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From Joel Koshy <jjkosh...@gmail.com>
Subject Re: [DISCUSS] Client-side Assignment for New Consumer
Date Fri, 28 Aug 2015 20:24:14 GMT
Another use-case I was thinking of was something like rack-aware
assignment of partitions to clients. This would require some
additional topic metadata to be propagated to and from the coordinator
and you would need some way to resolve conflicts for such strategies.
I think that could be addressed by attaching a generation id to the
metadata and use that (i.e., pick the highest) in order to resolve
conflicts without another round of join-group requests.

Likewise, without delete/recreate, partition counts are a sort of
generation id since they are non-decreasing. If we need to account for
delete/recreate that could perhaps be addressed by an explicit
(per-topic) generation id attached to each topic in the metadata blob.
Does that make sense? I think that covers my concerns wrt the
split-brain issues.

I'm still a bit wary of the n^2*m sized rebroadcast of all the
metadata - mainly because for various reasons at LinkedIn, we are
actually using large explicit whitelists (and not wildcards) in
several of our mirroring pipelines. At this point I feel that is a
reasonable cost to pay for having all the logic in one place (i.e.,
client side) but I would like to think a bit more on that.

Joel


On Fri, Aug 28, 2015 at 1:02 PM, Onur Karaman
<okaraman@linkedin.com.invalid> wrote:
> From what I understand, the "largest number of partitions" trick is based
> on the assumption that topics can only expand their partitions. What
> happens when a topic gets deleted and recreated? This breaks that
> assumption.
>
> On Fri, Aug 28, 2015 at 6:33 AM, Neha Narkhede <neha@confluent.io> wrote:
>
>> Thanks for re-reviewing Joel.
>>
>>
>>
>>
>>
>>
>> On Fri, Aug 28, 2015 at 2:51 AM -0700, "Joel Koshy" <jjkoshy.w@gmail.com>
>> wrote:
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> > I think we think this proposal addresses 100% of the split brain issues
>> > ever seen in the ZK-based protocol, but I think you think there are still
>> > issues. Can you explain what your thinking of and when you think it would
>> > happen? I want to make sure you aren't assuming client-side=>split-brain
>> > since I think that is totally not the case.
>>
>> Yes I had concluded that client-side assignment would still result in
>> split-brain wrt partition counts, but I overlooked a key sentence in
>> the wiki - i.e., that the assignment algorithm for consumers can just
>> use the largest number of partitions for each topic reported by any of
>> the consumers. i.e., I assumed that consumers would just fail
>> rebalance if the partition counts were inconsistent but that is not
>> the case since this conflict can be easily resolved as described
>> without further join-group requests. Sorry about that. There is still
>> the issue of the coordinator having to send back n*m worth of
>> metadata, but that was not my biggest concern. I'll look over it again
>> and reply back tomorrow.
>>
>> Joel
>>
>> On Thu, Aug 27, 2015 at 2:55 PM, Jay Kreps  wrote:
>> > Hey Joel,
>> >
>> > I really don't think we should do both. There are pros and cons but we
>> > should make a decision and work on operationalizing one approach. Much of
>> > really making something like this work is getting all the bugs out,
>> getting
>> > monitoring in place, getting rigorous system tests in place. Trying to do
>> > those things twice with the same resources will just mean we do them half
>> > as well. I also think this buys nothing from the user's point of
>> view--they
>> > want co-ordination that works correctly, the debate we are having is
>> purely
>> > a "how should we build that" debate. So this is really not the kind of
>> > thing we'd want to make pluggable and if we did that would just
>> complicate
>> > life for the user.
>> >
>> > I think we think this proposal addresses 100% of the split brain issues
>> > ever seen in the ZK-based protocol, but I think you think there are still
>> > issues. Can you explain what your thinking of and when you think it would
>> > happen? I want to make sure you aren't assuming client-side=>split-brain
>> > since I think that is totally not the case.
