**3.3.1 Computing Element**

12 Will-be-set-by-IN-TECH

Fig. 10. Schema of Grid services

• Virtual Data Toolkit [29]

model.

**3.2 Experiments' specific developments**

actually using the WLCG for their analysis.

• Globus Toolkit [27] developed by the Globus Alliance

tailored to better comply with their Computing models.

• OMII from the Open Middleware Infrastructure Institute [28]

All the LHC experiments created their own specific Computing models summarized in the individual Computing Technical Design Reports (TDRs). They do not rely only on the WLCG-provided middleware packages but are also developing some specific components

For example, the ALICE experiment has developed a grid middleware suite AliEn (AliCE Environment [30]), which provides a single interface for a transparent access to computing resources for the ALICE community. AliEn consists of a collection of components and services which will be described in the next section. AliEn, together with selected packages of the WLCG-provided middleware, gives a complete framework to the ALICE community to manage and process the data produced by the LHC according to the ALICE Computing

All the LHC experiments invested a considerable effort into shielding the users from the underlying complexity of the Grid machinery, trying to provide relatively simple entry points into the Grid. This effort has payed off and is reflected in a considerable number of physicists The Computing Element (CE) [31] is a middleware component/grid service providing an entry point to a grid site. It authenticates users and submits jobs to Worker Nodes (WN), aggregates and publishes information from the nodes. It includes a generic interface to the local cluster called Grid Gate (GG), Local Resource Management System (LRMS) and the collection of Worker Nodes.

Originally, the submission of jobs to CEs was performed by the Workload Management System (WMS) [32], a middleware component/grid service, that also monitors jobs status and retrieves their output. WLCG (gLite) CE is a computing resource access service using standard grid protocols. To improve the performance, the CREAM (Computing Resource Execution And Management) Computing Element [33] has replaced the gLite-CE in production since about 2009. It is a simple, lightweight service for job management operation at the Computing Element level. CREAM-CE accepts job submission requests (described with the same files as used for the Workload Management System) and other job management requests like, e.g., job monitoring. CREAM-CE can be used by a generic client, e.g., an end-user willing to directly submit jobs to a CREAM-CE, without the WMS component.
