Abstract. Programs often have a lot of duplicated code, which makes both understanding and maintenance more difficult. This problem can be alleviated by detecting duplicated code, extracting it into a separate new procedure, and replacing all the clones (the instances of the duplicated code) by calls to the new procedure. This paper describes the design and initial implementation of a tool that finds clones and displays them to the programmer. The novel aspect of our approach is the use of program dependence graphs (PDGs) and program slicing to find isomorphic PDG subgraphs that represent clones. The key benefits of this approach are that our tool can find non-contiguous clones (clones whose components do not occur as contiguous text in the program), clones in which matching statements have been reordered, and clones that are intertwined with each other. Furthermore, the clones that are found are likely to be meaningful computations, and thus good candidates for extraction.

No abstract

Replication is often used in many distributed systems to provide a higher level of performance, reliability and availability. Lazy replica update protocols, which propagate updates to replicas through independent transactions after the original transaction commits, have become popular with database vendors due to their superior performance characteristics. However, if lazy protocols are used indiscriminately, they can result in non-serializable executions. In this paper, we propose two new lazy update protocols that guarantee serializability but impose a much weaker requirement on data placement than earlier protocols. Further, many naturally occurring distributed systems, like distributed data warehouses, satisfy this requirement. We also extend our lazy update protocols to eliminate all requirements on data placement. The extension is a hybrid protocol that propagates as many updates as possible in a lazy fashion. We implemented our protocols on the Datablitz database system product developed at Bell Labs. We also conducted an extensive performance study which shows that our protocols outperform existing protocols over a wide range of workloads.

Abstract

Replication is often used in many distributed systems to provide a higher level of performance, reliability and availability. Lazy replica update protocols, which propagate updates to replicas through independent transactions after the original transaction commits, have become popular with database vendors due to their superior performance characteristics. However, if lazy protocols are used indiscriminately, they can result in non-serializable executions. In this paper, we propose two new lazy update protocols that guarantee serializability but impose a much weaker requirement on data placement than earlier protocols. Further, many naturally occurring distributed systems, like distributed data warehouses, satisfy this requirement. We also extend our lazy update protocols to eliminate all requirements on data placement. The extension is a hybrid protocol that propagates as many updates as possible in a lazy fashion. We implemented our protocols on the Datablitz database system product developed at Bell Labs. We also conducted an extensive performance study which shows that our protocols outperform existing protocols over a wide range of workloads.