1. A software update method, comprising: receiving, at a second computer system configured to distribute software, an update request from a first computer system, the first computer system configured to obtain software for a plurality of client computer systems, each client computer system having membership in one or more computer groups, wherein the update request identifies a composite group of software update packages for at least two client computer systems, each of the at least two computer systems having membership in different computer groups; identifying, by the second computer system, at least a portion of one or more sources from which the composite group of software updated packages are available; and sending, from the second computer system to the first computer system, a message comprising identified software update packages and their corresponding sources of availability.