1. A software update method, comprising: receiving, at a second computer system configured to distribute software, an update request from a local update server, the local update server configured to obtain software for a first client computer system having membership in a first computer group and a second client computer system having membership in a second computer group different than the first computer group, wherein the update request identifies a composite group of software update packages for the first client computer system and the second client computer system; 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; sending, from the second computer system to the local update server, a message comprising identified software update packages and their corresponding sources of availability; sending, from the second computer system to the local update server, a first catalog list identifying only applicable updates for the first client computer system based the first client computer system's membership in the first computer group; and sending, from the second computer system to the local update server, a second catalog list identifying only applicable updates for the second client computer system based the second client computer system's membership in the second computer group.