Re: Keeping state in a foreign data wrapper

On Mon, 2020-08-03 at 19:23 +0300, Stelios Sfakianakis wrote:
> I am trying to implement a FDW in Postgres for accessing a web api and I would
> like to keep information like for example the total number of requests submiited.
> Ideally these data should be kept “per-user” and of course with the proper locking
> to eliminate race conditions. So the question I have is how to design such a global
> (but per user and database) state, using the C FDW API of Postgres. I have gone
> through the API and for example I see various fdw_private fields in the query
> planning structures and callbacks but these do not seem to be relevant to my requirements.
> Another possiblity seems to be to use shared memory
> (https://www.postgresql.org/docs/12/xfunc-c.html#id-1.8.3.13.14) but it is even
> less clear how to do it.
>
> So is there any simple way to do implement such shared thread/process safe state?

I would create a table as part of the extension and use SPI to store
the data there.

Yours,
Laurenz Albe