> If the goal is to minimize the amount of redeliveries from broker ->
client, there are multiple ways to achieve that with the client based
approach
(eg. send message id and delay time instead of the full payload to
consumers
as Ivan proposed).
But the main reason to put this logic on client side was not adding delay
related logic on broker side, in order to do this optimisations the broker
must be aware of delayed message and only send message id and delay time
without payload.
> I don't necessarily agree with that. NTP is widely available
and understood. Any application that's doing anything time-related would
have
to make sure the clocks are reasonably synced.
Yep, that's true, but from my point of view a system that depends on client
side clock is weaker than a system that does this kind of calculation at
a more controlled environment aka backend. This adds one more factor that
depends on the user doing things right, which is not always the case.
One possible solution might be the broker send periodically its current
epoch time and the client do the calculations with this data, or send epoch
time initially at subscription and do the rest of calculations doing delta
of
time using the initial time from broker as a base (time flows equally for
both
the important thing is which one is positioned at the very present time).
Anyway this mentioned approach sound like an a hack just from the fact of
not doing the time calculations in the backend.
> Lastly, i do agree client side approaches have better scalability than
server side approaches in most cases. However I don’t believe that it is
the case here. And I don’t see anyone have a clear explanation on why a
broker approach is less scalable than the client side approach.
Yes, I agree with this. At least for fixed time delay at pr #3155.
The only remained concern to me would be Gc usage of stored positions
next to be expired, anyway, since the nature of a fixed delay and
from the fact that process a ledger tend to be in a sequentially manner,
we could store a range of positions id for some delta when intensive
traffic is going on, I believe I did this mention on the pr.
> Again, in general I'm more concerned of stuff that happens in broker
because
it will have to be scaled up 10s of thousands of times in a single
process, while in client typically the requirements are much simpler.
I agree that adding logic to broker should be considered with deep care,
but in this specific scenario at worst case we will only have one and only
one scheduled task per consumer which will take all expired positions
from a DelayQueue.
--
*Ezequiel Lovelle*
On Sat, 19 Jan 2019 at 01:02, Matteo Merli <matteo.me...@gmail.com> wrote:
> Just a quick correction:
>
> > And I don’t see anyone have a clear explanation on why a
> broker approach is less scalable than the client side approach.
>
> I haven't said it less or more scalable. I was meaning that it's
> "easier" to scale, in that we don't have to do lots of fancy stuff
> and add more and more control to make sure that the implementation
> will not become a concern point at scale (eg: limit the overall
> amount of memory used in broker, across all topics, and the
> impact on GC of these long-living objects).
>
> > However, clock skew in a brokerside approach
> is easier to manage and more predictable, but clock skew in a clientside
> approach is much harder to manage and more unpredictable
>
> I don't necessarily agree with that. NTP is widely available
> and understood.
> Any application that's doing anything time-related would have
> to make sure the clocks are reasonably synced.
>
> --
> Matteo Merli
> <matteo.me...@gmail.com>
>
> On Fri, Jan 18, 2019 at 7:46 PM Sijie Guo <guosi...@gmail.com> wrote:
> >
> > On Sat, Jan 19, 2019 at 9:45 AM Matteo Merli <matteo.me...@gmail.com>
> wrote:
> >
> > > Trying to group and compress responses here.
> > >
> > > > If consumer control the delayed message specific execution time we
> must
> > > trust clock of consumer, this can cause delayed message process ahead
> of
> > > time, some applications cannot tolerate this condition.
> > >
> > > This is a problem that cannot be solved.
> > > Even assuming the timestamps are assigned by brokers and are guaranteed
> > > to be monotonic, this won't prevent 2 brokers from having clock skews.
> > > That would results in different delivery delays.
> > >
> > > Similarly, the broker timestamp might be assigned later compared to
> when a
> > > publisher was "intending" to start the clock.
> > >
> > > Barring super-precise clock synchronization techniques (which are way
> out
> > > of the scope of this discussion), the only reasonable way to think
> about
> > > this is
> > > that delays needs to be orders of magnitudes bigger than the average
> clock
> > > skew experienced with common techniques (eg: NTP). NTP clock skew will
> > > generally be in the 10s of millis. Any delay > 1 seconds will hardly be
> > > noticeably affected by these skews.
> > >
> > > Additionally, any optimization on the timeouts handling (like the
> > > hash-wheel
> > > timer proposed in PIP-26) will trade off precision for efficiency. In
> that
> > > case,
> > > the delays are managed in buckets, and can result in higher delays that
> > > what was requested.
