Re: Event-driven programming?

On Sep 12, 3:05 pm, da...@lorenso.com ("D. Dante Lorenso") wrote:
> Pavel Stehule wrote:
> > 2007/9/12, Jay Dickon Glanville <dickon.glanvi...@gmail.com>:
> >> - I write a function (it doesn't matter what language it's in:
> >> PL/pgSQL, PL/Java, etc)
> >> - I register that function as a "post-commit" callback function
> >> - when a client commits a transaction, the function gets called, and
> >> the database passes the function some general information as to the
> >> content of the transaction
>
> >> Note how similar this process is to writing triggers.  The only
> >> problem I have with triggers is that events get generated per-table.
> >> I'd like to get notifications based on transactions, not table
> >> changes.
>
> >> What I'd like to be able to do with this event is to notify any
> >> applications of this change, so they can update their cached view of
> >> the database.
>
> Although I'm happy to use triggers as-is (not per transaction, etc) I've
> also wondered about firing events from the database.  I'm curious to
> know if anyone has attempted to write a trigger that will open a socket
> and send an event packet to an application server on the network.
>
> I've considered using a message queue like JMS to manage events on my
> network and have PostgreSQL fire off UDP messages to a socket server
> that would insert jobs into the message queue as triggers get fired in
> the database.  Doing this would be an alternative to storing the queue
> as a database table and having to use polling to constantly check the
> database for events in the queue.
>
> I am interested what anybody might contribute to this thread.  Let us
> know what you tried whether it worked or not, it might be useful.
>
> -- Dante

Depending on your reliability requirements UDP may not be a great
choice.

But, since you asked about what's been tried, my (successful so far)
production setup is along the lines of:

1. process A accepts multiple data flows, inserts "work to be done"
items into a table in batches and calls NOTIFY.
2. process B LISTENs for notifications (with a blocking read on the
socket connection to Postgres) and takes them as a signal to look for
"work items to be done". It also checks every N minutes of idle time
for "work items to be done" in case the NOTIFY/LISTEN mechanism is
broken (haven't seen that situation yet).

As for recovery, process B looks for work items on startup, then drops
into the LISTEN / blocking_read mode.