commit 9e18ddb094c17cfb790e6544a4fea6716b907784
author Jean de Klerk <deklerk@google.com> Fri Jun 07 17:02:15 2019 -0600
committer Jean de Klerk <deklerk@google.com> Thu Oct 10 10:03:16 2019 -0600
tree 30d2f1fb7cd37bef9a7e52c08d3bde407febe792
parent f013ab08d24b21e75e8e6aa7125de4336b705b1f

pubsub: add ordered keys

Pub/Sub ordered keys requires:

- Publish with a non-empty key gets added to a per-key queue, as implemented
by Bundler.HandlerLimit=1. This setting causes the bundler to only have one
outstanding bundle being handled at a time; further bundles form a queue.
- Receive with a message that has a non-empty key gets added to a per-key
queue, as implemented by a slice. When a worker operates on a key, it
processes all items in the key's slice until the slice is empty, at which point
it deletes the key to release the key's resources, and the worker returns to
the worker pool.

Functionally, for users, this should involve:

- Slightly slower processing speeds due to additional data structures
along the Publish and Receive paths. (we should do some performance
testing to vet this out)
- PublishSettings.NumGoroutines and ReceiveSettings.NumGoroutines should
both result in double the number of goroutines, since an additional data
structure (publish/receive scheduler) uses that value. Documentation has
been adjusted accordingly to describe it more as a scaling factor than
an exact number.
- Default ReceiveSettings.NumGoroutines is increased from 1 to 10. It's
generally a better experience to have multiple workers than one. And,
some tests - like TestStreamingPullFlowControl - implicitly require it.
- Since ordered keys require only a single outstanding RPC at once, it is
possible to send ordered key messages to Topic.Publish (and subsequently to
PublishScheduler.Add) faster than the bundler can publish them to the
Pub/Sub service, resulting in a backed up queue of Pub/Sub bundles. Each
item in the bundler queue is a goroutine, which means users may see thousands
of goroutines in debugging they might do.

TODO: add Resume method.

Change-Id: Ib710944d557970290cf12321d1fdbd9b1699231d