Channels

WebSphere® MQ uses two different types of channels:

• A message channel, which is a unidirectional communications link between two queue managers. WebSphere MQ uses message channels to transfer messages between the queue managers. To send messages in both directions, you must define a channel for each direction.
• An MQI channel, which is bidirectional and connects an application (MQI client) to a queue manager on a server machine. WebSphere MQ uses MQI channels to transfer MQI calls and responses between MQI clients and queue managers.

Do not confuse these two distinct types of channels.

When discussing message channels, the word channel is often used as a synonym for a channel definition. It is usually clear from the context whether we are talking about a complete channel, which has two ends, or a channel definition, which has only one end.

Message channels

Message channel definitions can be one of the following types:

Message channel definition type	Description
Sender	A sender channel is a message channel that the queue manager uses to send messages to other queue managers. To send messages using a sender channel, you must also create, on the other queue manager, a receiver channel with the same name as the sender channel. You can also use sender channels with requester channels if you are implementing a "callback" mechanism.
Server	A server channel is a message channel that the queue manager uses to send messages to other queue managers. To send messages using a server channel, you must also create, on the other queue manager, a receiver channel with the same name as the server channel. You can also use server channels with requester channels. In that case, the requester channel definition at the other end of the channel requests the server channel definition to start. The server sends messages to the requester. The server can also initiate the communication as long as the server knows the connection name of the partner channel.
Receiver	A receiver channel is a message channel that the queue manager uses to receive messages from other queue managers. To receive messages using a receiver channel, you must also create, on the other queue manager, a sender or a server channel with the same name as this receiver channel.
Requester	A requester channel is a message channel that the queue manager uses to send messages to other queue managers. To send messages using a requester channel, you must also create, on the other queue manager, a sender channel if you are implementing a callback mechanism, or a server channel.
Cluster-sender	A cluster-sender (CLUSSDR) channel definition defines the sending end of a channel on which a cluster queue manager can send cluster information to one of the full repositories. The cluster-sender channel is used to notify the repository of any changes to the queue manager's status, for example the addition or removal of a queue. It is also used to transmit messages. The full repository queue managers themselves have cluster-sender channels that point to each other. They use them to communicate cluster status changes to each other. It is of little importance which full repository a queue manager's CLUSSDR channel definition points to. After the initial contact has been made, further cluster queue manager objects are defined automatically as required so that the queue manager can send cluster information to every full repository, and messages to every queue manager. For more information, see Queue manager clusters.
Cluster-receiver	A cluster-receiver (CLUSRCVR) channel definition defines the receiving end of a channel on which a cluster queue manager can receive messages from other queue managers in the cluster. A cluster-receiver channel can also carry information about the cluster—information destined for the repository. By defining the cluster-receiver channel, the queue manager indicates to the other cluster queue managers that it is available to receive messages. You need at least one cluster-receiver channel for each cluster queue manager. For more information, see Queue manager clusters.

For each channel you must define both ends so that you have a channel definition for each end of the channel. The two ends of the channel must be compatible types.

You can have the following combinations of channel definitions:

• Sender–Receiver
• Server–Receiver
• Requester–Server
• Requester–Sender (callback)
• Cluster-sender–Cluster-receiver

Message channel agents

Each channel definition that you create belongs to a particular queue manager. A queue manager can have several channels of the same or different types. At each end of the channel is a program, the message channel agent (MCA). At one end of the channel, the caller MCA takes messages from the transmission queue and sends them through the channel. At the other end of the channel, the responder MCA receives the messages and delivers them to the remote queue manager.

A caller MCA can be associated with a sender, server, or requester channel. A responder MCA can be associated with any type of message channel.

WebSphere MQ supports the following combinations of channel types at the two ends of a connection:

Caller		Direction of message flow	Responder
Channel type	Listener required?		Listener required?	Channel type
Sender	No	Caller to Responder	Yes	Receiver
Server	No	Caller to Responder	Yes	Receiver
Server	No	Caller to Responder	Yes	Requester
Requester	No	Responder to Caller	Yes	Server
Requester	Yes	Responder to Caller	Yes	Sender

MQI channels

MQI channels can be one of the following types:

MQI channel type	Description
Server connection	A server connection channel is a bidirectional MQI channel that is used to connect a WebSphere MQ client to a WebSphere MQ server. The server connection channel is the server end of the channel.
Client connection	A client connection channel is a bidirectional MQI channel that is used to connect a WebSphere MQ client to a WebSphere MQ server. WebSphere MQ Explorer also uses client connections to connect to remote queue managers. The client connection channel is the client end of the channel. When you create a client-connection channel, a file is created on the computer that hosts the queue manager. You must then, copy the client-connection file to the WebSphere MQ Client computer.

For more information, see WebSphere MQ Intercommunication.