Send and receive exits
A send exit at one end of a channel and a receive exit at the other end normally work in pairs. A send exit is called just before an MCA issues a communications send to send data over a communications connection. A receive exit is called just after an MCA has regained control following a communications receive and has received data from a communications connection.
The WebSphere MQ channel protocol flows between two MCAs on a message channel contain control information as well as message data. Similarly, on an MQI channel, the flows contain control information as well as the parameters of MQI calls. Send and receive exits are called for all types of data.
Message data flows in only one direction on a message channel but, on an MQI channel, the input parameters of an MQI call flow in one direction and the output parameters flow in the other. On both message and MQI channels, control information flows in both directions. As a result, send and receive exits can be called at both ends of a channel.
Send and receive exits are not called for the initial data flows at channel startup and for the flows of security messages between the two security exits.
The unit of data that is transmitted in a single flow between two MCAs is called a transmission segment. Send and receive exits have access to each transmission segment. They can modify its contents and change its length. A send exit, however, must not change the first eight bytes of a transmission segment. These eight bytes form part of the WebSphere MQ channel protocol header. There are also restrictions on how much a send exit can increase the length of a transmission segment. In particular, a send exit cannot increase its length beyond the maximum that was negotiated between the two MCAs at channel startup.
On a message channel, if a message is too large to be sent in a single transmission segment, the sending MCA splits the message and sends it in more than one transmission segment. As a consequence, a send exit is called for each transmission segment containing a portion of the message and, at the receiving end, a receive exit is called for each transmission segment. The receiving MCA reconstitutes the message from the transmission segments after they have been processed by the receive exit.
Similarly, on an MQI channel, the input or output parameters of an MQI call are sent in more than one transmission segment if they are too large. This might occur, for example, on an MQPUT, MQPUT1, or MQGET call if the application data is sufficiently large.
Taking these considerations into account, it is more appropriate to use send and receive exits for purposes in which they do not need to understand the structure of the data they are handling and can therefore treat each transmission segment as a binary object.
A send or a receive exit can decide to close a channel.
The names of a send exit and a receive exit are specified as parameters in the channel definition at each end of a channel. We can also specify a list of send exits to be run in succession. Similarly, we can specify a list of receive exits.
For more information about send and receive exits, see Send and receive exits.
Parent topic:
Channel exit programs
sy11090_