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From: Becket Qin <becket.qin@gmail.com>
Date: Tue, 2 Jan 2018 23:49:08 -0800
Message-ID: <CABtAgwHE5fRBFC4uEU+SiwbOFhaxpop-wi8c6QGLqmim9QZTLA@mail.gmail.com>
Subject: Re: [DISCUSS] KIP-227: Introduce Incremental FetchRequests to
 Increase Partition Scalability
To: dev@kafka.apache.org
Content-Type: multipart/alternative; boundary="001a1143e3205a8daf0561da731c"
archived-at: Wed, 03 Jan 2018 07:49:36 -0000

--001a1143e3205a8daf0561da731c
Content-Type: text/plain; charset="UTF-8"

Thanks for the reply, Colin.

My concern for the reinitialization is potential churn rather than
efficiency. The current KIP proposal uses the time and priority based
protection to avoid thrashing, but it is not clear to me if that is
sufficient. For example, consider topic creation/deletion. In those cases,
a lot of the replica fetchers will potentially need to re-establish the
session. And there might be many client session got evicted. And thus again
need to re-establish sessions. This would involve two round trips (due to
InvalidFetchSessionException), potential metadata refresh and backoff.

Admittedly it is probably not going to be worse than what we have now, but
such uncertain impact still worries me. Are we going to have the follow up
optimization discussion before the implementation of this KIP or are we
going to do it after? In the past we used to have separate KIPs for a
complicated feature but implement them together. Perhaps we can do the same
here if you want to limit the scope of this KIP.

Thanks,

Jiangjie (Becket) Qin


On Tue, Jan 2, 2018 at 6:34 PM, Colin McCabe <cmccabe@apache.org> wrote:

> On Tue, Jan 2, 2018, at 04:46, Becket Qin wrote:
> > Hi Colin,
> >
> > Good point about KIP-226. Maybe a separate broker epoch is needed
> although
> > it is a little awkward to let the consumer set this. So was there a
> > solution to the frequent pause and resume scenario? Did I miss something?
> >
> > Thanks,
> > Jiangjie (Becket) Qin
>
> Hi Becket,
>
> Allowing sessions to be re-initialized (as the current KIP does) makes
> frequent pauses and resumes less painful, because the memory associated
> with the old session can be reclaimed.  The only cost is sending a full
> fetch request once when the pause or resume is activated.
>
> There are other follow-on optimizations that we might want to do later,
> like allowing new partitions to be added to existing fetch sessions without
> a re-initialization, that could make this even more efficient.  But that's
> not in the current KIP, in order to avoid expanding the scope too much.
>
> best,
> Colin
>
> >
> > On Wed, Dec 27, 2017 at 1:40 PM, Colin McCabe <cmccabe@apache.org>
> wrote:
> >
> > > On Sat, Dec 23, 2017, at 09:15, Becket Qin wrote:
> > > > Hi Colin,
> > > >
> > > > Thanks for the explanation. I want to clarify a bit more on my
> thoughts.
> > > >
> > > > I am fine with having a separate discussion as long as the follow-up
> > > > discussion will be incremental on top of this KIP instead of
> override the
> > > > protocol in this KIP.
> > >
> > > Hi Becket,
> > >
> > > Thanks for the clarification.  I do think that the changes we've been
> > > discussing would be incremental rather than completely replacing what
> we've
> > > talked about here.  See my responses inline below.
> > >
> > > >
> > > > I completely agree this KIP is useful by itself. That being said, we
> want
> > > > to avoid falling into a "local optimal" solution by just saying
> because
> > > it
> > > > solves the problem in this scope. I think we should also think if the
> > > > solution aligns with a "global optimal" (systematic optimal)
> solution as
> > > > well. That is why I brought up other considerations. If they turned
> out
> > > to
> > > > be orthogonal and should be addressed separately, that's good. But at
> > > least
> > > > it is worth thinking about the potential connections between those
> > > things.
> > > >
> > > > One example of such related consideration is the following two
> seemingly
> > > > unrelated things:
> > > >
> > > > 1. I might have missed the discussion, but it seems the concern of
> the
> > > > clients doing frequent pause and resume is still not addressed. Since
> > > this
> > > > is a pretty common use case for applications that want to have flow
> > > > control, or have prioritized consumption, or get consumption
> fairness, we
> > > > probably want to see how to handle this case. One of the solution
> might
> > > be
> > > > a long-lived session id spanning the clients' life time.
