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From "Pavel Moravec" <pmora...@redhat.com>
Subject Re: Review Request 19566: QPID-5642: Message sequence numbers (enabled with qpid.msg_sequence) should be persistent across broker restarts
Date Wed, 02 Apr 2014 09:19:10 GMT

> On April 1, 2014, 9:17 p.m., Alan Conway wrote:
> > Sorry, catching up. I think this patch is abusing the bootsequence which really
has nothing at all to do with exchange sequence numbers. It would work on a standalone (but
still be an abuse IMO) broker but it doesn't work for HA. The base for exchage sequence numbers
has to be synchronized across a HA cluster, whereas the number of times each broker has rebooted
*cannot* be synchronized without breaking its existing meaning as a reboot count. I'm also
a bit wary of the division of sequence number into 17 bits for "restart" count 47 bits for
"message" count. What are the grounds for that division? Is it safe to assume no overflows?
> > 
> > I see the goal of avoiding persisting the sequence number on every message, but
how about persisting it (per exchange) on some suitably large number of messages? E.g. persist
an initial next-sequence of 64k for each exchange, with the sequence number starting at 1.
Once the sequence number gets within 32k of next-sequence we increase next-sequence of by
64k etc. On reboot or on promotion of the primary we jump each exchange sequence number ahead
to the exchange's next-sequence number.
> > 
> > That lets us take smaller jumps on each restart/failover (16 bits vs. 47 bits) and
avoids using a single counter for every exchange which makes overflow less likely. The next-sequence
number for each exchange would need to be persisted, replicated on joining (both can be done
by putting it back in the args) and replicated via a management event when it is increased
on the primary. Jumping the next-sequence number ahead when we still have 32k messages left
to go means that we have time to store & replicate before we pass the new limit. The next-sequence
doesn't have to be precisely synchronized in a cluster provided it is always a good margin
bigger than the highest sequence number used by that exchange on the primary so far. During
promotion (before allowing backups to join) we would need to jump each exchange sequence ahead
to the next-sequence for the exchange and increase next-sequence by 64k to make sure the new
primary is using higher sequence numbers than the old.
> > 
> > Are we sure nobody assumes that exchange sequence numbers are sequential? The above
is fine for duplicate detection but are we sure nobody does missed message detection by checking
for gaps in the exchange sequence numbers? That would complicate everything quite a bit...
> > 
> > 
> >

This seems to be sound to me. To sum it up:
- exchanges with sequencing to have new argument qpid.next_sequenceNo, by default set to 64k
for new exchanges, or to the value read from store during startup (or being updated from primary
  - if the argument is missing in stored args, it will be automatically added there (to support
- when incrementing sequenceNo in preRoute, if args(qpid.next_sequenceNo) - sequenceNo ==
32k, then execute 3 steps:
  - increment qpid.next_sequenceNo by 64k
  - update store bdb for the exchange
  - if clustered broker and (primary or being promoted), notify backups about the increment
of qpid.next_sequenceNo
- when joining a cluster, active broker will update the joiner one automatically (via exchange
arguments) - no need to change
- when promoting a node to active:
  - set sequenceNo = args(qpid.next_sequenceNo)
  - execute the same 3 steps from "when incrementing sequenceNo"

Two questions:
1) Can a node being promoted already update backups? Or is there a way of postponing the update?
(see latest point above)
2) What if - after a new promotion - the broker sends few messages with sequenceNo say 128k
but dies before it really writes to the disk args(qpid.next_sequenceNo)=(128+64)k ? Can it
even happen?

Sequence number will then jump ahead by at most 96k whenever a standalone broker is restarted
or new promotion in cluster happens (or cold start of cluster happens). That means, there
can be 93824992236885 such "jump events" without sequence number overflow (or 93824671436104
"jump events" for steadily message flow of 1M messages per second for 10 years). More than
fair enough.

Comment to "Are we sure nobody assumes that exchange sequence numbers are sequential?" - Consumers
get messages from queues and no queue can be guaranteed to get _all_ messages routed by the
exchange (until all know the exchange is fanout or binding key is empty or so - but I think
nobody can build on this premise). So jumps in exchange sequence numbers can be always meant
as "the skipped messages were not meant for this queue".

- Pavel

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On April 1, 2014, 10:45 a.m., Pavel Moravec wrote:
> -----------------------------------------------------------
> This is an automatically generated e-mail. To reply, visit:
> https://reviews.apache.org/r/19566/
> -----------------------------------------------------------
> (Updated April 1, 2014, 10:45 a.m.)
> Review request for qpid, Alan Conway, Gordon Sim, and Kim van der Riet.
> Bugs: QPID-5642
>     https://issues.apache.org/jira/browse/QPID-5642
> Repository: qpid
> Description
> -------
> Elegant (but not performance optimal) way of patch:
> 1) In Exchange::PreRoute::PreRoute, update exchange in store (whole entry).
> 2) The update method is dealed very similarly like MessageStoreImpl::create(db_ptr db,
IdSequence& seq, const qpid::broker::Persistable& p) method, i.e. calls BufferValue
that calls Exchange::encode.
> Here the code can be unified by merging MessageStoreImpl::update intoMessageStoreImpl::create
method where the code almost duplicates.
> However I do not see the patch as performance efficient, as with every message preroute,
new qpid::framing::Buffer is filled in Exchange::encode method, data are copied from it to
char* BufferValue::data and even then they are really written to the BDB. While in fact we
just update the only one number in the Buffer.
> I tried to come up with less performance invasive approach (for those interested, see
https://bugzilla.redhat.com/attachment.cgi?id=877576&action=diff - if you dont have access
there, let me write), that keeps qpid::framing::Buffer for every durable exchange with sequencing
enabled, but it returned (among others) into the need of changing the way store encodes/decodes
Exchange instance (change in Exchange::encode / decode methods). What would make the broker
backward incompatible.
> Is the performance penalty (due to Exchange::encode method called for every message preroute)
> Is it worth merging MessageStoreImpl::update intoMessageStoreImpl::create method?
> Diffs
> -----
>   /trunk/qpid/cpp/src/qpid/broker/Exchange.cpp 1582719 
> Diff: https://reviews.apache.org/r/19566/diff/
> Testing
> -------
> - reproducer from JIRA verified
> - automated tests passed (except for those known to fail due to QPID-5641 (valgrind &
> Thanks,
> Pavel Moravec

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