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From: Becket Qin <becket.qin@gmail.com>
Date: Tue, 5 Dec 2017 21:38:05 -0800
Message-ID: <CABtAgwH8EqsX3cPZU37zb7QRtFr0Ngq1QvvtzH4qtmwANU1B4A@mail.gmail.com>
Subject: Re: [DISCUSS] KIP-227: Introduce Incremental FetchRequests to
 Increase Partition Scalability
To: dev@kafka.apache.org
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archived-at: Wed, 06 Dec 2017 05:38:24 -0000

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Hi Jun,

That is true, but in reality it seems rare that the fetch size is smaller
than index interval. In the worst case, we may need to do another look up.
In the future, when we have the mechanism to inform the clients about the
broker configurations, the clients may want to configure correspondingly as
well, e.g. max message size, max timestamp difference, etc.

On the other hand, we are not guaranteeing that the returned bytes in a
partition is always bounded by the per partition fetch size, because we are
going to return at least one message, so the per partition fetch size seems
already a soft limit. Since we are introducing a new fetch protocol and
this is related, it might be worth considering this option.

BTW, one reason I bring this up again was because yesterday we had a
presentation from Uber regarding the end to end latency. And they are
seeing this binary search behavior impacting the latency due to page in/out
of the index file.

Thanks,

Jiangjie (Becket) Qin


On Tue, Dec 5, 2017 at 5:55 PM, Jun Rao <jun@confluent.io> wrote:

> Hi, Jiangjie,
>
> Not sure returning the fetch response at the index boundary is a general
> solution. The index interval is configurable. If one configures the index
> interval larger than the per partition fetch size, we probably have to
> return data not at the index boundary.
>
> Thanks,
>
> Jun
>
> On Tue, Dec 5, 2017 at 4:17 PM, Becket Qin <becket.qin@gmail.com> wrote:
>
> > Hi Colin,
> >
> > Thinking about this again. I do see the reason that we want to have a
> epoch
> > to avoid out of order registration of the interested set. But I am
> > wondering if the following semantic would meet what we want better:
> >  - Session Id: the id assigned to a single client for life long time. i.e
> > it does not change when the interested partitions change.
> >  - Epoch: the interested set epoch. Only updated when a full fetch
> request
> > comes, which may result in the interested partition set change.
> > This will ensure that the registered interested set will always be the
> > latest registration. And the clients can change the interested partition
> > set without creating another session.
> >
> > Also I want to bring up the way the leader respond to the FetchRequest
> > again. I think it would be a big improvement if we just return the
> > responses at index entry boundary or log end. There are a few benefits:
> > 1. The leader does not need the follower to provide the offsets,
> > 2. The fetch requests no longer need to do a binary search on the index,
> it
> > just need to do a linear access to the index file, which is much cache
> > friendly.
> >
> > Assuming the leader can get the last returned offsets to the clients
> > cheaply, I am still not sure why it is necessary for the followers to
> > repeat the offsets in the incremental fetch every time. Intuitively it
> > should only update the offsets when the leader has wrong offsets, in most
> > cases, the incremental fetch request should just be empty. Otherwise we
> may
> > not be saving much when there are continuous small requests going to each
> > partition, which could be normal for some low latency systems.
> >
> > Thanks,
> >
> > Jiangjie (Becket) Qin
> >
> >
> >
> >
> > On Tue, Dec 5, 2017 at 2:14 PM, Colin McCabe <cmccabe@apache.org> wrote:
> >
> > > On Tue, Dec 5, 2017, at 13:13, Jan Filipiak wrote:
> > > > Hi Colin
> > > >
> > > > Addressing the topic of how to manage slots from the other thread.
> > > > With tcp connections all this comes for free essentially.
> > >
> > > Hi Jan,
> > >
> > > I don't think that it's accurate to say that cache management "comes
> for
> > > free" by coupling the incremental fetch session with the TCP session.
> > > When a new TCP session is started by a fetch request, you still have to
> > > decide whether to grant that request an incremental fetch session or
> > > not.  If your answer is that you always grant the request, I would
> argue
> > > that you do not have cache management.
> > >
> > > I guess you could argue that timeouts are cache management, but I don't
> > > find that argument persuasive.  Anyone could just create a lot of TCP
> > > sessions and use a lot of resources, in that case.  So there is
> > > essentially no limit on memory use.  In any case, TCP sessions don't
> > > help us implement fetch session timeouts.
> > >
> > > > I still would argue we disable it by default and make a flag in the
> > > > broker to ask the leader to maintain the cache while replicating and
> > > also only
> > > > have it optional in consumers (default to off) so one can turn it on
> > > > where it really hurts.  MirrorMaker and audit consumers prominently.
