A few additional questions:
1. Currently the IBP tells us what version of individual inter-broker RPCs
will be used. I think the plan in this KIP is to use ApiVersions request
instead to find the highest compatible version (just like clients). Do I
have that right?
2. The following wasn't very clear to me:
> Brokers will be able to observe changes to metadata.version by observing
the metadata log, and could then submit a new ApiVersionsRequest to the
other Kafka nodes.
Is the purpose of submitting new ApiVersions requests to update the
features or the RPC versions? Does metadata.version also influence the
versions that a broker advertises? It would help to have more detail about
this.
3. I imagine users will want to know before performing an upgrade whether
downgrading will be safe. Would the --dry-run flag tell them this?
Thanks,
Jason
On Wed, Nov 17, 2021 at 3:55 PM Colin McCabe <cmcc...@apache.org> wrote:
> On Wed, Nov 17, 2021, at 11:28, Jason Gustafson wrote:
> > Hi David,
> >
> > Forgive me if this ground has been covered already. Today, we have a few
> > other things that we have latched onto the IBP, such as upgrades to the
> > format of records in __consumer_offsets. I've been assuming that
> > metadata.version is not covering this. Is that right or is there some
> other
> > plan to take care of cases like this?
> >
>
> I think metadata.version could cover changes to things like
> __consumer_offsets, if people want it to. Or to put it another way, that is
> out of scope for this KIP.
>
> Like David said, basically it boils down to creating a feature flag for
> the new proposed __consumer_offsets version, or using a new
> IBP/metadata.version for it. Both approaches have pros and cons. Using an
> IBP/metadata.version bump reduces the size of the testing matrix. But using
> a feature flag allows people to avoid any bugs or pain associated with the
> change if they don't care about enabling it. This is basically the classic
> "should I use a feature flag or not?" discussion and we need to have it on
> a case-by-case basis.
>
> I think it's worth calling out that having a 1:1 mapping between IBP
> versions and metadata.versions will result in some metadata.versions that
> "don't do anything" (aka they do the same thing as the previous
> metadata.version). For example, if we change StopReplicaRequest again, that
> will not affect KRaft mode, but probably would require an IBP bump and
> hence a metadata.version bump. I think that's OK. It's not that different
> from updating your IBP and getting support for ZStandard, when your
> deployment doesn't use ZStandard compression.
>
> best,
> Colin
>
> > Thanks,
> > Jason
> >
> >
> >
> > On Wed, Nov 17, 2021 at 10:17 AM Jun Rao <j...@confluent.io.invalid>
> wrote:
> >
> >> Hi, Colin,
> >>
> >> Thanks for the reply.
> >>
> >> For case b, I am not sure that I understand your suggestion. Does "each
> >> subsequent level for metadata.version corresponds to an IBP version"
> mean
> >> that we need to keep IBP forever? Could you describe the upgrade
> process in
> >> this case?
> >>
> >> Jun
> >>
> >> On Tue, Nov 16, 2021 at 3:45 PM Colin McCabe <cmcc...@apache.org>
> wrote:
> >>
> >> > On Tue, Nov 16, 2021, at 15:13, Jun Rao wrote:
> >> > > Hi, David, Colin,
> >> > >
> >> > > Thanks for the reply.
> >> > >
> >> > > 16. Discussed with David offline a bit. We have 3 cases.
> >> > > a. We upgrade from an old version where the metadata.version has
> >> already
> >> > > been finalized. In this case it makes sense to stay with that
> feature
> >> > > version after the upgrade.
> >> >
> >> > +1
> >> >
> >> > > b. We upgrade from an old version where no metadata.version has been
> >> > > finalized. In this case, it makes sense to leave metadata.version
> >> > disabled
> >> > > since we don't know if all brokers have been upgraded.
> >> >
> >> > This is the scenario I was hoping to avoid by saying that ALL KRaft
> >> > clusters have metadata.version of at least 1, and each subsequent
> level
> >> for
> >> > metadata.version corresponds to an IBP version. The existing KRaft
> >> clusters
> >> > in 3.0 and earlier are preview (not for production) so I think this
> >> change
> >> > is OK for 3.x (given that it affects only KRaft). Then IBP is
> irrelevant
> >> > for KRaft clusters (the config is ignored, possibly with a WARN or
> ERROR
> >> > message generated if it is set).
