Storage elements are today designed to act as standalone services. In a grid Storage Elements can be discovered by querying the information system as described in the previous section. Since data must be replicated at many sites it is relatively easy to generate dark data: data that is stored within a storage element but of which the experiment is unaware.

Most large VOs in the grid use their own replica catalogues. These comprise of a large database of information stating on which storage elements the files are stored. By querying this information, the VO may determine how many replicas exist for each file. The WLCG middleware stack contains a simple replica catalogue, the LFC. The LFC is used in the Site Functional Tests when checking whether your site is functioning correctly.