Ensuring Semi-Atomicity in Heterogeneous Distributed Database Systems
نویسندگان
چکیده
Global transaction management in a distributed database system requires cooperation from local sites to ensure the consistent and reliable execution of global transactions. In a heterogeneous distributed database environment, various local sites make con icting assertions of autonomy over the execution of global transactions. The exible transaction approach increases the failure resilience of global transactions by allowing alternative (but in some sense equivalent) executions to be attempted when a local database system fails or some subtransaction of the global transaction aborts. In this paper, we de ne the concept of semi-atomicity, which is a correctness criterion related to semantic atomicity but which is applied speci cally to exible transactions. We o er a fundamental characterization of the exible transaction model as a set of partial orders of alternatives, each of which consists of a set of interdependent subtransactions. We precisely de ne semi-atomicity with respect to these transactions and characterize the restrictions that must be placed on a exible transaction for semi-atomicity to be ensured. We present an approach to global transaction commitment that preserves the semi-atomicity of exible transactions, allowing local sites to autonomously maintain serializability and recoverability. Our approach allows a subtransaction to commit before or after a commit/abort decision is made for its global transaction and uses compensation, retry, and switching to alternative executions to regain correctness. Since the subtransactions in an execution may depend on one another, we use commit dependencies to control the commitment order of the subtransactions in a exible transaction execution. Semi-atomicity enlarges the class of executable global transactions in a heterogeneous distributed database system. We will rst explore the exible transaction model in the heterogeneous distributed database setting and then demonstrate the application of semi-atomicity in the traditional distributed database environment. 2
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Position Statement to the Panel
Today’s database systems control the execution of concurrent transactions by ensuring the serializability condition. Serializability is easy to understand and allows to reason about a transaction independently of others. The application programmer only needs to specify the statements which constitute a single transaction. However, operations at the system-internal level are synchronized differe...
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