Grid is an infrastructure that involves the integrated and collaborative use of computers, networks, databases and scientific instruments owned and managed by many organizations. Grid applications often involve large amounts of data and/or computer resources that require a secure resource sharing throughout the organization. This makes grid operation and deployment a complex undertaking. Grid middleware provides users with seamless computer skills and uniform access to resources in the heterogeneous grid. A number of toolkits and systems have been developed, most of which are the result of academic research projects worldwide. This chapter focuses on four of these intermediaries: UNICORE, Globus, Legion and Grid bus. It also presents our implementation of a UNICORE resource broker because it did not support this functionality. A comparison of these systems is included based on the architecture, the implementation model and a number of other functions.
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