> > > >
> > > > 2. KAFKA-6029. The key problem is that the leader wants to know if a
> > > fetch
> > > > request is from a shutting down broker or from a restarted broker.
> > > >
> > > > The connection between those two issues is that both of them could be
> > > > addressed by having a life-long session id for each client (or
> fetcher,
> > > to
> > > > be more accurate). This may indicate that having a life long session
> id
> > > > might be a "global optimal" solution so it should be considered in
> this
> > > > KIP. Otherwise, a follow up KIP discussion for KAFKA-6029 may either
> > > > introduce a broker epoch unnecessarily (which will not be used by the
> > > > consumers at all) or override what we do in this KIP.
> > >
> > > Remember that a given follower will have more than one fetch session
> ID.
> > > Each fetcher thread will have its own session ID.  And we will
> eventually
> > > be able to dynamically add or remove fetcher threads using KIP-226.
> > > Therefore, we can't use fetch session IDs to uniquely identify a given
> > > broker incarnation.  Any time we increase the number of fetcher
> threads, a
> > > new fetch session ID will show up.
> > >
> > > If we want to know if a fetch request is from a shutting down broker or
> > > from a restarted broker, the most straightforward and robust way would
> > > probably be to add an incarnation number for each broker.  ZK can track
> > > this number.  This also helps with debugging and logging (you can tell
> > > "aha-- this request came from the second incarnation, not the first."
> > >
> > > > BTW, to clarify, the main purpose of returning the data at the index
> > > > boundary was to get the same benefit of efficient incremental fetch
> for
> > > > both low vol and high vol partitions, which is directly related to
> the
> > > > primary goal in this KIP. The other things (such as avoiding binary
> > > search)
> > > > are just potential additional gain, and they are also brought up to
> see
> > > if
> > > > that could be a "global optimal" solution.
> > >
> > > I still think these are separate.  The primary goal of the KIP was to
> make
> > > fetch requests where not all partitions are returning data more
> efficient.
> > > This isn't really related to the goal of trying to make accessing
> > > historical data more efficient.  In most cases, the data we're
> accessing is
> > > very recent data, and index lookups are not an issue.
> > >
> > > >
> > > > Some other replies below.
> > > > >In order for improvements to succeed, I think that it's important to
> > > > clearly define the scope and goals.  One good example of this was the
> > > > AdminClient KIP.  We deliberately avoiding ?>discussing new
> > > administrative
> > > > RPCs in that KIP, in order to limit the scope.  This kept the
> discussion
> > > > focused on the user interfaces and configuration, rather than on the
> > > > details of possible >new RPCs.  Once the KIP was completed, it was
> easy
> > > for
> > > > us to add new RPCs later in separate KIPs.
> > > > Hmm, why is AdminClient is related? All the discussion are about how
> to
> > > > make fetch more efficient, right?
> > > >
> > > > >Finally, it's not clear that the approach you are proposing is the
> right
> > > > way to go.  I think we would need to have a lot more discussion
> about it.
> > > > One very big disadvantage is that it couples >what we send back on
> the
> > > wire
> > > > tightly to what is on the disk.  It's not clear that we want to do
> that.
> > > > What if we want to change how things are stored in the future?  How
> does
> > > > this work with >clients' own concept of fetch sizes?  And so on, and
> so
> > > > on.  This needs its own discussion thread.
> > > > That might be true. However, the index file by definition is for the
> > > files
> > > > stored on the disk. So if we decide to change the storage layer to
> > > > something else, it seems natural to use some other suitable ways to
> get
> > > the
> > > > offsets efficiently.
> > > >
> > > > >There are a lot of simpler solutions that might work as well or
> better.
> > > > For example, each partition could keep an in-memory LRU cache of the
> most
> > > > recently used offset to file position >mappings.  Or we could have a
> > > thread
> > > > periodically touch the latest page or two of memory in the index
> file for
> > > > each partition, to make sure that it didn't fall out of the cache.
> In
> > > some
> > > > offline >discussions, some of these approaches have looked quite
> > > > promising.  I've even seen some good performance numbers for
> prototypes.
> > > > In any case, it's a separate problem which needs its >own KIP, I
> think.
> > > > Those are indeed separate discussions. I was not intended to discuss
> them
> > > > in this KIP. Sorry about the confusion.
> > > >
> > > > Thanks and Merry Christmas,
> > >
> > > Happy new year.  Sorry if some of my responses are delayed (I'm on
> > > vacation).
