Clustering refers to grouping one or more tc Runtime instances so that they appear to work as a single server. A cluster provides:
Session replication. When a client, typically using a browser, connects to a tc Runtime instance, tc Runtime creates a session Object that it uses to manage all subsequent interaction between itself and that client. Depending on how the Web application was programmed, the session Object can contain a lot of useful information, such as user security credentials, current items in a user's shopping cart, and so on. If the tc Runtime instance is part of a cluster, the session is automatically copied to each member of the cluster group, and is updated each time the session is modified, such as when the user adds a new item to their shopping cart. This means that if the first tc Runtime instance crashes, any tc Runtime instance in the group can immediately take over the session without interruption, completely hiding the server crash from the client who continues to work as if nothing had happened. This capability greatly increases the usability of Web applications.
Context attribute replication. A context represents a Web application that is deployed to a tc Runtime instance. In the same way that client sessions can be replicated, the Web application context itself can also be replicated to all members of a cluster group.
A tc Runtime cluster can be as small as two server instances hosted on the same computer to hundreds of tc Runtime instances hosted on many different computers of different operating systems.
Typically, you configure a tc Runtime cluster to use multicast for the communication between member servers. The cluster is then uniquely identified by the combination of its multicast IP address and port. Each member of the cluster must have the same multicast address and port configured so that the cluster can automatically discover each member and react appropriately if a member does not respond. You can create multiple clusters, such as one for testing and another for production, by configuring different multicast address/ports for each cluster.
In addition to creating a tc Runtime cluster, you might also want to configure a load balancer in front of the cluster so as to split up the incoming requests between multiple tc Runtime instances. Load balancing attempts to direct requests to the tc Runtime instance with the smallest load at that point in time. The load balancer can also detect when a tc Runtime instance has failed, in which case it stops directing requests to it until the tc Runtime instance restarts, adding to the high availability of tc Runtime.
See High Level Steps for Creating and Using tc Runtime Clusters for the basic steps to get started with tc Runtime clusters.