The AliEn Subsystem
AliEn is a lightweight Grid framework which is built around Open Source
components using the Web Services model. It has been initially developed by the
ALICE collaboration as a production environment for the simulation,
reconstruction, and analysis of Physics data in a distributed way. The
architecure of AliEn provided a blueprint and a starting point for developing
the gLite architecture. In gLite prototype, the AliEn components are used to
provide an initial implementation of components and services like a file and
metadata catalogue, task queue, package manager and various user interfaces in
the form of a command line prompt, an application API and corresponding Grid
Access Service (GAS).
Unlike most main-stream Grid-implementations AliEn has been based on the Web Services model and standard network protocols from the very beginning. Perl has been chosen as a scripting language mainly because of the availability of a large number of reusable Open Source modules, which provide complete cryptography support, a full featured SOAP platform and easy Web integration. At present, AliEn is using around 170 external components, most of them perl modules, but also third party Open Source packages.
The AliEn Workload Management uses a pull architecture (as opposed to the push mechanism traditionally implemented in other Grid systems). In this implementation the scheduling sevice does not need to know the status of all other resources in the system. The job description in the form of Job Description Language (the Condor ClassAd), is simply stored in a Task Queue database waiting for Computing Elements to connect and to advertise their status and capabilities. This results in a robust and fault tolerant system where resources can come and go at any point in time. Given this weak coupling between resources and the system, it is possible to abstract and interface an entire foreign Grid as a large Computing and Storage Element.
The user interacts with the system through a User Interface layer. The interaction can be instantiated from a command line interface with the most common Unix file system commands being implemented, by using a graphical user interface or via a generic Web portal which provides a convenient way to submit, inspect and manipulate a large number of jobs running at many sites. In order to gain access to AliEn at the application level an API is needed. Next to the perl module AliEn provides a C and C++ API.
The system has been put into production for ALICE users at the end of 2001. Currently ALICE is using AliEn mainly for distributed production of Monte-Carlo data, detector simulation and reconstruction at over 40 sites located on 4 continents. More than 400,000 ALICE-jobs have been run under AliEn control worldwide during recent current Physics Data Challenge excercises.
For more information, please see:
- ALICE, A Large Ion Collider Experiment http://alice.web.cern.ch/Alice/AliceNew/
- The Globus Alliance
- The Condor Project
- P.Buncic, A.J. Peters, P.Saiz: The AliEn System,
status and perspectives, Talk given at 2003 Conference for Computing in
High-Energy and Nuclear Physics (CHEP 03), La Jolla, California, 24-28 Mar
2003. Published in eConf C0303241:MOAT004,2003,
e-Print Archive: cs.dc/0306067
- Saiz, P. Buncic, A.J. Peters: AliEn Resource Brokers
Contributed to 2003 Conference for Computing in High-Energy and Nuclear Physics (CHEP 03), La Jolla, California, 24-28 Mar 2003. Published in eConf C0303241:TUAP002,2003
e-Print Archive: cs.dc/0306068
- A.J.Peters, P.Buncic, P.Saiz: AliEnFS - a Linux File
System for the AliEn Grid Services
Talk given at 2003 Conference for Computing in High-Energy and Nuclear Physics (CHEP 03), La Jolla, California, 24-28 Mar 2003. Published in eConf C0303241:THAT005,2003
e-Print Archive: cs.dc/0306071
- S.Bagnasco et.al.: AliEn-EDG interoperability in
Talk given at 2003 Conference for Computing in High-Energy and Nuclear Physics (CHEP 03), La Jolla, California, 24-28 Mar 2003. Published in eConf C0303241:TUcP005,2003
e-Print Archive: physics/0306103 http://arxiv.org/pdf/physics/0306103