I'm proposing a new generic serial:// transport for sending nanomsg (SP)
messages over {RS-232, RS-485, UART, radio, optical, etc} interfaces.
Basically, talking with low-cost/low-power microcontrollers in a non-IP
context.
Header/protocol wise, I think the closest existing transport is UDP (not
actually implemented in libnanomsg). However, because there is only a
single pipe/channel over which multiple message types might be sent, I
think the implementation might also have similarities with inproc and
tcpmux.
Here is a draft RFC in the form of a github gist:
https://gist.github.com/bnewbold/8fe816a5d70c6cbc9bb3
There was some previous discussion in the Crossroads project of serial
transports:
http://comments.gmane.org/gmane.network.crossroads.devel/622
I thought I remembered a ZeroMQ serial transport also when I looked years
ago, but I can't find it now.
I've been using this transport in development for about half a year now
for data acquisition and control. We've built distributed sensor systems
where data streams in from many tiny cheap synchronized sensors over UART
into a central Linux node which combines PUB messages into a single stream
available over TCP. Inbound REQ/REP requests to the linux node have a
per-sensor prefix stripped and are then dispatched to the end node for
control and configuration. We have also done low-throughput telemetry and
system control over a robust-but-extremely-slow low-frequency radio link.
FWIW, in these contexts over a very narrow pipe it's very important to
have "sender-side" filtering of PUB messages. Handling this at the
protocol layer seems to be hung up with concerns about reliable message
propagation. For now we've been using out-of-band RPC to turn topics on
and off at the source, but it would be much nicer to somehow do this as
part of the PUB/SUB protocol. Note that with UDP or serial there is no
connection context in which to even keep a list of subscribed topics
though!
I started trying to implement this transport in libnanomsg a while back,
but I didn't really understand how all the state machines worked even for
TCP so I gave up. We currently use a python proxy program to translate
between TCP (multiple sockets) and this serial transport. I know a tiny
bit of golang so I might try to do a reference implementation in mangos,
and maybe a limited no-heap Arduino library for basic PUB and REQ
functionality.
--bryan
PS, I generally try to lurk in the gitter channel, but I'll be AFK until
next Tuesday
