Re: Slony-I makes progress

Alex J. Avriette wrote:
> On Fri, Mar 05, 2004 at 12:47:23AM +0100, Jochem van Dieten wrote:
>
>> >I personally don't think that a GUI tool should be the province of the
>> >Slony project. Seriously. I think that Slony should focus on a
>
> I very much agree with this, but this is Jan's baby, so I didn't say
> anything. I have personally never used a GUI with a postgres database
> (well, okay, I used one for a bit to troubleshoot a problem my boss
> was having with a pg node once), and I don't really plan to. I guess
> I was unaware this is a common usage pattern.

I was explicitly asking for opinions and input. I don't want this to be
"my baby". In the end I am a developer, not a DBA. I know how to do it,
but don't have the ultimate wisdom about how to manage it.

>
>> >command-line api and catalogs, and allow the existing GUI projects to
>> >build a slony-supporting interface.
>>
>> Why a command line api? I believe it would make sense to be able
>> to configure and control all nodes of the entire system from psql
>> connected to any of the nodes. That would also facilitate the
>> existing GUI projects in adding a Slony-manager.
>
> In theory, most of the stuff that Slony is doing is within the
> database, and as such, could be configurable via stored procedures. I
> see a few problems with this.
>
> First off, it is not possible to configure external applications (such
> as erserver has a daemon) from within the database except through the
> modification of tables within the database which are monitored by said
> application.

Which is exactly the way the Slony node daemons communicate with each
other and the way most of the admin activity is actually communicated
into the system.

The communication channels are "event" tables. The node daemons use
listen and notify to send messages from on to another. Messages are only
exchanged over this when the replication cluster configuration is
changed or every 10 seconds to tell "new replication data has
accumulated, come and get it". So I think the listen/notify protocol
suits well for that.

Some of the functionality happening on an event is already put into
stored procedures, and the replication engine as well as the (to be)
admin tools just call those. But that doesn't mean that using psql will
do the job. There are certain operations that need to be initiated (the
corresponding SP called) on a particular node, not just on any available
one. Also, these stored procedures take arguments, most of which are
just the ID numbers of configuration objects. Not the ideal user interface.

>
> Second, it increases the footprint of Slony on the database. I am
> fairly uneasy about adding more tables, functions, and triggers to my
> (already quite taxed) production database. To add further functions for
> configuration, as well as related tables and triggers, makes my job
> managing the database more difficult. Additionally, those commands are
> queries. For something as trivial as configuration data, I would much
> rather not be issuing queries against an already very busy database. I
> am much more comfortable with the principle of external configuration
> files and programs.

All tables, sequences and stored procdures/functions related to the
Slony replication system reside is a separate namespace. I found out
lately that (without replicating sequences yet), the whole replication
system can be "cleanly" removed from a database with just a DROP SCHEMA
... CASCADE.

The problem I have with external configurations is that they collide
with the hot subscribe capability. If node-3 subscribes to a set from
node-1, getting the data cascaded over node-2, the event to enable that
subscription has to travel from 1 over 2 to 3. When that is received
there, 3 has to copy over the current status of the data from 2 and then
catch up by replicating all changes that have happened during this copy,
which for large data sets can take a while. So node-2 must be aware of
this happening and not throw away any replication log since node-3
started copying, unless it is confirmed received by 3. The knowledge
that 3 exists must also cause other forwarding nodes to keep the log.
Imagine that after 3 successfully copied the data, while he's catching
up node-2 dies. At that moment, 3 can be reconfigured to get the rest of
the log from 1, or anyone else who has it, so that the copy effort is
not lost ... which at the time a node is failing in the system would
just add to the pain of the DBA.

>
> Lastly, and I may be the black sheep here, I don't find sql to be
> particularly useful for doing things that require a complex grammar. In
> this instance, I don't want to have to do something like:
>
> production=# select slony_config_setval( 'log_dir', '/data/slony_logs');

It currently looks more like

select "_MyCluster".storePath(2, 3, 'dbname=mydb host=node2', 30);
select "_MyCluster".storeListen(2, 2, 3);

> to manage the configuration. Obviously, this could be worse than the
> above example.

So it "IS" worse! It is not supposed that the DBA uses the systems
internal API for configuration management. That is the whole reason for
the admin/config tools.

>
> I don't understand the opposition to an external set of tools (even a
> gui if need be). It seems to me, that until the postmaster has some
> kind of native replication, all replication efforts will be based on
> external programs. As such, they should be configured externally, and
> be treated as any other daemon would be.

There must be some external tools. And to be integrated into any
automated failover system, it needs to be commandline. So that one is a
given.

That still does not give an easy way to tell which of the existing
tables should be replicated, into how many independant sets they can be
divided, what nodes subscribe to what sets, what nodes do store and
forward of log data, all that stuff.

I have started on a small lex+yacc+libpq tool that will get me over the
immediate requirements I have to work on provider change and failover. I
will add that to the CVS (first as a subdirectory of ducttape) in a few
days.

Jan

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