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 Clock issues from client side being outdated and causing headache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
> > >
> >
>