> >> >
> >> > > c. We are starting from a brand new cluster and of course no
> >> > > metadata.version has been finalized. In this case, the KIP says it
> will
> >> > > pick the metadata.version in meta.properties. In the common case,
> >> people
> >> > > probably won't set the metadata.version in the meta.properties file
> >> > > explicitly. So, it will be useful to put a default (stable) version
> >> there
> >> > > when the meta.properties.
> >> >
> >> > Hmm. I was assuming that clusters where the admin didn't specify any
> >> > metadata.version during formatting would get the latest
> metadata.version.
> >> > Partly, because this is what we do for IBP today. It would be good to
> >> > clarify this...
> >> >
> >> > >
> >> > > Also, it would be useful to clarify that if a FeatureLevelRecord
> exists
> >> > for
> >> > > metadata.version, the metadata.version in meta.properties will be
> >> > ignored.
> >> > >
> >> >
> >> > Yeah, I agree.
> >> >
> >> > best,
> >> > Colin
> >> >
> >> > > Thanks,
> >> > >
> >> > > Jun
> >> > >
> >> > >
> >> > > On Tue, Nov 16, 2021 at 12:39 PM Colin McCabe <cmcc...@apache.org>
> >> > wrote:
> >> > >
> >> > >> On Fri, Nov 5, 2021, at 15:18, Jun Rao wrote:
> >> > >> > Hi, David,
> >> > >> >
> >> > >> > Thanks for the reply.
> >> > >> >
> >> > >> > 16. My first concern is that the KIP picks up meta.version
> >> > inconsistently
> >> > >> > during the deployment. If a new cluster is started, we pick up
> the
> >> > >> highest
> >> > >> > version. If we upgrade, we leave the feature version unchanged.
> >> > >>
> >> > >> Hi Jun,
> >> > >>
> >> > >> Thanks again for taking a look.
> >> > >>
> >> > >> The proposed behavior in KIP-778 is consistent with how it works
> >> today.
> >> > >> Upgrading the software is distinct from upgrading the IBP.
> >> > >>
> >> > >> I think it is important to keep these two operations ("upgrading
> >> > >> IBP/metadata version" and "upgrading software version") separate.
> If
> >> > they
> >> > >> are coupled it will create a situation where software upgrades are
> >> > >> difficult and dangerous.
> >> > >>
> >> > >> Consider a situation where you find some bug in your current
> software,
> >> > and
> >> > >> you want to upgrade to new software that fixes the bug. If upgrades
> >> and
> >> > IBP
> >> > >> bumps are coupled, you can't do this without also bumping the IBP,
> >> > which is
> >> > >> usually considered a high-risk change. That means that either you
> have
> >> > to
> >> > >> make a special build that includes only the fix (time-consuming and
> >> > >> error-prone), live with the bug for longer, or be very conservative
> >> > about
> >> > >> ever introducing new IBP/metadata versions. None of those are
> really
> >> > good
> >> > >> choices.
> >> > >>
> >> > >> > Intuitively, it seems that independent of how a cluster is
> deployed,
> >> > we
> >> > >> > should always pick the same feature version.
> >> > >>
> >> > >> I think it makes sense to draw a distinction between upgrading an
> >> > existing
> >> > >> cluster and deploying a new one. What most people want out of
> upgrades
> >> > is
> >> > >> that things should keep working, but with bug fixes. If we change
> >> that,
> >> > it
> >> > >> just makes people more reluctant to upgrade (which is always a
> >> > problem...)
> >> > >>
> >> > >> > I think we need to think this through in this KIP. My second
> concern
> >> > is
> >> > >> > that as a particular version matures, it's inconvenient for a
> user
> >> to
> >> > >> manually
> >> > >> > upgrade every feature version. As long as we have a path to
> achieve
> >> > that
> >> > >> in
> >> > >> > the future, we don't need to address that in this KIP.
> >> > >>
> >> > >> If people are managing a large number of Kafka clusters, they will
> >> want
> >> > to
> >> > >> do some sort of A/B testing with IBP/metadata versions. So if you
> have
> >> > 1000
> >> > >> Kafka clusters, you roll out the new IBP version to 10 of them and
> see
> >> > how
> >> > >> it goes. If that goes well, you roll it out to more, etc.
> >> > >>
> >> > >> So, the automation needs to be at the cluster management layer,
> not at
> >> > the
> >> > >> Kafka layer. Each Kafka cluster doesn't know how well things went
> in
> >> the
> >> > >> other 999 clusters. Automatically upgrading is a bad thing for the
> >> same
> >> > >> reason Kafka automatically upgrading its own software version would
> >> be a
> >> > >> bad thing -- it could lead to a disruption to a sensitive cluster
> at
> >> the
> >> > >> wrong time.
