Distributed Transactions

1
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.

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Transactions
Permanent Record
Commit / Abort
Shared variables
Server operation
Server operation
Server operation
Server operation
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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.

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Example 1
T1 starts (20) W(x1) OK R(x) OK T1
commits T2 starts(30) W(x2) OK T2
commits T3 starts (40) W(x3) OK R(x) T3
commits This is serializable. Think
of other examples too.
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Example 2
T1 starts (20) W(x1) OK R(x) NO T1
aborts T2 starts(30) W(x2) OK R(x)
T2 commits T3 starts (40) W(x3) OK T3
commits This is not serializable.
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Question
Transaction 1 Raise the Q score of all GRE
candidates from Iowa City by 10
points Transaction 2 Raise the Q score of all
students whose id ends with 035 by 5 points
Can we run these concurrently? Explain.
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Pitfalls in concurrency control

• Dirty read
• Lost update
• Premature write

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Lost update
Initially, B 1000

• Amys transaction Bobs transaction
• 1 Load B into local 4 Load B into local
• Add 250 to local 5 Add 250 to local
• 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

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Dirty read

• Initially B 1000
• Amys transaction Bobs transaction
• 1 Load B into local 4 Load B into local
• Add 250 to local 5 Add 250 to local
• 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

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Premature write

• Initially B 0
• Amys transaction Bobs transaction
• 1 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.

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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.

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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
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Serializability theorem

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

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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.

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Two-phase locking (2PL)
acquire
release
Growing phase
Shrinking phase
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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.

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Atomic Commit Protocols

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

S1
Servers may crash
S3
S2
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Requirements of Atomic Commit Protocols
S1

• 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.

Servers may crash
S3
S2
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One-phase Commit
server
participant
Commit
server
server
client
participant
coordinator
server
participant
If a participant deadlocks or faces a local
problem then the coordinator may never be able to
find it. Too simplistic.