Slony-I
------------------


Slony-I is a "master to multiple slaves" replication system with
cascading and failover.

The big picture for the development of Slony-I is a master-slave
system that includes all features and capabilities needed to replicate
large databases to a reasonably limited number of slave systems.

Slony-I is a system for data centers and backup sites, where the
normal mode of operation is that all nodes are available, and where
systems may be expected to be managed in a reasonably disciplined
fashion.

1.  Build and install

    See the INSTALL file nearby

2.  Creating a sample database with application

    See the SAMPLE file nearby

    There is also a "test" directory in the source tree which contains
    a variety of regression tests.  Looking at how they work may also
    be helpful.

3.  Help!  I ran into a problem.

    The file in the documentation area named "helpitsbroken.txt"
    contains a variety of notes describing problems that people have
    run into in setting up Slony-I instances.  This has been extended
    into a full scale FAQ.

    It may be worth checking there to see if the problem you are
    having has already been documented and diagnosed by someone else.

    There is an extensive "administrative guide" in the documentation
    tree that includes manual pages for binaries and for the Slonik
    language, as well as overviews of various aspects of how Slony-I
    works and should be used, including a "Best Practices" document
    that distills some of the hard-learned lessons and a FAQ.

    This "admin guide" may be found online at:
    <http://linuxdatabases.info/info/slony.html>

You may also wish to contact the Slony-I mailing list, findable at
<http://slony.info/>. or take a peek at who might be available to chat
on the IRC channel #slony on freenode.net.

-- Slony-I team

*** OUTSTANDING ISSUE WITH PostgreSQL 8.1 ***

PostgreSQL 8.1 is quite a lot more strict about what UTF-8 mappings of
Unicode characters it accepts as compared to version 8.0.

If you intend to use Slony-I to update an older database to 8.1, and
might have invalid UTF-8 values, you may be for an unpleasant
surprise.

Let us suppose we have a database running 8.0, encoding in UTF-8.
That database will accept the sequence '\060\242' as UTF-8 compliant,
even though it is really not.

If you replicate into a PostgreSQL 8.1 instance, it will complain
about this, either at subscribe time, where Slony-I will complain
about detecting an invalid Unicode sequence during the COPY of the
data, which will prevent the subscription from proceeding, or, upon
adding data, later, where this will hang up replication fairly much
irretrievably.  (You could hack on the contents of sl_log_1, but
that's *really* unattractive...)

There have been discussions as to what might be done about this.  No
compelling strategy has yet emerged, as all are unattractive.

If you are using Unicode with PostgreSQL 8.0, you run a considerable
risk of corrupting data.

If you use replication for a one-time conversion, there is a risk of
failure due to the issues mentioned earlier; if that happens, it
appears likely that the best answer is to fix the data on the 8.0
system, and retry.

In view of the risks, running replication between versions seems to be
something you should not keep running any longer than is necessary to
migrate to 8.1.

http://archives.postgresql.org/pgsql-hackers/2005-12/msg00181.php

Extracting from that:
---------------------------------------------------------------
Upgrading UNICODE databases to 8.1

Postgres 8.1 includes a number of bug-fixes and improvements to
Unicode and UTF-8 character handling.  Unfortunately previous releases
would accept character sequences that were not valid UTF-8.  This
may cause problems when upgrading your database using
pg_dump/pg_restore resulting in an error message like this:

  Invalid UNICODE byte sequence detected near byte ...

To convert your pre-8.1 database to 8.1 you may have to remove and/or
fix the offending characters.  One simple way to fix the problem is to
run your pg_dump output through the iconv command like this:

  iconv -c -f UTF8 -t UTF8 -o fixed.sql dump.sql

The -c flag tells iconv to omit invalid characters from output.

-- Paul Lindner
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