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From: Colin McCabe <cmccabe@apache.org>
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Subject: Re: [DISCUSS] KIP-227: Introduce Incremental FetchRequests to Increase
 Partition Scalability
In-Reply-To: <CAAaarBbjTJH3_Zwv6ZqCK-SMdWP0C_Nin8kpK57MfcOrcJXQUg@mail.gmail.com>
Date: Mon, 11 Dec 2017 15:18:35 -0800
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archived-at: Mon, 11 Dec 2017 23:18:45 -0000

On Mon, Dec 11, 2017, at 13:06, Dong Lin wrote:
> Hey Colin,
> 
> I went over the latest KIP wiki and have a few comments here.
> 
> 1) The KIP says that client ID is a string if the session belongs to a
> Kafka consumer. And it is a numerical follower Id if the session belongs
> to a follower. 

Hi Dong,

Right.  The issue is that replicas are identified by integers, whereas
consumers are identified by strings.

> Can we have a consistent type for the client Id?

We could use a string for both, perhaps?  Theoretically, a consumer
could also be named "broker 0" though, right?  So it would not be unique
any more.  Not sure what the best approach is here... what do you think?

> 2) "The numeric follower ID, if this fetch session belongs to a Kafka
> broker". If the broker has multiple replica fetcher thread, do they all
> have the same follower Id in teh leader broker?

Yes, every session created by the same replica will have the same
replica ID.  The fetch session ID will be different for each thread,
though.

> 3) One of the condition for evicting an existing session is that "The new
> session belongs to a follower, and the existing session belongs to a
> regular consumer". I am not sure the session from follower should also be
> restricted by the newly added config. It seems that we will always create
> lots for FetchRequest from follower brokers. Maybe the
> "max.incremental.fetch.session.cache.slots" should only be applies if the
> FetchRequest comes from a client consumer?

Well, replicas may sometimes go down.  In that case, when they come back
up, they will create new fetch sessions.  So we need to be able to evict
the old fetch sessions from the cache.  In the course of normal
operation, though, replicas should always have incremental fetch
sessions, because they are prioritized over consumers.

> 
> 4) Not sure I fully understand how the "The last dirty sequence number"
> is
> used. It is mentioned that "Let P1 have a last dirty sequence number of
> 100, and P2 have a last dirty sequence number of 101. An incremental
> fetch
> request with sequence number 100 will return information about both P1
> and
> P2." But would be the fetch offset for P2 in this case, if the last fetch
> offset stored in the Fetch Session for P2 is associated with the last
> dirty
> sequence number 101 for P2? My gut feel is that you would have to stored
> the fetch offset for sequence number 100 for P2 as well. Did I miss
> something here?

It's OK to return the latest available data.  So if someone sends a
fetch request with seqno 100, and then you get new data, and then the
fetch request with seqno 100 gets resent, it's OK to include the new
data in the second response, even though it wasn't in the first
response.  We are NOT implementing snapshots here, or anything like
that.  We are just trying to guard against the "lost updates" case. 
Also, keep in mind that the fetch offset is not updated until it is sent
in a fetch request, just like now.