> >> > >>
> >> > >> For people who are just managing one or two Kafka clusters,
> >> > automatically
> >> > >> upgrading feature versions isn't a big burden and can be done
> >> manually.
> >> > >> This is all consistent with how IBP works today.
> >> > >>
> >> > >> Also, we already have a command-line option to the feature tool
> which
> >> > >> upgrades all features to the latest available, if that is what the
> >> > >> administrator desires. Perhaps we could add documentation saying
> that
> >> > this
> >> > >> should be done as the last step of the upgrade.
> >> > >>
> >> > >> best,
> >> > >> Colin
> >> > >>
> >> > >> >
> >> > >> > 21. "./kafka-features.sh delete": Deleting a feature seems a bit
> >> weird
> >> > >> > since the logic is always there. Would it be better to use
> disable?
> >> > >> >
> >> > >> > Jun
> >> > >> >
> >> > >> > On Fri, Nov 5, 2021 at 8:11 AM David Arthur
> >> > >> > <david.art...@confluent.io.invalid> wrote:
> >> > >> >
> >> > >> >> Colin and Jun, thanks for the additional comments!
> >> > >> >>
> >> > >> >> Colin:
> >> > >> >>
> >> > >> >> > We've been talking about having an automated RPC compatibility
> >> > checker
> >> > >> >>
> >> > >> >> Do we have a way to mark fields in schemas as deprecated? It can
> >> > stay in
> >> > >> >> the RPC, it just complicates the logic a bit.
> >> > >> >>
> >> > >> >> > It would be nice if the active controller could validate that
> a
> >> > >> majority
> >> > >> >> of the quorum could use the proposed metadata.version. The
> active
> >> > >> >> controller should have this information, right? If we don't have
> >> > recent
> >> > >> >> information from a quorum of voters, we wouldn't be active.
> >> > >> >>
> >> > >> >> I believe we should have this information from the
> >> > ApiVersionsResponse.
> >> > >> It
> >> > >> >> would be good to do this validation to avoid a situation where a
> >> > >> >> quorum leader can't be elected due to unprocessable records.
> >> > >> >>
> >> > >> >> > Do we need delete as a command separate from downgrade?
> >> > >> >>
> >> > >> >> I think from an operator's perspective, it is nice to
> distinguish
> >> > >> between
> >> > >> >> changing a feature flag and unsetting it. It might be
> surprising to
> >> > an
> >> > >> >> operator to see the flag's version set to nothing when they
> >> requested
> >> > >> the
> >> > >> >> downgrade to version 0 (or less).
> >> > >> >>
> >> > >> >> > it seems like we should spell out that metadata.version
> begins at
> >> > 1 in
> >> > >> >> KRaft clusters
> >> > >> >>
> >> > >> >> I added this text:
> >> > >> >>
> >> > >> >> Introduce an IBP version to indicate the lowest software version
> >> that
> >> > >> >> > supports *metadata.version*. Below this IBP, the
> >> > *metadata.version* is
> >> > >> >> > undefined and will not be examined. At or above this IBP, the
> >> > >> >> > *metadata.version* must be *0* for ZooKeeper clusters and
> will be
> >> > >> >> > initialized as *1* for KRaft clusters.
> >> > >> >>
> >> > >> >>
> >> > >> >> > We probably also want an RPC implemented by both brokers and
> >> > >> controllers
> >> > >> >> that will reveal the min and max supported versions for each
> >> feature
> >> > >> level
> >> > >> >> supported by the server
> >> > >> >>
> >> > >> >> This is available in ApiVersionsResponse (we include the
> server's
> >> > >> supported
> >> > >> >> features as well as the cluster's finalized features)
> >> > >> >>
> >> > >> >> --------
> >> > >> >>
> >> > >> >> Jun:
> >> > >> >>
> >> > >> >> 12. I've updated the KIP with AdminClient changes
> >> > >> >>
> >> > >> >> 14. You're right, it looks like I missed a few sections
> regarding
> >> > >> snapshot
> >> > >> >> generation. I've corrected it
> >> > >> >>
> >> > >> >> 16. This feels more like an enhancement to KIP-584. I agree it
> >> could
> >> > be
> >> > >> >> useful, but perhaps we could address it separately from KRaft
> >> > upgrades?
> >> > >> >>
> >> > >> >> 20. Indeed snapshots are not strictly necessary during an
> upgrade,
> >> > I've
> >> > >> >> reworded this
> >> > >> >>
> >> > >> >>
> >> > >> >> Thanks!