> > >
> > > I agree with Jason's point from earlier in the thread.  Adding extra
> > > configuration knobs that aren't really necessary can harm usability.
> > > Certainly asking people to manually turn on a feature "where it really
> > > hurts" seems to fall in that category, when we could easily enable it
> > > automatically for them.
> > >
> > > >
> > > > Otherwise I left a few remarks in-line, which should help to
> understand
> > > > my view of the situation better
> > > >
> > > > Best Jan
> > > >
> > > >
> > > > On 05.12.2017 08:06, Colin McCabe wrote:
> > > > > On Mon, Dec 4, 2017, at 02:27, Jan Filipiak wrote:
> > > > >>
> > > > >> On 03.12.2017 21:55, Colin McCabe wrote:
> > > > >>> On Sat, Dec 2, 2017, at 23:21, Becket Qin wrote:
> > > > >>>> Thanks for the explanation, Colin. A few more questions.
> > > > >>>>
> > > > >>>>> The session epoch is not complex.  It's just a number which
> > > increments
> > > > >>>>> on each incremental fetch.  The session epoch is also useful
> for
> > > > >>>>> debugging-- it allows you to match up requests and responses
> when
> > > > >>>>> looking at log files.
> > > > >>>> Currently each request in Kafka has a correlation id to help
> match
> > > the
> > > > >>>> requests and responses. Is epoch doing something differently?
> > > > >>> Hi Becket,
> > > > >>>
> > > > >>> The correlation ID is used within a single TCP session, to
> uniquely
> > > > >>> associate a request with a response.  The correlation ID is not
> > > unique
> > > > >>> (and has no meaning) outside the context of that single TCP
> > session.
> > > > >>>
> > > > >>> Keep in mind, NetworkClient is in charge of TCP sessions, and
> > > generally
> > > > >>> tries to hide that information from the upper layers of the code.
> > So
> > > > >>> when you submit a request to NetworkClient, you don't know if
> that
> > > > >>> request creates a TCP session, or reuses an existing one.
> > > > >>>>> Unfortunately, this doesn't work.  Imagine the client misses an
> > > > >>>>> increment fetch response about a partition.  And then the
> > > partition is
> > > > >>>>> never updated after that.  The client has no way to know about
> > the
> > > > >>>>> partition, since it won't be included in any future incremental
> > > fetch
> > > > >>>>> responses.  And there are no offsets to compare, since the
> > > partition is
> > > > >>>>> simply omitted from the response.
> > > > >>>> I am curious about in which situation would the follower miss a
> > > response
> > > > >>>> of a partition. If the entire FetchResponse is lost (e.g.
> > timeout),
> > > the
> > > > >>>> follower would disconnect and retry. That will result in
> sending a
> > > full
> > > > >>>> FetchRequest.
> > > > >>> Basically, you are proposing that we rely on TCP for reliable
> > > delivery
> > > > >>> in a distributed system.  That isn't a good idea for a bunch of
> > > > >>> different reasons.  First of all, TCP timeouts tend to be very
> > > long.  So
> > > > >>> if the TCP session timing out is your error detection mechanism,
> > you
> > > > >>> have to wait minutes for messages to timeout.  Of course, we add
> a
> > > > >>> timeout on top of that after which we declare the connection bad
> > and
> > > > >>> manually close it.  But just because the session is closed on one
> > end
> > > > >>> doesn't mean that the other end knows that it is closed.  So the
> > > leader
> > > > >>> may have to wait quite a long time before TCP decides that yes,
> > > > >>> connection X from the follower is dead and not coming back, even
> > > though
> > > > >>> gremlins ate the FIN packet which the follower attempted to
> > > translate.
> > > > >>> If the cache state is tied to that TCP session, we have to keep
> > that
> > > > >>> cache around for a much longer time than we should.
> > > > >> Hi,
> > > > >>
> > > > >> I see this from a different perspective. The cache expiry time
> > > > >> has the same semantic as idle connection time in this scenario.
> > > > >> It is the time range we expect the client to come back an reuse
> > > > >> its broker side state. I would argue that on close we would get an
> > > > >> extra shot at cleaning up the session state early. As opposed to
> > > > >> always wait for that duration for expiry to happen.
> > > > > Hi Jan,
> > > > >
> > > > > The idea here is that the incremental fetch cache expiry time can
> be
> > > > > much shorter than the TCP session timeout.  In general the TCP
> > session
> > > > > timeout is common to all TCP connections, and very long.  To make
> > these
> > > > > numbers a little more concrete, the TCP session timeout is often
> > > > > configured to be 2 hours on Linux.  (See
> > > > > https://www.cyberciti.biz/tips/linux-increasing-or-
> > > decreasing-tcp-sockets-timeouts.html
> > > > > )  The timeout I was proposing for incremental fetch sessions was
> one
> > > or
> > > > > two minutes at most.
