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
