1. A system for providing event messages for requests to execute program code on a virtual compute system, the system comprising: an electronic data store configured to store at least programmatic event handling information; and an event triggering computing system comprising one or more hardware computing devices executing specific computer-executable instructions, the event triggering computing system in communication with a message queue system, the event triggering computing system configured to at least: detect an event on the event triggering computing system, the event associated with a first client; determine, based on the programmatic event handling information stored on the electronic data store, that the event is designated to trigger execution of a program code of a second client on a virtual compute system; in response to the determination that the event is designated to trigger execution of the program code of the second client, generate an event message comprising at least a client account identifier associated with the second client and programmatic information organized in a schema usable to execute the program code on behalf of the second client in a container created on a virtual machine instance on the virtual compute system; and provide the event message to the message queue system, wherein the message queue system is further in communication with the virtual compute system.