SET with expiry (EX, PX) in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Redis command sequence?

Consider the following Redis commands executed in order:

SET mykey "Hello" EX 10
TTL mykey

What will the TTL mykey command return immediately after setting the key?

Redis

SET mykey "Hello" EX 10
TTL mykey

A An integer close to 10 (e.g., 9 or 10)

B -1 (key exists but has no expiry)

C -2 (key does not exist)

D Syntax error

💡 Hint

EX sets the expiry time in seconds when setting the key.

The SET command with EX 10 sets the key to expire in 10 seconds. TTL returns the remaining time to live in seconds, so it will be close to 10 immediately after setting.

❓ query_result

intermediate

What happens when you use PX with SET in Redis?

You run the command:

SET session "abc123" PX 5000

What does the PX 5000 option do?

Redis

SET session "abc123" PX 5000

A Sets the key without expiry

B Sets the key to expire after 5000 seconds

C Sets the key to expire after 5000 milliseconds (5 seconds)

D Causes a syntax error

💡 Hint

PX sets expiry in milliseconds, EX sets expiry in seconds.

PX option sets the expiry time in milliseconds. PX 5000 means the key expires after 5 seconds.

📝 Syntax

advanced

Which SET command syntax is invalid in Redis?

Identify the invalid Redis SET command syntax among the options below:

A SET user "Alice" EX 20 PX 1000

B SET user "Alice" PX 1000

C SET user "Alice" EX 20

D SET user "Alice"

💡 Hint

You cannot use both EX and PX options together in one SET command.

Using both EX and PX options together in a single SET command is invalid and causes a syntax error.

❓ query_result

advanced

What is the result of this Redis command sequence?

Given the commands:

SET key1 "value1" EX 1
GET key1
WAIT 2000
GET key1

What will be the output of the second GET key1?

Redis

SET key1 "value1" EX 1
GET key1
WAIT 2000
GET key1

A "value1"

B nil

C Syntax error

D "" (empty string)

💡 Hint

EX 1 sets expiry to 1 second, WAIT 2000 waits 2 seconds.

The key expires after 1 second. After waiting 2 seconds, the key no longer exists, so GET returns nil.

🧠 Conceptual

expert

Why might you prefer PX over EX when setting expiry in Redis?

Choose the best reason why a developer might choose PX (milliseconds) instead of EX (seconds) when using the SET command with expiry.

A PX automatically renews the key expiry on access

B PX is faster to execute than EX

C PX stores expiry time as a timestamp instead of duration

D PX allows more precise expiry times for short-lived keys

💡 Hint

Think about the difference between seconds and milliseconds.

PX sets expiry in milliseconds, allowing finer control over expiry times, especially for very short durations.

Practice

1. What does the EX option do when used with the Redis SET command?

easy

A. Sets the key to expire after a specified number of seconds

B. Sets the key to expire after a specified number of milliseconds

C. Sets the key without any expiration

D. Deletes the key immediately after setting it

Solution

1. Step 1: Understand the EX option in Redis SET

   The EX option sets the expiration time in seconds for the key.

2. Step 2: Compare EX with other expiry options

   PX sets expiry in milliseconds, so EX is specifically for seconds.

3. Final Answer:

   Sets the key to expire after a specified number of seconds -> Option A

4. Quick Check:

   EX = seconds expiry [OK]

Hint: EX means seconds expiry, PX means milliseconds expiry [OK]

Common Mistakes:

• Confusing EX with PX and thinking EX is milliseconds
• Assuming EX sets no expiry
• Thinking EX deletes the key immediately

2. Which of the following is the correct syntax to set a key session with value abc123 that expires in 10 seconds using Redis?

easy

A. SET session abc123 EXPIRE 10

B. SET session EX 10 abc123

C. SET session abc123 PX 10

D. SET session abc123 EX 10

Solution

1. Step 1: Recall Redis SET command syntax with expiry

   The correct syntax is: SET key value EX seconds.

2. Step 2: Check each option for correct order and keyword

   SET session abc123 EX 10 matches the correct order and uses EX for seconds expiry.

3. Final Answer:

   SET session abc123 EX 10 -> Option D

4. Quick Check:

   SET key value EX seconds = correct syntax [OK]

Hint: Remember: SET key value EX seconds [OK]

Common Mistakes:

• Placing EX before the value
• Using PX instead of EX for seconds
• Using EXPIRE keyword inside SET command

3. What will be the output of the following Redis commands executed in order?

SET temp 100 PX 5000
TTL temp

medium

A. An integer close to 5

B. 5

C. 0

D. -1

Solution

1. Step 1: Understand PX expiry in SET command

   PX 5000 sets the key to expire in 5000 milliseconds (5 seconds).

2. Step 2: Interpret TTL command output

   TTL returns time to live in seconds. Since expiry was set in milliseconds, TTL rounds to nearest second.

3. Step 3: Predict TTL value

   TTL will return an integer close to 5 seconds, depending on execution time.

4. Final Answer:

   An integer close to 5 -> Option A

5. Quick Check:

   PX 5000 ms = ~5 seconds TTL [OK]

Hint: TTL returns seconds even if PX used for expiry [OK]

Common Mistakes:

• Expecting TTL to return milliseconds
• Thinking TTL returns 0 immediately
• Assuming TTL returns -1 for keys with expiry

4. Identify the error in this Redis command:

SET user123 "John" EXPIRE 20

medium

A. Value should not be in quotes

B. EXPIRE is not valid inside SET command

C. Missing quotes around key

D. EXPIRE should be PX for milliseconds

Solution

1. Step 1: Check Redis SET command syntax

   SET command uses EX or PX for expiry, not EXPIRE keyword.

2. Step 2: Identify misuse of EXPIRE keyword

   EXPIRE is a separate command, not an option inside SET.

3. Final Answer:

   EXPIRE is not valid inside SET command -> Option B

4. Quick Check:

   Use EX or PX inside SET, not EXPIRE [OK]

Hint: Use EX or PX with SET, EXPIRE is separate command [OK]

Common Mistakes:

• Using EXPIRE inside SET command
• Confusing EXPIRE command with expiry option
• Misplacing quotes around key or value

5. You want to store a temporary API token in Redis that expires exactly after 1500 milliseconds. Which command correctly sets the key api_token with value xyz789 and the required expiry?

hard

A. SET api_token xyz789 EX 1.5

B. SET api_token xyz789 EX 1500

C. SET api_token xyz789 PX 1500

D. SET api_token xyz789 PX 1.5

Solution

1. Step 1: Determine correct expiry option for milliseconds

   PX sets expiry in milliseconds, EX uses seconds only.

2. Step 2: Check value format for PX

   PX requires integer milliseconds, so 1500 is valid, decimals like 1.5 are invalid.

3. Step 3: Validate each option

   SET api_token xyz789 PX 1500 uses PX 1500 correctly; others use wrong units or decimals.

4. Final Answer:

   SET api_token xyz789 PX 1500 -> Option C

5. Quick Check:

   Milliseconds expiry = PX integer value [OK]

Hint: Use PX with integer milliseconds for precise expiry [OK]

Common Mistakes:

• Using EX with decimal seconds
• Using PX with decimal instead of integer
• Confusing EX and PX units