> >> > >> >> David
> >> > >> >>
> >> > >> >>
> >> > >> >> On Thu, Nov 4, 2021 at 6:51 PM Jun Rao <j...@confluent.io.invalid
> >
> >> > >> wrote:
> >> > >> >>
> >> > >> >> > Hi, David, Jose and Colin,
> >> > >> >> >
> >> > >> >> > Thanks for the reply. A few more comments.
> >> > >> >> >
> >> > >> >> > 12. It seems that we haven't updated the AdminClient
> accordingly?
> >> > >> >> >
> >> > >> >> > 14. "Metadata snapshot is generated and sent to the other
> >> inactive
> >> > >> >> > controllers and to brokers". I thought we wanted each broker
> to
> >> > >> generate
> >> > >> >> > its own snapshot independently? If only the controller
> generates
> >> > the
> >> > >> >> > snapshot, how do we force other brokers to pick it up?
> >> > >> >> >
> >> > >> >> > 16. If a feature version is new, one may not want to enable it
> >> > >> >> immediately
> >> > >> >> > after the cluster is upgraded. However, if a feature version
> has
> >> > been
> >> > >> >> > stable, requiring every user to run a command to upgrade to
> that
> >> > >> version
> >> > >> >> > seems inconvenient. One way to improve this is for each
> feature
> >> to
> >> > >> define
> >> > >> >> > one version as the default. Then, when we upgrade a cluster,
> we
> >> > will
> >> > >> >> > automatically upgrade the feature to the default version. An
> >> admin
> >> > >> could
> >> > >> >> > use the tool to upgrade to a version higher than the default.
> >> > >> >> >
> >> > >> >> > 20. "The quorum controller can assist with this process by
> >> > generating
> >> > >> a
> >> > >> >> > metadata snapshot after a metadata.version increase has been
> >> > >> committed to
> >> > >> >> > the metadata log. This snapshot will be a convenient way to
> let
> >> > broker
> >> > >> >> and
> >> > >> >> > controller components rebuild their entire in-memory state
> >> > following
> >> > >> an
> >> > >> >> > upgrade." The new version of the software could read both the
> new
> >> > and
> >> > >> the
> >> > >> >> > old version. Is generating a new snapshot during upgrade
> needed?
> >> > >> >> >
> >> > >> >> > Jun
> >> > >> >> >
> >> > >> >> >
> >> > >> >> > On Wed, Nov 3, 2021 at 5:42 PM Colin McCabe <
> cmcc...@apache.org>
> >> > >> wrote:
> >> > >> >> >
> >> > >> >> > > On Tue, Oct 12, 2021, at 10:34, Jun Rao wrote:
> >> > >> >> > > > Hi, David,
> >> > >> >> > > >
> >> > >> >> > > > One more comment.
> >> > >> >> > > >
> >> > >> >> > > > 16. The main reason why KIP-584 requires finalizing a
> feature
> >> > >> >> manually
> >> > >> >> > is
> >> > >> >> > > > that in the ZK world, the controller doesn't know all
> brokers
> >> > in a
> >> > >> >> > > cluster.
> >> > >> >> > > > A broker temporarily down is not registered in ZK. in the
> >> KRaft
> >> > >> >> world,
> >> > >> >> > > the
> >> > >> >> > > > controller keeps track of all brokers, including those
> that
> >> are
> >> > >> >> > > temporarily
> >> > >> >> > > > down. This makes it possible for the controller to
> >> > automatically
> >> > >> >> > > finalize a
> >> > >> >> > > > feature---it's safe to do so when all brokers support that
> >> > >> feature.
> >> > >> >> > This
> >> > >> >> > > > will make the upgrade process much simpler since no manual
> >> > >> command is
> >> > >> >> > > > required to turn on a new feature. Have we considered
> this?
> >> > >> >> > > >
> >> > >> >> > > > Thanks,
> >> > >> >> > > >
> >> > >> >> > > > Jun
> >> > >> >> > >
> >> > >> >> > > Hi Jun,
> >> > >> >> > >
> >> > >> >> > > I guess David commented on this point already, but I'll
> comment
> >> > as
> >> > >> >> well.