> > > > Currently this is taken care of by
> > > > connections.max.idle.ms on the broker and defaults to something of
> few
> > > > minutes.
> > >
> > > It is 10 minutes by default, which is longer than what we want the
> > > incremental fetch session timeout to be.  There's no reason to couple
> > > these two things.
> > >
> > > > Also something we could let the client change if we really wanted to.
> > > > So there is no need to worry about coupling our implementation to
> some
> > > > timeouts given by the OS, with TCP one always has full control over
> the
> > > worst
> > > > times + one gets the extra shot cleaning up early when the close
> comes
> > > through.
> > > > Which is the majority of the cases.
> > >
> > > In the majority of cases, the TCP session will be re-established.  In
> > > that case, we have to send a full fetch request rather than an
> > > incremental fetch request.
> > >
> > > >
> > > > >
> > > > >>> Secondly, from a software engineering perspective, it's not a
> good
> > > idea
> > > > >>> to try to tightly tie together TCP and our code.  We would have
> to
> > > > >>> rework how we interact with NetworkClient so that we are aware of
> > > things
> > > > >>> like TCP sessions closing or opening.  We would have to be
> careful
> > > > >>> preserve the ordering of incoming messages when doing things like
> > > > >>> putting incoming requests on to a queue to be processed by
> multiple
> > > > >>> threads.  It's just a lot of complexity to add, and there's no
> > > upside.
> > > > >> I see the point here. And I had a small chat with Dong Lin already
> > > > >> making me aware of this. I tried out the approaches and propose
> the
> > > > >> following:
> > > > >>
> > > > >> The client start and does a full fetch. It then does incremental
> > > fetches.
> > > > >> The connection to the broker dies and is re-established by
> > > NetworkClient
> > > > >> under the hood.
> > > > >> The broker sees an incremental fetch without having state =>
> returns
> > > > >> error:
> > > > >> Client sees the error, does a full fetch and goes back to
> > > incrementally
> > > > >> fetching.
> > > > >>
> > > > >> having this 1 additional error round trip is essentially the same
> as
> > > > >> when something
> > > > >> with the sessions or epoch changed unexpectedly to the client (say
> > > > >> expiry).
> > > > >>
> > > > >> So its nothing extra added but the conditions are easier to
> > evaluate.
> > > > >> Especially since we do everything with NetworkClient. Other
> > > implementers
> > > > >> on the
> > > > >> protocol are free to optimizes this and do not do the errornours
> > > > >> roundtrip on the
> > > > >> new connection.
> > > > >> Its a great plus that the client can know when the error is gonna
> > > > >> happen. instead of
> > > > >> the server to always have to report back if something changes
> > > > >> unexpectedly for the client
> > > > > You are assuming that the leader and the follower agree that the
> TCP
> > > > > session drops at the same time.  When there are network problems,
> > this
> > > > > may not be true.  The leader may still think the previous TCP
> session
> > > is
> > > > > active.  In that case, we have to keep the incremental fetch
> session
> > > > > state around until we learn otherwise (which could be up to that 2
> > hour
> > > > > timeout I mentioned).  And if we get a new incoming incremental
> fetch
> > > > > request, we can't assume that it replaces the previous one, because
> > the
> > > > > IDs will be different (the new one starts a new session).
> > > > As mentioned, no reason to fear some time-outs out of our control
> > > > >
> > > > >>> Imagine that I made an argument that client IDs are "complex" and
> > > should
> > > > >>> be removed from our APIs.  After all, we can just look at the
> > remote
> > > IP
> > > > >>> address and TCP port of each connection.  Would you think that
> was
> > a
> > > > >>> good idea?  The client ID is useful when looking at logs.  For
> > > example,
> > > > >>> if a rebalance is having problems, you want to know what clients
> > were
> > > > >>> having a problem.  So having the client ID field to guide you is
> > > > >>> actually much less "complex" in practice than not having an ID.
> > > > >> I still cant follow why the correlation idea will not help here.
> > > > >> Correlating logs with it usually works great. Even with primitive
> > > tools
> > > > >> like grep
> > > > > The correlation ID does help somewhat, but certainly not as much
> as a
> > > > > unique 64-bit ID.  The correlation ID is not unique in the broker,
> > just
> > > > > unique to a single NetworkClient.  Simiarly, the correlation ID is
> > not
> > > > > unique on the client side, if there are multiple Consumers, etc.
> > > > Can always bump entropy in correlation IDs, never had a problem
> > > > of finding to many duplicates. Would be a different KIP though.
