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(at)gmail(dot)com>
Sent: Monday, November 6, 2017 5:56 AM
To: Craig Ringer <craig(at)2ndquadrant(dot)com>
Cc: Satyanarayana Narlapuram <Satyanarayana(dot)Narlapuram(at)microsoft(dot)com>; PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [HACKERS] Client Connection redirection support for PostgreSQL

On Thu, Nov 2, 2017 at 4:33 PM, Craig Ringer <craig(at)2ndquadrant(dot)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 doing this 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 you have 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 replicate all of the data to the new instance, and then add a pg_hba.conf entry to redirect connections to that database to the new master (this would be even smoother if we had multi-master replication in core). So now that client is moved off to 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 this redirect functionality is useful, but I think the real win would be optionally using it in pgpool and pgbouncer.

--
Robert Haas
EnterpriseDB: https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.enterprisedb.com&data=02%7C01%7CSatyanarayana.Narlapuram%40microsoft.com%7Caafef2039b194d9c02c308d5251e12bb%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636455733453945798&sdata=8qystAJQ6UhnB7WRQh5i4nF8cyBUvKc9QIBfy59y%2FX8%3D&reserved=0
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