An imperative AMQP client API

[Justin Ross <ju...@gmail.com>]

Hi, everyone.  For a while now, some members of the Qpid community have
been working on a new style of messaging API.  It now has a reasonable
shape, and we want to share it and get your feedback.

We currently offer either JMS or Proton's reactive API.  These certainly
aren't going anywhere - they're important - but for some use cases, the
absence of a high-level command-oriented API for AMQP messaging is a source
of inconvenience.

This inconvenience comes in two forms.  First, JMS is helpfully imperative
(in part - it contains multitudes), but it doesn't expose some of the
things you can do with AMQP.  And it can't reasonably expose those things,
because that would break the contract of the JMS API.  Second, the Proton
APIs, since they are reactive, make it harder to handle cases where you
need to sequence the processing of events.

The imperative client API we are talking about here uses modern language
support for futures or coroutines.  Most of the API's operation is
asynchronous, but you can easily introduce blocking where you need to.

It's a client API only.  We think a comparably high-level server API would
be its own dedicated thing, functioning more like Python Flask or JAX-RS.
In any case, the Proton reactive API is already a good fit for writing
servers.

We have the outline of the API and some proof-of-concept implementations,
but much remains to be done.  We anticipate that this work will ultimately
become a sub-module of Proton, such as proton/client.

The API spec:
http://www.ssorj.net/pumpjack/client/index.html

I've worked on the Python prototypes, so I'll share them here.  I did these
some time ago, so they need updating for the latest API spec changes, but
they serve to show how the API can use futures or coroutines.  (For Python,
coroutines are the preferred approach.)

Python prototype (future-based):
https://github.com/ssorj/gambit/blob/futures/python/demo.py
https://github.com/ssorj/gambit/blob/futures/python/gambit.py

Python prototype (coroutine-based):
https://github.com/ssorj/gambit/blob/asyncio2/python/demo.py
https://github.com/ssorj/gambit/blob/asyncio2/python/gambit.py

Some of the other folks who have done exploratory work will follow up on
this thread to show what they've done.

Some caveats: this is in early stages, and the API will change as we
discuss it more.  There are also big outstanding pieces to look at, such as
reconnect and failover, to name just one.

Thanks for your time, and please let us know what you think.