Temporary data with TTL in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Redis command sequence?

Given the commands below, what will be the output of the final GET command?

Redis

SET session:12345 user1
EXPIRE session:12345 5
GET session:12345
# Wait 6 seconds
GET session:12345

A "user1" then "user1"

B nil then "user1"

C "user1" then nil

D nil then nil

💡 Hint

Think about what EXPIRE does and how TTL affects key availability.

The key session:12345 is set with value user1 and an expiration of 5 seconds. The first GET returns the value. After waiting 6 seconds, the key expires and the second GET returns nil.

🧠 Conceptual

intermediate

Which Redis command sets a key with a TTL in one atomic operation?

You want to store a temporary key that automatically expires after 10 seconds. Which command achieves this in one step?

A PSETEX key 10000 value

B

SET key value
EXPIRE key 10

C SETEX key 10 value

D SET key value EX 10

💡 Hint

Look for the command that combines setting and expiration in one call.

The SET command supports the EX option to set expiration in seconds atomically. SETEX also works but the question asks for one atomic command with TTL as an option.

📝 Syntax

advanced

Identify the syntax error in this Redis command for setting TTL

Which option contains a syntax error when trying to set a key with a TTL of 30 seconds?

Redis

SET mykey myvalue EXPIRE 30

A SETEX mykey 30 myvalue

B SET mykey myvalue EXPIRE 30

C SET mykey myvalue PX 30000

D SET mykey myvalue EX 30

💡 Hint

Check the correct option keyword for expiration in the SET command.

The SET command uses EX or PX to set expiration, not EXPIRE. Option B incorrectly uses EXPIRE as an argument.

❓ optimization

advanced

Best practice to update TTL without changing key value

You want to extend the TTL of an existing key without modifying its value. Which command is the best choice?

A EXPIRE key 60

B SET key value EX 60

C SETEX key 60 value

D PERSIST key

💡 Hint

Consider commands that only affect TTL without overwriting data.

EXPIRE updates the TTL of a key without changing its value. SET or SETEX overwrite the value. PERSIST removes TTL.

🔧 Debug

expert

Why does this Lua script fail to set TTL on a key in Redis?

Consider this Lua script run via EVAL: local key = KEYS[1] redis.call('EXPIRE', key, 10) redis.call('SET', key, 'temp') return redis.call('TTL', key) Why might the TTL returned be -1 (no expiration)?

A The key is set without expiration because SET overwrites TTL

B The EXPIRE command is ignored inside Lua scripts

C The script runs too fast for TTL to apply

D The key is overwritten after EXPIRE, removing TTL

💡 Hint

Think about the order of commands and how SET affects TTL.

The SET command without expiration removes any existing TTL. EXPIRE is called before SET, but then SET removes the TTL. Here, the script calls EXPIRE first, then SET, so TTL is lost. The question tests understanding that SET without expiration removes TTL.

Practice

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

easy

A. Deletes the key immediately after setting it

B. Sets a time-to-live (TTL) in seconds for the key, after which it expires

C. Sets the key to be persistent and never expire

D. Renames the key to a new name

Solution

1. Step 1: Understand the SET command with EX option

   The SET command stores a key-value pair. The EX option specifies the expiration time in seconds.

2. Step 2: Effect of EX on the key

   When EX is used, the key will automatically be deleted after the given time passes.

3. Final Answer:

   Sets a time-to-live (TTL) in seconds for the key, after which it expires -> Option B

4. Quick Check:

   EX = TTL in seconds [OK]

Hint: EX sets expiration time in seconds for keys [OK]

Common Mistakes:

• Thinking EX makes the key permanent
• Confusing EX with immediate deletion
• Assuming EX renames the key

2. Which of the following is the correct syntax to set a key session with value abc123 that expires in 120 seconds in Redis?

easy

A. SET session abc123 PX 120

B. SET session EX 120 abc123

C. SET EX 120 session abc123

D. SET session abc123 EX 120

Solution

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

   The correct syntax is SET key value EX seconds.

2. Step 2: Match the syntax with options

   SET session abc123 EX 120 matches the correct order: key, value, EX, seconds.

3. Final Answer:

   SET session abc123 EX 120 -> Option D

4. Quick Check:

   SET key value EX seconds [OK]

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

Common Mistakes:

• Placing EX before the key or value
• Using PX instead of EX for seconds
• Mixing order of arguments

3. Given the commands below, what will be the output of TTL user:123 immediately after execution?

SET user:123 "John" EX 60
TTL user:123

medium

A. 60

B. -1

C. -2

D. 0

Solution

1. Step 1: Understand the SET with EX 60

   The key user:123 is set with a TTL of 60 seconds.

2. Step 2: Check TTL command output

   The TTL command returns the remaining time to live in seconds. Immediately after setting, it should be 60.

3. Final Answer:

   60 -> Option A

4. Quick Check:

   TTL after SET with EX 60 = 60 [OK]

Hint: TTL shows seconds left, right after SET with EX it's full time [OK]

Common Mistakes:

• Expecting TTL to return -1 (no expiration)
• Confusing -2 (key does not exist) with TTL
• Thinking TTL returns 0 immediately

4. You run the following commands:

SET temp_key "data" EX 30
EXPIRE temp_key 60
TTL temp_key

What will TTL temp_key return?

medium

A. 60

B. 30

C. -1

D. -2

Solution

1. Step 1: SET with EX 30 sets TTL to 30 seconds

   The key temp_key initially has 30 seconds TTL.

2. Step 2: EXPIRE command updates TTL to 60 seconds

   The EXPIRE temp_key 60 resets the TTL to 60 seconds.

3. Step 3: TTL command returns updated TTL

   So, TTL temp_key returns 60 seconds remaining.

4. Final Answer:

   60 -> Option A

5. Quick Check:

   EXPIRE updates TTL, TTL shows new value [OK]

Hint: EXPIRE resets TTL, TTL shows latest expiration [OK]

Common Mistakes:

• Assuming TTL stays at original 30 seconds
• Confusing TTL -1 (no expiration) with updated TTL
• Thinking EXPIRE does not change TTL

5. You want to store a temporary login token in Redis that expires after 5 minutes. Which sequence of commands correctly sets the token and checks its remaining TTL after 3 minutes?

hard

A. SET login_token abc EX 300
SLEEP 180
EXISTS login_token

B. SET login_token abc PX 300
WAIT 180
EXISTS login_token

C. SET login_token abc EX 300
SLEEP 180
TTL login_token

D. SET login_token abc EX 300
WAIT 180
EXISTS login_token

Solution

1. Step 1: Use SET with EX 300 for 5 minutes expiration

   The command SET login_token abc EX 300 sets the token with 300 seconds TTL.

2. Step 2: Wait 180 seconds (3 minutes) and check TTL

   Using SLEEP 180 simulates waiting. Then TTL login_token shows remaining time, confirming token still exists.

3. Step 3: EXISTS only returns 1 or 0, TTL shows time left

   Checking TTL is better to confirm token is still valid and how long it lasts.

4. Final Answer:

   SET login_token abc EX 300 SLEEP 180 TTL login_token -> Option C

5. Quick Check:

   SET with EX + wait + TTL check = correct flow [OK]

Hint: Use SET with EX, wait, then TTL to check remaining time [OK]

Common Mistakes:

• Using PX instead of EX for seconds
• Using WAIT instead of SLEEP (WAIT waits for replicas, not sleep)
• Using EXISTS which only returns existence, not TTL