> > > > >
> > > > >>> Similarly, if metadata responses had epoch numbers (simple
> > > incrementing
> > > > >>> numbers), we would not have to debug problems like clients
> > > accidentally
> > > > >>> getting old metadata from servers that had been partitioned off
> > from
> > > the
> > > > >>> network for a while.  Clients would know the difference between
> old
> > > and
> > > > >>> new metadata.  So putting epochs in to the metadata request is
> much
> > > less
> > > > >>> "complex" operationally, even though it's an extra field in the
> > > request.
> > > > >>>    This has been discussed before on the mailing list.
> > > > >>>
> > > > >>> So I think the bottom line for me is that having the session ID
> and
> > > > >>> session epoch, while it adds two extra fields, reduces
> operational
> > > > >>> complexity and increases debuggability.  It avoids tightly
> coupling
> > > us
> > > > >>> to assumptions about reliable ordered delivery which tend to be
> > > violated
> > > > >>> in practice in multiple layers of the stack.  Finally, it  avoids
> > the
> > > > >>> necessity of refactoring NetworkClient.
> > > > >> So there is stacks out there that violate TCP guarantees? And
> > software
> > > > >> still works? How can this be? Can you elaborate a little where
> this
> > > > >> can be violated? I am not very familiar with virtualized
> > environments
> > > > >> but they can't really violate TCP contracts.
> > > > > TCP's guarantees of reliable, in-order transmission certainly can
> be
> > > > > violated.  For example, I once had to debug a cluster where a
> certain
> > > > > node had a network card which corrupted its transmissions
> > occasionally.
> > > > > With all the layers of checksums, you would think that this was not
> > > > > possible, but it happened.  We occasionally got corrupted data
> > written
> > > > > to disk on the other end because of it.  Even more frustrating, the
> > > data
> > > > > was not corrupted on disk on the sending node-- it was a bug in the
> > > > > network card driver that was injecting the errors.
> > > > true, but your broker might aswell read a corrupted 600GB as size
> from
> > > > the network and die with OOM instantly.
> > >
> > > If you read 600 GB as the size from the network, you will not "die with
> > > OOM instantly."  That would be a bug.  Instead, you will notice that
> 600
> > > GB is greater than max.message.bytes, and close the connection.
> > >
> > > > Optimizing for still having functional
> > > > software under this circumstances is not reasonable.
> > > > You want to get rid of such a
> > > > node ASAP and pray that zookeepers ticks get corrupted often enough
> > > > that it finally drops out of the cluster.
> > > >
> > > > There is a good reason that these kinda things
> > > > https://issues.apache.org/jira/browse/MESOS-4105
> > > > don't end up as kafka Jiras. In the end you can't run any software in
> > > > these containers anymore. Application layer checksums are a neat
> thing
> > to
> > > > fail fast but trying to cope with this probably causes more bad than
> > > > good.  So I would argue that we shouldn't try this for the fetch
> > > requests.
> > >
> > > One of the goals of Apache Kafka is to be "a streaming platform...
> > > [that] lets you store streams of records in a fault-tolerant way."  For
> > > more information, see https://kafka.apache.org/intro .
> Fault-tolerance
> > > is explicitly part of the goal of Kafka.  Prayer should be optional,
> not
> > > required, when running the software.
> > >
> > > Crashing because someone sent you a bad packet is not reasonable
> > > behavior.  It is a bug.  Similarly, bringing down the whole cluster,
> > > which could a hundred nodes, because someone had a bad network adapter
> > > is not reasonable behavior.  It is perhaps reasonable for the cluster
> to
> > > perform worse when hardware is having problems.  But that's a different
> > > discussion.
> > >
> > > best,
> > > Colin
> > >
> > > >
> > > >
> > > > >
> > > > > However, my point was not about TCP's guarantees being violated.
> My
> > > > > point is that TCP's guarantees are only one small building block to
> > > > > build a robust distributed system.  TCP basically just says that if
> > you
> > > > > get any bytes from the stream, you will get the ones that were sent
> > by
> > > > > the sender, in the order they were sent.  TCP does not guarantee
> that
> > > > > the bytes you send will get there.  It does not guarantee that if
> you
> > > > > close the connection, the other end will know about it in a timely
> > > > > fashion.
> > > > These are very powerful grantees and since we use TCP we should
> > > > piggy pack everything that is reasonable on to it. IMO there is no
> > > > need to reimplement correct sequencing again if you get that from
> > > > your transport layer. It saves you the complexity, it makes
> > > > you application behave way more naturally and your api easier to
> > > > understand.
> > > >
> > > > There is literally nothing the Kernel wont let you decide
> > > > especially not any timings. Only noticeable exception being TIME_WAIT
> > > > of usually 240 seconds but that already has little todo with the
> broker
> > > > itself and
> > > > if we are running out of usable ports because of this then expiring
> > > > fetch requests
> > > > wont help much anyways.
