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From "Sean Policarpio (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (KAFKA-2260) Allow specifying expected offset on produce
Date Thu, 14 Dec 2017 02:54:00 GMT

    [ https://issues.apache.org/jira/browse/KAFKA-2260?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16290260#comment-16290260

Sean Policarpio commented on KAFKA-2260:

We also desire optimistic concurrency control for our Kafka application. Understandably, relying
on Kafka to manage offsets should be the defacto strategy, but our use case requires more
transactionality before allowing Kafka to accept data we produce. Ideally, it would be great
if the API could let us fail fast when we attempt to push data that we assume was still sequential.

> Allow specifying expected offset on produce
> -------------------------------------------
>                 Key: KAFKA-2260
>                 URL: https://issues.apache.org/jira/browse/KAFKA-2260
>             Project: Kafka
>          Issue Type: Improvement
>            Reporter: Ben Kirwin
>            Priority: Minor
>         Attachments: KAFKA-2260.patch, expected-offsets.patch
> I'd like to propose a change that adds a simple CAS-like mechanism to the Kafka producer.
This update has a small footprint, but enables a bunch of interesting uses in stream processing
or as a commit log for process state.
> h4. Proposed Change
> In short:
> - Allow the user to attach a specific offset to each message produced.
> - The server assigns offsets to messages in the usual way. However, if the expected offset
doesn't match the actual offset, the server should fail the produce request instead of completing
the write.
> This is a form of optimistic concurrency control, like the ubiquitous check-and-set --
but instead of checking the current value of some state, it checks the current offset of the
> h4. Motivation
> Much like check-and-set, this feature is only useful when there's very low contention.
Happily, when Kafka is used as a commit log or as a stream-processing transport, it's common
to have just one producer (or a small number) for a given partition -- and in many of these
cases, predicting offsets turns out to be quite useful.
> - We get the same benefits as the 'idempotent producer' proposal: a producer can retry
a write indefinitely and be sure that at most one of those attempts will succeed; and if two
producers accidentally write to the end of the partition at once, we can be certain that at
least one of them will fail.
> - It's possible to 'bulk load' Kafka this way -- you can write a list of n messages consecutively
to a partition, even if the list is much larger than the buffer size or the producer has to
be restarted.
> - If a process is using Kafka as a commit log -- reading from a partition to bootstrap,
then writing any updates to that same partition -- it can be sure that it's seen all of the
messages in that partition at the moment it does its first (successful) write.
> There's a bunch of other similar use-cases here, but they all have roughly the same flavour.
> h4. Implementation
> The major advantage of this proposal over other suggested transaction / idempotency mechanisms
is its minimality: it gives the 'obvious' meaning to a currently-unused field, adds no new
APIs, and requires very little new code or additional work from the server.
> - Produced messages already carry an offset field, which is currently ignored by the
server. This field could be used for the 'expected offset', with a sigil value for the current
behaviour. (-1 is a natural choice, since it's already used to mean 'next available offset'.)
> - We'd need a new error and error code for a 'CAS failure'.
> - The server assigns offsets to produced messages in {{ByteBufferMessageSet.validateMessagesAndAssignOffsets}}.
After this changed, this method would assign offsets in the same way -- but if they don't
match the offset in the message, we'd return an error instead of completing the write.
> - To avoid breaking existing clients, this behaviour would need to live behind some config
flag. (Possibly global, but probably more useful per-topic?)
> I understand all this is unsolicited and possibly strange: happy to answer questions,
and if this seems interesting, I'd be glad to flesh this out into a full KIP or patch. (And
apologies if this is the wrong venue for this sort of thing!)

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