Disclosed herein are system, method, and computer program product embodiments for message handling related to non-parallelizable functionality. An embodiment operates by determining a plurality of diagnostic messages corresponding to processing a request that includes both a first set and a second set of diagnostic messages. Diagnostic information corresponding to the first set of diagnostic messages, from previously processed requests, is determined to be stored in a cache. Diagnostic information corresponding to the second set of diagnostic messages is retrieved from the disk storage. A diagnostic report associated with the processing of the request including diagnostic information from both the first set and the second set of diagnostic messages is generated and returned.