Subject: Re: "Dynamic routing" to different databases
To: pgsql-admin@postgresql.org
References: <560BFE58.9030201@evaristesys.com>
From: Alex Balashov <abalashov@evaristesys.com>
Message-ID: <560C6F2D.1090107@evaristesys.com>
Date: Wed, 30 Sep 2015 19:24:29 -0400
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After some more research, my conclusion - unless anyone has better 
suggestions - is to just bite the bullet and partition these multiple 
databases into schemas.

That's going to take a lot of code modification, but not nearly as much 
as a full-on application-layer multitenant alternative.

On 09/30/2015 11:23 AM, Alex Balashov wrote:

> Hello all,
>
> We have a rather complex service delivery application that is presently
> designed with a "single-tenant" use case in mind: one self-contained
> PostgreSQL database with all necessary tables, and one service consuming
> that database, configured via a static connection string.
>
> We are trying to figure out the most economical technical possibilities
> for retrofitting "multi-tenant" functionality into it; you might think
> of it as a kind of virtual hosting, where different requests coming into
> the consuming service, segregated by, say, request URI domain, result in
> very different behaviour.
>
> Obviously, one way to do this is to add an additional layer of
> abstraction at the database schema and the application level itself,
> creating nested entities and so forth. This is impractical, however; it
> would be nothing short of a wholesale overhaul of the entire codebase
> (which consists in large part of PostgreSQL stored procedures for the
> business layer).
>
> So, what we're trying to figure out is the feasibility of grafting
> multi-tenant functionality into the existing paradigm. For instance, we
> could have a "meta-data" database with a table that defines DNS domains
> and maps them to other database names/connection info on the same
> server. We could then create a new and self-contained database for each
> "tenant". I think that would make the most sense by far.
>
> The major limitation is that the consuming service can connect to one
> database and one database only, so what is needed is some way to present
> foreign databases through one PostgreSQL connection handle to one
> particular database.
>
> The dblink facility seems to provide ways of doing this, but what I am
> not clear on is: 1) how transparently the foreign database can be
> rendered, vs. having to graft in lots of dblink_*() calls and 2) whether
> there is a significant performance hit in using dblink, assuming the
> "remote" tenant databases are located on the same cluster. Can this be
> done using foreign data wrappers or something of the sort, for instance?
>
> Failing that, is there some functionality in PgPool or another
> connection proxy which can effectively take
>
>     SELECT * FROM tbl META_ATTRIB = 'domain.com';
>
> and transform this into
>
>     SELECT * FROM tbl;
>
> on the appropriate database for 'domain.com', which is a mapping that
> would be maintained in something that is, at least in principle,
> dynamically reloadable?
>
> Advice much appreciated!
>
> -- Alex
>


-- 
Alex Balashov | Principal | Evariste Systems LLC
303 Perimeter Center North, Suite 300
Atlanta, GA 30346
United States

Tel: +1-800-250-5920 (toll-free) / +1-678-954-0671 (direct)
Web: http://www.evaristesys.com/, http://www.csrpswitch.com/


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