> > >
> > > > 1. Fixed timeout, e.g.(with 10s, 30s, 10min delayed), this is the
> largest
> > > proportion in throughput of delayed message . A subscription with a
> fixed
> > > delayed time can approach to this scene.
> > >
> > > I don't think that for fixed delays, any server-side implementation
> > > would provide
> > > any advantage compared to doing:
> > >
> > > ```
> > > while (true) {
> > > Message msg = consumer.receive();
> > > long delayMillis = calculateDelay(msg)
> > > if (delayMillis > 0) {
> > > Thread.sleep(delayMillis);
> > > }
> > >
> > > // Do something
> > > consumer.acknowledge(msg);
> > > }
> > > ```
> > >
> > > This will not need any support from broker. Also, there will be no
> > > redeliveries.
> > >
> > > It could be wrapped in the client API, although I don't see that as
> > > big of a problem.
> > >
> > > > My concern of this category of approaches is "bandwidth" usage. It is
> > > basically trading bandwidth for complexity.
> > >
> > > With mixed delays on a single topic, in any case there has to be some
> kind
> > > of time-based sorting of the messages that needs to happen either at
> broker
> > > or at client.
> > >
> > > Functionally, I believe that either place is equivalent (from a user
> > > point of view),
> > > barring the different implementation requirements.
> > >
> > > In my view, the bigger cost here is not bandwidth but rather the disk
> > > IO, that will
> > > happen exactly in the same way in both cases. Messages can be cached,
> > > up to a certain point, either in broker or in client library. After
> > > that, in both cases,
> > > the messages will have to be fetched from bookies.
> > >
> > > Also, when implementing the delay feature in the client, the existing
> > > flow control
> > > mechanism is naturally applied to limit the overall amount of
> information
> > > that
> > > we have to keep track (the "currently tracked" messages). Some other
> > > mechanism
> > > would have to be done in the broker as well.
> > >
> > > Again, in general I'm more concerned of stuff that happens in broker
> > > because
> > > it will have to be scaled up 10s of thousands of times in a single
> > > process, while
> > > in client typically the requirements are much simpler.
> > >
> > > If the goal is to minimize the amount of redeliveries from broker ->
> > > client, there
> > > are multiple ways to achieve that with the client based approach (eg.
> send
> > > message id and delay time instead of the full payload to consumers as
> Ivan
> > > proposed).
> > >
> > > This seems to be simpler and with less overhead than having to persist
> > > the whole
> > > hashweel timer state into a ledger.
> >
> >
> > I agree with that there are many optimizations can be applied at a client
> > side approach. In a stable world, these approaches are technically
> > equivalent.
> >
> > However I do not agree with a few points:
> >
> > First, based on my past production experiences, network bandwidth on
> broker
> > is the bigger cost than io cost in a multi subscription case. Also, I
> have
> > heard a few production users have experienced latency issues where broker
> > network bandwidth is saturated. So any mechanisms that rely on
> redeliveries
> > are a big red flag to me.
> >
> > Secondly, currently pulsar is using more bandwidth on brokers, than
> > bandwidth on bookies. It is not a balanced state. I am more leaning
> towards
> > an approach that can leverage bookies’ idle bandwidth, rather than
> > potentially using more bandwidth on brokers.
> >
> > Thirdly, in my view, clock skew concern is not a technical issue, but a
> > management issue. As what Ivan and you have pointed out, there are many
> > ways on addressing clock skew. However, clock skew in a brokerside
> approach
> > is easier to manage and more predictable, but clock skew in a clientside
> > approach is much harder to manage and more unpredictable. This
> > unpredictability can significantly change the io or network pattern when
> > things go bad. When such unpredictability happens, it can cause bad
> things
> > and saturating broker network in a redeliver-ish approach. If we are
> > building a distributed system that can handle this unpredictability, a
> > broker-side approach is much more friendly to managebility and incident
> > management.
> >
> > Lastly, i do agree client side approaches have better scalability than
> > server side approaches in most cases. However I don’t believe that it is
> > the case here. And I don’t see anyone have a clear explanation on why a
> > broker approach is less scalable than the client side approach.
> >
> > Anyway, for managebility, bandwidth usage, client simplicity, I am more
> in
> > favor of a broker side approach, or at least an approach that is not
> > redelivery based. However since the feature is requested by Penghui
> > and Ezequiel,
> > I am also fine with this client side approach if they are okay with that.
> >
> > - Sijie
> >
> >
> >
> >
> > >
> > >
> > >
> > > --
> > > Matteo Merli
> > > <matteo.me...@gmail.com>
> > >
> > >
> > > On Fri, Jan 18, 2019 at 6:35 AM Ezequiel Lovelle
> > > <ezequiellove...@gmail.com> wrote:
> > > >
> > > > Hi All! and sorry for delay :)
> > > >
> > > > Probably I'm going to say some things already said, so sorry
> beforehand.
