**3. Middleware**

As we already mentioned, the Worldwide LHC Computing Grid is a distributed computing infrastructure that spans over five continents managing resources distributed across the world (due to funding, operability and access reasons). The resources operated by the WLCG belong either to the two main global grids, EGI [16] and OSG [17], or to other collaborating regional or national grids. To make this diverse variety of resources globally available for all the WLCG users, the WLCG has been developing its own middleware, a software layer that "brings all the resources together": a collection of programs, services and protocols to manage and operate the entire WLCG infrastructure (see Figure 9).