>> >
>> > With respect to "herd issues" I actually think all the proposals address
>> > this by scaling the co-ordinator out to all nodes and making the
>> > co-ordination vastly cheaper. No proposal, of course, gets rid of the
>> fact
>> > that all clients rejoin at once when there is a membership change, but
>> that
>> > is kind of fundamental to the problem.
>> >
>> > -Jay
>> >
>> > On Thu, Aug 27, 2015 at 2:02 PM, Joel Koshy  wrote:
>> >
>> >> I actually feel these set of tests (whatever they may be) are somewhat
>> >> irrelevant here. My main concern with the current client-side proposal
>> >> (i.e., without Becket's follow-up suggestions) is that it makes a
>> >> significant compromise to the original charter of the new consumer -
>> >> i.e., reduce/eliminate herd and split brain problems in both group
>> >> management and partition assignment. I understand the need for
>> >> client-side partition assignment in some use cases (which we are also
>> >> interested in), but I also think we should make every effort to keep
>> >> full server-side coordination for the remaining (majority) of use
>> >> cases especially if it does not complicate the protocol. The proposed
>> >> changes do not complicate the protocol IMO - i.e., there is no further
>> >> modification to the request/response formats beyond the current
>> >> client-side proposal. It only involves a trivial reinterpretation of
>> >> the content of the protocol metadata field.
>> >>
>> >> Joel
>> >>
>> >> On Wed, Aug 26, 2015 at 9:33 PM, Neha Narkhede  wrote:
>> >> > Hey Becket,
>> >> >
>> >> > In that case, the broker side partition assignment would be ideal
>> because
>> >> >> it avoids
>> >> >> issues like metadata inconsistency / split brain / exploding
>> >> subscription
>> >> >> set propagation.
>> >> >
>> >> >
>> >> > As per our previous discussions regarding each of those concerns
>> >> (referring
>> >> > to this email thread, KIP calls and JIRA comments), we are going to
>> run a
>> >> > set of tests using the LinkedIn deployment numbers that we will wait
>> for
>> >> > you to share. The purpose is to see if those concerns are really
>> valid or
>> >> > not. I'd prefer to see that before making any more changes that will
>> >> > complicate the protocol.
>> >> >
>> >> > On Wed, Aug 26, 2015 at 4:57 PM, Jiangjie Qin > >
>> >> > wrote:
>> >> >
>> >> >> Hi folks,
>> >> >>
>> >> >> After further discussion in LinkedIn, we found that while having
a
>> more
>> >> >> general group management protocol is very useful, the vast majority
>> of
>> >> the
>> >> >> clients will not use customized partition assignment strategy.
In
>> that
>> >> >> case, the broker side partition assignment would be ideal because
it
>> >> avoids
>> >> >> issues like metadata inconsistency / split brain / exploding
>> >> subscription
>> >> >> set propagation.
>> >> >>
>> >> >> So we have the following proposal that satisfies the majority of
the
>> >> >> clients' needs without changing the currently proposed binary
>> protocol.
>> >> >> i.e., Continue to support broker-side assignment if the assignment
>> >> strategy
>> >> >> is recognized by the coordinator.
>> >> >>
>> >> >> 1. Keep the binary protocol as currently proposed.
>> >> >>
>> >> >> 2. Change the way we interpret ProtocolMetadata:
>> >> >> 2.1 On consumer side, change partition.assignment.strategy to
>> >> >> partition.assignor.class. Implement the something like the following
>> >> >> PartitionAssignor Interface:
>> >> >>
>> >> >> public interface PartitionAssignor {
>> >> >>   List protocolTypes();
>> >> >>   byte[] protocolMetadata();
>> >> >>   // return the Topic->List map that are assigned to this
>> >> >> consumer.
