Key expiry for memory management in Redis - Step-by-Step Execution

Concept Flow - Key expiry for memory management

Set Key with Expiry

↓

Key Stored in Memory

↓

Time Passes

↓

Check Expiry Time

↓

Key Accessible

↩Back to Time Passes

This flow shows how Redis stores a key with an expiry time, checks if the key has expired over time, and deletes it to free memory if expired.

Execution Sample

Redis

SET session:user123 "data" EX 5
GET session:user123
(wait 6 seconds)
GET session:user123

Set a key with 5 seconds expiry, get it before and after expiry to see memory management in action.

Execution Table

Step	Command	Action	Result	Memory State
1	SET session:user123 "data" EX 5	Store key with 5s expiry	OK	session:user123 stored with expiry=5s
2	GET session:user123	Check key existence and expiry	"data"	Key exists, not expired
3	(wait 6 seconds)	Time passes beyond expiry	N/A	Expiry time reached
4	GET session:user123	Check key existence and expiry	(nil)	Key deleted from memory due to expiry

💡 Key expired after 5 seconds, so GET returns nil and key is removed from memory

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
session:user123	does not exist	exists with expiry=5s	exists with expiry=5s	expiry reached	deleted

Key Moments - 2 Insights

Why does the GET command return nil after waiting 6 seconds?

Because the key's expiry time of 5 seconds has passed (see execution_table step 3), Redis automatically deletes the key to free memory.

Does the key get deleted exactly at 5 seconds?

Redis checks expiry lazily on access or via periodic cleanup, so the key is deleted when accessed after expiry or during cleanup (step 4 shows deletion on GET).

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what is the value returned by GET?

A "data"

B nil

C Error

D "" (empty string)

💡 Hint

Check the 'Result' column at step 2 in execution_table

At which step does the key get deleted from memory?

A Step 1

B Step 2

C Step 4

D Step 3

💡 Hint

Look at the 'Memory State' column in execution_table for when the key is deleted

If the expiry was set to 10 seconds instead of 5, what would happen at step 4?

A GET returns nil because key is expired

B GET returns "data" because key is not expired

C GET returns error

D Key is deleted immediately at step 1

💡 Hint

Refer to variable_tracker and execution_table timing of expiry

Concept Snapshot

SET key with EX seconds sets expiry time
Key remains accessible until expiry
After expiry, key is deleted automatically
GET returns nil if key expired
Expiry helps free memory by removing unused keys

Full Transcript

This visual execution shows how Redis manages memory by expiring keys. First, a key is set with a 5-second expiry. The key is stored in memory and accessible immediately. After waiting 6 seconds, the expiry time is reached. When the key is accessed again, Redis detects the expiry and deletes the key from memory, returning nil. This automatic deletion frees memory by removing unused keys. Redis expiry is checked lazily on access or during periodic cleanup.

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