Sendmail Clustering

Sendmail plays a variety of roles, all critical to the proper flow of email. It listens to the network for incoming mail, transports mail messages to other servers, and delivers local mail to a local program.

The LifeKeeper for Linux Sendmail Recovery Kit provides a mechanism to recover Sendmail from a failed primary server on a backup server in a LifeKeeper environment. Both LifeKeeper and Sendmail ensure data integrity throughout the course of the failover process without significant lost time or human intervention.

A typical LifeKeeper Sendmail configuration is shown below:

The above illustration shows the use of a shared storage solution. LifeKeeper however does not require a shared storage solution, and can be configured to undertake data replication to a remote server. In this case, if failover occurs, the backup server will have an identical copy of the information, and can become the primary storage which the failed server can synchronise with when it comes back online in a backup role.

Because the replication allows for two machines to be geographically distant, it is now possible to have a true offsite disaster recovery solution operating over a WAN under normal conditions. This ensures that in the event of an outage on one site, the application can become available on a remote site, and remote clients can continue to connect - normally without even being aware that one site has gone down.

The configuration above illustrates how Sendmail works in a LifeKeeper environment.

Sendmail does not currently support multiple instances of itself on the same machine, so it is not possible to cluster it in an Active/Active configuration.

One server acts as a primary mailhub or mailserver, while the other server acts as a backup mailhub or mailserver. The primary server runs the LifeKeeper - protected Sendmail daemon and accepts and transports email over the network and locally, while the backup server does not run the Sendmail daemon. In the event of a failure, the backup server will take over the role of the primary server.

The LifeKeeper Sendmail Recovery Kit will handle the change of control of the configuration files and tables, and will restart the LifeKeeper-protected daemon on the backup server. Once the primary server is restored, it will function in a backup capacity.

The Sendmail Recovery Kit also requires a free/unused shared file system. The shared file system is used by the Sendmail Recovery Kit for the storage of the Sendmail configuration file (sendmail.cf), the Sendmail source files and database tables (for example, alias, domaintable, genericstable, etc.). The shared file system is required so that both the primary and backup servers can run the Sendmail daemon with the same configuration.

Original Source: www.openminds.co.uk