Chap 16 Transactions Transactions n n n Transaction

Chap 16. Transactions

Transactions n n n Transaction: sequence of operations such that the entire sequence appears as one indivisible operation Indivisibility is maintained even in the case of concurrency and failures Example: q Begin transaction withdraw x from account A; deposit x to account B; End transaction

Transaction: Implementing the following primitives n n begin_transaction end_transaction : commit the transaction abort_transaction : all values prior to the transaction are restored read, write : the program can read or write objects

ACID properties n n Atomicity: all or nothing Consistency: should not violate integrity constraints of the system Isolation: transactions are isolated from the effects of concurrent transactions Durability: once a transaction has been committed it remains permanent even if there are failures

Concurrency Control T 1 n T 2 May lead to violation of isolation if both are executed simultaneously

Two-Phase locking n n Lock data before accessing Unlock data only at the end

Dealing with failures n Private workspace : q q n Copy the objects that have been updated by the transaction Discard copy if transaction aborts, overwrite original if transaction commits Logging: q Maintain a trail of all writes so that they can be undone if necessary

Distributed commit n n Agreement: No two processes decide on different outcomes of the transaction Validity: If any process starts with abort then abort is the only possible final outcome Weak termination: If there are no failures, all processes eventually decide Non-blocking: All processes eventually decide

Two phase protocol satisfying first 3 properties n n n Phase 1 The coordinator sends a request message to all participants On receiving a request message, each participant replies with either a ‘yes’ or a ‘no’ message. q A ‘yes’ message signifies that the participant can commit all the actions performed at its site.

Two Phase Protocol (contd. ) n n Phase 2 The coordinator waits to receive messages from all participants. q n If all of them are ‘yes’, then the coordinator sends the final. Commit message. Otherwise, it sends a final. Abort message. The participant carries out the action associated with the message received from the coordinator.

//Coordinator public class Two. Phase. Coord extends Process { boolean global. Commit = false; boolean done. Phase 1 = false; boolean no. Received = false; int num. Participants; int num. Replies = 0; public Two. Phase. Coord(Linker init. Comm) { super(init. Comm); num. Participants = N - 1; } public synchronized void do. Coordinator() { // Phase 1 broadcast. Msg("request", my. Id); while (!done. Phase 1) my. Wait(); // Phase 2 if (no. Received) broadcast. Msg("final. Abort", my. Id); else { global. Commit = true; broadcast. Msg("final. Commit", my. Id); } } public synchronized void handle. Msg(Msg m, int src, String tag) { if (tag. equals("yes")) { num. Replies++; if (num. Replies == num. Participants) { done. Phase 1 = true; notify(); } } else if (tag. equals("no")) { no. Received = true; done. Phase 1 = true; notify(); } } }

public class Two. Phase. Participant extends Process { boolean local. Commit; boolean global. Commit; boolean done = false; boolean has. Proposed = false; public Two. Phase. Participant(Linker init. Comm) { super(init. Comm); } public synchronized void propose(boolean vote) { local. Commit = vote; has. Proposed = true; notify(); } public synchronized boolean decide() { while (!done) my. Wait(); return global. Commit; } public synchronized void handle. Msg(Msg m, int src, String tag) { while (!has. Proposed) my. Wait(); if (tag. equals("request")) { if (local. Commit) send. Msg(src, "yes"); else send. Msg(src, "no"); } else if (tag. equals("final. Commit")) { global. Commit = true; done = true; notify(); } else if (tag. equals("final. Abort")) { global. Commit = false; done = true; notify(); } } }

Analysis n n If coordinator fails in the second phase before informing any participant then all participants have to wait for the coordinator to recover Hence this protocol is blocking