1. A method, including: receiving at least one rule specification, that specifies criteria for determining one or more output values that depend on input data; providing an interface for identifying conditions for generating log messages; generating output records by transforming input data according to the rule, including: determining that at least one of the conditions has occurred, tracing the transforming, and in response to determining that at least one of the conditions has occurred, generating log messages based on the tracing, the log messages including information not included in the output records; providing the output records on a first channel; and providing the log messages on a second channel different from the first channel.