postgresql/contrib/dbmirror/README.dbmirror

DBMirror - PostgreSQL Database Mirroring
===================================================


DBMirror is a database mirroring system developed for the PostgreSQL
database Written and maintained by Steven Singer(ssinger@navtechinc.com)


(c) 2001-2004 Navtech Systems Support Inc.
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Overview
--------------------------------------------------------------------

The mirroring system is trigger based and provides the following key features:

-Support for multiple mirror slaves
-Transactions are maintained
-Per table selection of what gets mirrored.


The system is based on the idea that a master database exists where all
edits are made to the tables being mirrored.   A trigger attached to the
tables being mirrored runs logging information about the edit to 
the Pending table and  PendingData table. 

A perl script(DBMirror.pl) runs continuously for each slave database(A database
that the change is supposed to be mirrored to) examining the Pending
table; searching for transactions that need to be sent to that particular slave 
database.  Those transactions are then mirrored to the slave database and
the MirroredTransaction table is updated to reflect that the transaction has
been sent.

If the transaction has been sent to all know slave hosts (All entries
in the MirrorHost table) then all records of it are purged from the
Pending tables.

Requirements:
---------------------------------
-PostgreSQL-7.4 (Older versions are no longer supported)
-Perl 5.6 or 5.8 (Other versions might work)
-PgPerl (http://gborg.postgresql.org/project/pgperl/projdisplay.php)


Upgrading from versions prior to 8.0
---------------------------------------
Users upgrading from a version of dbmirror prior to the one shipped with 
Postgresql 8.0 will need to perform the following steps

1. Dump the database then drop it (dropdb no not use the -C option)
2. Create database with createdb.
3. Run psql databasename -f MirrorSetup.sql
4. Restore the database(do not use the -C option of pg_dump/pg_restore)
5. run the SQL commands: DROP "Pending";DROP "PendingData"; DROP "MirrorHost";
   DROP "MirroredTransaction";

The above steps are needed A) Because the names of the tables used by dbmirror
to store data have changed and B) In order for sequences to be mirrored properly
all serial types must be recreated.



Installation Instructions
------------------------------------------------------------------------



1) Compile pending.c

The file pending.c contains the recordchange trigger.  This runs every
time a row inside of a table being mirrored changes.


To build the trigger run  make on the "Makefile" in the DBMirror directory.

PostgreSQL-8.0  Make Instructions:

  If you have already run "configure" in the top (pgsql) directory 
  then run "make" in the dbmirror directory to compile the trigger.


You should now have a file named pending.so that contains the trigger.

Install this file in your Postgresql lib directory (/usr/local/pgsql/lib)


2) Run MirrorSetup.sql

This file contains SQL commands to setup the Mirroring environment.  
This includes

-Telling PostgreSQL about the "recordchange" trigger function.
-Creating the dbmirror_Pending,dbmirror_PendingData,dbmirror_MirrorHost, 
dbmirror_MirroredTransaction tables


To execute the script use psql as follows 

"psql -f MirrorSetup.sql  MyDatabaseName"

where MyDatabaseName is the name of the database you wish to install mirroring
on(Your master).


3) Create slaveDatabase.conf files.

Each slave database needs its own configuration file for the 
DBMirror.pl script.  See slaveDatabase.conf for a sample.

The master settings refer to the master database(The one that is
being mirrored).

The slave settings refer to the database that the data is being
mirrored to.

The slaveName setting in the configuration file must match the slave
name specified in the dbmirror_MirrorHost table.

DBMirror.pl can be run in two modes of operation: 

 A) It can connect directly to the slave database.  To do this specify
 a slave database name and optional host and port along with a username
 and password.  See slaveDatabase.conf for details.


 The master user must have sufficient permissions to modify the Pending
 tables and to read all of the tables being mirrored.

 The slave user must have enough permissions on the slave database to
 modify(INSERT,UPDATE,DELETE) any tables on the slave system that are being
 mirrored. 

 B) The SQL statements that should be executed on the slave can be
 written to files which can then be executed slave database through
 psql.  This would be suitable for setups where their is no direct
 connection between the slave database and the master. A file is
 generated for each transaction in the directory specified by
 TransactionFileDirectory. The file name contains the date/time the
 file was created along with the transaction id.


4) Add the trigger to tables.

Execute the SQL code in AddTrigger.sql once for each table that should
be mirrored.   Replace MyTableName with the name of the table that should
be mirrored.

NOTE: DBMirror requires that every table being mirrored have a primary key
defined.

5)  Create the slave database.

The DBMirror system keeps the contents of mirrored tables identical on the
master and slave databases.  When you first install the mirror triggers the
master and slave databases must be the same.

If you are starting with an empty master database then the slave should
be empty as well.  Otherwise use pg_dump to ensure that the slave database
tables are initially identical to the master.

6) Add entries in the MirrorHost table.

Each slave database must have an entry in the MirrorHost table.

The name of the host in the MirrorHost table must exactly match the
slaveHost variable for that slave in the configuration file.

For example
INSERT INTO "MirrorHost" ("SlaveName") VALUES ('backup_system');


6)  Start DBMirror.pl


DBMirror.pl is the perl script that handles the mirroring.  

It requires the Perl library Pg(See http://gborg.postgresql.org/project/pgperl/projdisplay.php)

It takes its configuration file as an argument(The one from step 3)
One instance of DBMirror.pl runs for each slave machine that is receiving
mirrored data.

Any errors are printed to standard out and emailed to the address specified in
the configuration file. 

DBMirror can be run from the master, the slave, or a third machine as long
as it is able to access both the master and slave databases(not
required if SQL files are being generated)

7) Periodically run clean_pending.pl 
clean_pending.pl cleans out any entries from the Pending tables that
have already been mirrored to all hosts in the MirrorHost table.
It uses the same configuration file as DBMirror.pl.

Normally DBMirror.pl will clean these tables as it goes but in some 
circumstances this will not happen.

For example if a transaction has been mirrored to all slaves except for
one, then that host is removed from the MirrorHost table(It stops being
a mirror slave) the transactions that had already been mirrored to 
all the other hosts will not be deleted from the Pending tables by 
DBMirror.pl since DBMirror.pl will run against these transactions again
since they have already been sent to all the other hosts.

clean_pending.pl will remove these transactions.

TODO(Current Limitations)
----------
-Support for selective mirroring based on the content of data.
-Support for BLOB's.
-Support for multi-master mirroring with conflict resolution.
-Better support for dealing with Schema changes.



Significant Changes Since 7.4
----------------
-Support for mirroring SEQUENCE's
-Support for unix domain sockets
-Support for outputting slave SQL statements to a file
-Changed the names of replication tables are now named
dbmirror_pending etc..



Credits
-----------
Achilleus Mantzios <achill@matrix.gatewaynet.com>




Steven Singer
Navtech Systems Support Inc.
ssinger@navtechinc.com