Distributed Transactions - SCHOOL OF COMPUTER SCIENCE, Carnegie Mellon

Distributed Transactions - SCHOOL OF COMPUTER SCIENCE, Carnegie Mellon

9 Pages · 2001 · 55 KB · English

Distributed Transactions Two Issues: Develop an atomic commit protocol a cooperative procedure used by a set of servers involved in a distributed transaction

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 Distributed Transaction Management Anastassia Ailamaki http://wwwcscmuedu/~natassa 2 Distributed Transactions Tw o Issues: Develop an atomic commit protocol a cooperative procedure used by a set of servers involved in a distributed transaction enable the serv ers to r each a joint decision as to whether a transaction can be c ommitted or aborted Deal w ith distributed Deadlock each member of a group of transactions is waiting for some other member to release a lock 3 Distributed Transactions  4 Atomic Commit Protocol Transaction atomicity:either all of its op erations ar e carried out or none of them In a distributed environment, all the servers involved a transaction mustagreeon the final outcome of the transaction Ie, a transaction must either commit or abort all all servers Why do we need an atomic commitment protocol?uncertainty of the servers’ decisions on the transaction committhe server’s decision is affected by the concurrency control, server and network failure 5 TwoPhase Commit Protocol In the second phase: every server carries out joint decision one server v otes to abort => abort tr ansactio all servers vote to commit => commit transaction The problem:How to ensur e that all of the s er v er s vot e + that th ey all r eac h th e s am e decision Answ er:It is simple if no errors occur, but the pr ot oc ol mus t wor k c or r ec tly ev en wh en s er v er fails, messages are lost, etc 6 TwoPhase Commit Protocol Simp les t and m os t wid ely us ed c omm it pr ot oc ol In the first phase: each server v otes for the transaction to be c ommitted or aborted once a s erver has voted to commit a transaction, it is not allowed to abort it even if it fails and restarts in the interim the server voting to commit must ensure that the updated data have been saved in the stable storage and enters the prepared state  7 2PC live Coordinator Subordinate PREPARE VOT E YES/NO COMMIT /ABORT ACK commit*/abort* commit*/abort*commit*/abort* commit*/abort*prepare*/abort* prepare*/abort*prepare*/abort* prepare*/abort* commit*/abort* commit*/abort*commit*/abort* commit*/abort* end endend end 8 2PC principles of operation 4 types of messages: pr ep ar e, v ote y/n, c ommit/ ab or t, ac k 4 types of log r ec or ds : pr ep ar e* , c ommit* , ab or t* , end Subordinates forcewrite log records – why? ( nev er as k c oor d in at or ab out that info) W hy are ACKs required? (to ensure ever yone knows final outcome) 9 Blocking There are various stag es at which a server cannot progress its part of the protocol until it receives another message Example: if a server has v oted Yes and is waiting for the decision of the coordinator the server is blocked until it gets the commit decision because it cannot decide unilaterally But, the data items held cannot be released for use by other transactions If the coordinator has failed, the server must wait for the decision until the coordinator recovers Timeouts at the coordinator in the first phase may av oid the long waiting due to the long delay of s erver’s response  10 Summary thus far Committing Tr ans action: SubordinateWrites 2 records (prepare*, commit*)Sends 2 messages (YES vote and ACK) CoordinatorWrites 2 records (commit*, end)Sends 2 msgs to each subord (prepare and commit) If ev er ything g oes w ell:3(N1) messagesThe ACK messages are not counted since the protocol can function correctly without them 11 2PC and Failures Assumptions recovery exists both sides all failed nodes ultimately recover What happens if recovery finds node inpreparedstate transaction alive at crash,nolog information commitorabortstate (periodically polls coordinator to find what happened) (don’t know, don’t care, undo, write abort record) (periodically send commit or abort to noack subords) 12 2PC and Failures (cont) Coor din at or n otic es s ubor din at e failur e If subordinatehas not sent vote If subordinatehas not sent ACK Sub or d in at e n ot ic es c oor d in at or failur e If subordinatehas not sent vote (not prepared) If subordinateis in prepared state coordinator aborts transaction coordinator hands Xtion ov er

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