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G4G0-1/Semester 2/Database Systems/Week 7/Week 7 Database Systems.md

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Deadlock

An impasse that may result when two or more transactions are waiting for locks held by the other to be released. ex.

Breaking a Deadlock

Abort one or more transactions. If deadlock occurs, DBMS will

  • Automatically abort one transaction
  • Release locks, allow other transactions to continue
  • Restart aborted transaction

Handling Deadlock

  • Timeouts
  • Prevention
  • Detection and recovery

Timeouts

Transactions that request a lock will only wait a system-defined amount of time. If lock not granted within the period,

  • Lock request times out
  • DBMS assumes deadlock
  • Transaction aborted and restarted

Prevention

DBMS looks ahead to see if a transaction causes deadlock. Order transactions using transaction timestamps with the following methods:

Wait-Die

Only older transaction can wait for a younger one. If a younger transaction requests a lock held by an older one, the younger one is aborted then restarted with the same timestamp, so will eventually become the oldest active transaction and will not die.

Wound-Wait

If an older transaction requests a lock held by a younger one, the younger one is aborted.

Summary of Prevention Techniques

Deadlock free Wait-Die, transactions only wait for younger transactions, no cycle Wound-Wait, transactions only wait for older transactions, no cycle Both may cause transactions to be aborted and restarted needlessly, even if they would not cause a deadlock.

Deadlock Prevention and Recovery

DBMS allows deadlock to occur, recognises and breaks it. DBMS uses a wait-for-graph (WFG) that shows transaction dependencies A WFG is generated by creating a node for each transaction, and an edge ( Ti -> Tj ). If Ti is waiting to lock an item locked by Tj. Deadlock exists if the WFG contains a cycle. A WFG is created at regular intervals

Wait-For-Graph Example 1

Example 2

Edge Ti -> Tj, if Ti waiting to lock item locked by Tj Since the WFG contains a cycle, deadlock exists.

Example 3

  • Graph contains several cycles, deadlock exists

Recovery

Several issues that DBMS needs to address

  • Selecting deadlock victim that will minimise cost of breaking deadlock
    • May be better to abort transaction that has just started, rather than one that has been running longer.
    • May be better to abort transaction that makes little change to database, rather than one that makes a significant change
  • Avoiding starvation
    • Occurs when same transaction is chosen as victim, preventing completion
    • DBMS may try to avoid this by storing a count of abortions of a transaction.

Tutorial

  1. .
    1. Deadlock occurs when one or more transactions are held by each other, and neither can continue
    2. The DBMS detects deadlock by utilising a WFG, and checking for cycles
    3. The only way to break a deadlock is by aborting one or more transaction.
  2. .
Transaction Locked Waiting For
T1 X
T2 Z
T3 Z

  1. There is no deadlock since there are no cycles.

  2. There may be a deadlock, since there are multiple cycles in the WFG

Database Recovery Part 1