Database Replication with Slony-I
Listing 4. promote.sh
#!/bin/bash CLUSTER=sql_cluster H1=master.example.com H2=slave.example.com U=postgres DB1=contactdb DB2=contactdb su - postgres -c slonik <<_EOF_ cluster name = $CLUSTER; node 1 admin conninfo = 'dbname=$DB1 host=$H1 user=$U'; node 2 admin conninfo = 'dbname=$DB2 host=$H2 user=$U'; failover (id = 1, backup node = 2); drop node (id = 1, event node = 2);
From Listing 4, the failover Slonik command is used to indicate that the node with id = 1, the node running on master.example.com, has failed, and that the node with id = 2 will take over all sets from the failed node. The second command, drop node, is used to remove the node with id = 1 from the replication system completely. Eventually, you might want to bring back the failed node in the cluster. To do this, you must configure it as a slave and let Slony-I replicate any missing information. Eventually, you can proceed with a switchback to the initial master node by locking the set (lock set), waiting for all events to complete (wait for event), moving the set to a new origin (move set) and waiting for a confirmation that the last command has completed. Please refer to the Slonik Command Summary for more information on those commands.
Replicating databases using Slony-I is relatively simple. Combined with the Linux-HA Heartbeat, this allows you to offer high availability of your database services. Although the combination of Slony-I and Linux HA-Heartbeat is an attractive solution, it is important to note that this is not a substitute for good hardware for your database servers.
Even with its small limitations, like not being able to propagate schema changes or replicate large objects, Slony-I is a great alternative to both rserv and ERServer and is now, in fact, the preferred solution for replicating PostgreSQL databases. Slony-II even supports synchronous multimaster replication and is already on the design table.
To conclude, I would like to thank Jan Wieck, the author of Slony-I, for reviewing this article.
Resources for this article: /article/8202.
Ludovic Marcotte (firstname.lastname@example.org) holds a Bachelor's degree in Computer Science from the University of Montréal. He is currently a software architect for Inverse, Inc., an IT consulting company located in downtown Montréal.
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