Re: Details about libpq cross-version compatibility

Thank you Heikki for your answer!

I can write a paragraph, (hence I did never do that => where can I find
info to contribute on the doc?)

But so far it's not quite clear to me:

You write "That really shouldn't be necessary", when I write that we have
to ship different drivers for each major PostgreSQL version...

Is this what I should document? Based on assumptions?

Further: Today we use a simple naming convention:

dbmpgs83x => for PostgreSQL 8.3.x
dbmpgs84x => for PostgreSQL 8.4.x
dbmpgs90x => for PostgreSQL 9.0.x
dbmpgs91x => for PostgreSQL 9.1.x
dbmpgs92x => for PostgreSQL 9.2.x

While we need to ship 5 drivers, this is clear and simple to understand.

If a future major version (for ex 9.3) implements a new strategic feature
that makes libpq headers incompatible with prior versions, what shall we
do with our convention? Shall we ship 9.x (where 0<=x<=2) drivers and 9.3+
drivers? What if a next major version implements again another feature
that makes headers incompatible again?

I think I am asking for is an official statement that clearly says that
libpq headers are only compatible for a given set of minor versions, like
(8.4.1, 8.4.2, 8.4.3, ...) or (9.2.1, 9.2.2, ...)

I believe also that the libpq library should automatically check for
compatiblity, based on PG_MAJORVERSION definition in pg_config.h ...

Seb

On 09/13/2012 09:01 AM, Heikki Linnakangas wrote:
> On 12.09.2012 18:21, Sebastien FLAESCH wrote:
>> I believe there is some info missing in the page describing libpq
>> programs build:
>>
>> http://www.postgresql.org/docs/9.2/interactive/libpq-build.html
>>
>> I am working with PostgreSQL for several years now (since V7),
>> maintaining a set
>> of "db drivers" for our system, that can be used with the different
>> PostgreSQL
>> versions.
>>
>> We need to build our binaries for different versions of PostgreSQL,
>> today it's
>> from 8.3 to 9.2 ...
>>
>> What is the compatibility rule regarding this?
>>
>> Can I compile / link with a PostgreSQL 9.1 environment and deploy on 9.2
>> ???
>>
>> Can I compile / link with a PostgreSQL 8.x environment and deploy on 9.x
>> ???
>
> If your binary is just a client application that uses libpq to speak to
> the server, you can use any (supported) version of libpq with any
> (supported) server version. There hasn't been any incompatible protocol
> changes since version 7.4, which introduced the protocol version 3, and
> even that was done in a backwards-compatible fashion so that even 9.2
> server still supports the old version 2 of the protocol.
>
> Now, another question is what happens if you compile your binaries
> against e.g 9.1 version of libpq, but at runtime, you have 9.2 libpq
> installed. I think that should work too, but not vice versa, if you use
> any new functions from the 9.2 libpq library. In general, you should
> have the same or higher version of libpq at runtime that the binaries
> were built with.
>
> The version number bump from 8.x to 9.0 was made because 9.0 introduced
> many new features, but from a compatibility policy point-of-view, 8.4 ->
> 9.0 is no different than, say, going from 8.3 to 8.4.
>
>> The problem is that we have to ship several versions of the drivers
>> for all
>> the PostgreSQL versions that are in the support live cycle, today this is
>> from 8.3 to 9.2 ...
>
> That really shouldn't be necessary.
>
>> So far, we followed the version policy as described here:
>>
>> http://www.postgresql.org/support/versioning
>>
>> I think it's ok to follow this rule, but would feel much better if it was
>> clearly stated in the "Building libpq Programs" page.
>
> Agreed. Either that page, or the libpq top page at
> http://www.postgresql.org/docs/devel/static/libpq.html, or a new
> "cross-version compatibility" section under that. Could you write up a
> few paragraphs for the manual please? That would be much appreciated,
> you'd be in a good position to do that since you're facing the problem
> right now.
>
> - Heikki
>