>> >> >>   List assignPartitions(String protocolType, byte[]
>> >> >> responseProtocolMetadata);
>> >> >> }
>> >> >>
>> >> >> public abstract class AbstractPartitionAssignor implements
>> >> >> PartitionAssignor {
>> >> >>   protected final KafkaConsumer consumer;
>> >> >>   AbstractPartitionAssignor(KafkaConsumer consumer) {
>> >> >>     this.consumer = consumer;
>> >> >>   }
>> >> >> }
>> >> >>
>> >> >> 2.2 The ProtocolMetadata in JoinGroupRequest will be
>> >> >> partitionAssignor.protocolMetadata(). When partition.assignor.class
>> is
>> >> >> "range" or "roundrobin", the ProtocolMetadata in JoinGroupRequest
>> will
>> >> be a
>> >> >> JSON subscription set. ("range", "roundrobin" will be reserved
>> words, we
>> >> >> can also consider reserving some Prefix such as "broker-" to be
more
>> >> clear)
>> >> >> 2.3 On broker side when ProtocolType is "range" or "roundroubin",
>> >> >> coordinator will parse the ProtocolMetadata in the JoinGroupRequest
>> and
>> >> >> assign the partitions for consumers. In the JoinGroupResponse,
the
>> >> >> ProtocolMetadata will be the global assignment of partitions.
>> >> >> 2.4 On client side, after receiving the JoinGroupResponse,
>> >> >> partitionAssignor.assignPartitions() will be invoked to return
the
>> >> actual
>> >> >> assignment. If the assignor is RangeAssignor or RoundRobinAssignor,
>> they
>> >> >> will parse the assignment from the ProtocolMetadata returned by
>> >> >> coordinator.
>> >> >>
>> >> >> This approach has a few merits:
>> >> >> 1. Does not change the proposed binary protocol, which is still
>> general.
>> >> >> 2. The majority of the consumers will not suffer from inconsistent
>> >> metadata
>> >> >> / split brain / exploding subscription set propagation. This is
>> >> >> specifically to deal with the issue that the current proposal caters
>> to
>> >> a
>> >> >> 20% use-case while adversely impacting the more common 80% use-cases.
>> >> >> 3. Easy to implement. The only thing needed is implement a
>> partitioner
>> >> >> class. For most users, the default range and roundrobin partitioner
>> are
>> >> >> good enough.
>> >> >>
>> >> >> Thoughts?
>> >> >>
>> >> >> Thanks,
>> >> >>
>> >> >> Jiangjie (Becket) Qin
>> >> >>
>> >> >> On Tue, Aug 18, 2015 at 2:51 PM, Jason Gustafson
>> >> >> wrote:
>> >> >>
>> >> >> > Follow-up from the kip call:
>> >> >> >
>> >> >> > 1. Onur brought up the question of whether this protocol provides
>> >> enough
>> >> >> > coordination capabilities to be generally useful in practice
(is
>> that
>> >> >> > accurate, Onur?). If it doesn't, then each use case would
probably
>> >> need a
>> >> >> > dependence on zookeeper anyway, and we haven't really gained
>> anything.
>> >> >> The
>> >> >> > group membership provided by this protocol is a useful primitive
>> for
>> >> >> > coordination, but it's limited in the sense that everything
shared
>> >> among
>> >> >> > the group has to be communicated at the time the group is
created.
>> If
>> >> any
>> >> >> > shared data changes, then the only way the group can ensure
>> agreement
>> >> is
>> >> >> to
>> >> >> > force a rebalance. This is expensive since all members must
stall
>> >> while
>> >> >> the
>> >> >> > rebalancing takes place. As we have also seen, there is a
practical
>> >> limit
>> >> >> > on the amount of metadata that can be sent through this protocol
>> when
>> >> >> > groups get a little larger. This protocol is therefore not
>> suitable to
>> >> >> > cases which require frequent communication or which require
a large
>> >> >> amount
>> >> >> > of data to be communicated. For the use cases listed on the
wiki,
>> >> neither
>> >> >> > of these appear to be an issue, but there may be other limitations
>> >> which
>> >> >> > would limit reuse of the protocol. Perhaps it would be sufficient
>> to
>> >> >> sketch
>> >> >> > how these cases might work?