> > >
> > > cheers,
> > > Colin
> > >
> > > >
> > > > Jiangjie (Becket) Qin
> > > >
> > > >
> > > > On Sat, Dec 23, 2017 at 1:16 AM, Colin McCabe <cmccabe@apache.org>
> > > wrote:
> > > >
> > > > > On Fri, Dec 22, 2017, at 14:31, Becket Qin wrote:
> > > > > > >>
> > > > > > >> The point I want to make is that avoiding doing binary search
> on
> > > index
> > > > > > >> file and avoid reading the log segments during fetch has some
> > > > > additional
> > > > > > >> benefits. So if the solution works for the current KIP, it
> might
> > > be a
> > > > > > >> better choice.
> > > > > >
> > > > > > >Let's discuss this in a follow-on KIP.
> > > > > >
> > > > > > If the discussion will potentially change the protocol in the
> current
> > > > > > proposal. Would it be better to discuss it now instead of in a
> > > follow-up
> > > > > > KIP so we don't have some protocol that immediately requires a
> > > change.
> > > > >
> > > > > Hi Becket,
> > > > >
> > > > > I think that the problem that you are discussing is different than
> the
> > > > > problem this KIP is designed to address.  This KIP is targeted at
> > > > > eliminating the wastefulness of re-transmitting information about
> > > > > partitions that haven't changed in every FetchRequest and
> > > FetchResponse.
> > > > > The problem you are discussing is dealing with situations where the
> > > index
> > > > > file or the data file is not in the page cache, and therefore we
> take a
> > > > > page fault when doing an index lookup.
> > > > >
> > > > > This KIP is useful and valuable on its own.  For example, if you
> have
> > > > > brokers in a public cloud in different availability zones, you may
> > > wish to
> > > > > minimize the network traffic between them.  Therefore, you don't
> want
> > > every
> > > > > FetchRequest between brokers to be a full FetchRequest.  In that
> case,
> > > this
> > > > > KIP is very valuable.
> > > > >
> > > > > In order for improvements to succeed, I think that it's important
> to
> > > > > clearly define the scope and goals.  One good example of this was
> the
> > > > > AdminClient KIP.  We deliberately avoiding discussing new
> > > administrative
> > > > > RPCs in that KIP, in order to limit the scope.  This kept the
> > > discussion
> > > > > focused on the user interfaces and configuration, rather than on
> the
> > > > > details of possible new RPCs.  Once the KIP was completed, it was
> easy
> > > for
> > > > > us to add new RPCs later in separate KIPs.
> > > > >
> > > > > While it's clear that there is probably even more we could do to
> > > optimize
> > > > > fetch requests, making them incremental seems like a good first
> cut.  I
> > > > > deliberately avoided changing the replication protocol in this KIP,
> > > because
> > > > > I think that it's a big enough change as-is.  If we want to change
> the
> > > > > replication protocol in the future, there is nothing preventing
> us...
> > > and
> > > > > this change will be a useful starting point.
> > > > >
> > > > > Finally, it's not clear that the approach you are proposing is the
> > > right
> > > > > way to go.  I think we would need to have a lot more discussion
> about
> > > it.
> > > > > One very big disadvantage is that it couples what we send back on
> the
> > > wire
> > > > > tightly to what is on the disk.  It's not clear that we want to do
> > > that.
> > > > > What if we want to change how things are stored in the future?  How
> > > does
> > > > > this work with clients' own concept of fetch sizes?  And so on,
> and so
> > > on.
> > > > > This needs its own discussion thread.
> > > > >
> > > > > There are a lot of simpler solutions that might work as well or
> better.
> > > > > For example, each partition could keep an in-memory LRU cache of
> the
> > > most
> > > > > recently used offset to file position mappings.  Or we could have a
> > > thread
> > > > > periodically touch the latest page or two of memory in the index
> file
> > > for
> > > > > each partition, to make sure that it didn't fall out of the
> cache.  In
> > > some
> > > > > offline discussions, some of these approaches have looked quite
> > > promising.
> > > > > I've even seen some good performance numbers for prototypes.  In
> any
> > > case,
> > > > > it's a separate problem which needs its own KIP, I think.
> > > > >
> > > > > best,
> > > > > Colin
> > > > >
> > > > > >
> > > > > >
> > > > > > On Tue, Dec 19, 2017 at 9:26 AM, Colin McCabe <colin@cmccabe.xyz
> >
> > > wrote:
> > > > > >
> > > > > > > On Tue, Dec 19, 2017, at 02:16, Jan Filipiak wrote:
> > > > > > > > Sorry for coming back at this so late.