> > > >
> > > > I hope I could strengthen the trust you have in userland TCP
> connection
> > > > management. It is really powerful and can be exploited for maximum
> > gains
> > > > without much risk in my opinion.
> > > >
> > > >
> > > >
> > > > > It does not guarantee that the bytes will be received in a
> > > > > certain timeframe, and certainly doesn't guarantee that if you
> send a
> > > > > byte on connection X and then on connection Y, that the remote end
> > will
> > > > > read a byte on X before reading a byte on Y.
> > > > Noone expects this from two independent paths of any kind.
> > > >
> > > > >
> > > > > best,
> > > > > Colin
> > > > >
> > > > >> Hope this made my view clearer, especially the first part.
> > > > >>
> > > > >> Best Jan
> > > > >>
> > > > >>
> > > > >>> best,
> > > > >>> Colin
> > > > >>>
> > > > >>>
> > > > >>>> If there is an error such as NotLeaderForPartition is
> > > > >>>> returned for some partitions, the follower can always send a
> full
> > > > >>>> FetchRequest. Is there a scenario that only some of the
> partitions
> > > in a
> > > > >>>> FetchResponse is lost?
> > > > >>>>
> > > > >>>> Thanks,
> > > > >>>>
> > > > >>>> Jiangjie (Becket) Qin
> > > > >>>>
> > > > >>>>
> > > > >>>> On Sat, Dec 2, 2017 at 2:37 PM, Colin McCabe<cmccabe@apache.org
> >
> > > wrote:
> > > > >>>>
> > > > >>>>> On Fri, Dec 1, 2017, at 11:46, Dong Lin wrote:
> > > > >>>>>> On Thu, Nov 30, 2017 at 9:37 AM, Colin McCabe<
> > cmccabe@apache.org>
> > > > >>>>> wrote:
> > > > >>>>>>> On Wed, Nov 29, 2017, at 18:59, Dong Lin wrote:
> > > > >>>>>>>> Hey Colin,
> > > > >>>>>>>>
> > > > >>>>>>>> Thanks much for the update. I have a few questions below:
> > > > >>>>>>>>
> > > > >>>>>>>> 1. I am not very sure that we need Fetch Session Epoch. It
> > > seems that
> > > > >>>>>>>> Fetch
> > > > >>>>>>>> Session Epoch is only needed to help leader distinguish
> > between
> > > "a
> > > > >>>>> full
> > > > >>>>>>>> fetch request" and "a full fetch request and request a new
> > > > >>>>> incremental
> > > > >>>>>>>> fetch session". Alternatively, follower can also indicate "a
> > > full
> > > > >>>>> fetch
> > > > >>>>>>>> request and request a new incremental fetch session" by
> > setting
> > > Fetch
> > > > >>>>>>>> Session ID to -1 without using Fetch Session Epoch. Does
> this
> > > make
> > > > >>>>> sense?
> > > > >>>>>>> Hi Dong,
> > > > >>>>>>>
> > > > >>>>>>> The fetch session epoch is very important for ensuring
> > > correctness.  It
> > > > >>>>>>> prevents corrupted or incomplete fetch data due to network
> > > reordering
> > > > >>>>> or
> > > > >>>>>>> loss.
> > > > >>>>>>>
> > > > >>>>>>> For example, consider a scenario where the follower sends a
> > fetch
> > > > >>>>>>> request to the leader.  The leader responds, but the response
> > is
> > > lost
> > > > >>>>>>> because of network problems which affected the TCP session.
> In
> > > that
> > > > >>>>>>> case, the follower must establish a new TCP session and
> re-send
> > > the
> > > > >>>>>>> incremental fetch request.  But the leader does not know that
> > the
> > > > >>>>>>> follower didn't receive the previous incremental fetch
> > > response.  It is
> > > > >>>>>>> only the incremental fetch epoch which lets the leader know
> > that
> > > it
> > > > >>>>>>> needs to resend that data, and not data which comes
> afterwards.
> > > > >>>>>>>
> > > > >>>>>>> You could construct similar scenarios with message
> reordering,
> > > > >>>>>>> duplication, etc.  Basically, this is a stateful protocol on
> an
> > > > >>>>>>> unreliable network, and you need to know whether the follower
> > > got the
> > > > >>>>>>> previous data you sent before you move on.  And you need to
> > > handle
> > > > >>>>>>> issues like duplicated or delayed requests.  These issues do
> > not
> > > affect
> > > > >>>>>>> the full fetch request, because it is not stateful-- any full
> > > fetch
> > > > >>>>>>> request can be understood and properly responded to in
> > isolation.
> > > > >>>>>>>
> > > > >>>>>> Thanks for the explanation. This makes sense. On the other
> hand
> > I
> > > would
> > > > >>>>>> be interested in learning more about whether Becket's solution
> > > can help
> > > > >>>>>> simplify the protocol by not having the echo field and whether
> > > that is
> > > > >>>>>> worth doing.