>> >> >> >
>> >> >> > 2. We talked a little bit about the issue of metadata churn.
Becket
>> >> >> brought
>> >> >> > up the interesting point that not only do we depend on topic
>> metadata
>> >> >> > changing relatively infrequently, but we also expect timely
>> agreement
>> >> >> among
>> >> >> > the brokers on what that metadata is. To resolve this, we
can have
>> the
>> >> >> > consumers fetch metadata from the coordinator. We still depend
on
>> >> topic
>> >> >> > metadata not changing frequently, but this should resolve
any
>> >> >> disagreement
>> >> >> > among the brokers themselves. In fact, since we expect that
>> >> disagreement
>> >> >> is
>> >> >> > relatively rare, we can have the consumers fetch from the
>> coordinator
>> >> >> only
>> >> >> > when when a disagreement occurs. The nice thing about this
>> proposal is
>> >> >> that
>> >> >> > it doesn't affect the join group semantics, so the coordinator
>> would
>> >> >> remain
>> >> >> > oblivious to the metadata used by the group for agreement.
Also, if
>> >> >> > metadata churn becomes an issue, it might be possible to have
the
>> >> >> > coordinator provide a snapshot for the group to ensure that
a
>> >> generation
>> >> >> > would be able to reach agreement (this would probably require
>> adding
>> >> >> > groupId/generation to the metadata request).
>> >> >> >
>> >> >> > 3. We talked briefly about support for multiple protocols
in the
>> join
>> >> >> group
>> >> >> > request in order to allow changing the assignment strategy
without
>> >> >> > downtime. I think it's a little doubtful that this would get
much
>> use
>> >> in
>> >> >> > practice, but I agree it's a nice option to have on the table.
An
>> >> >> > alternative, for the sake of argument, is to have each member
>> provide
>> >> >> only
>> >> >> > one version of the protocol, and to let the coordinator choose
the
>> >> >> protocol
>> >> >> > with the largest number of supporters. All members which can't
>> support
>> >> >> the
>> >> >> > selected protocol would be kicked out of the group. The drawback
>> in a
>> >> >> > rolling upgrade is that the total capacity of the group would
be
>> >> >> > momentarily halved. It would also be a little tricky to handle
the
>> >> case
>> >> >> of
>> >> >> > retrying when a consumer is kicked out of the group. We wouldn't
>> want
>> >> it
>> >> >> to
>> >> >> > be able to effect a rebalance, for example, if it would just
be
>> kicked
>> >> >> out
>> >> >> > again. That would probably complicate the group management
logic on
>> >> the
>> >> >> > coordinator.
>> >> >> >
>> >> >> >
>> >> >> > Thanks,
>> >> >> > Jason
>> >> >> >
>> >> >> >
>> >> >> > On Tue, Aug 18, 2015 at 11:16 AM, Jiangjie Qin
>> >> > >> >
>> >> >> > wrote:
>> >> >> >
>> >> >> > > Jun,
>> >> >> > >
>> >> >> > > Yes, I agree. If the metadata can be synced quickly there
should
>> >> not be
>> >> >> > an
>> >> >> > > issue. It just occurred to me that there is a proposal
to allow
>> >> >> consuming
>> >> >> > > from followers in ISR, that could potentially cause more
frequent
>> >> >> > metadata
>> >> >> > > change for consumers. Would that be an issue?
>> >> >> > >
>> >> >> > > Thanks,
>> >> >> > >
>> >> >> > > Jiangjie (Becket) Qin
>> >> >> > >
>> >> >> > > On Tue, Aug 18, 2015 at 10:22 AM, Jason Gustafson <
>> >> jason@confluent.io>
>> >> >> > > wrote:
>> >> >> > >
>> >> >> > > > Hi Jun,
>> >> >> > > >
>> >> >> > > > Answers below:
>> >> >> > > >
>> >> >> > > > 1. When there are multiple common protocols in the
>> >> JoinGroupRequest,
>> >> >> > > which
>> >> >> > > > one would the coordinator pick?
