lect-14 - Computer Science 425 Distributed Systems Lecture...

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2006, 2007 M. T. Harandi Lecture 14- 1 Computer Science 425 Distributed Systems Lecture 14 Concurrency Control - II
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2006, 2007 M. T. Harandi Lecture 14- 2 : Each server is responsible for applying concurrency control to its objects. : Servers are collectively responsible for serial equivalence of operations. : Locks are held locally, and cannot be released until all servers involved in a transaction have committed or aborted. : Locks are retained during 2PC protocol : Since lock managers work independently, deadlocks are very likely. Locks in Distributed Transactions
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2006, 2007 M. T. Harandi Lecture 14- 3 : The wait-for graph in a distributed set of transactions is held partially by each server : To find cycles in a distributed wait-for graph, we can use a central coordinator: : Each server reports updates of its wait-for graph : The coordinator constructs a global graph and checks for cycles : Centralized deadlock detection suffers from usual comm. problems. : In edge chasing servers collectively make the global wait-for graph and detect deadlocks : : Servers forward “probe” messages to servers in the edges of wait-for graph, pushing the graph forward, until cycle is found. Distributed Deadlocks
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2006, 2007 M. T. Harandi Lecture 14- 4 Edge Chasing Initiation: When a server S 1 notes that a transaction U is waiting for an object held by another transaction V, where V is waiting to access an object at another server S 2 , it initiates detection by sending <U V> to S 2 . Detection: Servers receive probes and decide whether deadlock has occurred and whether to forward the probes. Resolution: When a cycle is detected, a transaction in the cycle is aborted to break the deadlock.
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2006, 2007 M. T. Harandi Lecture 14- 5 Example: Edge Chasing T U Wait for Held by Held by Wait for B V Held by Wait for A C X Y Z X: U V Y: T U Z: V T LOCAL Wait-for GRAPHS T Wait for Held by Held by Wait for B V Held by Wait for A C X Y Z U V X: U V T Y: T U V Z: T U V T deadlock detected
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2006, 2007 M. T. Harandi
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lect-14 - Computer Science 425 Distributed Systems Lecture...

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