Lock Conversion in Transactions

Lock Conversion

• Sometimes, transactions can convert their closure type from writing to reading and vice versa, based on specific needs.

Lock Conversion (cont.)

• Procedure T, having element x under read_lock(x), can switch to write_lock(x) if T is the only procedure reading x.
• The switch to the highest upgrade, requires acknowledgement before the procedure T is allowed to proceed.

Lock Conversion (cont.)

• Transaction T, when writing on element x, can directly transform the state.

• The direct transformation is called conversion to the bottom, known as downgrade.

• Allowing closure transfers may cause failures in sequential operations.

• This can impact data validity.

Transactions Example

• Initial values: x:=20 and y:=30

• If the T1 procedure precedes T2, the results are x = 50 and y = 80.

• If T2 precedes T1, the results are x = 70 and y = 50.

• Implementation where T1 is: read_lock(y); read_item(y); unlock(y); read_lock(x); read_item(x); unlock_item(x); write_lock(y); read_item(y); y := y + x; write_item(y); unlock(y); write_lock(x); read_item(x); x := x + y; write_item(x); unlock_item(x); the results will be x = 50 and y = 50

Two Phase Locking (2PL)

• Two Phase Locking (2PL) involves read_lock and write_lock preceding the first operation to nullify the closure unlock within the procedure

• The action is divided into two parts.

• Expanding Phase: Acquiring closures on data elements without releasing them.

• Shrinking Phase: Canceling closure of data elements without acquiring new closures.

• Upgrading_lock is in the Expanding phase.

• Downgrade_lock is in the Shrinking Phase.

Impacts of 2PL

• Applying 2PL to all transactions ensures sequential transaction conduct without needing to ensure the implementation of sequential.
• Potential time loss may occur due to the procedure closing data elements before starting operations.
• For example procedure T may have closed element x and another process L requires x also.
• Process L must wait for process T even if T is not yet operating on element x
• This is called Basic 2PL.

2PL Types

• Basic 2PL.
• Static 2PL: The closure of all elements required for operations must occur before the procedure begins.
• The procedure waits if it cannot close all elements until all necessary data elements are available.
• Strict 2PL: Closed data elements for writing can only be released after a commit or abort.
• Rigorous 2PL: All closed data elements (reading and writing) can only be released after a commit or abort.