>> >> >> > > >
>> >> >> > > > I was intending to use the list to indicate preference.
If all
>> >> group
>> >> >> > > > members support protocols ["A", "B"] in that order,
then we
>> will
>> >> >> choose
>> >> >> > > > "A." If some support ["B", "A"], then we would either
choose
>> >> based on
>> >> >> > > > respective counts or just randomly. The main use
case of
>> >> supporting
>> >> >> the
>> >> >> > > > list is for rolling upgrades when a change is made
to the
>> >> assignment
>> >> >> > > > strategy. In that case, the new assignment strategy
would be
>> >> listed
>> >> >> > first
>> >> >> > > > in the upgraded client. I think it's debatable whether
this
>> >> feature
>> >> >> > would
>> >> >> > > > get much use in practice, so we might consider dropping
it.
>> >> >> > > >
>> >> >> > > > 2. If the protocols don't agree, the group construction
fails.
>> >> What
>> >> >> > > exactly
>> >> >> > > > does it mean? Do we send an error in every JoinGroupResponse
>> and
>> >> >> remove
>> >> >> > > all
>> >> >> > > > members in the group in the coordinator?
>> >> >> > > >
>> >> >> > > > Yes, that is right. It would be handled similarly
to
>> inconsistent
>> >> >> > > > assignment strategies in the current protocol. The
coordinator
>> >> >> returns
>> >> >> > an
>> >> >> > > > error in each join group response, and the client
propagates
>> the
>> >> >> error
>> >> >> > to
>> >> >> > > > the user.
>> >> >> > > >
>> >> >> > > > 3. Consumer embedded protocol: The proposal has
two different
>> >> formats
>> >> >> > of
>> >> >> > > > subscription depending on whether wildcards are
used or not.
>> This
>> >> >> > seems a
>> >> >> > > > bit complicated. Would it be better to always use
the metadata
>> >> hash?
>> >> >> > The
>> >> >> > > > clients know the subscribed topics already. This
way, the
>> client
>> >> code
>> >> >> > > > behaves the same whether wildcards are used or not.
>> >> >> > > >
>> >> >> > > > Yeah, I think this is possible (Neha also suggested
it). I
>> haven't
>> >> >> > > updated
>> >> >> > > > the wiki yet, but the patch I started working on
uses only the
>> >> >> metadata
>> >> >> > > > hash. In the case that an explicit topic list is
provided, the
>> >> hash
>> >> >> > just
>> >> >> > > > covers the metadata for those topics.
>> >> >> > > >
>> >> >> > > >
>> >> >> > > > Thanks,
>> >> >> > > > Jason
>> >> >> > > >
>> >> >> > > >
>> >> >> > > >
>> >> >> > > > On Tue, Aug 18, 2015 at 10:06 AM, Jun Rao
>> >> wrote:
>> >> >> > > >
>> >> >> > > > > Jason,
>> >> >> > > > >
>> >> >> > > > > Thanks for the writeup. A few comments below.
>> >> >> > > > >
>> >> >> > > > > 1. When there are multiple common protocols
in the
>> >> >> JoinGroupRequest,
>> >> >> > > > which
>> >> >> > > > > one would the coordinator pick?
>> >> >> > > > > 2. If the protocols don't agree, the group
construction
>> fails.
>> >> What
>> >> >> > > > exactly
>> >> >> > > > > does it mean? Do we send an error in every
JoinGroupResponse
>> and
>> >> >> > remove
>> >> >> > > > all
>> >> >> > > > > members in the group in the coordinator?
>> >> >> > > > > 3. Consumer embedded protocol: The proposal
has two different
>> >> >> formats
>> >> >> > > of
>> >> >> > > > > subscription depending on whether wildcards
are used or not.
>> >> This
>> >> >> > > seems a
>> >> >> > > > > bit complicated. Would it be better to always
use the
>> metadata
>> >> >> hash?
