Composing Distributed Services for Selection and Retrieval of Event Data in the ATLAS Experiment
Elisabeth Vinek, Florbela Tique Aires Viegas · Journal of Physics Conference Series · 2011
The ATLAS experiment uses TAGs, which are basic quantities describing events recorded at the ATLAS detector, to index and optimize the access to the full event data. TAGs allow physicists to pre-select data based on queries of any of the TAG attributes. TAGs are stored in relational databases deployed at various locations, and several distributed services are in place to access and extract them. When a user starts a request, typically several of those services need to be selected and composed in order to properly respond to the request. As there can be several deployments of the same service, differing only in Quality-of-Service (QoS) attributes, there has to be a mechanism to select one deployment for each service required. In this publication, we describe the selection model and the underlying optimization considerations. The objective is not only to optimize the access for a single user, but to give to as many users as possible efficient access to the TAGs. The approach also takes into account the dynamic aspects of the distributed TAG system, where whole sites or single service deployments can be down or decommissioned. A QoS registry has been designed and implemented, to provide the data needed as input to the optimization itself.