> > > >
> > > > The two main needed features I think are the proposed:
> > > > A. Producer delay PIP-26. B. Consumers delay PR #3155
> > > >
> > > > Of course PIP-26 would result in consumers receiving delayed messages
> > > > but the important thing here is one of them made the decision about
> > > delay.
> > > >
> > > > First, the easy one, PR #3155. Consumers delay:
> > > >
> > > > As others have stated before, this is a more trivial approach because
> > > > of the nature of having the exactly same period of delay for each
> message
> > > > which is predictable.
> > > >
> > > > I agree that adding logic at broker should be avoided, but, for this
> > > > specific feature #3155 which I don't think is complex I believe there
> > > > are others serious advantages:
> > > >
> > > > 1. Simplicity at client side, we don't need to add any code which is
> > > > less error prone.
> > > > 2. Clock issues from client side being outdated and causing headache
> > > > to users detecting this.
> > > > 3. Avoids huge overhead delivering non expired messages across the
> > > > network unnecessary.
> > > > 4. Consumers are free to decide to consume messages with delay
> > > regardless
> > > > of the producer.
> > > > 5. Delay is uniform for all messages, which sometimes is the
> solution
> > > > to the problem rather than arbitrary delays.
> > > >
> > > > I think that would be great if pulsar can provide this kind of
> features
> > > > without relaying on users needing to know heavy details about the
> > > > mechanism.
> > > >
> > > > For PIP-26:
> > > >
> > > > I think we can offer this with the purpose of message's with a more
> long
> > > > delay in terms of time? hours / days?
> > > >
> > > > So, if this is the case, we can assume a small granularity of time
> like
> > > > 1 minute making ledger's representing 1 minute of time and truncating
> > > > each time of message for it corresponding minute and storing in that
> > > > special ledger.
> > > > Users wanting to receive a messages scheduled for some days in future
> > > > rarely would care of a margin of error of 1 minute.
> > > >
> > > > Of course we need somehow make the broker aware of this in order to
> only
> > > > process ledger's for current corresponding minute and consume it.
> > > > And the broker would be the one subject to close current minute
> truncated
> > > > processed ledger.
> > > >
> > > > One problem I can think about this approach, is it painful for
> Bookkeeper
> > > > to having a lot of opened ledgers? (one for each minute per topic)
> > > >
> > > > Another problem here might be what happen if consumer was not
> started?
> > > > At startup time the broker should looking for potentially older
> ledger's
> > > > than its current time and this might be expensive.
> > > >
> > > > Other more trivial issue, we might need to refactor current mechanism
> > > > which deletes closed ledgers older than the configured time on name
> > > space.
> > > >
> > > > As a final note I think that would be great to have both features in
> > > pulsar
> > > > but sometimes not everything desired is achievable.
> > > > And please correct me if I said something senseless.
> > > >
> > > > --
> > > > *Ezequiel Lovelle*
> > > >
> > > >
> > > > On Fri, 18 Jan 2019 at 05:51, PengHui Li <codelipeng...@gmail.com>
> > > wrote:
> > > >
> > > > > > So rather than specifying the absolute timestamp that the message
> > > > > > should appear to the user, the dispatcher can specify the
> relative
> > > > > > delay after dispatch that it should appear to the user.
> > > > >
> > > > > As matteo said the worst case would be that the applied delay to be
> > > higher
> > > > > for some of the messages, if specify the relative delay to
> consumer,
> > > > > if consumer offline for a period of time, consumer will receive
> many
> > > > > delayed messages
> > > > > after connect to broker again will cause the worst case more
> serious.
> > > It's
> > > > > difficult to keep
> > > > > consumers always online.
> > > > >
> > > > > In my personal perspective, i refer to use `delay level topic` to
> > > approach
> > > > > smaller delays scene.
> > > > > e.g(10s-topic, 30s-topic), this will not be too much topic. And we
> are
> > > > > using dead letter topic to simulate
> > > > > delay message feature, delayed topics has different delay level.
> > > > >
> > > > > For very long delays scene, in our practice, user may cancel it or
> > > restart
> > > > > it.
> > > > > After previous discussions, i agree that PIP-26 will make broker
> > > > > more complexity.
> > > > > So I had the idea to consider as a separate mechanism.
> > > > >
> > > > >
> > > > > Sijie Guo <guosi...@gmail.com> 于2019年1月18日周五 下午3:22写道：
> > > > >
> > > > > > On Fri, Jan 18, 2019 at 2:51 PM Ivan Kelly <iv...@apache.org>
> wrote:
> > > > > >
> > > > > > > One thing missing from this discussion is details on the
> motivating
> > > > > > > use-case. How many delayed messages per second are we
> expecting?
