WATCH for optimistic locking in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Redis transaction with WATCH?

Consider the following Redis commands executed in sequence:

1. SET balance 100
2. WATCH balance
3. GET balance (returns 100)
4. MULTI
5. DECRBY balance 50
6. EXEC

Assuming no other client modifies balance between WATCH and EXEC, what will be the value of balance after EXEC?

Redis

SET balance 100
WATCH balance
GET balance
MULTI
DECRBY balance 50
EXEC

A 50

B 100

C EXEC returns null and balance remains 100

D Syntax error due to wrong command order

💡 Hint

WATCH monitors keys for changes; if no changes occur, EXEC applies queued commands.

Since no other client changed 'balance' after WATCH, the transaction succeeds and DECRBY reduces balance from 100 to 50.

❓ query_result

intermediate

What happens if a key changes after WATCH but before EXEC?

Given these commands:

1. SET stock 10
2. WATCH stock
3. Another client executes DECR stock
4. MULTI
5. DECR stock
6. EXEC

What will EXEC return and what is the value of stock?

Redis

SET stock 10
WATCH stock
-- another client DECR stock
MULTI
DECR stock
EXEC

A EXEC returns null and stock is 9

B EXEC returns 8 and stock is 8

C EXEC returns 9 and stock is 9

D EXEC returns error due to key change

💡 Hint

If a watched key changes, EXEC aborts and returns null.

Because another client changed 'stock' after WATCH, EXEC aborts and returns null. The stock value is 9 after the other client's DECR.

📝 Syntax

advanced

Identify the syntax error in this Redis transaction using WATCH

Which option contains a syntax error that will cause the Redis transaction to fail?

Redis

WATCH key1
MULTI
INCR key1
EXEC

A WATCH followed by MULTI and then EXEC is correct

B Calling EXEC before MULTI

C Using INCR inside MULTI without WATCH

D Using WATCH after MULTI

💡 Hint

WATCH must be called before MULTI to monitor keys.

WATCH must be called before MULTI. Calling WATCH after MULTI is invalid and causes a syntax error.

❓ optimization

advanced

How to minimize transaction aborts with WATCH in Redis?

You have a high contention key 'counter' updated by many clients using WATCH and MULTI. Which approach reduces transaction aborts best?

A Use longer transactions with multiple commands inside MULTI

B Increase the number of keys watched to detect conflicts earlier

C Use Lua scripts to perform atomic increments instead of WATCH

D Remove WATCH and rely on optimistic concurrency without monitoring

💡 Hint

Lua scripts run atomically without needing WATCH.

Lua scripts execute atomically, avoiding transaction aborts caused by WATCH conflicts.

🧠 Conceptual

expert

Why does Redis WATCH provide optimistic locking?

Select the best explanation for how WATCH implements optimistic locking in Redis.

A WATCH queues commands and executes them atomically without conflicts

B WATCH monitors keys and aborts transactions if keys change, allowing safe retries

C WATCH locks keys exclusively preventing other clients from modifying them

D WATCH duplicates keys to a temporary store for isolated transactions

💡 Hint

Optimistic locking means detecting conflicts rather than preventing them.

WATCH detects if keys change after being watched and aborts the transaction to avoid conflicts, enabling optimistic concurrency control.

Practice

1. What is the main purpose of the WATCH command in Redis optimistic locking?

easy

A. To monitor keys for changes before executing a transaction

B. To lock keys permanently for exclusive access

C. To delete keys automatically after a timeout

D. To create backups of keys before modification

Solution

1. Step 1: Understand the role of WATCH in Redis

   WATCH is used to monitor keys for any changes before a transaction starts.

2. Step 2: Identify what happens if keys change

   If any watched key changes before EXEC, the transaction is canceled to avoid conflicts.

