Transaction execution model in Redis - Mini Project: Build & Apply

Redis Transaction Execution Model

📖 Scenario: You are managing a simple Redis database for a small online store. You want to ensure that multiple commands that update the stock and sales count happen together without interference from other operations.

🎯 Goal: Build a Redis transaction that safely updates the stock and sales count for a product in one atomic operation.

📋 What You'll Learn

Create a Redis key for product stock with an initial value

Create a Redis key for product sales count with an initial value

Start a Redis transaction using MULTI

Queue commands to decrement stock and increment sales count

Execute the transaction with EXEC

💡 Why This Matters

🌍 Real World

Redis transactions help ensure that multiple related updates happen together without interference, which is important in inventory and sales systems.

💼 Career

Understanding Redis transactions is useful for backend developers and database administrators working with real-time data and caching systems.

1

Set initial product stock and sales count

Create two Redis keys: product_stock with value 100 and product_sales with value 0 using the SET command.

Redis

# Your code here

Hint

Use SET command to assign values to keys.

2

Start the transaction

Start a Redis transaction by writing the MULTI command.

Redis

SET product_stock 100
SET product_sales 0
# Your code here

Hint

Use MULTI to begin a transaction block.

3

Queue stock decrement and sales increment commands

Inside the transaction, queue the commands DECR product_stock to reduce stock by 1 and INCR product_sales to increase sales count by 1.

Redis

SET product_stock 100
SET product_sales 0
MULTI
# Your code here

Hint

Use DECR and INCR commands to queue changes inside the transaction.

4

Execute the transaction

Complete the transaction by writing the EXEC command to execute all queued commands atomically.

Redis

SET product_stock 100
SET product_sales 0
MULTI
DECR product_stock
INCR product_sales
# Your code here

Hint

Use EXEC to run all commands queued in the transaction.

Practice

1. What is the purpose of the MULTI command in Redis transactions?

easy

A. To discard all queued commands in a transaction

B. To immediately execute all queued commands

C. To start a transaction and queue commands for atomic execution

D. To check the status of the current transaction

Solution

1. Step 1: Understand the role of MULTI

   The MULTI command starts a transaction and queues subsequent commands instead of executing them immediately.

2. Step 2: Differentiate MULTI from other commands

   EXEC runs the queued commands, DISCARD cancels them, and no command checks status directly.

3. Final Answer:

   To start a transaction and queue commands for atomic execution -> Option C

4. Quick Check:

   MULTI = start transaction [OK]

Hint: MULTI always starts queuing commands, not executing them [OK]

Common Mistakes:

• Confusing MULTI with EXEC
• Thinking MULTI executes commands immediately
• Assuming MULTI discards commands

2. Which of the following is the correct sequence to execute a Redis transaction?

easy

A. EXEC, MULTI, commands

B. commands, MULTI, EXEC

C. commands, EXEC, MULTI

D. MULTI, commands, EXEC

Solution

1. Step 1: Recall the transaction command order

   First, MULTI starts the transaction, then commands are queued, and finally EXEC runs them.

2. Step 2: Verify the sequence

   Only MULTI, commands, EXEC follows the correct order: MULTI, then commands, then EXEC.

3. Final Answer:

   MULTI, commands, EXEC -> Option D

4. Quick Check:

   Order = MULTI -> commands -> EXEC [OK]

Hint: MULTI starts, EXEC ends transaction execution [OK]

Common Mistakes:

• Starting with EXEC instead of MULTI
• Running commands before MULTI
• Mixing order of commands and EXEC

3. Consider the following Redis transaction commands:

MULTI
SET key1 value1
INCR counter
EXEC

What will be the result of the EXEC command if counter does not exist before the transaction?

medium

A. ["OK", "(error) counter is not an integer"]

B. ["OK", 1]

C. Syntax error

D. Transaction discarded

Solution

1. Step 1: Analyze commands in the transaction

   SET key1 value1 sets a string key, and INCR counter increments the counter key. If counter does not exist, Redis treats it as 0 before incrementing.

2. Step 2: Understand Redis transaction behavior

   Redis queues commands and executes them atomically. Since counter is missing, INCR will create it with value 1. Both commands succeed.

3. Final Answer:

   ["OK", 1] -> Option B

4. Quick Check:

   INCR missing key = 1 [OK]

Hint: INCR creates missing keys with value 1 [OK]

Common Mistakes:

• Expecting error if key is missing
• Thinking transaction rolls back on error
• Confusing syntax error with runtime error

4. You run this Redis transaction:

MULTI
SET key1 value1
INCR key1
EXEC

What is the expected outcome and why?

medium

A. EXEC returns an error for INCR because key1 is not an integer

B. Both commands succeed; key1 becomes 'value1' then incremented

C. Transaction is discarded due to type error

D. Only SET executes; INCR is ignored

Solution

1. Step 1: Examine commands and data types

   SET key1 value1 sets key1 to a string. INCR key1 expects an integer but key1 is a string.

2. Step 2: Understand Redis transaction error handling

   Redis queues commands and executes all on EXEC. It does not rollback on errors. INCR fails with a type error, but SET succeeds.

3. Final Answer:

   EXEC returns an error for INCR because key1 is not an integer -> Option A

4. Quick Check:

   Type error in transaction = error on EXEC [OK]

Hint: Redis transactions don't rollback on command errors [OK]

Common Mistakes:

• Assuming transaction rolls back on error
• Expecting INCR to convert string automatically
• Thinking only first command runs

5. You want to increment a counter only if a key lock does not exist, using Redis transactions. Which sequence correctly ensures atomicity?

hard

A. WATCH lock
MULTI
INCR counter
EXEC

B. MULTI
SET lock 1
INCR counter
EXEC

C. MULTI
INCR counter
GET lock
EXEC

D. MULTI
GET lock
INCR counter
EXEC

Solution

1. Step 1: Understand the need for atomic check and increment

   You must check if lock exists and increment counter only if it does not, atomically.

2. Step 2: Use WATCH to monitor lock

   WATCH lock watches the key for changes. If lock changes before EXEC, the transaction aborts, ensuring atomicity.

3. Step 3: MULTI queues commands and EXEC runs them if lock unchanged

   This sequence ensures the increment happens only if lock was not modified.

4. Final Answer:

   WATCH lock MULTI INCR counter EXEC -> Option A

5. Quick Check:

   WATCH + MULTI = atomic check and update [OK]

Hint: Use WATCH before MULTI to ensure atomic conditional updates [OK]

Common Mistakes:

• Not using WATCH for conditional logic
• Checking key inside MULTI without WATCH
• Assuming MULTI alone ensures atomic condition