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.
Fast/Flexible Linux OS Recovery
On Demand Now
In this live one-hour webinar, learn how to enhance your existing backup strategies for complete disaster recovery preparedness using Storix System Backup Administrator (SBAdmin), a highly flexible full-system recovery solution for UNIX and Linux systems.
Join Linux Journal's Shawn Powers and David Huffman, President/CEO, Storix, Inc.
Free to Linux Journal readers.Register Now!
- The Qt Company's Qt Start-Up
- Devuan Beta Release
- May 2016 Issue of Linux Journal
- EnterpriseDB's EDB Postgres Advanced Server and EDB Postgres Enterprise Manager
- The US Government and Open-Source Software
- Open-Source Project Secretly Funded by CIA
- The Death of RoboVM
- The Humble Hacker?
- BitTorrent Inc.'s Sync
- New Container Image Standard Promises More Portable Apps
In modern computer systems, privacy and security are mandatory. However, connections from the outside over public networks automatically imply risks. One easily available solution to avoid eavesdroppers’ attempts is SSH. But, its wide adoption during the past 21 years has made it a target for attackers, so hardening your system properly is a must.
Additionally, in highly regulated markets, you must comply with specific operational requirements, proving that you conform to standards and even that you have included new mandatory authentication methods, such as two-factor authentication. In this ebook, I discuss SSH and how to configure and manage it to guarantee that your network is safe, your data is secure and that you comply with relevant regulations.Get the Guide