[zeromq-dev] Discard old messages on XPUB before transmission

[Stephen Lord]

I have often though that the HWM settings on a socket could have a variant where they drop the oldest message rather than now queuing  a new message. The documentation at least is not clear on does it drop the oldest or newest message when the limit is reached. Having explicit control over this would allow some different behaviors, one being discard old messages if they are superseded by new ones. The addition of a specific error return on a send to indicate messages are being dropped might also help, and an getsockopt to see if messages have been dropped.

Note this is just a suggestion, not something I have the time to actually go try and implement.

Steve Lord

On Jul 18, 2013, at 9:45 AM, CFK <cfkaran2 at gmail.com>
 wrote:

> Hi all, I'm trying to figure out what socket options I need to set under ZMQ 3.2.3 to force ZMQ XPUB sockets to drop messages if they are old.  I have time-sensitive messages, any/all of which can be dropped if they are older than some amount (set by the option) before ZMQ starts actual transmission. Is there an option for this?  From my testing, ZMQ_SNDTIMEO does not seem to do this, and none of the other options I've read about seem to be applicable. As it stands, I'm using pyzmq's MessageTracker interface, along with a hackish thread interface, to queue all messages I plan on sending until the currently sent message is actually transmitted, before starting on the next message.  This is suboptimal, and I'd like to do something better.
> 
> Note that any solution MUST work with XPUB/XSUB as they operate over UDP (via PGM).  I'm operating on a lossy wireless network, and cannot tolerate how long TCP takes to timeout a connection, or how long it takes to figure out a connection is back up.
> 
> Thanks,
> Cem Karan
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