Key expiry for memory management in Redis

Introduction

Key expiry helps automatically remove old or unused data from memory. This keeps the database fast and saves space.

You want to store temporary login sessions that expire after some time.

You need to cache data that should refresh regularly.

You want to limit memory use by deleting old keys automatically.

You want to set reminders or time-limited offers in your app.

You want to avoid manual cleanup of expired data.

Syntax

Redis

EXPIRE key seconds
TTL key
PERSIST key

EXPIRE sets a time (in seconds) after which the key is deleted.

TTL shows how many seconds remain before the key expires.

PERSIST removes the expiration from the key, making it persistent.

Examples

Set the key 'session123' to expire in 300 seconds (5 minutes).

Redis

EXPIRE session123 300

Check how many seconds remain before 'session123' expires.

Redis

TTL session123

Remove the expiry from 'session123' so it stays forever.

Redis

PERSIST session123

Sample Program

This sets a key 'tempKey' with value 'hello', sets it to expire in 10 seconds, then checks how many seconds remain.

Redis

SET tempKey "hello"
EXPIRE tempKey 10
TTL tempKey

Important Notes

Expiry time counts down only when the key exists.

If you set EXPIRE on a key that does not exist, it returns 0 (failure).

Use PERSIST to cancel expiry if needed.

Summary

Key expiry automatically deletes keys after a set time.

Use EXPIRE to set expiry, TTL to check time left, and PERSIST to cancel expiry.

This helps manage memory by removing old data.

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