Is the protocol bump caused by the behavior change or the new error code?
1) IMO, error_codes are data, and clients can expect to receive errors
that they don't understand (i.e. unknown errors). AFAIK, clients don't
break on unknown errors, they are simple more challenging to debug. If
we document the new behavior, then its definitely debuggable and
fixable.
2) The behavior change is basically a deprecation - i.e. acks > 1 were
never documented, and are not supported by Kafka clients starting with
version 0.8.2. I'm not sure this requires a protocol bump either,
although its a better case than new error codes.
Thanks,
Gwen
On Thu, Jan 15, 2015 at 10:10 AM, Joe Stein <joe.st...@stealth.ly> wrote:
> Looping in the mailing list that the client developers live on because they
> are all not on dev (though they should be if they want to be helping to
> build the best client libraries they can).
>
> I whole hardily believe that we need to not break existing functionality of
> the client protocol, ever.
>
> There are many reasons for this and we have other threads on the mailing
> list where we are discussing that topic (no pun intended) that I don't want
> to re-hash here.
>
> If we change wire protocol functionality OR the binary format (either) we
> must bump version AND treat version as a feature flag with backward
> compatibility support until it is deprecated for some time for folks to deal
> with it.
>
> match version = {
> case 0: keepDoingWhatWeWereDoing()
> case 1: doNewStuff()
> case 2: doEvenMoreNewStuff()
> }
>
> has to be a practice we adopt imho ... I know feature flags can be construed
> as "messy code" but I am eager to hear another (better? different?) solution
> to this.
>
> If we don't do a feature flag like this specifically with this change then
> what happens is that someone upgrades their brokers with a rolling restart
> in 0.8.3 and every single one of their producer requests start to fail and
> they have a major production outage. eeeek!!!!
>
> I do 100% agree that > 1 makes no sense and we *REALLY* need people to start
> using 0,1,-1 but we need to-do that in a way that is going to work for
> everyone.
>
> Old producers and consumers must keep working with new brokers and if we are
> not going to support that then I am unclear what the use of "version" is
> based on our original intentions of having it because of the 0.7=>-0.8. We
> said no more breaking changes when we did that.
>
> - Joe Stein
>
> On Thu, Jan 15, 2015 at 12:38 PM, Ewen Cheslack-Postava <e...@confluent.io>
> wrote:
>>
>> Right, so this looks like it could create an issue similar to what's
>> currently being discussed in
>> https://issues.apache.org/jira/browse/KAFKA-1649 where users now get
>> errors
>> under conditions when they previously wouldn't. Old clients won't even
>> know
>> about the error code, so besides failing they won't even be able to log
>> any
>> meaningful error messages.
>>
>> I think there are two options for compatibility:
>>
>> 1. An alternative change is to remove the ack > 1 code, but silently
>> "upgrade" requests with acks > 1 to acks = -1. This isn't the same as
>> other
>> changes to behavior since the interaction between the client and server
>> remains the same, no error codes change, etc. The client might just see
>> some increased latency since the message might need to be replicated to
>> more brokers than they requested.
>> 2. Split this into two patches, one that bumps the protocol version on
>> that
>> message to include the new error code but maintains both old (now
>> deprecated) and new behavior, then a second that would be applied in a
>> later release that removes the old protocol + code for handling acks > 1.
>>
>> 2 is probably the right thing to do. If we specify the release when we'll
>> remove the deprecated protocol at the time of deprecation it makes things
>> a
>> lot easier for people writing non-java clients and could give users better
>> predictability (e.g. if clients are at most 1 major release behind
>> brokers,
>> they'll remain compatible but possibly use deprecated features).
>>
>>
>> On Wed, Jan 14, 2015 at 3:51 PM, Gwen Shapira <gshap...@cloudera.com>
>> wrote:
>>
>> > Hi Kafka Devs,
>> >
>> > We are working on KAFKA-1697 - remove code related to ack>1 on the
>> > broker. Per Neha's suggestion, I'd like to give everyone a heads up on
>> > what these changes mean.
>> >
>> > Once this patch is included, any produce requests that include
>> > request.required.acks > 1 will result in an exception. This will be
>> > InvalidRequiredAcks in new versions (0.8.3 and up, I assume) and
>> > UnknownException in existing versions (sorry, but I can't add error
>> > codes retroactively).
>> >
>> > This behavior is already enforced by 0.8.2 producers (sync and new),
>> > but we expect impact on users with older producers that relied on acks
>> > > 1 and external clients (i.e python, go, etc).
>> >
>> > Users who relied on acks > 1 are expected to switch to using acks = -1
>> > and a min.isr parameter than matches their user case.
>> >
>> > This change was discussed in the past in the context of KAFKA-1555
>> > (min.isr), but let us know if you have any questions or concerns
>> > regarding this change.
>> >
>> > Gwen
>> >
>>
>>
>>
>> --
>> Thanks,
>> Ewen
>
>
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