PERSIST to remove expiry in Redis

Introduction

Sometimes you want to keep a key forever in Redis without it disappearing after a set time. PERSIST helps by removing any expiration time from a key.

You set a temporary key but then decide to keep it permanently.

You want to stop a countdown timer on a key so it doesn't expire.

You accidentally set an expiry and want to undo it.

You want to make sure important data stays in Redis without disappearing.

Syntax

Redis

PERSIST key

This command removes the expiration from the given key.

If the key does not have an expiration, PERSIST does nothing and returns 0.

Examples

Removes the expiration from 'mykey' so it stays forever.

Redis

PERSIST mykey

Stops the expiration timer on 'session123' key.

Redis

PERSIST session123

Sample Program

We create a key 'tempkey' with value 'hello'. Then we set it to expire in 10 seconds. TTL shows the time left. PERSIST removes the expiry. TTL after PERSIST returns -1, meaning no expiry.

Redis

SET tempkey "hello"
EXPIRE tempkey 10
TTL tempkey
PERSIST tempkey
TTL tempkey

Important Notes

TTL command returns -1 if the key has no expiration.

PERSIST returns 1 if expiration was removed, 0 if key had no expiration or does not exist.

Removing expiry means the key will stay until deleted manually.

Summary

PERSIST removes the expiration time from a Redis key.

Use it to keep keys permanently after setting expiry.

Check TTL before and after to see the effect.

Practice

1. What does the Redis command PERSIST key do?

easy

A. Renames the key to 'persisted_key'

B. Sets a new expiration time on the key

C. Deletes the key immediately

D. Removes the expiration time from the key, making it permanent

Solution

1. Step 1: Understand the purpose of PERSIST

   The PERSIST command removes any expiration time set on a Redis key, so the key no longer expires.

2. Step 2: Compare with other commands

   Unlike EXPIRE which sets a timeout, PERSIST cancels it. It does not delete or rename the key.

3. Final Answer:

   Removes the expiration time from the key, making it permanent -> Option D

4. Quick Check:

   PERSIST removes expiry = Removes the expiration time from the key, making it permanent [OK]

Hint: PERSIST cancels expiry, keeps key forever [OK]

Common Mistakes:

• Confusing PERSIST with EXPIRE
• Thinking PERSIST deletes the key
• Assuming PERSIST renames the key

2. Which of the following is the correct syntax to remove expiry from a Redis key named session123?

easy

A. PERSIST session123

B. REMOVEEXPIRY session123

C. EXPIRE session123 0

D. TTL session123

Solution

1. Step 1: Recall the correct command for removing expiry

   The Redis command to remove expiry is PERSIST key. So for key 'session123', it is PERSIST session123.

2. Step 2: Check other options

   REMOVEEXPIRY is not a Redis command. EXPIRE with 0 does not remove expiry but sets it to expire immediately. TTL only checks expiry time.

3. Final Answer:

   PERSIST session123 -> Option A

4. Quick Check:

   Correct syntax for removing expiry = PERSIST session123 [OK]

Hint: Use PERSIST key to remove expiry [OK]

Common Mistakes:

• Using EXPIRE with 0 to remove expiry
• Trying non-existent commands like REMOVEEXPIRY
• Confusing TTL with expiry removal

3. Given the following Redis commands executed in order:

SET mykey "hello"
EXPIRE mykey 10
TTL mykey
PERSIST mykey
TTL mykey

What will be the output of the two TTL commands?

medium

A. 10 and -1

B. -1 and 10

C. 10 and 10

D. -1 and -1

Solution

1. Step 1: Analyze the first TTL after EXPIRE

   After setting key 'mykey' and applying EXPIRE 10, TTL returns the remaining seconds, which is 10.

2. Step 2: Analyze the second TTL after PERSIST

   PERSIST removes expiry, so TTL returns -1 indicating no expiration.

3. Final Answer:

   10 and -1 -> Option A

4. Quick Check:

   TTL before persist = 10, after persist = -1 [OK]

Hint: TTL shows seconds left; PERSIST makes TTL -1 [OK]

Common Mistakes:

• Expecting TTL to stay 10 after PERSIST
• Confusing TTL -1 meaning
• Thinking PERSIST sets TTL to 0

4. You run the command PERSIST mykey but it returns 0. What is the most likely reason?

medium

A. The key does not exist

B. The key has no expiry set

C. The key is already persistent

D. The command syntax is incorrect

Solution

1. Step 1: Understand PERSIST return values

   PERSIST returns 1 if expiry was removed, 0 if key had no expiry or key does not exist.

2. Step 2: Differentiate between no expiry and missing key

   If key does not exist, PERSIST returns 0, but usually you check key existence first. If key exists but no expiry, PERSIST returns 0 because nothing to remove.

3. Final Answer:

   The key has no expiry set -> Option B

4. Quick Check:

   PERSIST returns 0 if no expiry to remove [OK]

Hint: PERSIST returns 0 if no expiry on key [OK]

Common Mistakes:

• Assuming 0 means syntax error
• Confusing no expiry with key missing
• Thinking 0 means key deleted

5. You have a Redis key cache:item with an expiry of 30 seconds. You want to keep it permanently only if its current TTL is more than 10 seconds. Which sequence of commands achieves this safely?

hard

A. Run EXPIRE cache:item 0 to remove expiry

B. Run PERSIST cache:item directly without checking TTL

C. Use TTL cache:item, then if result > 10, run PERSIST cache:item

D. Rename the key to a new key without expiry

Solution

1. Step 1: Check current TTL of the key

   Use TTL cache:item to get remaining expiry time in seconds.

2. Step 2: Conditionally remove expiry

   If TTL is more than 10, run PERSIST cache:item to remove expiry and keep key permanently.

3. Step 3: Avoid wrong commands

   Running PERSIST without checking TTL removes expiry regardless. EXPIRE with 0 does not remove expiry but sets immediate expiry. Renaming does not affect expiry.

4. Final Answer:

   Use TTL, then if > 10, run PERSIST -> Option C

5. Quick Check:

   Check TTL before PERSIST to keep key only if TTL > 10 [OK]

Hint: Check TTL first, then PERSIST if TTL > 10 [OK]

Common Mistakes:

• Running PERSIST blindly without TTL check
• Using EXPIRE 0 to remove expiry
• Thinking renaming removes expiry