If you have any concerns, please let me know! > > > > > > > > > > Thanks, > > > > > -John > > > > > > > > > > > > > > > > > > >
