Because Holmes relies on physical evidence, his contact with suspects is usually brief and impersonal; he works almost in isolation, with little contact with others. Poirot, on the other hand, mingles with the suspects on a daily basis, sometimes becoming a temporary part of the household until the mystery is solved.

Similarly, the APPC/MVS diagnostic tools either work independently of TP processing, or can be integrated into the TP's design. You might use the TP message log to collect write-to-programmer messages only during testing; collecting such footprints is usually a temporary task, rather than being a permanent, integral part of the TP's design. Calls to Error_Extract, however, can be permanently integrated into the design of any APPC/MVS TP. Whenever the TP's processing is based on the results of a call to an APPC/MVS or CPI-C service, the TP can call Error_Extract immediately after detecting a non-zero value for the service return code. With the additional diagnostic information that Error_Extract returns, the TP can more efficiently process unsuccessful calls, even if it does nothing more than pass that information to its caller or an end user.

A call to Error_Extract does not have to be a permanent part of the TP's design, but inserting calls for testing and then removing them for production would be relatively inefficient and time-consuming. Instead, you could simply turn on the API trace facility during testing, and turn it off during production. Because the API trace facility is provided through a REXX exec instead of a callable service, you can control tracing activity in several ways, without invoking the exec directly in the TP's code. This flexibility, along with the amount of trace data collected, makes the API trace facility easier and more efficient to use.