> > > And
> > > > > > > what is the payload size?
> > > > > > >
> > > > > > > > If consumer control the delayed message specific execution
> time
> > > we
> > > > > must
> > > > > > > > trust clock of consumer, this can cause delayed message
> process
> > > ahead
> > > > > > of
> > > > > > > > time, some applications cannot tolerate this condition.
> > > > > > >
> > > > > > > This can be handled in a number of ways. Consumer clocks can be
> > > skewed
> > > > > > > with regard to other clocks, but it is generally safe to assume
> > > that
> > > > > > > clocks advance at the same rate, especially at the granularity
> of a
> > > > > > > couple of hours.
> > > > > > > So rather than specifying the absolute timestamp that the
> message
> > > > > > > should appear to the user, the dispatcher can specify the
> relative
> > > > > > > delay after dispatch that it should appear to the user.
> > > > > > >
> > > > > > > > > My concern of this category of approaches is "bandwidth"
> > > usage. It
> > > > > is
> > > > > > > > > basically trading bandwidth for complexity.
> > > > > > > >
> > > > > > > > @Sijie Guo <si...@apache.org> Agree with you, such an
> trading
> > > can
> > > > > > cause
> > > > > > > the
> > > > > > > > broker's out going network to be more serious.
> > > > > > >
> > > > > > > I don't think PIP-26's approach may not use less bandwidth in
> this
> > > > > > > regard. With PIP-26, the msg ids are stored in a ledger, and
> when
> > > the
> > > > > > > timeout triggers it dispatches? Are all the delayed message
> being
> > > > > > > cached at the broker? If so, that is using a lot of memory, and
> > > it's
> > > > > > > exactly the kind of memory usage pattern that is very bad for
> JVM
> > > > > > > garbage collection. If not, then you have to read the message
> back
> > > in
> > > > > > > from bookkeeper, so the bandwidth usage is the same, though on
> a
> > > > > > > different path.
> > > > > > >
> > > > > > > In the client side approach, the message could be cached to
> avoid a
> > > > > > > redispatch. When I was discussing with Matteo, we discussed
> this.
> > > The
> > > > > > > redelivery logic has to be there in any case, as any cache
> (broker
> > > or
> > > > > > > client side) must have a limited size.
> > > > > > > Another option would be to skip sending the payload for delayed
> > > > > > > messages, and only send it when the client request redelivery,
> but
> > > > > > > this has the same issue with regard to the entry likely
> falling out
> > > > > > > the cache at the broker-side.
> > > > > >
> > > > > >
> > > > > > There are bandwidth usage at either approaches for sure. The main
> > > > > > difference between broker-side and client-side approaches is
> which
> > > part
> > > > > of
> > > > > > the bandwidth is used.
> > > > > >
> > > > > > In the broker-side approach, it is using the bookies egress and
> > > broker
> > > > > > ingress bandwidth. In a typical pulsar deployment, bookies
> egress is
> > > > > mostly
> > > > > > idle unless there are consumers falling behind.
> > > > > >
> > > > > > In the client-side approach, it is using broker’s egress
> bandwidth
> > > and
> > > > > > potentially bookies’ egress bandwidth. Brokers’ egress is
> critical
> > > since
> > > > > it
> > > > > > is shared across consumers. So if the broker egress is doubled,
> it
> > > is a
> > > > > red
> > > > > > flag.
> > > > > >
> > > > > > Although I agree the bandwidth usage depends on workloads. But in
> > > theory,
> > > > > > broker-side approach is more friendly to resource usage and a
> better
> > > > > > approach to use the resources in a multi layered architecture.
> > > Because it
> > > > > > uses less bandwidth at broker side. A client side can cause more
> > > > > bandwidth
> > > > > > usage at broker side.
> > > > > >
> > > > > > Also as what penghui pointed out, clock screw can be another
> factor
> > > > > causing
> > > > > > more traffic in a fanout case. In a broker-side approach, the
> > > deferred is
> > > > > > done in a central point, so when the deferred time point kicks
> in,
> > > broker
> > > > > > just need to read the data one time from bookies. However in a
> > > > > client-side
> > > > > > approach, the messages are asked by different subscriptions,
> > > different
> > > > > > subscription can ask the deferred message at any time based on
> their
> > > > > > clocks.
> > > > > >
> > > > > >
> > > > > >
> > > > > > >
> > > > > > > -Ivan
> > > > > > >
> > > > > >
> > > > >
> > >
>
