[GENERAL] Inter-app communication via DB

Hi list.

One pattern I've used is for apps to communicate events to each other
through the database.

ie:

- App 1 sents a boolean value to True
- App 2 queries the field every 10s, sets the value to False, and does
something.

Is this reasonable, or should apps avoid this pattern?

I have seen the NOTIFY and LISTEN SQL statements. However:

1) App 2 might not be running at the time (eg: it's launched from
cron, or it was temporarily stopped), and the expectation is that App
2 will run the special logic when it is started.

2) App 2 is usually single-threaded, and needs to do other things in
it's main thread besides wait for a DB notification.

I also know of RPC, but haven't used it before, and don't see a need
if you can use the above pattern.

I would use RPC (or unix signals if on the same host) if App 2 needed
to respond quickly, and I didn't want to hammer the DB & network with
constant polling.

Any comments?

David.

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