There are some considerations that you should understand before using user exit programs for DDM.
If the user exit program is a CL program that creates an i5/OS™ exception, an inquiry message is sent to the server operator on the target server if, for the target job, the job attribute INQMSGRPY is *RQD (the default) or *SYSRPYL with no value in the reply list for this message. The user exit program waits for a response to the message on the target server, which causes the source job to wait also.
There are other potential situations in which waiting could occur. For example, if lengthy wait values are specified on the WAIT parameter of the Allocate Object (ALCOBJ) or Receive Message (RCVMSG) command, both the source and target jobs wait up to the maximum time specified for an object lock to be obtained or a message to be received by the target job.