The version control service for the ATLAS data acquisition configuration files

The ATLAS experiment at the LHC in Geneva uses a complex and highly distributed Trigger and Data Acquisition system, involving a very large number of computing nodes and custom modules. The configuration of the system is specified by schema and data in more than 1000 XML files, with various experts responsible for updating the files associated with their components. Maintaining an error free and consistent set of XML files proved a major challenge. Therefore a special service was implemented; to validate any modifications; to check the authorization of anyone trying to modify a file; to record who had made changes, plus when and why; and to provide tools to compare different versions of files and to go back to earlier versions if required. This paper provides details of the implementation and exploitation experience, that may be interesting for other applications using many human-readable files maintained by different people, where consistency of the files and traceability of modifications are key requirements.