> >> > >> >> > I
> >> > >> >> > > always had the perception that users viewed rolls as
> >> potentially
> >> > >> risky
> >> > >> >> > and
> >> > >> >> > > were looking for ways to reduce the risk. Not enabling
> features
> >> > >> right
> >> > >> >> > away
> >> > >> >> > > after installing new software seems like one way to do
> that. If
> >> > we
> >> > >> had
> >> > >> >> a
> >> > >> >> > > feature to automatically upgrade during a roll, I'm not sure
> >> > that I
> >> > >> >> would
> >> > >> >> > > recommend that people use it, because if something fails, it
> >> > makes
> >> > >> it
> >> > >> >> > > harder to tell if the new feature is at fault, or something
> >> else
> >> > in
> >> > >> the
> >> > >> >> > new
> >> > >> >> > > software.
> >> > >> >> > >
> >> > >> >> > > We already tell users to do a "double roll" when going to a
> new
> >> > IBP.
> >> > >> >> > (Just
> >> > >> >> > > to give background to people who haven't heard that phrase,
> the
> >> > >> first
> >> > >> >> > roll
> >> > >> >> > > installs the new software, and the second roll updates the
> >> IBP).
> >> > So
> >> > >> >> this
> >> > >> >> > > KIP-778 mechanism is basically very similar to that, except
> the
> >> > >> second
> >> > >> >> > > thing isn't a roll, but just an upgrade command. So I think
> >> this
> >> > is
> >> > >> >> > > consistent with what we currently do.
> >> > >> >> > >
> >> > >> >> > > Also, just like David said, we can always add auto-upgrade
> >> later
> >> > if
> >> > >> >> there
> >> > >> >> > > is demand...
> >> > >> >> > >
> >> > >> >> > > best,
> >> > >> >> > > Colin
> >> > >> >> > >
> >> > >> >> > >
> >> > >> >> > > >
> >> > >> >> > > > On Thu, Oct 7, 2021 at 5:19 PM Jun Rao <j...@confluent.io>
> >> > wrote:
> >> > >> >> > > >
> >> > >> >> > > >> Hi, David,
> >> > >> >> > > >>
> >> > >> >> > > >> Thanks for the KIP. A few comments below.
> >> > >> >> > > >>
> >> > >> >> > > >> 10. It would be useful to describe how the controller
> node
> >> > >> >> determines
> >> > >> >> > > the
> >> > >> >> > > >> RPC version used to communicate to other controller
> nodes.
> >> > There
> >> > >> >> seems
> >> > >> >> > > to
> >> > >> >> > > >> be a bootstrap problem. A controller node can't read the
> log
> >> > and
> >> > >> >> > > >> therefore the feature level until a quorum leader is
> >> elected.
> >> > But
> >> > >> >> > leader
> >> > >> >> > > >> election requires an RPC.
> >> > >> >> > > >>
> >> > >> >> > > >> 11. For downgrades, it would be useful to describe how to
> >> > >> determine
> >> > >> >> > the
> >> > >> >> > > >> downgrade process (generating new snapshot, propagating
> the
> >> > >> >> snapshot,
> >> > >> >> > > etc)
> >> > >> >> > > >> has completed. We could block the UpdateFeature request
> >> until
> >> > the
> >> > >> >> > > process
> >> > >> >> > > >> is completed. However, since the process could take time,
> >> the
> >> > >> >> request
> >> > >> >> > > could
> >> > >> >> > > >> time out. Another way is through DescribeFeature and the
> >> > server
> >> > >> only
> >> > >> >> > > >> reports downgraded versions after the process is
> completed.
> >> > >> >> > > >>
> >> > >> >> > > >> 12. Since we are changing UpdateFeaturesRequest, do we
> need
> >> to
> >> > >> >> change
> >> > >> >> > > the
> >> > >> >> > > >> AdminClient api for updateFeatures too?
> >> > >> >> > > >>
> >> > >> >> > > >> 13. For the paragraph starting with "In the absence of an
> >> > >> operator
> >> > >> >> > > >> defined value for metadata.version", in KIP-584, we
> >> described
> >> > >> how to
> >> > >> >> > > >> finalize features with New cluster bootstrap. In that
> case,
> >> > it's
> >> > >> >> > > >> inconvenient for the users to have to run an admin tool
> to
> >> > >> finalize
> >> > >> >> > the
> >> > >> >> > > >> version for each feature. Instead, the system detects
> that
> >> the
> >> > >> >> > /features
> >> > >> >> > > >> path is missing in ZK and thus automatically finalizes
> every
> >> > >> feature
> >> > >> >> > > with
> >> > >> >> > > >> the latest supported version. Could we do something
> similar
> >> in
> >> > >> the
> >> > >> >> > KRaft
> >> > >> >> > > >> mode?