> > > > >>>>> Hi Dong,
> > > > >>>>>
> > > > >>>>> I commented about this in the other thread.  A solution which
> > > doesn't
> > > > >>>>> maintain session information doesn't work here.
> > > > >>>>>
> > > > >>>>>>>> 2. It is said that Incremental FetchRequest will include
> > > partitions
> > > > >>>>> whose
> > > > >>>>>>>> fetch offset or maximum number of fetch bytes has been
> > changed.
> > > If
> > > > >>>>>>>> follower's logStartOffet of a partition has changed, should
> > this
> > > > >>>>>>>> partition also be included in the next FetchRequest to the
> > > leader?
> > > > >>>>>>> Otherwise, it
> > > > >>>>>>>> may affect the handling of DeleteRecordsRequest because
> leader
> > > may
> > > > >>>>> not
> > > > >>>>>>> know
> > > > >>>>>>>> the corresponding data has been deleted on the follower.
> > > > >>>>>>> Yeah, the follower should include the partition if the
> > > logStartOffset
> > > > >>>>>>> has changed.  That should be spelled out on the KIP.  Fixed.
> > > > >>>>>>>
> > > > >>>>>>>> 3. In the section "Per-Partition Data", a partition is not
> > > considered
> > > > >>>>>>>> dirty if its log start offset has changed. Later in the
> > section
> > > > >>>>>>> "FetchRequest
> > > > >>>>>>>> Changes", it is said that incremental fetch responses will
> > > include a
> > > > >>>>>>>> partition if its logStartOffset has changed. It seems
> > > inconsistent.
> > > > >>>>> Can
> > > > >>>>>>>> you update the KIP to clarify it?
> > > > >>>>>>>>
> > > > >>>>>>> In the "Per-Partition Data" section, it does say that
> > > logStartOffset
> > > > >>>>>>> changes make a partition dirty, though, right?  The first
> > bullet
> > > point
> > > > >>>>>>> is:
> > > > >>>>>>>
> > > > >>>>>>>> * The LogCleaner deletes messages, and this changes the log
> > > start
> > > > >>>>> offset
> > > > >>>>>>> of the partition on the leader., or
> > > > >>>>>>>
> > > > >>>>>> Ah I see. I think I didn't notice this because statement
> assumes
> > > that the
> > > > >>>>>> LogStartOffset in the leader only changes due to LogCleaner.
> In
> > > fact the
> > > > >>>>>> LogStartOffset can change on the leader due to either log
> > > retention and
> > > > >>>>>> DeleteRecordsRequest. I haven't verified whether LogCleaner
> can
> > > change
> > > > >>>>>> LogStartOffset though. It may be a bit better to just say
> that a
> > > > >>>>>> partition is considered dirty if LogStartOffset changes.
> > > > >>>>> I agree.  It should be straightforward to just resend the
> > > partition if
> > > > >>>>> logStartOffset changes.
> > > > >>>>>
> > > > >>>>>>>> 4. In "Fetch Session Caching" section, it is said that each
> > > broker
> > > > >>>>> has a
> > > > >>>>>>>> limited number of slots. How is this number determined? Does
> > > this
> > > > >>>>> require
> > > > >>>>>>>> a new broker config for this number?
> > > > >>>>>>> Good point.  I added two broker configuration parameters to
> > > control
> > > > >>>>> this
> > > > >>>>>>> number.
> > > > >>>>>>>
> > > > >>>>>> I am curious to see whether we can avoid some of these new
> > > configs. For
> > > > >>>>>> example, incremental.fetch.session.cache.slots.per.broker is
> > > probably
> > > > >>>>> not
> > > > >>>>>> necessary because if a leader knows that a FetchRequest comes
> > > from a
> > > > >>>>>> follower, we probably want the leader to always cache the
> > > information
> > > > >>>>>> from that follower. Does this make sense?
> > > > >>>>> Yeah, maybe we can avoid having
> > > > >>>>> incremental.fetch.session.cache.slots.per.broker.
> > > > >>>>>
> > > > >>>>>> Maybe we can discuss the config later after there is agreement
> > on
> > > how the
> > > > >>>>>> protocol would look like.
> > > > >>>>>>
> > > > >>>>>>
> > > > >>>>>>>> What is the error code if broker does
> > > > >>>>>>>> not have new log for the incoming FetchRequest?
> > > > >>>>>>> Hmm, is there a typo in this question?  Maybe you meant to
> ask
> > > what
> > > > >>>>>>> happens if there is no new cache slot for the incoming
> > > FetchRequest?
