Hi all,
I wanted to let you know that I have made the following minor changes to
the KIP-584 write up. The purpose is to ensure the design is correct for a
few things which came up during implementation:
1. Feature version data type has been made to be int16 (instead of int64).
The reason is two fold:
a. Usage of int64 felt overkill. Feature version bumps are infrequent
(since these bumps represent breaking changes that are generally
infrequent). Therefore int16 is big enough to support version bumps of a
particular feature.
b. The int16 data type aligns well with existing API versions data
type. Please see the file
'<kafka>/clients/src/main/resources/common/message/ApiVersionsResponse.json'.
2. Finalized feature version epoch data type has been made to be int32
(instead of int64). The reason is that the epoch value is the value of ZK
node version, whose data type is int32.
3. Introduced a new 'status' field in the '/features' ZK node schema. The
purpose is to implement Colin's earlier point for the strategy for
transitioning from not having a /features znode to having one. An
explanation has been provided in the following section of the KIP detailing
the different cases:
https://cwiki.apache.org/confluence/display/KAFKA/KIP-584%3A+Versioning+scheme+for+features#KIP584:Versioningschemeforfeatures-FeatureZKnodestatus
.
Please let me know if you have any questions or concerns.
Cheers,
Kowshik
Cheers,
Kowshik
On Tue, Apr 28, 2020 at 11:24 PM Kowshik Prakasam <kpraka...@confluent.io>
wrote:
> Hi all,
>
> This KIP vote has been open for ~12 days. The summary of the votes is that
> we have 3 binding votes (Colin, Guozhang, Jun), and 3 non-binding votes
> (David, Dhruvil, Boyang). Therefore, the KIP vote passes. I'll mark KIP as
> accepted and start working on the implementation.
>
> Thanks a lot!
>
>
> Cheers,
> Kowshik
>
> On Mon, Apr 27, 2020 at 12:15 PM Colin McCabe <cmcc...@apache.org> wrote:
>
>> Thanks, Kowshik. +1 (binding)
>>
>> best,
>> Colin
>>
>> On Sat, Apr 25, 2020, at 13:20, Kowshik Prakasam wrote:
>> > Hi Colin,
>> >
>> > Thanks for the explanation! I agree with you, and I have updated the
>> > KIP.
>> > Here is a link to relevant section:
>> >
>> https://cwiki.apache.org/confluence/display/KAFKA/KIP-584%3A+Versioning+scheme+for+features#KIP-584:Versioningschemeforfeatures-Controller:ZKnodebootstrapwithdefaultvalues
>> >
>> >
>> > Cheers,
>> > Kowshik
>> >
>> > On Fri, Apr 24, 2020 at 8:50 PM Colin McCabe <cmcc...@apache.org>
>> wrote:
>> >
>> > > On Fri, Apr 24, 2020, at 00:01, Kowshik Prakasam wrote:
>> > > > (Kowshik): Great point! However for case #1, I'm not sure why we
>> need to
>> > > > create a '/features' ZK node with disabled features. Instead, do
>> you see
>> > > > any drawback if we just do not create it? i.e. if IBP is less than
>> 2.6,
>> > > the
>> > > > controller treats the case as though the versioning system is
>> completely
>> > > > disabled, and would not create a non-existing '/features' node.
>> > >
>> > > Hi Kowshik,
>> > >
>> > > When the IBP is less than 2.6, but the software has been upgraded to a
>> > > state where it supports this KIP, that
>> > > means the user is upgrading from an earlier version of the
>> software. In
>> > > this case, we want to start with all the features disabled and allow
>> the
>> > > user to enable them when they are ready.
>> > >
>> > > Enabling all the possible features immediately after an upgrade could
>> be
>> > > harmful to the cluster. On the other hand, for a new cluster, we do
>> want
>> > > to enable all the possible features immediately . I was proposing
>> this as a
>> > > way to distinguish the two cases (since the new cluster will never be
>> > > started with an old IBP).
>> > >
>> > > > Colin MccCabe wrote:
>> > > > > And now, something a little bit bigger (sorry). For finalized
>> > > features,
>> > > > > why do we need both min_version_level and max_version_level?
>> Assuming
>> > > that
>> > > > > we want all the brokers to be on the same feature version level,
>> we
>> > > really only care
>> > > > > about three numbers for each feature, right? The minimum
>> supported
>> > > version
>> > > > > level, the maximum supported version level, and the current active
>> > > version level.
>> > > >
>> > > > > We don't actually want different brokers to be on different
>> versions of
>> > > > > the same feature, right? So we can just have one number for
>> current
>> > > > > version level, rather than two. At least that's what I was
>> thinking
>> > > -- let
>> > > > > me know if I missed something.
>> > > >
>> > > > (Kowshik): It is my understanding that the "current active version
>> level"
>> > > > that you have mentioned, is the "max_version_level". But we still
>> > > > maintain/publish both min and max version levels, because, the
>> detail
>> > > about
>> > > > min level is useful to external clients. This is described below.
>> > > >
>> > > > For any feature F, think of the closed range: [min_version_level,
>> > > > max_version_level] as the range of finalized versions, that's
>> guaranteed
>> > > to
>> > > > be supported by all brokers in the cluster.
