1. A system for processing event messages for requests to execute program code on one or more virtual machine instances, the system comprising: a frontend computing system comprising one or more hardware computing devices executing specific computer-executable instructions, the frontend computing system in communication with (i) a virtual compute system adapted to provide compute capacity on a per-request basis and (ii) one or more other services, the frontend computing system configured to at least: receive a plurality of code execution requests associated with a first program code, each code execution request including an identity of a user account, an identity of the first program code, and one or more first input parameters to the first program code, wherein the user account is associated with the first program code and a set of execution parameters used for executing program codes associated with the user account; process the plurality of code execution requests at least by attempting to execute the first program code at a first time period using the one or more first input parameters and the set of execution parameters; determine that one or more requests from the plurality of code execution requests were not processed successfully due to an error during execution of the first program code; route the one or more requests to a separate storage area for additional processing, the separate storage area storing a plurality of requests that were not processed successfully by the frontend computing system; determine whether a request stored in the separate storage area is to be processed again by the frontend computing system; and in response to determining that the request is to be processed again by the frontend computing system, process the request at least by attempting to execute the first program code at a second time period subsequent to the first time period.