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205 lines
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205 lines
7.0 KiB
Plaintext
DBMirror - Postgres Database Mirroring
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===================================================
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DBMirror is a database mirroring system developed for the Postgres
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database Written and maintened by Steven Singer(ssinger@navtechinc.com)
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(c) 2001-2002 Navtech Systems Support Inc.
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Released under the GNU Public License version 2. See COPYING.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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Overrview
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--------------------------------------------------------------------
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The mirroring system is trigger based and provides the following key features:
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-Support for multiple mirror slaves
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-Transactions are maintained
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-Per table selection of what gets mirrored.
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The system is based on the idea that a master database exist where all
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edits are made to the tables being mirrored. A trigger attatched to the
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tables being mirrored runs logging information about the edit to
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the Pending table and PendingData table.
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A perl script(DBMirror.pl) runs continiously for each slave database(A database
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that the change is supposed to be mirrored to) examining the Pending
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table; searching for transactions that need to be sent to that particular slave
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database. Those transactions are then mirrored to the slave database and
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the MirroredTransaction table is updated to reflect that the transaction has
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been sent.
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If the transaction has been sent to all know slave hosts (All entries
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in the MirrorHost table) then all records of it are purged from the
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Pending tables.
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Installation Instructions
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------------------------------------------------------------------------
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1) Compile pending.c
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The file pending.c contains the recordchange trigger. This runs every
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time a row inside of a table being mirrored changes.
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To build the trigger run make on the "Makefile" in the DBMirror directory.
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The Makefile supplied assumes that the postgres include files are in
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/usr/local/pgsql/include/server.
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Postgres-7.1.x installations should change this to
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/usr/local/pgsql/include (The server part is for 7.2+)
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If you have installed the postgres include files to another location then
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modify the Makefile to reflect this.
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The trigger requires that all postgres headers be installed, this is
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accomplished in postgresql(7.1 or 7.2) by running "make install-all-headers"
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in the postgres source directory.
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The Makefile should create a file named pending.so that contains the trigger.
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Install this file in /usr/local/pgsql/lib (or another suitable location).
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If you choose a different location the MirrorSetup.sql script will need
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to be modified to reflect your new location. The CREATE FUNCTION command
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in the MirrorSetup.sql script associates the trigger function with the
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pending.so shared library. Modify the arguments to this command if you
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choose to install the trigger elsewhere.
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2) Run MirroSetup.sql
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This file contains SQL commands to setup the Mirroring environment.
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This includes
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-Telling Postgres about the "recordchange" trigger function.
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-Creating the Pending,PendingData, MirrorHost, MirroredTransaction tables
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To execute the script use psql as follows
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"psql -f MirrorSetup.sql MyDatabaseName"
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where MyDatabaseName is the name of the database you wish to install mirroring
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on(Your master).
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3) Create slaveDatabase.conf files.
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Each slave database needs its own configuration file for the
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DBMirror.pl script. See slaveDatabase.conf for a sample.
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The master settings refer to the master database(The one that is
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being mirrored).
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The slave settings refer to the database that the data is being mirrored to.
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The slaveHost parameter must refer to the machine name of the slave (Either
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a resolvable hostname or an IP address). The value for slave host
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must match the Hostname field in the MirrorHost table(See step 6).
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The master user must have sufficient permissions to modify the Pending
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tables and to read all of the tables being mirrored.
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The slave user must have enough permissions on the slave database to
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modify(INSERT,UPDATE,DELETE) any tables on the slave system that are being
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mirrored.
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4) Add the trigger to tables.
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Execute the SQL code in AddTrigger.sql once for each table that should
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be mirrored. Replace MyTableName with the name of the table that should
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be mirrored.
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5) Create the slave database.
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The DBMirror system keeps the contents of mirrored tables identical on the
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master and slave databases. When you first install the mirror triggers the
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master and slave databases must be the same.
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If you are starting with an empty master database then the slave should
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be empty as well. Otherwise use pg_dump to ensure that the slave database
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tables are initially identical to the master.
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6) Add entries in the MirrorHost table.
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Each slave database must have an entry in the MirrorHost table.
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The name of the host in the MirrorHost table must exactly match the
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slaveHost variable for that slave in the configuration file.
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For example
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INSERT INTO "MirrorHost" ("HostName") VALUES ('mySlaveMachine.mycompany.com');
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6) Start DBMirror.pl
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DBMirror.pl is the perl script that handles the mirroring.
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It requires the Perl library Pg(See src/interfaces/perl5 in the postgres
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source distribution).
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It takes its configuration file as an argument(The one from step 3)
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One instance of DBMirror.pl runs for each slave machine that is receiving
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mirrored data.
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Any errors are printed to standard out and emailed to the address specified in
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the configuration file.
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DBMirror can be run from the master, the slave, or a third machine as long
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as it is able to access both the master and slave databases.
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7) Periodically run clean_pending.pl
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clean_pending.pl cleans out any entries from the Pending tables that
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have already been mirrored to all hosts in the MirrorHost table.
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It uses the same configuration file as DBMirror.pl.
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Normally DBMirror.pl will clean these tables as it goes but in some
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circumstances this will not happen.
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For example if a transaction has been mirrored to all slaves except for
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one, then that host is removed from the MirrorHost table(It stops being
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a mirror slave) the transactions that had already been mirrored to
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all the other hosts will not be deleted from the Pending tables by
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DBMirror.pl since DBMirror.pl will run against these transactions again
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since they have already been sent to all the other hosts.
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clean_pending.pl will remove these transactions.
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TODO(Current Limitations)
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----------
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-Support for selective mirroring based on the content of data.
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-Support for BLOB's.
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-Support for conflict resolution.
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-Batching SQL commands in DBMirror for better performance over WAN's.
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-Better support for dealing with Schema changes.
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Tested Platforms:
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------------------
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DBMirror has been tested on the following configurations but should
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work on any platform with Postgres >= 7.1 and Perl 5.6.
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RedHat Linux 7.1 & 6.2
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-Postgres 7.1.2
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-Perl 5.6
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Mandrake Linux 8.0(Limited Testing)
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-Postgres 7.2
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-Perl 5.6
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Steven Singer
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Navtech Systems Support Inc.
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ssinger@navtechinc.com
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