48.1. Introduction to Webmin Clustering

Webmin has several modules that make it easy to perform tasks on several machines at once. There are known as a cluster. A large organization might have tens or hundreds of servers that need some software package installed, UNIX user created, or Webmin module added. The cluster modules make this easy. Each corresponds to one of the single-machine modules, but allows the same tasks to be performed on more than one system at a time.

For a system to be part of a cluster, it must have Webmin installed, even if you never actually login to it directly. One of the cluster modules on a single host contacts all of the others using Webmin's RPC (Remote Procedure Call) protocol and instructs them to carry out certain tasks. This master host might be part of the cluster and instruct itself to perform the same tasks, or it may be totally independent.

On the master system, the Webmin Servers Index module (covered in Chapter 53) must first be used to register all of the other managed servers. For each managed server, the root or admin username and password must be specified so that the master knows how to log in. Once this is done, each of the cluster modules can be set up to manage some or all of the registered systems.

Because Webmin's RPC mechanism allows any file to be accessed or any command run on a server, only the users root and admin are allowed to receive RPC calls on a managed system by default. This means that entering some other user in the Webmin Servers Index module for a managed server will not work, unless that user has been specifically configured to be able to accept RPC logins. Section 52.5 “Editing Module Access Control” explains how to set this up.

The RPC protocol that the master system uses to control managed hosts is unique to Webmin, and is not based on any other similar protocol, such as Sun's RPC, SOAP, or RMI. It has two different modes: the old mode in which only HTTP requests are used to send commands, and a newer mode in which a permanent TCP connection is used. The latter method is faster and more reliable, but may fail if a firewall is blocking traffic between the master and managed hosts. It uses ports 10001 and above, by default, whereas the old protocol just uses the port on which Webmin accepts normal connections (usually 10000). Chapter 53 explains how to select a mode for a server in more detail, while Chapter 56 covers the internal workings of the protocol.