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Introduction

Computing for the ATLAS experiment at the LHC proton-proton collider at CERN will pose a number of new, yet unsolved technological and organisational challenges:

• The CPU power required would currently not be affordable. For the offline processing, it is estimated that 2.5x105 SPECint95 (107 MIPS) will be needed. To collect this computer power today, it would take 50,000 of the most powerful PCs. We count on the continuing improvement of the price-performance ratio of CPU power and of the corresponding disks and memory.

• The data volume produced by the experiment of about 1 Pbyte (1015 bytes) per year requires new methods for data reduction, data selection, and data access for  physics analysis. The basic consideration is that every physicist in ATLAS must have the best possible access to the data necessary for the analysis, irrespective of his/her location. The proposed scheme consists of archiving the raw daat  (1 Pbyte/year) selected by the Level-3 trigger system. A first event reconstruction will be performed at CERN, on all data a few hours after data taking. For this processing, basic calibration and alignment have to be available. The purpose of this processing is to determine physics quantities for use in analysis and to allow event classification according to physics channels. The data produced in the processing have to be accessible at the event level and even below that at the physics object level. We are considering an object-oriented database system for this purpose. One copy of the data will be held at CERN. We also consider replicating some or all of the data at a small number of regional centres.

• The world-wide collaboration will require performant wide-area networks for data access and the physics analysis. Big improvements in the price-performance evolution of networks are hoped for in view of thede-regulation of the PTTs, the evolution of the Internet and the wide-spread use of networks for multi-media applications such as video on demand.

 The effort required to develop and maintain the ATLAS software will be enormous. Because of the dependence of the whole experiment?s success and due to the long lifetime of about 20 years of the project, the software quality requirements will have to be very high. For the whole ATLAS software development, up to 1000 person-years will be required. It appears that the overall manpower is available within the collaboration. A complication is that the workforce is very much spread geographically and that many developers will be students who can spend only a few years in the project.