I'd now like to turn to the "initial value" behaviour for exchanges. Can
anyone explain what exactly this means? Is the mechanism documented
anywhere? Obviously, I understand that an exchange with this behaviour
will store some values in addition to forwarding them to queues but what
exactly does it keep? Only one message for the entire exchange? Or one
per routing key? Or something else entirely?
Maybe should explain exactly what I'm trying to do, so that it will be
easier to say if this behaviour is appropriate for my needs:
I won't go into details, but essentially I'm setting up a server, or
publisher if you like, that will distribute different status readings
from external hardware to one or more client application. Or actually,
the setup already exists, but using a different communication
mechanism... There are several different, but identical (and
identifiable), hardware units, several different status values for each
of them, and several samples over time for each value. In normal
operation, all the values should simply be forwarded to every client,
which is the easy bit. But, I also want to support clients that start up
after the data has started appearing. When they do, they should get
exactly one sample of each status value for each of the hardware units
from the data distributed earlier - specifically, the latest instance of
everything. This is important to get a complete state within a
reasonable amount of time, as some of the data is sampled at a low rate.
What setup(s) would be appropriate in this case? My current prototype
implementation has:
* One topic exchange.
* One routing key (topic) per status value.
* A hardware unit identifier stored as a custom message property.
* A last-value queue per topic, using the above mentioned property
as "last value queue key". This is shared between clients, which
will only ever browse messages.
It seems like this might be a way forward, but it does have some issues,
notably that it only works correctly if the queues are created and
correctly bound to the exchange before the publisher is started, and are
kept that way throughout its lifetime.
Would an initial value exchange be appropriate in such a scenario? And,
do you think it would make it easier to get a robust setup? I mean,
ideally, the publisher should re-issue the last samples if a problem has
occurred. This is no problem as such as the data is still available on
the server-side, but how do I know that I have to re-send it?
TIA,
- Toralf
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