PhEDEx is the data-placement management tool for the CMS experiment at the LHC. It manages the scheduling of all large-scale WAN transfers in CMS, ensuring reliable delivery of the data. It consists of several components:

• an Oracle database, hosted at CERN
• a website and data-service, which users (humans or machine) use to interact with and control PhEDEx
• a set of central agents that deal with routing, request-management, bookkeeping and other
• activities. These agents are also hosted at CERN, though they could be run anywhere. The key point is that there is only one set of central agents per PhEDEx instance
• a set of site-agents, one set for every site that receives data

PhEDEx maintains knowledge and history of transfer performance, and the central agents use that information to choose among source replicas when a user makes a request (users specify the destination, PhEDEx chooses the source).

The central agents then queue the transfer to be processed by the site agents. PhEDEx operates in a data-pull mode, the destination site pulls the data to itself when it is ready. This gives the sites more control over the activity at their site, so they can ensure that neither their network nor their storage are overloaded.