On 1/17/20 3:48 AM, Yuval Lifshitz wrote:
On Thu, Jan 16, 2020 at 10:06 PM Matt Benjamin <mbenjami@xxxxxxxxxx
<mailto:mbenjami@xxxxxxxxxx>> wrote:
Well, the need for reservation is general, arising from need for
reliable delivery.
agree, reservations are needed for any queue in order to allow push
back from failed endpoints.
the reserve/commit mechanism would allow us to reliably signal the
client that notifications triggered by the operation would fail.
To be more specific, this reserve/commit mechanism is only a requirement
for a strictly-bounded queue. And while a strict bound may be desirable
for several reasons, I don't see it as a requirement for reliable
delivery. Reliable delivery just means that we either a) notify the
endpoint directly and receive an ack or b) record an intent to notify
before responding the client.
Separately, I have some concerns with separating the 'reserve' request
from the one that actually 'commits' a notification to this queue.
Mainly, the osd request to reserve might succeed, but the request to
commit may fail or hang - for example, if the object's pg is no longer
available. Or maybe radosgw crashes before it's able to commit, and
(depending on implementation) the reserved size may leak. But the point
is that a successful reservation can't guarantee a successful commit.
You could substitute the fifo abstraction if 128M
is too small.
Matt
On Thu, Jan 16, 2020 at 1:23 PM Casey Bodley <cbodley@xxxxxxxxxx
<mailto:cbodley@xxxxxxxxxx>> wrote:
>
>
> On 1/16/20 12:13 PM, Yuval Lifshitz wrote:
> > two updates on the design (after some discussions):
> >
> > (1) "best effort queue" (stretch goal) is probably not needed:
> > - cls queue performance should be high enough when put on
fast media pool
> > - the "acl level" settings allow for existing mechanism to
perform as
> > "best effort" and non-blocking for topics that does not need
delivery
> > guarantees
> >
> > (2) since the cls queue does not allow for random access (without
> > linear search) the retries will have to be implemented based
only on
> > the end of the queue. This means that we must assume that the
acks or
> > nack arrive in the same order in which the notifications were set.
> > This is true only for a specific endpoint (e.g. a specific kafka
> > broker) which means that there will have to be a separate cls
queue
> > instance for each endpoint
> >
> >
> > On Tue, Jan 14, 2020 at 3:47 PM Yuval Lifshitz
<ylifshit@xxxxxxxxxx <mailto:ylifshit@xxxxxxxxxx>
> > <mailto:ylifshit@xxxxxxxxxx <mailto:ylifshit@xxxxxxxxxx>>> wrote:
> >
> > Dear Community,
> > Would like to share some design ideas around the above topic.
> > Feedback is welcomed!
> >
> > Current State
> >
> > - in "pull mode" [1] we have the same guarantees as the
multisite
> > syncing mechanism (guarantee against HW/SW failures). On
top of
> > that, if writing the event to RADOS fails, this trickle
back as
> > sync failure, which means that the master zone will try to
sync
> > the pubsub zone
> >
> > - in "push mode" [2] we send the notification from the ops
context
> > that triggered the notification. The original operation is
blocked
> > until we get a reply from the endpoint. As part of the
> > configuration for the endpoint, we also configure the "ack
level",
> > indicating whether we block until we get a reply from the
endpoint
> > or not.
> > Since the operation response is not sent back to the
client until
> > the endpoint acks, this method guarantees against any
failure in
> > the radosgw (at the cost of adding latency to the operation).
> > This, however, does not guarantee delivery if the endpoint
is down
> > or disconnected. The endpoint we interact with (rabbitmq,
kafka) ,
> > usually have built in redundancy mechanism, but this does not
> > cover the case where there is a network disconnect between our
> > gateways and these systems.
> > In some cases we can get a nack from the endpoint,
indicating that
> > our message would never reach the endpoint. But we can
only log
> > these cases:
> > - we cannot fail the operation that triggered us, because
we send
> > the notification only after the actual operation (e.g. "put
> > object") was done (=no atomicity)
> > - no retry mechanism (in theory, we can add one)
> >
> > Next Phase Requirements
> >
> > We would like to add delivery guarantee to "push mode" for
> > endpoint failures. For that we would use a message queue
with the
> > following features:
> > - rados backed, so it would survive HW/SW failures
> > - blocking only on local read/writes (so it introduces smaller
> > latency than over-the-wire endpoint acks)
> > - has reserve/commit semantics, so we can "reserve" before the
> > operation (e.g. "put object") was done, and fail it if we
cannot
> > reserve a slot on the queue, and commit the notification
to the
> > queue only after the operation was successful (and
unreserve if
> > the operation failed)
> >
> I guess this reservation piece is only a requirement because of the
> choice of cls_queue, which resides in a single rados object and so
> enforces a bound on the total space used. The maximum size is
> configurable, but can't exceed osd_max_object_size=128M. How many
> notifications could we fit within that the 128M limit?
attached an example of a notification, which is 700 bytes long. if
longer bucket and object names are used, together with metadata it may
go up to 1KB. which would allow for 128K notifications (per endpoint).
not sure what is the official operations per second we support per RGW
(for small objects)?
but if we assume 3.5K requests/sec [1] we can handle a 35 sec
disconnect before we start pushing back on the client.
what would probably make sense here is that we will define multiple
queues per endpoint, that would scale with the number of RGWs.
[1]
https://www.slideshare.net/alohamora/ceph-object-storage-performance-secrets-and-ceph-data-lake-solution-81010759
I worry that
> clusters at a sufficient scale could fill that pretty quickly if the
> notification endpoint is unavailable or slow, and that would leave
> radosgw unable to satisfy any requests that would generate a
notification.
>
> > - we would have a retry mechanism based on the queue,
which means
> > that if a notification was successfully pushed into the
queue, we
> > can assume it would (eventually) be successfully delivered
to the
> > endpoint
> >
> > Proposed Solution
> >
> > - use the cls_queue [3] (cls_queue is not omap based,
hence, no
> > builtin iops limitations)
> > - add reserve/commit functionality (probably store that
info in
> > the queue head)
> > - a dedicated thread(s) should be reading requests from
the queue,
> > sending the notifications to the endpoints, and waiting for
> > the replies (if needed) - this should be done via coroutines
> > - acked requests are removed from the queue, nacked or
> > timed-out requests should be retried (at least for a while)
> > - both mechanism would coexist, as this would be
configurable per
> > topic
> > - as a stretch goal, we may add a "best effort queue".
This would
> > be similar to the cls_queue solution, but won't address
> > radosgw failures (as the queue would be in-memory), only
endpoint
> > failures/disconnects
> > - for now, this mechanism won't be supported for pushing
events
> > from the pubsub zone (="pull+push mode"), but might be
added if
> > users would find it useful
> >
> > Yuval
> >
> > [1] https://docs.ceph.com/docs/master/radosgw/pubsub-module/
> > [2] https://docs.ceph.com/docs/master/radosgw/notifications/
> > [3] https://github.com/ceph/ceph/tree/master/src/cls/queue
> >
> >
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--
Matt Benjamin
Red Hat, Inc.
315 West Huron Street, Suite 140A
Ann Arbor, Michigan 48103
http://www.redhat.com/en/technologies/storage
tel. 734-821-5101
fax. 734-769-8938
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