Assuming that strict two-phase locking is in use, describe how the actions of the two-phase commit protocol relate to the concurrency control actions of each individual server. How does distributed deadlock detection fit in?
What will be an ideal response?
Each individual server sets locks on its own data items according to the requests it receives, making transactions wait when others hold locks.
When the coordinator in the two-phase commit protocol sends the doCommit or doAbort request for a particular transaction, each individual server (including the coordinator) first carries out the commit or abort action and then releases all the local locks held by that transaction.
(Workers in the uncertain state may hold locks for a very long time if the coordinator fails)
Only transactions that are waiting on locks can be involved in deadlock cycles. When a transaction is waiting on a lock it has not yet reached the client request to commit, so a transaction in a deadlock cycle cannot be involved in the two-phase commit protocol.
When a transaction is aborted to break a cycle, the coordinator is informed by the deadlock detector. The coordinator then sends the doAbort to the workers.
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