Hi Alexander,
Thanks for the KIP! This seems like a great proposal. I have the same
opinion as John on the Configuration part though. I think the 2 level
config and its behaviour based on the setting/unsetting of the flag seems
confusing to me as well. Since the KIP seems specifically centred around
RocksDB it might be better to add it at the Supplier level as John
suggested.
On similar lines, this config name => *statestore.transactional.mechanism *may
also need rethinking as the value assigned to it(rocksdb_indexbatch)
implicitly seems to assume that rocksdb is the only statestore that Kafka
Stream supports while that's not the case.
Also, regarding the potential memory pressure that can be introduced by
WriteBatchIndex, do you think it might make more sense to include some
numbers/benchmarks on how much the memory consumption might increase?
Lastly, the read_uncommitted flag's behaviour on IQ may need more
elaboration.
These points aside, as I said, this is a great proposal!
Thanks!
Sagar.
On Tue, May 24, 2022 at 10:35 PM John Roesler <vvcep...@apache.org> wrote:
> Thanks for the KIP, Alex!
>
> I'm really happy to see your proposal. This improvement fills a
> long-standing gap.
>
> I have a few questions:
>
> 1. Configuration
> The KIP only mentions RocksDB, but of course, Streams also ships with an
> InMemory store, and users also plug in their own custom state stores. It is
> also common to use multiple types of state stores in the same application
> for different purposes.
>
> Against this backdrop, the choice to configure transactionality as a
> top-level config, as well as to configure the store transaction mechanism
> as a top-level config, seems a bit off.
>
> Did you consider instead just adding the option to the
> RocksDB*StoreSupplier classes and the factories in Stores ? It seems like
> the desire to enable the feature by default, but with a feature-flag to
> disable it was a factor here. However, as you pointed out, there are some
> major considerations that users should be aware of, so opt-in doesn't seem
> like a bad choice, either. You could add an Enum argument to those
> factories like `RocksDBTransactionalMechanism.{NONE,
>
> Some points in favor of this approach:
> * Avoid "stores that don't support transactions ignore the config"
> complexity
> * Users can choose how to spend their memory budget, making some stores
> transactional and others not
> * When we add transactional support to in-memory stores, we don't have to
> figure out what to do with the mechanism config (i.e., what do you set the
> mechanism to when there are multiple kinds of transactional stores in the
> topology?)
>
> 2. caching/flushing/transactions
> The coupling between memory usage and flushing that you mentioned is a bit
> troubling. It also occurs to me that there seems to be some relationship
> with the existing record cache, which is also an in-memory holding area for
> records that are not yet written to the cache and/or store (albeit with no
> particular semantics). Have you considered how all these components should
> relate? For example, should a "full" WriteBatch actually trigger a flush so
> that we don't get OOMEs? If the proposed transactional mechanism forces all
> uncommitted writes to be buffered in memory, until a commit, then what is
> the advantage over just doing the same thing with the RecordCache and not
> introducing the WriteBatch at all?
>
> 3. ALOS
> You mentioned that a transactional store can help reduce duplication in
> the case of ALOS. We might want to be careful about claims like that.
> Duplication isn't the way that repeated processing manifests in state
> stores. Rather, it is in the form of dirty reads during reprocessing. This
> feature may reduce the incidence of dirty reads during reprocessing, but
> not in a predictable way. During regular processing today, we will send
> some records through to the changelog in between commit intervals. Under
> ALOS, if any of those dirty writes gets committed to the changelog topic,
> then upon failure, we have to roll the store forward to them anyway,
> regardless of this new transactional mechanism. That's a fixable problem,
> by the way, but this KIP doesn't seem to fix it. I wonder if we should make
> any claims about the relationship of this feature to ALOS if the real-world
> behavior is so complex.
>
> 4. IQ
> As a reminder, we have a new IQv2 mechanism now. Should we propose any
> changes to IQv1 to support this transactional mechanism, versus just
> proposing it for IQv2? Certainly, it seems strange only to propose a change
> for IQv1 and not v2.
>
> Regarding your proposal for IQv1, I'm unsure what the behavior should be
> for readCommitted, since the current behavior also reads out of the
> RecordCache. I guess if readCommitted==false, then we will continue to read
> from the cache first, then the Batch, then the store; and if
> readCommitted==true, we would skip the cache and the Batch and only read
> from the persistent RocksDB store?
>
> What should IQ do if I request to readCommitted on a non-transactional
> store?
>
> Thanks again for proposing the KIP, and my apologies for the long reply;
> I'm hoping to air all my concerns in one "batch" to save time for you.
>
> Thanks,
> -John
>
> On Tue, May 24, 2022, at 03:45, Alexander Sorokoumov wrote:
> > Hi all,
> >
> > I've written a KIP for making Kafka Streams state stores transactional
> and
> > would like to start a discussion:
> >
> >
> https://cwiki.apache.org/confluence/display/KAFKA/KIP-844%3A+Transactional+State+Stores
> >
> > Best,
> > Alex
>
