Ten years of European Grids: What have we learnt?

The European DataGrid project started in 2001, and was followed by the three phases of EGEE and the recent transition to EGI. This paper discusses the history of both middleware development and Grid operations in these projects, and in particular the impact on the development of the LHC Computing Grid. It considers to what extent the initial ambitions have been realised, which aspects have been successful and what lessons can be derived from the things which were less so, both in technical and sociological terms. In particular it considers the middleware technologies used for data management, workload management, information systems and security, and the difficulties of operating a highly distributed worldwide production infrastructure, drawing on practical experience with many aspects of the various Grid projects over the last decade.