Distributed Transactions

What is a transaction?(A sequence of server operations that must be carried out atomically)

ACID properties - what are these

(Atomicity, Consistency, Isolation, Durability)

What is a distributed transaction?-Involves objects managed by multiple servers communicating with one

another.

Transactions

Permanent Record

Server operation

Server operation

Server operation

Server operation

Sharedvariables

Commit / Abort

Concurrency control

The goal of concurrency control is to guarantee that

when multiple transactions are concurrently

executed, the net effect should be equivalent to

executing them in some serial order. This is the

essence of the serializability property.

Example 1

T1 starts (20) W(x:=1) [OK] R(x) [OK] T1 commits

T2 starts(30) W(x:=2) [OK] T2 commits

T3 starts (40) W(x:=3) [OK] R(x) T3 commits

This is serializable. Think of other examples too.

Example 2

T1 starts (20) W(x:=1) [OK] R(x) [NO] T1 aborts

T2 starts(30) W(x:=2) [OK] R(x) T2 commits

T3 starts (40) W(x:=3) [OK] T3 commits

This is not serializable.

Question

Transaction 1 Raise the Q score of all GRE candidates from Iowa City by 10 points

Transaction 2Raise the Q score of all students whose id ends with 035 by 5 points

Can we run these concurrently? Explain.

Pitfalls in concurrency control

Dirty read Lost update Premature write

Lost update

Amy’s transaction Bob’s transaction

1 Load B into local 4 Load B into local 2 Add $250 to local 5 Add $250 to local3 Store local to B 6 Store local to B

What if the interleaving is 1 4 2 5 3 6 ? The final value of B is $1250, although it should have been $1500

Initially, B= $1000

Dirty read

Initially B= $1000

Amy’s transaction Bob’s transaction1 Load B into local 4 Load B into local 2 Add $250 to local 5 Add $250 to local3 Store local to B 6 Store local to B

ABORT COMMIT

Execute the actions in the sequence 1 2 3 4 5 6. The final result is still $1500, although it should have been $1250

Premature write

{Initially B = 0}

Amy’s transaction Bob’s transaction1 B:= $500 2 B := $1000

3 COMMIT

4 ABORT

B changes to 0. This could have been avoided if the second transaction postponed its commit UNTIL the first transaction commits or aborts.

Locks

Locks are commonly used to implement serrializability of concurrent transactions. Operations on shared objects are in conflict when one of them is a write operation. Each transaction must acquire the corresponding exclusive lock before executing an action.

Locks can be fine grained. Note that there is no conflict between two reads.

Serializability

The serialization graph is a directed graph (V, E) where V is the set of transactions, and E is the set of directed edges between transactions - a directed edge from a transaction Tj

to a transaction Tk implies that Tk applied a lock only after Tj

released the corresponding lock.

Tj Tk

Serializability theorem

For a set of concurrent transaction, the serializability property holds if and only if the corresponding serialization graph is acyclic

Two-phase locking (2PL)

Phase 1. Acquire all locks needed to execute the transaction. The locks will be acquired one after another, and this phase is called the growing phase or acquisition phase

Phase 2. Release all locks acquired so far. This is called the shrinking phase or the release phase.

Two-phase locking (2PL)

Growing phase Shrinking phase

acquire release

2PL

Theorem. 2PL guarantees serializability.

Proof. Suppose that the theorem is not correct. Then the serialization graph must contain a cycle …Tj Tk … Tm Tj

…This implies that Tj must have released a lock (that was later

acquired by Tk) and then acquired a lock (that was released by Tm).

However this violates the condition of two-phase locking that rules out any locking once a lock has been released.

Atomic Commit Protocols

Network of servers

The initiator of a transaction is called the coordinator, and the remianing servers are participants

S1

S3S2

Servers may crash

Requirements of Atomic Commit Protocols Network of servers

Termination. All non-faulty servers must eventually reach an irrevocable decision.

Agreement. If any server decides to commit, then every server must have voted to commit.

Validity. If all servers vote commit and there is no failure, then all servers must commit.

S1

S3S2

Servers may crash

One-phase Commit

server

coordinator

client

server

server

server

participant

participant

participant

Commit

If a participant deadlocks or faces a local problem then thecoordinator may never be able to find it. Too simplistic.