3. Final Answer:

   To monitor keys for changes before executing a transaction -> Option A

4. Quick Check:

   WATCH detects key changes before transaction [OK]

Hint: WATCH checks keys before transaction to avoid conflicts [OK]

Common Mistakes:

• Thinking WATCH locks keys permanently
• Assuming WATCH deletes keys
• Believing WATCH creates backups

2. Which of the following is the correct sequence of Redis commands to safely update a key counter using optimistic locking?

easy

A. MULTI, WATCH counter, INCR counter, EXEC

B. WATCH counter, MULTI, INCR counter, EXEC

C. WATCH counter, INCR counter, MULTI, EXEC

D. INCR counter, WATCH counter, MULTI, EXEC

Solution

1. Step 1: Identify correct order for optimistic locking

   You must first WATCH the key to monitor changes, then start MULTI for transaction.

2. Step 2: Confirm command order

   The correct order is WATCH counter, then MULTI, then commands like INCR, finally EXEC.

3. Final Answer:

   WATCH counter, MULTI, INCR counter, EXEC -> Option B

4. Quick Check:

   WATCH before MULTI for optimistic locking [OK]

Hint: Always WATCH before MULTI to start transaction safely [OK]

Common Mistakes:

• Starting MULTI before WATCH
• Running commands before MULTI
• Not using EXEC at the end

3. Consider this Redis transaction sequence:

WATCH balance
MULTI
INCR balance
EXEC

If another client modifies balance after WATCH but before EXEC, what will be the result of EXEC?

medium

A. EXEC will fail and return null

B. EXEC will succeed and increment balance

C. EXEC will throw a syntax error

D. EXEC will retry automatically

Solution

1. Step 1: Understand WATCH behavior on key change

   If a watched key changes before EXEC, the transaction is aborted to prevent conflicts.

2. Step 2: Identify EXEC result when aborting

   When aborted, EXEC returns null (or nil) indicating the transaction did not run.

3. Final Answer:

   EXEC will fail and return null -> Option A

4. Quick Check:

   Key changed after WATCH causes EXEC to return null [OK]

Hint: If watched key changes, EXEC returns null, no update done [OK]

Common Mistakes:

• Assuming EXEC still increments key
• Expecting syntax error on EXEC
• Thinking EXEC retries automatically

4. You wrote this Redis code:

WATCH stock
MULTI
DECR stock
EXEC

But sometimes the stock count is not decremented even though no errors occur. What is the most likely cause?

medium

A. EXEC command was missing

B. The DECR command syntax is incorrect

C. The stock key was modified by another client after WATCH

D. WATCH does not support DECR command

Solution

1. Step 1: Analyze why stock is not decremented

   If another client changes the watched key after WATCH, EXEC aborts the transaction silently.

2. Step 2: Confirm command correctness

   DECR syntax is correct and EXEC is present, so no syntax errors.

3. Final Answer:

   The stock key was modified by another client after WATCH -> Option C

4. Quick Check:

   Concurrent key change aborts transaction silently [OK]

Hint: Concurrent changes abort EXEC silently, no decrement done [OK]

Common Mistakes:

• Blaming DECR syntax
• Forgetting EXEC command
• Thinking WATCH blocks commands

5. You want to safely increment a shared counter only if it is less than 10 using Redis optimistic locking. Which code snippet correctly implements this logic?

hard

A. WATCH counter
if val < 10:
val = GET counter
MULTI
INCR counter
EXEC

B. MULTI
WATCH counter
val = GET counter
if val < 10:
INCR counter
EXEC

C. WATCH counter
MULTI
val = GET counter
if val < 10:
INCR counter
EXEC

D. WATCH counter
val = GET counter
MULTI
if val < 10:
INCR counter
EXEC

Solution

1. Step 1: Understand optimistic locking with conditional logic

   You must WATCH the key, then GET its value before starting MULTI to ensure value is current.

2. Step 2: Confirm correct command order

   WATCH first, then GET value, then MULTI, then conditionally INCR, finally EXEC.

3. Final Answer:

   WATCH counter val = GET counter MULTI if val < 10: INCR counter EXEC -> Option D

4. Quick Check:

   GET before MULTI after WATCH ensures correct conditional update [OK]

Hint: GET value after WATCH but before MULTI for safe conditional update [OK]

Common Mistakes:

• Starting MULTI before GET
• Getting value after MULTI
• Not watching key before GET