> > 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 > > > > > > > >
