On Sat, Jul 8, 2023, at 02:41, Erik van Oosten wrote: > Hi Colin, > > Thanks for your thoughts and taking the time to reply. > > Let me take away your concerns. None of your worries are an issue with > the algorithm described in KIP-944. Here it goes: > > > It's not clear ot me that it's safe to access the Kafka consumer or > > producer concurrently from different threads. > > Concurrent access is /not/ a design goal of KIP-944. In fact, it goes > through great lengths to make sure that this cannot happen. > > *The only design goal is to allow callbacks to call the consumer from > another thread.* > > To make sure there are no more misunderstandings about this, I have > added this goal to the KIP. >
Hi Erik, Sorry, I spoke imprecisely. My concern is not concurrent access, but multithreaded access in general. Basically cache line visibility issues. > > This is true even if the accesses happen at different times, because > > modern CPUs require memory barriers to guarantee inter-thread visibilty > > of loads and stores. > > In KIP-944, the callback thread can only delegate to another thread > after reading from and writing to a threadlocal variable, providing the > barriers right there. > I don't see any documentation that accessing thread local variables provides a total store or load barrier. Do you have such documentation? It seems like if this were the case, we could eliminate volatile variables from most of the code base. > > I know that there are at least a few locks in the consumer code now, > > due to our need to send heartbeats from a worker thread. I don't think > > those would be sufficient to protect a client that is making calls from > > random threads. > > In the current implementation the consumer is also invoked from random > threads. If it works now, it should continue to work. > I'm not sure what you're referring to. Can you expand on this? > > There has been some discussion of moving to a more traditional model > > where people make calls to the client and the clients passes the given > > data to a single background worker thread. This would avoid a lot lof > > the footguns of the current model and probably better reflect how people > > actually use the client. > > That is awesome. However, I'd rather not wait for that. > > > Another issue is that neither the producer nor the consumer is fully > > nonblocking. There are some corner cases where we do in fact block. From > > memory, the producer blocks in some "buffer full" cases, and the > > consumer blocks sometimes when fetching metadata. > > I am aware of that. This is not an issue; all async runtimes can > cooperate with blocking code. > Hmm, not sure what you mean by "cooperate with blocking code." If you have 10 green threads you're multiplexing on to one CPU thread, and that CPU thread gets blocked because of what one green thread is doing, the other 9 green threads are blocked too, right? I guess it's "just" a performance problem, but it still seems like it could be a serious one. > > I suspect it would be more appropriate for Kotlin coroutines, Zio > > coroutines and so on to adopt this "pass messages to and from a > > background worker thread" model than to try to re-engineer the Kafka > > client ot work from random threads. > > In both zio-kafka and fs2-kafka this is already the approach we are taking. > > Unfortunately, the Kafka consumer forces us to perform some work in > callbacks: > > * commit completed callback: register that the callback is complete, > * partition revoked callback: in this callback we need to submit > commits from everything consumed and processed so far, using > timeouts if processing takes to long. In an async runtime, this is > an inherently multi-threaded process. Especially, we cannot do > timeouts without involving multiple threads. > I don't see why this has to be "inherently multi-threaded." Why can't we have the other threads report back what messages they've processed to the worker thread. Then it will be able to handle these callbacks without involving the other threads. regards, Colin > I have extended the KIP's motivation to explain the major use case. > > Please read KIP-944 again. Even though the description is extensive > (this callback from callback stuff is tricky), you will find that my > goals are modest. > > Also the implementation is just a few lines. With understanding of the > idea it should not be a lot of work to follow it. > > Kind regards, > Erik. > > > Op 07-07-2023 om 19:57 schreef Colin McCabe: >> Hi Erik, >> >> It's not clear ot me that it's safe to access the Kafka consumer or producer >> concurrently from different threads. There are data structures that aren't >> protected by locks, so I wouldn't necessarily expect accessing and mutating >> them in a concurrent way to work. This is true even if the accesses happen >> at different times, because modern CPUs require memory barriers to guarantee >> inter-thread visibilty of loads and stores. >> >> I am writing this is without doing a detailed dive into the code (I haven't >> been into the consumer / producer code in a bit.) Someone who has worked >> more on the consumer recently might be able to give specific examples of >> things that wouldn't work. >> >> I know that there are at least a few locks in the consumer code now, due to >> our need to send heartbeats from a worker thread. I don't think those would >> be sufficient to protect a client that is making calls from random threads. >> >> There has been some discussion of moving to a more traditional model where >> people make calls to the client and the clients passes the given data to a >> single background worker thread. This would avoid a lot lof the footguns of >> the current model and probably better reflect how people actually use the >> client. >> >> Another issue is that neither the producer nor the consumer is fully >> nonblocking. There are some corner cases where we do in fact block. From >> memory, the producer blocks in some "buffer full" cases, and the consumer >> blocks sometimes when fetching metadata. >> >> I suspect it would be more appropriate for Kotlin coroutines, Zio coroutines >> and so on to adopt this "pass messages to and from a background worker >> thread" model than to try to re-engineer the Kafka client ot work from >> random threads. >> >> There is actually somed good advice about how to handle multiple threads in >> the KafkaConsumer.java header file itself. Check the sections "One Consumer >> Per Thread" and "Decouple Consumption and Processing." What I'm recommending >> here is essentially the latter. >> >> I do understand that it's frustrating to not get a quick response. However, >> overall I think this one needs a lot more discussion before getting anywhere >> near a vote. I will leave a -1 just as a procedural step. Maybe some of the >> people working in the client area can also chime in. >> >> best, >> Colin >> >> >> On Thu, Jul 6, 2023, at 12:02, Erik van Oosten wrote: >>> Dear PMCs, >>> >>> So far there have been 0 responses to KIP-944. I understand this may not >>> be something that keeps you busy, but this KIP is important to people >>> that use async runtimes like Zio, Cats and Kotlin. >>> >>> Is there anything you need to come to a decision? >>> >>> Kind regards, >>> Erik. >>> >>> >>> Op 05-07-2023 om 11:38 schreef Erik van Oosten: >>>> Hello all, >>>> >>>> I'd like to call a vote on KIP-944 Support async runtimes in consumer. >>>> It has has been 'under discussion' for 7 days now. 'Under discussion' >>>> between quotes, because there were 0 comments so far. I hope the KIP >>>> is clear! >>>> >>>> KIP description:https://cwiki.apache.org/confluence/x/chw0Dw >>>> >>>> Kind regards, >>>> Erik. >>>> >>>> > -- > Erik van Oosten > e.vanoos...@grons.nl > https://day-to-day-stuff.blogspot.com
