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 (email@example.com) 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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One of the best things about the UNIX environment (aside from being stable and efficient) is the vast array of software tools available to help you do your job. Traditionally, a UNIX tool does only one thing, but does that one thing very well. For example, grep is very easy to use and can search vast amounts of data quickly. The find tool can find a particular file or files based on all kinds of criteria. It's pretty easy to string these tools together to build even more powerful tools, such as a tool that finds all of the .log files in the /home directory and searches each one for a particular entry. This erector-set mentality allows UNIX system administrators to seem to always have the right tool for the job.
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