Re: Using PostgreSQL for service discovery and health-check

On 2/9/23 07:30, Dominique Devienne wrote:
> Hi. I'm requesting advice on something I'm about to start on.
>
> In a normally 2-tier system, where "desktop" native apps connect
> directly to PostgreSQL to access data, some things must be mediated via
> a middle-tier service (i.e. 3-tier architecture). That service is HTTP
> based thus clients (the "desktop" native apps) must know the URL
> (host+port+path) of a server providing the requested service.

Is there more then one server providing the same service?

>
> Since clients must already have access to PostgreSQL to operate, I
> wanted to add a table in PostgreSQL for services, and server(s) on
> startup (probably using a random port) would register in that table, and
> deregister on shutdown. Also, since crashes are a fact of life, the
> server would regularly update the DB with a "heartbeat" on some
> frequency (e.g. 1s?), so clients would select (or see) only registered
> services with a "fresh enough" heartbeat timestamp.

Would it no be easier to not have random ports and just attempt
connections to the servers either:

1) In the client with reattempt to different port on failure.

2) From Postgres server and update table to have current up servers.

>
> That's basically my plan. Now come the questions:
> 1) will updating a row every second (for example) create issues?
> 2) if yes to #1 above, what would be good mitigation tactics? Use
> different table for service vs heartbeat? Special kind of table? or
> configured in a particular way?
> 3) if a service crashes, it won't remove its row(s), obviously. What
> kind of mechanism exists to "reap" "zombie" services?
> 4) Related to #3 above, I think built-in "cron"-like services are only
> available via extensions, not in PostgreSQL proper. Why? Seems like such
> an essential service.
> 5) Which cron-like extension to use? Especially since we run both
> on-prem but also in managed-PostgreSQL on the cloud?
>
> I'd appreciate community input. Thanks, --DD
>
> PS: Note that there could be more than 1 server registered, for the same
> service, possibly on the same machine, for redundancy. But I think
> that's mostly orthogonal to my questions above.

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com