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This Concept Map, created with IHMC CmapTools, has information related to: ch14 dist conc, concurrency control in distributed transactions Timestamp ordering concurrency control Single server transactions coordinator issues a unique timestamp to each transaction before it starts serial equivalence ensured by committing objects in order of timestamps Distributed transactions the first coordinator accessed by a transaction issues a globally unique timestamp as before the timestamp is passed with each object access the servers are jointly responsible for ensuring serial equivalence that is if T access an object before U, then T is before U at all objects coordinators agree on timestamp ordering a timestamp consists of a pair <local timestamp, server-id>. the agreed ordering of pairs of timestamps is based on a comparison in which the server-id part is less significant – they should relate to time, concurrency control in distributed transactions Locking In a distributed transaction, the locks on an object are held by the server that manages it. The local lock manager decides whether to grant a lock or make the requesting transaction wait. it cannot release any locks until it knows that the transaction has been committed or aborted at all the servers involved in the transaction. the objects remain locked and are unavailable for other transactions during the atomic commit protocol an aborted transaction releases its locks after phase 1 of the protocol., concurrency control in distributed transactions ???? Each server manages a set of objects and is responsible for ensuring that they remain consistent when accessed by concurrent transactions therefore, each server is responsible for applying concurrency control to its own objects. the members of a collection of servers of distributed transactions are jointly responsible for ensuring that they are performed in a serially equivalent manner therefore if transaction T is before transaction U in their conflicting access to objects at one of the servers then they must be in that order at all of the servers whose objects are accessed in a conflicting manner by both T and U