问题
I'm currently evaluating options for designing/implementing Event Sourcing + CQRS architectutral approach to system design. Since we want to use Apache Kafka for other aspects (normal pubsub messaging + stream processing), the next logical question would be, "Can we use the Apache Kafka store as event store for CQRS"?, or more importantly would that be a smart desition?
Right now I'm unsure about this. This source seems to support it: https://www.confluent.io/blog/okay-store-data-apache-kafka/
This other source recommends against that: https://medium.com/serialized-io/apache-kafka-is-not-for-event-sourcing-81735c3cf5c
In my current tests/experiments, I'm having problems similar to those described by the 2nd source, those are:
- recomposing an entity: Kafka doesn't seem to support fast retrieval/searching of specific events within a topic (for example: all commands related to an order's history - necessary for the reconstruction of the entity's instance, seems to require the scan of all the topic's events and filter only those matching some entity instance identificator, which is a no go). [This other person seems to have arrived to a similar conclusion: Query Kafka topic for specific record -- that is, it is just not possible (without relying on some hacky trick)]
- - write consistency: Kafka doesn't support transactional atomicity on their store, so it seems a common practice to just put a DB with some locking approach (usually optimistic locking) before asynchronously exporting the events to the Kafka qeue (I can live with this though, the first problem is much more crucial to me).
- The partition problem: On the Kafka documentation, it is mentioned that "order guarantee", exists only within a "Topic's partition". At the same time they also say that the partition is the basic unit of parallelism, in other words, if you want to parallelize work, spread the messages accross partitions (and brokers of course). But this is a problem, because an "Event store" in an event sourced system needs the order guarantee, so this means I'm forced to use only 1 partition for this use case if I absolutely need the order guarantee. Is this correct?
Even though this question is a bit open, It really is like that: Have you used Kafka as your main event store on an event sourced system? How have you dealt with the problem of recomposing entity instances out of their command history (given that the topic has millions of entries scaning all the set is not an option)? Did you use only 1 partition sacrificing potential concurrent consumers (given that the order guarantee is restricted to a specific topic partition)?
Any specific or general feedback would the greatly appreciated, as this is a complex topic witn several considerations.
Thanks in advance.
EDIT There was a similar discussion 6 years ago here: Using Kafka as a (CQRS) Eventstore. Good idea? Consensus back then was also divided, and alot of people that suggest this approach is convenient, mention how Kafka deals natively with huge amounts of real time data. Nevertheless the problem (for me at least) isn't related to that, but is more related to how unconvenient are Kafka's capabilities to rebuild an Entity's state- Either by modeling topics as Entities instances (where the exponential explosion in topics amount is undesired), or by modelling topics es entity Types (where amounts of events within the topic make reconstruction very slow/unpractical).
回答1:
your understanding is mostly correct:
- kafka has no search. definitely not by key. there's a seek to timestamp, but its imperfect and not good for what youre trying to do.
- kafka actually supports a limited form of transactions (see exactly once) these days, although if you interact with any other system outside of kafka they will be of no use.
- the unit of anything in kafka (event ordering, availability, replication) is a partition. there are no guarantees across partitions of the same topic.
all these dont stop applications from using kafka as the source of truth for their state, so long as:
- your problem can be "sharded" into topic partitions so you dont care about order of events across partitions
- youre willing to "replay" an entire partition if/when you lose your local state as bootstrap.
- you use log compacted topics to try and keep a bound on their size (because you will need to replay them to bootstrap, see above point)
both samza and (IIUC) kafka-streams back their state stores with log-compacted kafka topics. internally to kafka offset and consumer group management is stored as a log compacted topic with brokers holding a "materialized view" in memory - when ownership of a partition of __consumer_offsets moves between brokers the new leader replays the partition to rebuild this view.
来源：https://stackoverflow.com/questions/58763727/is-it-ok-to-use-apache-kafka-infinite-retention-policy-as-a-base-for-an-event