>> >> >> > > The
>> >> >> > > > > clients know the subscribed topics already.
This way, the
>> client
>> >> >> code
>> >> >> > > > > behaves the same whether wildcards are used
or not.
>> >> >> > > > >
>> >> >> > > > > Jiangjie,
>> >> >> > > > >
>> >> >> > > > > With respect to rebalance churns due to topics
being
>> >> >> created/deleted.
>> >> >> > > > With
>> >> >> > > > > the new consumer, the rebalance can probably
settle within
>> 200ms
>> >> >> when
>> >> >> > > > there
>> >> >> > > > > is a topic change. So, as long as we are not
changing topic
>> more
>> >> >> > than 5
>> >> >> > > > > times per sec, there shouldn't be constant
churns, right?
>> >> >> > > > >
>> >> >> > > > > Thanks,
>> >> >> > > > >
>> >> >> > > > > Jun
>> >> >> > > > >
>> >> >> > > > >
>> >> >> > > > >
>> >> >> > > > > On Tue, Aug 11, 2015 at 1:19 PM, Jason Gustafson
<
>> >> >> jason@confluent.io
>> >> >> > >
>> >> >> > > > > wrote:
>> >> >> > > > >
>> >> >> > > > > > Hi Kafka Devs,
>> >> >> > > > > >
>> >> >> > > > > > One of the nagging issues in the current
design of the new
>> >> >> consumer
>> >> >> > > has
>> >> >> > > > > > been the need to support a variety of
assignment
>> strategies.
>> >> >> We've
>> >> >> > > > > > encountered this in particular in the
design of copycat and
>> >> the
>> >> >> > > > > processing
>> >> >> > > > > > framework (KIP-28). From what I understand,
Samza also has
>> a
>> >> >> number
>> >> >> > > of
>> >> >> > > > > use
>> >> >> > > > > > cases with custom assignment needs. The
new consumer
>> protocol
>> >> >> > > supports
>> >> >> > > > > new
>> >> >> > > > > > assignment strategies by hooking them
into the broker. For
>> >> many
>> >> >> > > > > > environments, this is a major pain and
in some cases, a
>> >> >> > non-starter.
>> >> >> > > It
>> >> >> > > > > > also challenges the validation that the
coordinator can
>> >> provide.
>> >> >> > For
>> >> >> > > > > > example, some assignment strategies call
for partitions to
>> be
>> >> >> > > assigned
>> >> >> > > > > > multiple times, which means that the coordinator
can only
>> >> check
>> >> >> > that
>> >> >> > > > > > partitions have been assigned at least
once.
>> >> >> > > > > >
>> >> >> > > > > > To solve these issues, we'd like to propose
moving
>> assignment
>> >> to
>> >> >> > the
>> >> >> > > > > > client. I've written a wiki which outlines
some protocol
>> >> changes
>> >> >> to
>> >> >> > > > > achieve
>> >> >> > > > > > this:
>> >> >> > > > > >
>> >> >> > > > > >
>> >> >> > > > >
>> >> >> > > >
>> >> >> > >
>> >> >> >
>> >> >>
>> >>
>> https://cwiki.apache.org/confluence/display/KAFKA/Kafka+Client-side+Assignment+Proposal
>> >> >> > > > > > .
>> >> >> > > > > > To summarize briefly, instead of the coordinator
assigning
>> the
>> >> >> > > > partitions
>> >> >> > > > > > itself, all subscriptions are forwarded
to each member of
>> the
>> >> >> group
>> >> >> > > > which
>> >> >> > > > > > then decides independently which partitions
it should
>> consume.
