The REPEATABLE READ isolation level can lead to deadlocks in two common scenarios:
Scenario 1:
• Transaction A reads a data item, then tries to write to it.
• Transaction B reads the same data item, then tries to write to it.
Here’s what happens:
1. Both transactions start by acquiring S-locks (shared locks) on the data item for their read operations.
2. When they try to write, both need to upgrade their S-locks to X-locks (exclusive locks).
3. Transaction A can’t upgrade because Transaction B is holding an S-lock, and Transaction B can’t upgrade because Transaction A is holding an S-lock.
4. Neither can proceed, causing a deadlock.
Scenario 2:
Similar to Scenario 1, but the deadlock arises because transactions read and then write to the same data item. The sequence of acquiring an S-lock for the read and then requesting an X-lock for the write creates contention.
To avoid these deadlocks:
1. Minimize read-before-write patterns in transactions.
2. Use X-locks directly if you know a write will follow the read.
3. Implement retry mechanisms to handle deadlocks when they occur.
The REPEATABLE READ isolation level can lead to deadlocks in two common scenarios:
Scenario 1:
• Transaction A reads a data item, then tries to write to it.
• Transaction B reads the same data item, then tries to write to it.
Here’s what happens:
1. Both transactions start by acquiring S-locks (shared locks) on the data item for their read operations.
2. When they try to write, both need to upgrade their S-locks to X-locks (exclusive locks).
3. Transaction A can’t upgrade because Transaction B is holding an S-lock, and Transaction B can’t upgrade because Transaction A is holding an S-lock.
4. Neither can proceed, causing a deadlock.
Scenario 2:
Similar to Scenario 1, but the deadlock arises because transactions read and then write to the same data item. The sequence of acquiring an S-lock for the read and then requesting an X-lock for the write creates contention.
To avoid these deadlocks:
1. Minimize read-before-write patterns in transactions.
2. Use X-locks directly if you know a write will follow the read.
3. Implement retry mechanisms to handle deadlocks when they occur.
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