Systems and methods are described for transforming a data set within a data source into a series of task calls to an on-demand code execution environment or other distributed code execution environment. Such environments utilize pre-initialized virtual machine instances to enable execution of user-specified code in a rapid manner, without delays typically caused by initialization of the virtual machine instances, and are often used to process data in near-real time, as it is created. However, limitations in computing resources may inhibit a user from utilizing an on-demand code execution environment to simultaneously process a large, existing data set. The present application provides a task generation system that can iteratively retrieve data items from an existing data set and generate corresponding task calls to the on-demand computing environment, while ensuring that at least one task call for each data item within the existing data set is made.