>> >> >> The
>> >> >> > > > > protocol
>> >> >> > > > > > provides a mechanism for the coordinator
to validate that
>> all
>> >> >> > > consumers
>> >> >> > > > > use
>> >> >> > > > > > the same assignment strategy, but it does
not ensure that
>> the
>> >> >> > > resulting
>> >> >> > > > > > assignment is "correct." This provides
a powerful
>> capability
>> >> for
>> >> >> > > users
>> >> >> > > > to
>> >> >> > > > > > control the full data flow on the client
side. They control
>> >> how
>> >> >> > data
>> >> >> > > is
>> >> >> > > > > > written to partitions through the Partitioner
interface and
>> >> they
>> >> >> > > > control
>> >> >> > > > > > how data is consumed through the assignment
strategy, all
>> >> without
>> >> >> > > > > touching
>> >> >> > > > > > the server.
>> >> >> > > > > >
>> >> >> > > > > > Of course nothing comes for free. In particular,
this
>> change
>> >> >> > removes
>> >> >> > > > the
>> >> >> > > > > > ability of the coordinator to validate
that commits are
>> made
>> >> by
>> >> >> > > > consumers
>> >> >> > > > > > who were assigned the respective partition.
This might not
>> be
>> >> too
>> >> >> > bad
>> >> >> > > > > since
>> >> >> > > > > > we retain the ability to validate the
generation id, but it
>> >> is a
>> >> >> > > > > potential
>> >> >> > > > > > concern. We have considered alternative
protocols which
>> add a
>> >> >> > second
>> >> >> > > > > > round-trip to the protocol in order to
give the coordinator
>> >> the
>> >> >> > > ability
>> >> >> > > > > to
>> >> >> > > > > > confirm the assignment. As mentioned above,
the
>> coordinator is
>> >> >> > > somewhat
>> >> >> > > > > > limited in what it can actually validate,
but this would
>> >> return
>> >> >> its
>> >> >> > > > > ability
>> >> >> > > > > > to validate commits. The tradeoff is that
it increases the
>> >> >> > protocol's
>> >> >> > > > > > complexity which means more ways for the
protocol to fail
>> and
>> >> >> > > > > consequently
>> >> >> > > > > > more edge cases in the code.
>> >> >> > > > > >
>> >> >> > > > > > It also misses an opportunity to generalize
the group
>> >> membership
>> >> >> > > > protocol
>> >> >> > > > > > for additional use cases. In fact, after
you've gone to the
>> >> >> trouble
>> >> >> > > of
>> >> >> > > > > > moving assignment to the client, the main
thing that is
>> left
>> >> in
>> >> >> > this
>> >> >> > > > > > protocol is basically a general group
management
>> capability.
>> >> This
>> >> >> > is
>> >> >> > > > > > exactly what is needed for a few cases
that are currently
>> >> under
>> >> >> > > > > discussion
>> >> >> > > > > > (e.g. copycat or single-writer producer).
We've taken this
>> >> >> further
>> >> >> > > step
>> >> >> > > > > in
>> >> >> > > > > > the proposal and attempted to envision
what that general
>> >> protocol
>> >> >> > > might
>> >> >> > > > > > look like and how it could be used both
by the consumer and
>> >> for
>> >> >> > some
>> >> >> > > of
>> >> >> > > > > > these other cases.
>> >> >> > > > > >
>> >> >> > > > > > Anyway, since time is running out on the
new consumer, we
>> have
>> >> >> > > perhaps
>> >> >> > > > > one
>> >> >> > > > > > last chance to consider a significant
change in the
>> protocol
>> >> like
>> >> >> > > this,
>> >> >> > > > > so
>> >> >> > > > > > have a look at the wiki and share your
thoughts. I've no
>> doubt
>> >> >> that
>> >> >> > > > some
>> >> >> > > > > > ideas seem clearer in my mind than they
do on paper, so ask
>> >> >> > questions
>> >> >> > > > if
>> >> >> > > > > > there is any confusion.
>> >> >> > > > > >
>> >> >> > > > > > Thanks!
>> >> >> > > > > > Jason
>> >> >> > > > > >
>> >> >> > > > >
>> >> >> > > >
>> >> >> > >
>> >> >> >
>> >> >>
>> >> >
>> >> >
>> >> >
>> >> > --
>> >> > Thanks,
>> >> > Neha
>> >>
>>

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