best,
Colin

> 
> Thanks,
> Dong
> 
> On Sun, Dec 10, 2017 at 11:15 PM, Becket Qin <becket.qin@gmail.com>
> wrote:
> 
> > Hi Jun,
> >
> > I see. Yes, that makes sense. Are we going to do that only for the fetches
> > whose per partition fetch size cannot reach the first index entry after the
> > fetch position, or are we going to do that for any fetch? If we do that for
> > any fetch, then we will still need to read the actual log segment, which
> > could be expensive if the data is no longer in the cache. This hurts
> > performance if some fetches are on the old log segments.
> >
> > I took a quick look on the clusters we have. The idle topic ratio varies
> > depending on the usage of the cluster. For our metric cluster and database
> > replication clusters almost all the topics are actively used. For tracking
> > clusters, ~70% topics have data coming in at different rate. For other
> > clusters such as queuing and data deployment. There are more idle topics
> > and the traffic is more bursty (I don't have the exact number here).
> >
> > Thanks,
> >
> > Jiangjie (Becket) Qin
> >
> > On Sun, Dec 10, 2017 at 10:17 PM, Colin McCabe <cmccabe@apache.org> wrote:
> >
> > > On Fri, Dec 8, 2017, at 16:56, Jun Rao wrote:
> > > > Hi, Jiangjie,
> > > >
> > > > What I described is almost the same as yours. The only extra thing is
> > to
> > > > scan the log segment from the identified index entry a bit more to
> > find a
> > > > file position that ends at a message set boundary and is less than the
> > > > partition level fetch size. This way, we still preserve the current
> > > > semantic of not returning more bytes than fetch size unless there is a
> > > > single message set larger than the fetch size.
> > > >
> > > > In a typically cluster at LinkedIn, what's the percentage of idle
> > > > partitions?
> > >
> > > Yeah, that would be a great number to get.
> > >
> > > Of course, KIP-227 will also benefit partitions that are not completely
> > > idle.  For instance, a partition that's getting just one message a
> > > second will appear in many fetch requests, unless every other partition
> > > in the system is also only getting a low rate of incoming messages.
> > >
> > > regards,
> > > Colin
> > >
> > > >
> > > > Thanks,
> > > >
> > > > Jun
> > > >
> > > >
> > > > On Wed, Dec 6, 2017 at 6:57 PM, Becket Qin <becket.qin@gmail.com>
> > wrote:
> > > >
> > > > > Hi Jun,
> > > > >
> > > > > Yes, we still need to handle the corner case. And you are right, it
> > is
> > > all
> > > > > about trade-off between simplicity and the performance gain.
> > > > >
> > > > > I was thinking that the brokers always return at least
> > > > > log.index.interval.bytes per partition to the consumer, just like we
> > > will
> > > > > return at least one message to the user. This way we don't need to
> > > worry
> > > > > about the case that the fetch size is smaller than the index
> > interval.
> > > We
> > > > > may just need to let users know this behavior change.
> > > > >
> > > > > Not sure if I completely understand your solution, but I think we are
> > > > > thinking about the same. i.e. for the first fetch asking for offset
> > > x0, we
> > > > > will need to do a binary search to find the position p0. and then the
> > > > > broker will iterate over the index entries starting from the first
> > > index
> > > > > entry whose offset is greater than p0 until it reaches the index
> > > entry(x1,
> > > > > p1) so that p1 - p0 is just under the fetch size, but the next entry
> > > will
> > > > > exceed the fetch size. We then return the bytes from p0 to p1.
> > > Meanwhile
> > > > > the broker caches the next fetch (x1, p1). So when the next fetch
> > > comes, it
> > > > > will just iterate over the offset index entry starting at (x1, p1).
> > > > >
> > > > > It is true that in the above approach, the log compacted topic needs
> > > to be
> > > > > handled. It seems that this can be solved by checking whether the
> > > cached
> > > > > index and the new log index are still the same index object. If they
> > > are
> > > > > not the same, we can fall back to binary search with the cached
> > > offset. It
> > > > > is admittedly more complicated than the current logic, but given the
> > > binary
> > > > > search logic already exists, it seems the additional object sanity
> > > check is
> > > > > not too much work.
> > > > >
> > > > > Not sure if the above implementation is simple enough to justify the
> > > > > performance improvement. Let me know if you see potential complexity.
> > > > >
> > > > > Thanks,
> > > > >
> > > > > Jiangjie (Becket) Qin
> > > > >
> > > > >
> > > > >
> > > > >
> > > > >
> > > > > On Wed, Dec 6, 2017 at 4:48 PM, Jun Rao <jun@confluent.io> wrote:
> > > > >
> > > > > > Hi, Becket,
> > > > > >
> > > > > > Yes, I agree that it's rare to have the fetch size smaller than
> > index
> > > > > > interval. It's just that we still need additional code to handle
> > the
> > > rare
> > > > > > case.
> > > > > >
> > > > > > If you go this far, a more general approach (i.e., without
> > returning
> > > at
> > > > > the
> > > > > > index boundary) is the following. We can cache the following
> > > metadata for
> > > > > > the next fetch offset: the file position in the log segment, the
> > > first
> > > > > > index slot at or after the file position. When serving a fetch
> > > request,
> > > > > we
> > > > > > scan the index entries from the cached index slot until we hit the
> > > fetch
> > > > > > size. We can then send the data at the message set boundary and
> > > update
> > > > > the
> > > > > > cached metadata for the next fetch offset. This is kind of
> > > complicated,
> > > > > but
> > > > > > probably not more than your approach if the corner case has to be
> > > > > handled.
> > > > > >
> > > > > > In both the above approach and your approach, we need the
> > additional
> > > > > logic
> > > > > > to handle compacted topic since a log segment (and therefore its
> > > index)
> > > > > can
> > > > > > be replaced between two consecutive fetch requests.
> > > > > >
> > > > > > Overall, I agree that the general approach that you proposed
> > applies
> > > more
> > > > > > widely since we get the benefit even when all topics are high
> > volume.
> > > > > It's
> > > > > > just that it would be better if we could think of a simpler
> > > > > implementation.
> > > > > >
> > > > > > Thanks,
> > > > > >
> > > > > > Jun
> > > > > >
> > > > > > On Tue, Dec 5, 2017 at 9:38 PM, Becket Qin <becket.qin@gmail.com>
> > > wrote:
> > > > > >
> > > > > > > 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
> > > > > > > > > > >
> > > > > > > > > >
> > > > > > > > >
> > > > > > > >
> > > > > > >
> > > > > >
> > > > >
> > >
> >