> > > > >>>>>>> That's not an error-- the incremental fetch session ID just
> > gets
> > > set to
> > > > >>>>>>> 0, indicating no incremental fetch session was created.
> > > > >>>>>>>
> > > > >>>>>> Yeah there is a typo. You have answered my question.
> > > > >>>>>>
> > > > >>>>>>
> > > > >>>>>>>> 5. Can you clarify what happens if follower adds a partition
> > to
> > > the
> > > > >>>>>>>> ReplicaFetcherThread after receiving LeaderAndIsrRequest?
> Does
> > > leader
> > > > >>>>>>>> needs to generate a new session for this
> ReplicaFetcherThread
> > or
> > > > >>>>> does it
> > > > >>>>>>> re-use
> > > > >>>>>>>> the existing session?  If it uses a new session, is the old
> > > session
> > > > >>>>>>>> actively deleted from the slot?
> > > > >>>>>>> The basic idea is that you can't make changes, except by
> > sending
> > > a full
> > > > >>>>>>> fetch request.  However, perhaps we can allow the client to
> > > re-use its
> > > > >>>>>>> existing session ID.  If the client sets sessionId = id,
> epoch
> > =
> > > 0, it
> > > > >>>>>>> could re-initialize the session.
> > > > >>>>>>>
> > > > >>>>>> Yeah I agree with the basic idea. We probably want to
> understand
> > > more
> > > > >>>>>> detail about how this works later.
> > > > >>>>> Sounds good.  I updated the KIP with this information.  A
> > > > >>>>> re-initialization should be exactly the same as an
> > initialization,
> > > > >>>>> except that it reuses an existing ID.
> > > > >>>>>
> > > > >>>>> best,
> > > > >>>>> Colin
> > > > >>>>>
> > > > >>>>>
> > > > >>>>>>>> BTW, I think it may be useful if the KIP can include the
> > example
> > > > >>>>> workflow
> > > > >>>>>>>> of how this feature will be used in case of partition change
> > > and so
> > > > >>>>> on.
> > > > >>>>>>> Yeah, that might help.
> > > > >>>>>>>
> > > > >>>>>>> best,
> > > > >>>>>>> Colin
> > > > >>>>>>>
> > > > >>>>>>>> Thanks,
> > > > >>>>>>>> Dong
> > > > >>>>>>>>
> > > > >>>>>>>>
> > > > >>>>>>>> On Wed, Nov 29, 2017 at 12:13 PM, Colin McCabe<
> > > cmccabe@apache.org>
> > > > >>>>>>>> wrote:
> > > > >>>>>>>>
> > > > >>>>>>>>> I updated the KIP with the ideas we've been discussing.
> > > > >>>>>>>>>
> > > > >>>>>>>>> best,
> > > > >>>>>>>>> Colin
> > > > >>>>>>>>>
> > > > >>>>>>>>> On Tue, Nov 28, 2017, at 08:38, Colin McCabe wrote:
> > > > >>>>>>>>>> On Mon, Nov 27, 2017, at 22:30, Jan Filipiak wrote:
> > > > >>>>>>>>>>> Hi Colin, thank you  for this KIP, it can become a really
> > > > >>>>> useful
> > > > >>>>>>> thing.
> > > > >>>>>>>>>>> I just scanned through the discussion so far and wanted
> to
> > > > >>>>> start a
> > > > >>>>>>>>>>> thread to make as decision about keeping the
> > > > >>>>>>>>>>> cache with the Connection / Session or having some sort
> of
> > > UUID
> > > > >>>>>>> indN
> > > > >>>>>>>>> exed
> > > > >>>>>>>>>>> global Map.
> > > > >>>>>>>>>>>
> > > > >>>>>>>>>>> Sorry if that has been settled already and I missed it.
> In
> > > this
> > > > >>>>>>> case
> > > > >>>>>>>>>>> could anyone point me to the discussion?
> > > > >>>>>>>>>> Hi Jan,
> > > > >>>>>>>>>>
> > > > >>>>>>>>>> I don't think anyone has discussed the idea of tying the
> > cache
> > > > >>>>> to an
> > > > >>>>>>>>>> individual TCP session yet.  I agree that since the cache
> is
> > > > >>>>>>> intended to
> > > > >>>>>>>>>> be used only by a single follower or client, it's an
> > > interesting
> > > > >>>>>>> thing
> > > > >>>>>>>>>> to think about.
> > > > >>>>>>>>>>
> > > > >>>>>>>>>> I guess the obvious disadvantage is that whenever your TCP
> > > > >>>>> session
> > > > >>>>>>>>>> drops, you have to make a full fetch request rather than
> an
> > > > >>>>>>> incremental
> > > > >>>>>>>>>> one.  It's not clear to me how often this happens in
> > practice
> > > --
> > > > >>>>> it
> > > > >>>>>>>>>> probably depends a lot on the quality of the network.