> > > > > > > >
> > > > > > > >
> > > > > > > >
> > > > > > > > On 11.12.2017 07:12, Colin McCabe wrote:
> > > > > > > > > On Sun, Dec 10, 2017, at 22:10, Colin McCabe wrote:
> > > > > > > > >> On Fri, Dec 8, 2017, at 01:16, Jan Filipiak wrote:
> > > > > > > > >>> Hi,
> > > > > > > > >>>
> > > > > > > > >>> sorry for the late reply, busy times :-/
> > > > > > > > >>>
> > > > > > > > >>> I would ask you one thing maybe. Since the timeout
> > > > > > > > >>> argument seems to be settled I have no further argument
> > > > > > > > >>> form your side except the "i don't want to".
> > > > > > > > >>>
> > > > > > > > >>> Can you see that connection.max.idle.max is the exact
> time
> > > > > > > > >>> that expresses "We expect the client to be away for this
> > > long,
> > > > > > > > >>> and come back and continue"?
> > > > > > > > >> Hi Jan,
> > > > > > > > >>
> > > > > > > > >> Sure, connection.max.idle.max is the exact time that we
> want
> > > to
> > > > > keep
> > > > > > > > >> around a TCP session.  TCP sessions are relatively cheap,
> so
> > > we
> > > > > can
> > > > > > > > >> afford to keep them around for 10 minutes by default.
> > > Incremental
> > > > > > > fetch
> > > > > > > > >> state is less cheap, so we want to set a shorter timeout
> for
> > > it.
> > > > > We
> > > > > > > > >> also want new TCP sessions to be able to reuse an existing
> > > > > incremental
> > > > > > > > >> fetch session rather than creating a new one and waiting
> for
> > > the
> > > > > old
> > > > > > > one
> > > > > > > > >> to time out.
> > > > > > > > >>
> > > > > > > > >>> also clarified some stuff inline
> > > > > > > > >>>
> > > > > > > > >>> Best Jan
> > > > > > > > >>>
> > > > > > > > >>>
> > > > > > > > >>>
> > > > > > > > >>>
> > > > > > > > >>> On 05.12.2017 23:14, Colin McCabe 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.
> > > > > > > > >>> First I would say, the client has a big say in this. If
> the
> > > > > client
> > > > > > > > >>> is not going to issue incremental he shouldn't ask for a
> > > cache
> > > > > > > > >>> when the client ask for the cache we still have all
> options
> > > to
> > > > > deny.
> > > > > > > > >> To put it simply, we have to have some cache management
> above
> > > and
> > > > > > > beyond
> > > > > > > > >> just giving out an incremental fetch session to anyone who
> > > has a
> > > > > TCP
> > > > > > > > >> session.  Therefore, caching does not become simpler if
> you
> > > > > couple the
> > > > > > > > >> fetch session to the TCP session.
> > > > > > > > Simply giving out an fetch session for everyone with a
> > > connection is
> > > > > too
> > > > > > > > simple,
> > > > > > > > but I think it plays well into the idea of consumers
> choosing to
> > > use
> > > > > the
> > > > > > > > feature
> > > > > > > > therefore only enabling where it brings maximum gains
> > > > > > > > (replicas,MirrorMakers)
> > > > > > > > >>
> > > > > > > > >>>> 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.
> > > > > > > > >>> We still have all the options denying the request to
> keep the
> > > > > state.
> > > > > > > > >>> What you want seems like a max connections / ip
> safeguard.
> > > > > > > > >>> I can currently take down a broker with to many
> connections
> > > > > easily.
> > > > > > > > >>>
> > > > > > > > >>>
> > > > > > > > >>>>> 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.
> > > > > > > > >>> This doesn't make much sense to me.
> > > > > > > > >> There are no tradeoffs to think about from the client's
> point
> > > of
> > > > > view:
> > > > > > > > >> it always wants an incremental fetch session.  So there
> is no
> > > > > benefit
> > > > > > > to
> > > > > > > > >> making the clients configure an extra setting.  Updating
> and
> > > > > managing
> > > > > > > > >> client configurations is also more difficult than managing
> > > broker
> > > > > > > > >> configurations for most users.
> > > > > > > > >>
> > > > > > > > >>> You also wanted to implement
> > > > > > > > >>> a "turn of in case of bug"-knob. Having the client
> indicate
> > > if
> > > > > the
> > > > > > > > >>> feauture will be used seems reasonable to me.,
> > > > > > > > >> True.  However, if there is a bug, we could also roll
> back the
> > > > > client,
> > > > > > > > >> so having this configuration knob is not strictly
> required.
