Temporary data with TTL in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does TTL stand for in Redis?

TTL stands for Time To Live. It is the duration a key will exist before Redis automatically deletes it.

beginner

How do you set a key with a TTL in Redis?

Use the SET command with the EX option, like SET key value EX 10 to set a key that expires in 10 seconds.

beginner

What happens when a Redis key's TTL expires?

Redis automatically deletes the key, so it no longer exists in the database.

beginner

How can you check the remaining TTL of a key in Redis?

Use the TTL key command. It returns the remaining time in seconds before the key expires.

intermediate

Can you update the TTL of an existing key in Redis?

Yes, use the EXPIRE key seconds command to set or update the TTL of an existing key.

Which Redis command sets a key with a TTL of 30 seconds?

A GET mykey EX 30

B SET mykey myvalue EX 30

C TTL mykey 30

D EXPIRE mykey 30

The SET command with EX 30 sets the key and its TTL in one step.

What does the Redis command TTL mykey return if the key does not have a TTL?

A -1

B 0

C null

D -2

Redis returns -1 if the key exists but has no expiration set.

If a key's TTL expires, what happens to the key?

A It is deleted automatically

B It remains but becomes read-only

C Its value is set to null

D It is moved to a backup database

Redis deletes keys automatically when their TTL expires.

Which command updates the TTL of an existing key to 60 seconds?

A SET key value EX 60

B TTL key 60

C EXPIRE key 60

D GET key EX 60

The EXPIRE command sets or updates the TTL of an existing key.

How can you create a key that never expires in Redis?

A Use TTL command with -1

B Use EXPIRE with 0 seconds

C Use SET with EX 0

D Set the key without any TTL

Keys without TTL never expire unless deleted manually.

Explain how Redis uses TTL to manage temporary data.

Think about how Redis automatically removes data after some time.

Describe how to set, check, and update TTL for a Redis key.

Focus on the commands that control expiration.

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