Two-phase locking (2PL) in DBMS Theory - Mini Project: Build & Apply

Understanding Two-Phase Locking (2PL) in Databases

📖 Scenario: You are learning how databases keep data safe when many users try to change data at the same time. Two-phase locking (2PL) is a method that helps avoid problems by controlling when locks on data are taken and released.Imagine a library where many people want to borrow books. To keep things fair and safe, the library uses a rule: first, you must get all the books you want before you start reading, and you cannot take any new books after you start reading. This is like two-phase locking.

🎯 Goal: Build a simple step-by-step example that shows how two-phase locking works by creating a list of transactions, setting a lock phase, applying the locking rules, and then completing the transaction.

📋 What You'll Learn

Create a list of transactions with exact names and actions

Add a variable to track the current phase of locking

Use a loop to apply locking rules to each transaction

Complete the transaction by releasing locks in the correct phase

💡 Why This Matters

🌍 Real World

Two-phase locking is used in real databases to make sure that when many users change data at the same time, the data stays correct and safe.

💼 Career

Understanding 2PL helps database administrators and developers design systems that avoid data errors and improve reliability.

1

Create the list of transactions

Create a list called transactions with these exact entries: 'T1: read A', 'T2: write B', 'T3: read B', 'T4: write A'.

DBMS Theory

# Create the list of transactions
# Your code here

Need a hint?

Use square brackets to create a list and put each transaction as a string inside quotes.

2

Set the locking phase variable

Add a variable called locking_phase and set it to the string 'growing' to represent the first phase of two-phase locking.

DBMS Theory

transactions = ['T1: read A', 'T2: write B', 'T3: read B', 'T4: write A']
# Set the locking phase variable
# Your code here

Need a hint?

Use a simple assignment to create the variable and set its value to 'growing'.

3

Apply locking rules to transactions

Use a for loop with variable transaction to go through transactions. Inside the loop, add a comment line # Acquire lock for {transaction} to show the locking action during the growing phase.

DBMS Theory

transactions = ['T1: read A', 'T2: write B', 'T3: read B', 'T4: write A']
locking_phase = 'growing'
# Loop through transactions to acquire locks
# Your code here

Need a hint?

Use a for loop and inside it write a comment showing the lock acquisition for each transaction.

4

Complete the transaction by releasing locks

Change the locking_phase variable to 'shrinking'. Then add a for loop with variable transaction to go through transactions. Inside the loop, add a comment line # Release lock for {transaction} to show the unlocking action during the shrinking phase.

DBMS Theory

transactions = ['T1: read A', 'T2: write B', 'T3: read B', 'T4: write A']
locking_phase = 'growing'
for transaction in transactions:
    # Acquire lock for {transaction}

# Change phase to shrinking and release locks
# Your code here

Need a hint?

Assign 'shrinking' to locking_phase and use a for loop with a comment inside to show lock release.