Leasing prioritized tasks

Methods, systems, and apparatus, including computer programs encoded on computer storage media, for leasing tasks from multiple prioritized groups. One of the methods includes separating tasks into multiple groups where each task is associated with a respective group identification. The tasks are prioritized in each of the multiple groups where each task is associated with a respective priority identification. A request is received from a worker process for a number of tasks within a group, and a number of the tasks within the group are leased to the worker process for a first time period such that the leased tasks are unavailable to other worker processes during the first time period.