> >> > >> >> > > >>
> >> > >> >> > > >> 14. After the quorum leader generates a new snapshot,
> how do
> >> > we
> >> > >> >> force
> >> > >> >> > > >> other nodes to pick up the new snapshot?
> >> > >> >> > > >>
> >> > >> >> > > >> 15. I agree with Jose that it will be useful to describe
> >> when
> >> > >> >> > > generating a
> >> > >> >> > > >> new snapshot is needed. To me, it seems the new snapshot
> is
> >> > only
> >> > >> >> > needed
> >> > >> >> > > >> when incompatible changes are made.
> >> > >> >> > > >>
> >> > >> >> > > >> 7. Jose, what control records were you referring?
> >> > >> >> > > >>
> >> > >> >> > > >> Thanks,
> >> > >> >> > > >>
> >> > >> >> > > >> Jun
> >> > >> >> > > >>
> >> > >> >> > > >>
> >> > >> >> > > >> On Tue, Oct 5, 2021 at 8:53 AM David Arthur <
> >> > >> davidart...@apache.org
> >> > >> >> >
> >> > >> >> > > >> wrote:
> >> > >> >> > > >>
> >> > >> >> > > >>> Jose, thanks for the thorough review and comments!
> >> > >> >> > > >>>
> >> > >> >> > > >>> I am out of the office until next week, so I probably
> won't
> >> > be
> >> > >> able
> >> > >> >> > to
> >> > >> >> > > >>> update the KIP until then. Here are some replies to your
> >> > >> questions:
> >> > >> >> > > >>>
> >> > >> >> > > >>> 1. Generate snapshot on upgrade
> >> > >> >> > > >>> > > Metadata snapshot is generated and sent to the other
> >> > nodes
> >> > >> >> > > >>> > Why does the Active Controller need to generate a new
> >> > snapshot
> >> > >> >> and
> >> > >> >> > > >>> > force a snapshot fetch from the replicas (inactive
> >> > controller
> >> > >> and
> >> > >> >> > > >>> > brokers) on an upgrade? Isn't writing the
> >> > FeatureLevelRecord
> >> > >> good
> >> > >> >> > > >>> > enough to communicate the upgrade to the replicas?
> >> > >> >> > > >>>
> >> > >> >> > > >>>
> >> > >> >> > > >>> You're right, we don't necessarily need to _transmit_ a
> >> > >> snapshot,
> >> > >> >> > since
> >> > >> >> > > >>> each node can generate its own equivalent snapshot
> >> > >> >> > > >>>
> >> > >> >> > > >>> 2. Generate snapshot on downgrade
> >> > >> >> > > >>> > > Metadata snapshot is generated and sent to the other
> >> > >> inactive
> >> > >> >> > > >>> > controllers and to brokers (this snapshot may be
> lossy!)
> >> > >> >> > > >>> > Why do we need to send this downgraded snapshot to the
> >> > >> brokers?
> >> > >> >> The
> >> > >> >> > > >>> > replicas have seen the FeatureLevelRecord and noticed
> the
> >> > >> >> > downgrade.
> >> > >> >> > > >>> > Can we have the replicas each independently generate a
> >> > >> downgraded
> >> > >> >> > > >>> > snapshot at the offset for the downgraded
> >> > FeatureLevelRecord?
> >> > >> I
> >> > >> >> > > assume
> >> > >> >> > > >>> > that the active controller will guarantee that all
> >> records
> >> > >> after
> >> > >> >> > the
> >> > >> >> > > >>> > FatureLevelRecord use the downgraded version. If so,
> it
> >> > would
> >> > >> be
> >> > >> >> > good
> >> > >> >> > > >>> > to mention that explicitly.
> >> > >> >> > > >>>
> >> > >> >> > > >>>
> >> > >> >> > > >>> Similar to above, yes a broker that detects a downgrade
> via
> >> > >> >> > > >>> FeatureLevelRecord could generate its own downgrade
> >> snapshot
> >> > and
> >> > >> >> > reload
> >> > >> >> > > >>> its
> >> > >> >> > > >>> state from that. This does get a little fuzzy when we
> >> > consider
> >> > >> >> cases
> >> > >> >> > > where
> >> > >> >> > > >>> brokers are on different software versions and could be
> >> > >> generating
> >> > >> >> a
> >> > >> >> > > >>> downgrade snapshot for version X, but using different
> >> > versions
> >> > >> of
> >> > >> >> the
> >> > >> >> > > >>> code.
> >> > >> >> > > >>> It might be safer to let the controller generate the
> >> > snapshot so
> >> > >> >> each
> >> > >> >> > > >>> broker (regardless of software version) gets the same
> >> > records.
