Figure 1 shows the possible sources of jobs entered into JES2. Each of the input sources (known internally as devices) is represented by a Processor Control Element ($PCE) and a Device Control Table ($DCT). The $PCE is the dispatchable element used by the JES2 dispatcher and the $DCT contains the device (input source) information.

Figure 1. Job input sources

When designing input service exits, be aware that jobs can be entered from a number of input sources. Consider whether the source of a job could affect the exit processing. For example, in the case of a spool offload job receiver, an individual job could be submitted more than once. This could be an important consideration if the purpose of the exit is to add a JCL or JECL statement. A test for a spool offload device ($DCT) may be in order to see if the additional statement already exists. Also, some exit-provided functions may not apply to all job sources. For example, you might want to bypass started tasks or time sharing sessions when enforcing installations standards. When using spool offload to selectively reload jobs, Exits 2-3-4 will be taken even for jobs that are not selected. This is because the work selection takes place after the JCL has been received.

There are jobs ($JQEs) that do not originate through input service, for example, the system log ($SYSLOG), the JES2 trace facility ($TRCLOG), and remote message spooling ($RMTMSG) that are created internally and do not have JCL associated with them. Additionally, there are jobs created for NJE and spool offload SYSOUT receivers and NJE store-and-forward jobs. These are also specially created jobs that do not go through input service and therefore input service exits are not taken for these special jobs.