Key expiry for memory management in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set a key with an expiry time of 60 seconds.

Redis

SET mykey "value" EX [1]

Drag options to blanks, or click blank then click option'

A0

B30

C60

D120

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 as expiry disables expiry, so the key never expires.

Confusing milliseconds with seconds.

The EX option sets the expiry time in seconds. Here, 60 seconds is the correct expiry time.

2fill in blank

medium

Complete the code to set a key to expire in 5 minutes using the PX option (milliseconds).

Redis

SET session_token "abc123" PX [1]

Drag options to blanks, or click blank then click option'

A300000

B3000

C5000

D60000

Attempts:

3 left

💡 Hint

Common Mistakes

Using 3000 instead of 300000, confusing seconds and milliseconds.

Using PX with seconds value instead of milliseconds.

The PX option sets expiry in milliseconds. 5 minutes = 5 * 60 * 1000 = 300000 ms.

3fill in blank

hard

Fix the error in the command to set a key to expire in 10 seconds.

Redis

SET user_id "12345" EX [1]

Drag options to blanks, or click blank then click option'

A-10

B10

C0

D10000

Attempts:

3 left

💡 Hint

Common Mistakes

Using 10000 which is milliseconds, not seconds.

Using negative or zero values which are invalid.

The EX option expects seconds, so 10 is correct. 10000 ms = 10 s, but EX takes seconds directly.

4fill in blank

hard

Fill both blanks to set a key with expiry and then check its remaining TTL.

Redis

SET cache_key "data" EX [1]
TTL [2]

Drag options to blanks, or click blank then click option'

A120

Bcache_key

Cmykey

D60

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different key name in TTL than the one set.

Using wrong expiry time units.

First, set the key cache_key to expire in 120 seconds. Then, check TTL of cache_key.

5fill in blank

hard

Fill all three blanks to set a key with expiry, update its expiry, and check the new TTL.

Redis

SET temp_key "value" EX [1]
EXPIRE [2] [3]
TTL temp_key

Drag options to blanks, or click blank then click option'

Atemp_key

B300

C600

Dcache_key

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong key name in EXPIRE command.

Confusing expiry times or units.

Set temp_key to expire in 300 seconds, then update expiry to 600 seconds using EXPIRE, and finally check TTL.

Practice

1. What does the EXPIRE command do in Redis?

easy

A. Sets a time limit after which a key will be automatically deleted

B. Immediately deletes a key from the database

C. Shows the remaining time before a key expires

D. Removes the expiration time from a key

Solution

1. Step 1: Understand the purpose of EXPIRE

   The EXPIRE command sets a countdown timer on a key, after which Redis deletes it automatically.

2. Step 2: Differentiate from other commands

   Unlike DEL which deletes immediately, EXPIRE schedules deletion after a set time.

3. Final Answer:

   Sets a time limit after which a key will be automatically deleted -> Option A

4. Quick Check:

   EXPIRE = set auto-delete timer [OK]

Hint: EXPIRE sets timer; key deletes after time [OK]

Common Mistakes:

• Confusing EXPIRE with DEL command
• Thinking EXPIRE shows time left instead of setting it
• Assuming EXPIRE removes expiration instead of adding it

2. Which of the following is the correct syntax to set a key session to expire in 120 seconds?

easy

A. SETEX session 120

B. EXPIRE 120 session

C. TTL session 120

D. EXPIRE session 120

Solution

1. Step 1: Recall EXPIRE command syntax

   The correct syntax is EXPIRE key seconds, so EXPIRE session 120 sets expiry to 120 seconds.

2. Step 2: Check other options

   EXPIRE 120 session reverses arguments, A is incomplete syntax for SETEX, D is invalid usage of TTL.

3. Final Answer:

   EXPIRE session 120 -> Option D

4. Quick Check:

   EXPIRE key seconds = EXPIRE session 120 [OK]

Hint: EXPIRE syntax: EXPIRE key seconds [OK]

Common Mistakes:

• Swapping key and seconds in EXPIRE
• Using TTL to set expiry instead of check
• Confusing SETEX syntax

3. Given the commands:

SET user:1 "Alice"
EXPIRE user:1 10
TTL user:1

What will the TTL user:1 command return immediately after setting expiry?

medium

A. -1

B. Error

C. 10

D. 0

Solution

1. Step 1: Understand TTL after EXPIRE

   After setting EXPIRE to 10 seconds, TTL returns the remaining time in seconds, which is 10 immediately.

2. Step 2: Interpret other options

   -1 means no expiry, 0 means expired, Error is invalid here.

3. Final Answer:

   10 -> Option C

4. Quick Check:

   TTL after EXPIRE = expiry seconds [OK]

Hint: TTL shows seconds left after EXPIRE set [OK]

Common Mistakes:

• Expecting TTL to return -1 immediately
• Thinking TTL returns 0 right after EXPIRE
• Confusing TTL with DEL command

4. You run the command EXPIRE mykey 30 but TTL mykey returns -2. What is the likely problem?

medium

A. The key mykey has no expiry set

B. The key mykey does not exist

C. The EXPIRE command syntax is incorrect

D. The Redis server is down

Solution

1. Step 1: Understand TTL return values

   TTL returns -2 if the key does not exist in Redis.

2. Step 2: Analyze the scenario

   Since TTL returns -2, it means mykey is missing, so EXPIRE had no effect.

3. Final Answer:

   The key mykey does not exist -> Option B

4. Quick Check:

   TTL = -2 means key missing [OK]

Hint: TTL -2 means key missing, not syntax error [OK]

Common Mistakes:

• Assuming -2 means no expiry set
• Thinking EXPIRE syntax is wrong
• Ignoring that key might be deleted

5. You want to keep a Redis key cache:data but remove its expiry to prevent automatic deletion. Which command should you use?

hard

A. PERSIST cache:data

B. EXPIRE cache:data 0

C. DEL cache:data

D. TTL cache:data

Solution

1. Step 1: Identify command to remove expiry

   The PERSIST command removes the expiry time from a key, making it permanent.

2. Step 2: Check other options

   EXPIRE with 0 deletes immediately, DEL deletes key, TTL only checks expiry time.

3. Final Answer:

   PERSIST cache:data -> Option A

4. Quick Check:

   PERSIST removes expiry timer [OK]

Hint: Use PERSIST to cancel expiry, keep key [OK]

Common Mistakes:

• Using EXPIRE 0 which deletes key immediately
• Confusing DEL with removing expiry
• Using TTL expecting it to remove expiry