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Subject: Re: [DISCUSS] KIP-227: Introduce Incremental FetchRequests to Increase
 Partition Scalability
Date: Sun, 03 Dec 2017 12:55:41 -0800
archived-at: Sun, 03 Dec 2017 20:55:49 -0000

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.

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.

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.

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.

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
> > > > > > > >
> > > > > >
> > > >
> >