>> > > > - "max_version_level" is the finalized highest common version
>> among all
>> > > > brokers,
>> > > > - "min_version_level" is the finalized lowest common version among
>> all
>> > > > brokers.
>> > > >
>> > > > Next, think of "client" here as the "user of the new feature
>> versions
>> > > > system". Imagine that such a client learns about finalized feature
>> > > > versions, and exercises some logic based on the version. These
>> clients
>> > > can
>> > > > be of 2 types:
>> > > > 1. Some part of the broker code itself could behave like a client
>> trying
>> > > to
>> > > > use some feature that's "internal" to the broker cluster. Such a
>> client
>> > > > would learn the latest finalized features via ZK.
>> > > > 2. An external system (ex: Streams) could behave like a client,
>> trying to
>> > > > use some "external" facing feature. Such a client would learn latest
>> > > > finalized features via ApiVersionsRequest. Ex: group_coordinator
>> feature
>> > > > described in the KIP.
>> > > >
>> > > > Next, imagine that for F, the max_version_level is successfully
>> bumped by
>> > > > +1 (via Controller API). Now it is guaranteed that all brokers (i.e.
>> > > > internal clients) understand max_version_level + 1. However, it is
>> still
>> > > > not guaranteed that all external clients have support for (or have
>> > > > activated) the logic for the newer version. Why? Because, this is
>> > > > subjective as explained next:
>> > > >
>> > > > 1. On one hand, imagine F as an internal feature only relevant to
>> > > Brokers.
>> > > > The binary for the internal client logic is controlled by Broker
>> cluster
>> > > > deployments. When shipping a new Broker release, we wouldn't bump
>> max
>> > > > "supported" feature version for F by 1, unless we have introduced
>> some
>> > > new
>> > > > logic (with a potentially breaking change) in the Broker.
>> Furthermore,
>> > > such
>> > > > feature logic in the broker should/will not be implemented in a way
>> that
>> > > it
>> > > > would activate logic for an older feature version after it has
>> migrated
>> > > to
>> > > > using the logic for a newer feature version (because this could
>> break the
>> > > > cluster!). For these cases, max_version_level will be very useful
>> for
>> > > > decision making.
>> > > >
>> > > > 2. On the other hand, imagine F as an external facing feature.
>> External
>> > > > clients are not within the control of Broker cluster. An external
>> client
>> > > > may not have upgraded it's code (yet) to use 'max_version_level +
>> 1'.
>> > > But,
>> > > > the Kafka cluster could have been deployed with support for
>> > > > 'max_version_level + 1' of an external facing feature. Now, these
>> > > external
>> > > > clients (until an upgrade) are benefitted in learning "what is the
>> lowest
>> > > > common version for F among all brokers?". This is where the
>> > > > "min_version_level" becomes useful. Using this, a client could
>> learn the
>> > > > specific supported versions that's lower than max_version_level
>> (instead
>> > > of
>> > > > assuming that all brokers support the range: [1,
>> max_version_level]). For
>> > > > example, if the cluster deprecates "min_version_level", then the
>> client
>> > > > becomes aware because it periodically learns the latest
>> > > "min_version_level"
>> > > > via ApiVersionsRequest.
>> > > >
>> > >
>> > > Thanks for the explanation. I agree that this does make sense when
>> you
>> > > take the client perspective into account.
>> > >
>> > > best,
>> > > Colin
>> > >
>> > >
>> > > >
>> > > > Cheers,
>> > > > Kowshik
>> > > >
>> > > >
>> > > >
>> > > >
>> > > > On Thu, Apr 23, 2020 at 12:07 PM Colin McCabe <cmcc...@apache.org>
>> > > wrote:
>> > > >
>> > > > > Hi Kowshik,
>> > > > >
>> > > > > Thanks again for working on this-- it looks great. I went over
>> the KIP
>> > > > > again and have a few more comments.
>> > > > >
>> > > > > ===
>> > > > >
>> > > > > It would be good to note that deprecating a feature version (in
>> other
>> > > > > words, increasing minVersionLevel on the broker) is an
>> incompatible
>> > > change,
>> > > > > which requires a major release of Kafka.
>> > > > >
>> > > > > ===
>> > > > >
>> > > > > I think the strategy for transitioning from not having a /features
>> > > znode
>> > > > > to having one could use some work. The current proposal is to
>> wait for
>> > > all
>> > > > > the brokers to fill in their feature znodes and then pick the
>> highest
>> > > > > common versions. But that requires blocking in the controller
>> startup
>> > > code
>> > > > > until the whole cluster is active (technically, a point in time
>> which
>> > > we
>> > > > > never really know that we have reached...)
>> > > > >
>> > > > > Instead, I think it would be better to have a strategy like this:
>> > > > > 1. If the controller comes up and there is no /features znode AND
>> the
>> > > IBP
>> > > > > is less than 2.6, create a /features znode where all the features
>> are
>> > > > > disabled.
