In order to join a group and send messages, a process has to create a channel. A channel is like a socket. When a client connects to a channel, it gives the the name of the group it would like to join. Thus, a channel is (in its connected state) always associated with a particular group. The protocol stack takes care that channels with the same group name find each other: whenever a client connects to a channel given group name G, then it tries to find existing channels with the same name, and joins them, resulting in a new view being installed (which contains the new member). If no members exist, a new group will be created.
A state transition diagram for the major states a channel can assume are shown in fig. 3.2.
When a channel is first created, it is in the unconnected state. An attempt to perform certain operations which are only valid in the connected state (e.g. send/receive messages) will result in an exception. After a successful connection by a client, it moves to the connected state. Now channels will receive messages, views and suspicions from other members and may send messages to other members or to the group. Getting the local address of a channel is guaranteed to be a valid operation in this state (see below). When the channel is disconnected, it moves back to the unconnected state. Both a connected and unconnected channel may be closed, which makes the channel unusable for further operations. Any attempt to do so will result in an exception. When a channel is closed directly from a connected state, it will first be disconnected, and then closed.
The methods available for creating and manipulating channels are discussed now.
