Systems for processing requests to execute a program code of a user use a message queue service to store requests when there are not enough resources to process the requests. The message queue service distributes the stored requests across multiple queues; each queue is associated with a context of the underlying events that generate the requests. A context describes one or more attributes of the event, such as information identifying the requestor or the event source. Thus, queued messages are divided into different queues based on user ID, event source or location, event type, etc. The message queue service then selects from the queues when requested to deliver a queued request. This provides a balanced delivery of requests from different requestors or groups of requestors.