Re: libpq compression

On 09.02.2019 1:38, Tomas Vondra wrote:
> On 2/8/19 11:10 PM, Konstantin Knizhnik wrote:
>>
>> On 08.02.2019 21:57, Andres Freund wrote:
>>> On 2019-02-08 12:15:58 +0300, Konstantin Knizhnik wrote:
>>>> Frankly speaking, I do not think that such flexibility in choosing
>>>> compression algorithms is really needed.
>>>> I do not expect that there will be many situations where old client
>>>> has to
>>>> communicate with new server or visa versa.
>>>> In most cases both client and server belongs to the same postgres
>>>> distributive and so implements the same compression algorithm.
>>>> As far as we are compressing only temporary data (traffic), the
>>>> problem of
>>>> providing backward compatibility seems to be not so important.
>>> I think we should outright reject any patch without compression type
>>> negotiation.
>> Does it mean that it is necessary to support multiple compression
>> algorithms and make it possible to perform switch between them at
>> runtime?
> IMHO the negotiation should happen at connection time, i.e. the server
> should support connections compressed by different algorithms. Not sure
> if that's what you mean by runtime.
>
> AFAICS this is quite close to how negotiation of encryption algorithms
> works, in TLS and so on. Client specifies supported algorithms, server
> compares that to list of supported algorithms, deduces the encryption
> algorithm and notifies the client.
>
> To allow fall-back to uncompressed connection, use "none" as algorithm.
> If there's no common algorithm, fail.

It is good analogue with SSL.
Yes, SSL protocol provides several ways of authentication, encryption,...
And there are several different libraries implementing SSL.
But Postgres is using only one of them: OpenSSL.
If I want to use some other library (for example to make it possible to 
serialize and pass SSL session state to other
process), then there is no way to achieve it.

Actually zstd also includes implementations of several compression 
algorithms and it choose one of them best fitting particular data 
stream. As in case of SSL, choice of algorithm is performed internally 
inside zstd - not at libpq level.

Sorry, if my explanation about static and dynamic (at runtime) choice 
were not correct.
This is how compression is toggled now:

#if HAVE_LIBZSTD
ZpqStream*
zpq_create(zpq_tx_func tx_func, zpq_rx_func rx_func, void *arg)
{
...
}
#endif

So if Postgres was configured with zstd, then this implementation is 
included inclient and server Postgres libraries.
If postgres is configures with zlib, them  zlib implementation will be used.
This is similar with using compression and most of other configurable 
features in Postgres.

If we want to provide dynamic choice at runtime, then we need to have 
array with available compression algorithms:

#if HAVE_LIBZSTD
static ZpqStream*
zstd_create(zpq_tx_func tx_func, zpq_rx_func rx_func, void *arg)
{
...
}
#endif

ZpqCompressorImpl compressorImpl[] =
{
#if HAVE_LIBZSTD
{zstd_create, zstd_read,zstd_write,...},
#endif
#if HAVE_ZLIB
{zlib_create, zlib_read,zslib_write,...},
#endif
...
}

And the most interesting case is that if we load library dynamically.
Each implementation is generated in separate library (for  example 
libpztd.so).
In this case we need to somehow specify available libraries.
For example by placing them in separate directory, or specifying list of 
libraries in postgresql.conf.
Then we try to load this library using dlopen.  Such library has 
external dependencies of correspondent compressor library (for example 
-lz). The library can be successfully loaded if there correspond 
compressor implementation was install at the system.
This is most flexible approach allowing to provide custom implementation 
of compressors.
Compression implementation can be organized as Postgres extension and 
its PG_init function registers this implementation in some list.

This is what I am asking about.
Right now approach 1) is implemented: compression algorithm is defined 
by configure.
It is no so difficult to extend it to support multiple algorithms.
And the most flexible but more sophisticated is to load libraries 
dynamically.

>
>> Right now compression algorithm is linked statically.
>> Negotiation of compression type is currently performed but it only
>> checks that server and client are implementing the same algorithm and
>> disables compression if it is not true.
>>
> I don't think we should automatically fall-back to disabled compression,
> when a client specifies compression algorithm.

Compression is disabled only when client and server were configured with 
different compression algorithms (i.e. zstd and zlib).

>
>> If we are going to support multiple compression algorithms, do we need
>> dynamic loading of correspondent compression libraries or static linking
>> is ok? In case of dynamic linking we need to somehow specify information
>> about available compression algorithms.
>> Some special subdirectory for them so that I can traverse this directory
>> and try to load correspondent libraries?
>>
>> Only I find it too complicated for the addressed problem?
>>
> I don't think we need dynamic algorithms v1, but IMHO it'd be pretty
> simple to do - just add a shared_preload_library which registers it in a
> list in memory.

I do not think that it is necessary to include such libraries in 
preload_shared_libraries list.
It can be done lazily only of compression is requested by client.
Also please notice that we need to load compression library both at 
server and client sides.
preload_shared_libraries works only for postmaster.