Sherlock Holmes always concentrates fiercely on physical evidence; he never visits the scene of the crime without his magnifying glass. Hercule Poirot, in marked contrast, disdains crawling in the dirt, on hands and knees; instead, he prefers to mingle with the suspects, collecting significant words and phrases that explain people's behavior and psychology.

As with Holmes and Poirot, each APPC/MVS diagnostic tool collects different types of clues. Error_Extract provides detailed information about the error that occurred on a single call to an APPC/MVS or CPI-C service, whereas API trace provides the details about each service call made by a particular TP, regardless of whether an error occurred. When an error does occur on a specific call, the API trace data for that call automatically includes the information that Error_Extract would return for that service. Because API trace provides the same, and more, information as Error_Extract, API trace is the preferred detective for most situations.

API trace and Error_Extract provide diagnostic information for most, but not all, APPC/MVS and CPI-C callable services. See Table 1 for a list of the services that are supported by either API trace, Error_Extract, or both.

In contrast, the TP message log provides information through messages generated while a TP runs, such as system messages about resources allocated by the TP. The message log could contain messages about a conversation call, but only if the application programmer deliberately codes the TP to issue write-to-programmer messages to record its progress. Although useful, these messages cannot contain the detailed information that APPC/MVS provides through the API trace facility.