- Wait-for graph:
- Wait-for graph is
a graph where:
- Node represents a
transaction
- Edge i ⇒ j
represents the fact that:
- The transaction i is waiting for a lock held by the transaction j
Example:
- Node represents a
transaction
- Wait-for graph is
a graph where:
- The wait-for graph can be
constructed using the
information stored in the
lock table entries
Example:
represents the fact that:
- Transaction T3 is
waiting on
the transactions
T1 and
T2
- Transaction T3 is
waiting on
the transactions
T1 and
T2
- Deadlock occurs in a
circilar wait situation:
Each transaction is waiting for some transaction to complete
Result:
- No transaction can
proceed forward
This situation called:
- Deadlock
- No transaction can
proceed forward
- Deadlock detection and
wait-for graphs:
- If the wait-for graph has
no cycles then:
- There is no deadlock
Example:
Completion order:
- Transaction T1 will complete first (and releases its locks)
- Then Transaction T3 can obtain its locks after transaction T1 unlocks and complete next (and releases its locks)
- Finally Transaction T2 can obtain its locks after transaction T3 unlocks and complete next (and releases its locks)
- If the wait-for graph has a
cycle:
- There is deadlock
Example:
- If the wait-for graph has
no cycles then:
- A transaction scheduler that uses
locking to
ensure serializability must:
- Maintain a
wait-for graph on
all transaction
- If the wait-for graph contains
a cycle:
- The scheduler will abort one of the transactions in the cycle
- Maintain a
wait-for graph on
all transaction
- Example:
- Transactions:
- Example of
schedule
(in time steps):
- The corresponding
wait-for graph:
The transaction manager will now have to abort some transaction T1, T2 and T3 that are involved in the deadlock
- Transactions:
- Commonly used
policies:
- Last transaction that performed a lock operation
(fair):
- The last transaction that
makes a
lock request and
caused a
deadlock
will be aborted
(This transaction has caused the deadlock !!!)
- The last transaction that
makes a
lock request and
caused a
deadlock
will be aborted
- Last starting transaction
(least wastage, efficient):
- Each transaction is
given a timestamp indicating
the order in which the
transactions has
started execution
- The transaction with the highest timestamp (= "youngest" transaction) is aborted
- Each transaction is
given a timestamp indicating
the order in which the
transactions has
started execution
- Last transaction that performed a lock operation
(fair):