Highly Available LDAP
If updates have been made to the LDAP namespace while the master LDAP server is down, the LDAP databases must be resynchronized prior to restarting the master server. There are two methods for doing this. If a service interruption is possible, the databases can be hand-copied after the LDAP server has been stopped. (Data files are kept by default in /usr/local/var.)
You also can use OpenLDAP replication to restore the database without the service interruption. First, start the LDAP server on the former master node as a slave. Then start slurpd on the current master. Changes received while the former master was out of service are pushed from the new master. Finally, stop the slave LDAP server on the former master node, and start Heartbeat. This results in a failback to the original configuration.
This article outlines a simple example of using open-source software to create some highly available basic network services. Network services including LDAP seldom require huge servers. The additional reliability provided by clustering and the duplication of servers and data files can increase overall service availability. The system worked under all tests, with a failover of less than 15 seconds in all cases. Given a good understanding of system loads and utilization, failover time could be reduced below this threshold.
Thanks to Alan Robertson, IBM Linux Technology Center, for his helpful comments and review.
The foregoing article is based on laboratory tests undertaken in a laboratory environment. Results in particular customer installations may vary based on a number of factors, including workload and configuration in each particular installation. Therefore, the above information is provided on an AS IS basis. The WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE EXPRESSLY DISCLAIMED. Use of this information is at user's sole risk.
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