> > > > > > > > >>
> > > > > > > > >>>>> 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.
> > > > > > > > >>> I actually have a hard time believing this. Do you have
> any
> > > > > numbers
> > > > > > > of
> > > > > > > > >>> any existing production system? Is it the virtualisation
> > > layer
> > > > > > > cutting
> > > > > > > > >>> all the connections?
> > > > > > > > >>> We see this only on application crashes and restarts
> where
> > > the
> > > > > app
> > > > > > > needs
> > > > > > > > >>> todo the full anyways
> > > > > > > > >>> as it probably continues with stores offsets.
> > > > > > > > >> Yes, TCP connections get dropped.  It happens very often
> in
> > > > > production
> > > > > > > > >> clusters, actually.  When I was working on Hadoop, one of
> the
> > > most
> > > > > > > > >> common questions I heard from newcomers was "why do I see
> so
> > > many
> > > > > > > > >> EOFException messages in the logs"?  The other thing that
> > > happens
> > > > > a
> > > > > > > lot
> > > > > > > > >> is DNS outages or slowness.  Public clouds seem to have
> even
> > > more
> > > > > > > > >> unstable networks than the on-premise clusters.  I am not
> > > sure why
> > > > > > > that
> > > > > > > > >> is.
> > > > > > > > Hadoop has a wiki page on exactly this
> > > > > > > > https://wiki.apache.org/hadoop/EOFException
> > > > > > > >
> > > > > > > > besides user errors they have servers crashing and actually
> loss
> > > of
> > > > > > > > connection high on their list.
> > > > > > > > In the case of "server goes away" the cache goes with it. So
> > > nothing
> > > > > to
> > > > > > > > argue about the cache beeing reused by
> > > > > > > > a new connection.
> > > > > > > >
> > > > > > > > Can you make an argument at which point the epoch would be
> > > updated
> > > > > > > > broker side to maximise re-usage of the cache on
> > > > > > > > lost connections. In many cases the epoch would go out of
> sync
> > > and we
> > > > > > > > would need a full fetch anyways. Am I mistaken here?
> > > > > > >
> > > > > > > The current proposal is that the server can accept multiple
> > > requests
> > > > > in a
> > > > > > > row with the same sequence number.
> > > > > > >
> > > > > > > Colin
> > > > > > >
> > > > > > > >
> > > > > > > >
> > > > > > > >
> > > > > > > >
> > > > > > > > >>
> > > > > > > > >>>>>>>> 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.
> > > > > > > > >>> We only check max.message.bytes to late to guard against
> > > consumer
> > > > > > > > >>> stalling.
> > > > > > > > >>> we dont have a notion of max.networkpacket.size before we
> > > > > allocate
> > > > > > > the
> > > > > > > > >>> bytebuffer to read it into.
> > > > > > > > >> "network packets" are not the same thing as "kafka
> RPCs."  One
> > > > > Kafka
> > > > > > > RPC
> > > > > > > > >> could take up mutiple ethernet packets.
> > > > > > > > >>
> > > > > > > > >> Also, max.message.bytes has nothing to do with "consumer
> > > > > stalling" --
> > > > > > > > >> you are probably thinking about some of the fetch request
> > > > > > > > >> configurations.  max.message.bytes is used by the RPC
> system
> > > to
> > > > > figure
> > > > > > > > >> out whether to read the full incoming RP
> > > > > > > > > Whoops, this is incorrect.  I was thinking about
> > > > > > > > > "socket.request.max.bytes" rather than "max.message.bytes."
> > > Sorry
> > > > > > > about
> > > > > > > > > that.  See Ismael's email as well.
> > > > > > > > >
> > > > > > > > > best,
> > > > > > > > > Colin
> > > > > > > > >
> > > > > > > > >> best,
> > > > > > > > >> Colin
> > > > > > > > >>
> > > > > > > > >>>>> 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.
> > > > > > > > >>> Yes, we need to fail ASAP when we read corrupted
> packages. It
> > > > > seemed
> > > > > > > > >>> to me like you tried to make the case for pray and try to
> > > stay
> > > > > alive.
> > > > > > > > >>> Fault
> > > > > > > > >>> tolerance here means. I am a fishy box i am going to let
> a
> > > good
> > > > > box
> > > > > > > > >>> handle
> > > > > > > > >>> it and be silent until i get fixed up.
> > > > > > > > >>>> 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.
> > > > > > > > >>> See above.
> > > > > > > > >>>> 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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