vault backup: 2024-02-27 16:57:07

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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. ![](Pasted%20image%2020240227130252.png)
+
+## 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.
+![](Pasted%20image%2020240227131306.png)
+
+#### Wound-Wait
+
+If an older transaction requests a lock held by a younger one, the younger one is aborted.
+![](Pasted%20image%2020240227131321.png)
+
+#### 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
+
+![](Pasted%20image%2020240227131936.png)
+![](Pasted%20image%2020240227131954.png)
+![](Pasted%20image%2020240227132007.png)
+
+#### Example 2
+
+![](Pasted%20image%2020240227132741.png)
+Edge Ti -> Tj, if Ti waiting to lock item locked by Tj
+![](Pasted%20image%2020240227132816.png)
+Since the WFG contains a cycle, deadlock exists.
+
+#### Example 3
+
+![](Pasted%20image%2020240227133521.png)
+
+- ![](Pasted%20image%2020240227133544.png)
+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           |
+![](Pasted%20image%2020240227161426.png)
+
+3. ![](Pasted%20image%2020240227161946.png)
+There is no deadlock since there are no cycles.
+
+4. ![](Pasted%20image%2020240227162324.png)
+There may be a deadlock, since there are multiple cycles in the WFG
+
+# Database Recovery Part 1