> From
> > a
> > > > >>>>> code
> > > > >>>>>>>>>> perspective, it might also be a bit difficult to access
> data
> > > > >>>>>>> associated
> > > > >>>>>>>>>> with the Session from classes like KafkaApis (although we
> > > could
> > > > >>>>>>> refactor
> > > > >>>>>>>>>> it to make this easier).
> > > > >>>>>>>>>>
> > > > >>>>>>>>>> It's also clear that even if we tie the cache to the
> > session,
> > > we
> > > > >>>>>>> still
> > > > >>>>>>>>>> have to have limits on the number of caches we're willing
> to
> > > > >>>>> create.
> > > > >>>>>>>>>> And probably we should reserve some cache slots for each
> > > > >>>>> follower, so
> > > > >>>>>>>>>> that clients don't take all of them.
> > > > >>>>>>>>>>
> > > > >>>>>>>>>>> Id rather see a protocol in which the client is hinting
> the
> > > > >>>>> broker
> > > > >>>>>>>>> that,
> > > > >>>>>>>>>>> he is going to use the feature instead of a client
> > > > >>>>>>>>>>> realizing that the broker just offered the feature
> > > (regardless
> > > > >>>>> of
> > > > >>>>>>>>>>> protocol version which should only indicate that the
> > feature
> > > > >>>>>>>>>>> would be usable).
> > > > >>>>>>>>>> Hmm.  I'm not sure what you mean by "hinting."  I do think
> > > that
> > > > >>>>> the
> > > > >>>>>>>>>> server should have the option of not accepting incremental
> > > > >>>>> requests
> > > > >>>>>>> from
> > > > >>>>>>>>>> specific clients, in order to save memory space.
> > > > >>>>>>>>>>
> > > > >>>>>>>>>>> This seems to work better with a per
> > > > >>>>>>>>>>> connection/session attached Metadata than with a Map and
> > > could
> > > > >>>>>>> allow
> > > > >>>>>>>>> for
> > > > >>>>>>>>>>> easier client implementations.
> > > > >>>>>>>>>>> It would also make Client-side code easier as there
> > wouldn't
> > > > >>>>> be any
> > > > >>>>>>>>>>> Cache-miss error Messages to handle.
> > > > >>>>>>>>>> It is nice not to have to handle cache-miss responses, I
> > > agree.
> > > > >>>>>>>>>> However, TCP sessions aren't exposed to most of our
> > > client-side
> > > > >>>>> code.
> > > > >>>>>>>>>> For example, when the Producer creates a message and hands
> > it
> > > > >>>>> off to
> > > > >>>>>>> the
> > > > >>>>>>>>>> NetworkClient, the NC will transparently re-connect and
> > > re-send a
> > > > >>>>>>>>>> message if the first send failed.  The higher-level code
> > will
> > > > >>>>> not be
> > > > >>>>>>>>>> informed about whether the TCP session was re-established,
> > > > >>>>> whether an
> > > > >>>>>>>>>> existing TCP session was used, and so on.  So overall I
> > would
> > > > >>>>> still
> > > > >>>>>>> lean
> > > > >>>>>>>>>> towards not coupling this to the TCP session...
> > > > >>>>>>>>>>
> > > > >>>>>>>>>> best,
> > > > >>>>>>>>>> Colin
> > > > >>>>>>>>>>
> > > > >>>>>>>>>>>     Thank you again for the KIP. And again, if this was
> > > clarified
> > > > >>>>>>> already
> > > > >>>>>>>>>>> please drop me a hint where I could read about it.
> > > > >>>>>>>>>>>
> > > > >>>>>>>>>>> Best Jan
> > > > >>>>>>>>>>>
> > > > >>>>>>>>>>>
> > > > >>>>>>>>>>>
> > > > >>>>>>>>>>>
> > > > >>>>>>>>>>>
> > > > >>>>>>>>>>> On 21.11.2017 22:02, Colin McCabe wrote:
> > > > >>>>>>>>>>>> Hi all,
> > > > >>>>>>>>>>>>
> > > > >>>>>>>>>>>> I created a KIP to improve the scalability and latency
> of
> > > > >>>>>>>>> FetchRequest:
> > > > >>>>>>>>>>>> https://cwiki.apache.org/confluence/display/KAFKA/KIP-
> > > > >>>>>>>>> 227%3A+Introduce+Incremental+FetchRequests+to+Increase+
> > > > >>>>>>>>> Partition+Scalability
> > > > >>>>>>>>>>>> Please take a look.
> > > > >>>>>>>>>>>>
> > > > >>>>>>>>>>>> cheers,
> > > > >>>>>>>>>>>> Colin
> > > >
> > >
> >
>

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