>> > > > > 2. If the controller comes up and there is no /features znode AND
>> the
>> > > IBP
>> > > > > is greater than or equal to 2.6, create a /features znode where
>> all the
>> > > > > features are enabled at the highest versions supported by the
>> > > controller.
>> > > > >
>> > > > > People upgrading from the pre-KIP-584
>> > > > > <https://issues.apache.org/jira/browse/KIP-584> world will end
>> up in
>> > > case
>> > > > > #1 because they have to do a double roll to upgrade, and during
>> the
>> > > first
>> > > > > roll, the IBP is unchanged. People creating new clusters from
>> scratch
>> > > will
>> > > > > end up in case #2, which is what we want since we don't want a
>> brand
>> > > new
>> > > > > cluster to be using old feature flag versions.
>> > > > >
>> > > > > ===
>> > > > >
>> > > > > UpdateFeaturesResponse#ErrorMessage should specify
>> nullableVersions
>> > > since
>> > > > > null is a valid value here
>> > > > >
>> > > > > Also, in the message format, the tags we use need to start at 0.
>> > > > >
>> > > > > ===
>> > > > >
>> > > > > I don't think we need the FEATURE_UPDATE_IN_PROGRESS error code.
>> The
>> > > > > controller is basically single-threaded and will only do one of
>> these
>> > > > > operations at once. Even if it weren't, though, we could simply
>> block
>> > > the
>> > > > > second operation behind the first one.
>> > > > >
>> > > > > ===
>> > > > >
>> > > > > For updateFeatures, it would be good to specify that if a single
>> > > feature
>> > > > > version update in the batch can't be done, none of them are
>> done. I
>> > > think
>> > > > > this was the intention, but I wasn't able to find it spelled out
>> > > (maybe i
>> > > > > missed it).
>> > > > >
>> > > > > ===
>> > > > >
>> > > > > And now, something a little bit bigger (sorry). For finalized
>> > > features,
>> > > > > why do we need both min_version_level and max_version_level?
>> Assuming
>> > > that
>> > > > > we want all the brokers to be on the same feature version level,
>> we
>> > > really
>> > > > > only care about three numbers for each feature, right? The
>> minimum
>> > > > > supported version level, the maximum supported version level, and
>> the
>> > > > > current active version level.
>> > > > >
>> > > > > We don't actually want different brokers to be on different
>> versions of
>> > > > > the same feature, right? So we can just have one number for
>> current
>> > > > > version level, rather than two. At least that's what I was
>> thinking
>> > > -- let
>> > > > > me know if I missed something.
>> > > > >
>> > > > > best,
>> > > > > Colin
>> > > > >
>> > > > >
>> > > > > On Tue, Apr 21, 2020, at 13:01, Dhruvil Shah wrote:
>> > > > > > Thanks for the KIP! +1 (non-binding)
>> > > > > >
>> > > > > > On Tue, Apr 21, 2020 at 6:09 AM David Jacot <
>> dja...@confluent.io>
>> > > wrote:
>> > > > > >
>> > > > > > > Great KIP, thanks! +1 (non-binding)
>> > > > > > >
>> > > > > > > On Fri, Apr 17, 2020 at 8:56 PM Guozhang Wang <
>> wangg...@gmail.com>
>> > > > > wrote:
>> > > > > > >
>> > > > > > > > Thanks for the great KIP Kowshik, +1 (binding).
>> > > > > > > >
>> > > > > > > > On Fri, Apr 17, 2020 at 11:22 AM Jun Rao <j...@confluent.io>
>> > > wrote:
>> > > > > > > >
>> > > > > > > > > Hi, Kowshik,
>> > > > > > > > >
>> > > > > > > > > Thanks for the KIP. +1
>> > > > > > > > >
>> > > > > > > > > Jun
>> > > > > > > > >
>> > > > > > > > > On Thu, Apr 16, 2020 at 11:14 AM Kowshik Prakasam <
>> > > > > > > > kpraka...@confluent.io>
>> > > > > > > > > wrote:
>> > > > > > > > >
>> > > > > > > > > > Hi all,
>> > > > > > > > > >
>> > > > > > > > > > I'd like to start a vote for KIP-584
>> > > > > <https://issues.apache.org/jira/browse/KIP-584>. The link to the
>> KIP
>> > > can
>> > > > > be
>> > > > > > > found
>> > > > > > > > > > here:
>> > > > > > > > > >
>> > > > > > > > > >
>> > > > > > > > >
>> > > > > > > >
>> > > > > > >
>> > > > >
>> > >
>> https://cwiki.apache.org/confluence/display/KAFKA/KIP-584%3A+Versioning+scheme+for+features
>> > > > > > > > > > .
>> > > > > > > > > >
>> > > > > > > > > > Thanks!
>> > > > > > > > > >
>> > > > > > > > > >
>> > > > > > > > > > Cheers,
>> > > > > > > > > > Kowshik
>> > > > > > > > > >
>> > > > > > > > >
>> > > > > > > >
>> > > > > > > >
>> > > > > > > > --
>> > > > > > > > -- Guozhang
>> > > > > > > >
>> > > > > > >
>> > > > > >
>> > > > >
>> > > >
>> > >
>> >
>>
>
