RE: [HACKERS] Client Connection redirection support for PostgreSQL

I simplified the patch and for now just allowed one server. Please find the attached patches, and the commit message.

Thanks,
Satya

-----Original Message-----
From: Robert Haas <robertmhaas@gmail.com> 
Sent: Monday, November 6, 2017 5:56 AM
To: Craig Ringer <craig@2ndquadrant.com>
Cc: Satyanarayana Narlapuram <Satyanarayana.Narlapuram@microsoft.com>; PostgreSQL-development
<pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Client Connection redirection support for PostgreSQL

On Thu, Nov 2, 2017 at 4:33 PM, Craig Ringer <craig@2ndquadrant.com> wrote:
>> Add the ability to the PostgreSQL server instance to route the 
>> traffic to a different server instance based on the rules defined in 
>> server’s pg_bha.conf configuration file. At a high level this enables 
>> offloading the user requests to a different server instance based on 
>> the rules defined in the pg_hba.conf configuration file.
>
> pg_hba.conf is "host based access [control]" . I'm not sure it's 
> really the right place.

Well, we could invent someplace else, but I'm not sure I quite see the point (full disclosure: I suggested the idea of
doingthis via pg_hba.conf in an off-list discussion).
 

I do think the functionality is useful, for the same reasons that HTTP redirects are useful.  For example, let's say
youhave all of your databases for various clients on a single instance.  Then, one client starts using a lot more
resources,so you want to move that client to a separate instance on another VM.  You can set up logical replication to
replicateall of the data to the new instance, and then add a pg_hba.conf entry to redirect connections to that database
tothe new master (this would be even smoother if we had multi-master replication in core).  So now that client is moved
offto another machine in a completely client-transparent way.  I think that's pretty cool.
 

> When this has come up before, one of the issues has been determining 
> what exactly should constitute "read only" vs "read write" for the 
> purposes of redirecting work.

Yes, that needs some thought.

> Backends used just for a redirect would be pretty expensive though.

Not as expensive as proxying the whole connection, as pgpool and other systems do today.  I think the in-core use of
thisredirect functionality is useful, but I think the real win would be optionally using it in pgpool and pgbouncer.
 

--
Robert Haas
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