> >> > >> >> > However,
> >> > >> >> > > >>> for
> >> > >> >> > > >>> upgrades (or downgrades) we expect the whole cluster to
> be
> >> > >> running
> >> > >> >> > the
> >> > >> >> > > >>> same
> >> > >> >> > > >>> software version before triggering the metadata.version
> >> > change,
> >> > >> so
> >> > >> >> > > perhaps
> >> > >> >> > > >>> this isn't a likely scenario. Thoughts?
> >> > >> >> > > >>>
> >> > >> >> > > >>>
> >> > >> >> > > >>> 3. Max metadata version
> >> > >> >> > > >>> > >For the first release that supports
> metadata.version, we
> >> > can
> >> > >> >> > simply
> >> > >> >> > > >>> > initialize metadata.version with the current (and
> only)
> >> > >> version.
> >> > >> >> > For
> >> > >> >> > > >>> future
> >> > >> >> > > >>> > releases, we will need a mechanism to bootstrap a
> >> > particular
> >> > >> >> > version.
> >> > >> >> > > >>> This
> >> > >> >> > > >>> > could be done using the meta.properties file or some
> >> > similar
> >> > >> >> > > mechanism.
> >> > >> >> > > >>> The
> >> > >> >> > > >>> > reason we need the allow for a specific initial
> version
> >> is
> >> > to
> >> > >> >> > support
> >> > >> >> > > >>> the
> >> > >> >> > > >>> > use case of starting a Kafka cluster at version X
> with an
> >> > >> older
> >> > >> >> > > >>> > metadata.version.
> >> > >> >> > > >>>
> >> > >> >> > > >>>
> >> > >> >> > > >>> I assume that the Active Controller will learn the
> metadata
> >> > >> version
> >> > >> >> > of
> >> > >> >> > > >>> > the broker through the BrokerRegistrationRequest. How
> >> will
> >> > the
> >> > >> >> > Active
> >> > >> >> > > >>> > Controller learn about the max metadata version of the
> >> > >> inactive
> >> > >> >> > > >>> > controller nodes? We currently don't send a
> registration
> >> > >> request
> >> > >> >> > from
> >> > >> >> > > >>> > the inactive controller to the active controller.
> >> > >> >> > > >>>
> >> > >> >> > > >>>
> >> > >> >> > > >>> This came up during the design, but I neglected to add
> it
> >> to
> >> > the
> >> > >> >> KIP.
> >> > >> >> > > We
> >> > >> >> > > >>> will need a mechanism for determining the supported
> >> features
> >> > of
> >> > >> >> each
> >> > >> >> > > >>> controller similar to how brokers use
> >> > BrokerRegistrationRequest.
> >> > >> >> > > Perhaps
> >> > >> >> > > >>> controllers could write a FeatureLevelRecord (or
> similar)
> >> to
> >> > the
> >> > >> >> > > metadata
> >> > >> >> > > >>> log indicating their supported version. WDYT?
> >> > >> >> > > >>>
> >> > >> >> > > >>> Why do you need to bootstrap a particular version? Isn't
> >> the
> >> > >> intent
> >> > >> >> > > >>> > that the broker will learn the active metadata
> version by
> >> > >> reading
> >> > >> >> > the
> >> > >> >> > > >>> > metadata before unfencing?
> >> > >> >> > > >>>
> >> > >> >> > > >>>
> >> > >> >> > > >>> This bootstrapping is needed for when a KRaft cluster is
> >> > first
> >> > >> >> > > started. If
> >> > >> >> > > >>> we don't have this mechanism, the cluster can't really
> do
> >> > >> anything
> >> > >> >> > > until
> >> > >> >> > > >>> the operator finalizes the metadata.version with the
> tool.
> >> > The
> >> > >> >> > > >>> bootstrapping will be done by the controller and the
> >> brokers
> >> > >> will
> >> > >> >> see
> >> > >> >> > > this
> >> > >> >> > > >>> version as a record (like you say). I'll add some text
> to
> >> > >> clarify
> >> > >> >> > this.
> >> > >> >> > > >>>
> >> > >> >> > > >>>
> >> > >> >> > > >>> 4. Reject Registration - This is related to the bullet
> >> point
> >> > >> above.
> >> > >> >> > > >>> > What will be the behavior of the active controller if
> the
> >> > >> broker
> >> > >> >> > > sends
> >> > >> >> > > >>> > a metadata version that is not compatible with the
> >> cluster
> >> > >> wide
> >> > >> >> > > >>> > metadata version?
> >> > >> >> > > >>>
> >> > >> >> > > >>>
> >> > >> >> > > >>> If a broker starts up with a lower supported version
> range
> >> > than
> >> > >> the
> >> > >> >> > > >>> current
> >> > >> >> > > >>> cluster metadata.version, it should log an error and
> >> > shutdown.
> >> > >> This
> >> > >> >> > is
> >> > >> >> > > in
> >> > >> >> > > >>> line with KIP-584.
> >> > >> >> > > >>>
> >> > >> >> > > >>> 5. Discover upgrade
> >> > >> >> > > >>> > > This snapshot will be a convenient way to let broker
> >> and
> >> > >> >> > controller
> >> > >> >> > > >>> > components rebuild their entire in-memory state
> following
> >> > an
> >> > >> >> > upgrade.
> >> > >> >> > > >>> > Can we rely on the presence of the FeatureLevelRecord
> for
> >> > the
> >> > >> >> > > metadata
> >> > >> >> > > >>> > version for this functionality? If so, it avoids
> having
> >> to
> >> > >> reload
> >> > >> >> > the
> >> > >> >> > > >>> > snapshot.
> >> > >> >> > > >>>
> >> > >> >> > > >>>
> >> > >> >> > > >>> For upgrades, yes probably since we won't need to
> "rewrite"
> >> > any
> >> > >> >> > > records in
> >> > >> >> > > >>> this case. For downgrades, we will need to generate the
> >> > snapshot
> >> > >> >> and
> >> > >> >> > > >>> reload
> >> > >> >> > > >>> everything.
> >> > >> >> > > >>>
> >> > >> >> > > >>> 6. Metadata version specification
> >> > >> >> > > >>> > > V4(version=4, isBackwardsCompatible=false,
> >> > description="New
> >> > >> >> > > metadata
> >> > >> >> > > >>> > record type Bar"),
> >> > >> >> > > >>>
> >> > >> >> > > >>>
> >> > >> >> > > >>> Very cool. Do you have plans to generate Apache Kafka
> HTML
> >> > >> >> > > >>> > documentation for this information? Would be helpful
> to
> >> > >> display
> >> > >> >> > this
> >> > >> >> > > >>> > information to the user using the kafka-features.sh
> and
> >> > >> feature
> >> > >> >> > RPC?
> >> > >> >> > > >>>
> >> > >> >> > > >>>
> >> > >> >> > > >>> Hm good idea :) I'll add a brief section on
> documentation.
> >> > This
> >> > >> >> would
> >> > >> >> > > >>> certainly be very useful
> >> > >> >> > > >>>
> >> > >> >> > > >>> 7.Downgrade records
> >> > >> >> > > >>> > I think we should explicitly mention that the
> downgrade
> >> > >> process
> >> > >> >> > will
> >> > >> >> > > >>> > downgrade both metadata records and controller
> records.
> >> In
> >> > >> >> KIP-630
> >> > >> >> > we
> >> > >> >> > > >>> > introduced two control records for snapshots.
> >> > >> >> > > >>>
> >> > >> >> > > >>>
> >> > >> >> > > >>> Yes, good call. Let me re-read that KIP and include some
> >> > >> details.
> >> > >> >> > > >>>
> >> > >> >> > > >>> Thanks again for the comments!
> >> > >> >> > > >>> -David
> >> > >> >> > > >>>
> >> > >> >> > > >>>
> >> > >> >> > > >>> On Wed, Sep 29, 2021 at 5:09 PM José Armando García
> Sancio
> >> > >> >> > > >>> <jsan...@confluent.io.invalid> wrote:
> >> > >> >> > > >>>
> >> > >> >> > > >>> > One more comment.
> >> > >> >> > > >>> >
> >> > >> >> > > >>> > 7.Downgrade records
> >> > >> >> > > >>> > I think we should explicitly mention that the
> downgrade
> >> > >> process
> >> > >> >> > will
> >> > >> >> > > >>> > downgrade both metadata records and controller
> records.
> >> In
> >> > >> >> KIP-630
> >> > >> >> > we
> >> > >> >> > > >>> > introduced two control records for snapshots.
> >> > >> >> > > >>> >
> >> > >> >> > > >>>
> >> > >> >> > > >>
> >> > >> >> > >
> >> > >> >> >
> >> > >> >>
> >> > >> >>
> >> > >> >> --
> >> > >> >> -David
> >> > >> >>
> >> > >>
> >> >
> >>
>
