Key Expiration & TTL

11. Key Expiration & TTL

What is TTL?

TTL (Time To Live) defines how long a key exists in Redis before it is automatically deleted. This is one of Redis’s most powerful features for cache management.

Analogy: TTL is like a best-before date on food. Once expired, Redis automatically throws it away — you don’t have to clean it up manually.

TTL Lifecycle

flowchart LR
    Create["SET otp:user:42 '895621' EX 300<br/>Key created with 300s TTL
⏱️ Timer starts now"] --> Active["Key is alive<br/>TTL reports: 287 seconds remaining<br/>GET returns the value"]
    Active --> Check{Does TTL
reach 0?}
    Check -->|Not yet ⏳| Active
    Check -->|Yes! 🕐| Expired["Key auto-deleted by Redis<br/>TTL reports: -2 (key not found)<br/>GET returns nil"]

    Refresh["User activity: EXTEND TTL<br/>EXPIRE otp:user:42 300<br/>Reset timer ⏰"] -.-> Active
    Delete["Manual delete<br/>DEL otp:user:42<br/>Remove immediately ❌"] -.-> Expired
    Persist["Make permanent<br/>PERSIST otp:user:42<br/>Removes TTL 🏠"] -.-> Active

    style Create fill:#7c3aed,color:#fff
    style Active fill:#3b82f6,color:#fff
    style Check fill:#f59e0b,color:#fff
    style Expired fill:#ef4444,color:#fff
    style Refresh fill:#059669,color:#fff
    style Delete fill:#ef4444,color:#fff
    style Persist fill:#10b981,color:#fff

Warning: Keys without TTL will accumulate in memory indefinitely and eventually cause Redis to run out of RAM.

Core Commands

# Set a key with TTL in SECONDS
SET otp:user:42 "895621" EX 300      # Expires in 5 minutes

# Set a key with TTL in MILLISECONDS
SET temp:token "abc123" PX 10000     # Expires in 10 seconds

# Check remaining TTL (in seconds)
TTL otp:user:42
# Output: (integer) 287  (287 seconds remaining)
# Output: -1  (key exists, no expiry set)
# Output: -2  (key does not exist)

# Check remaining TTL in milliseconds
PTTL otp:user:42

# Add expiry to an existing key
SET api:response:home '{"data": [...]}'
EXPIRE api:response:home 60         # Expire in 60 seconds

# Make a key permanent (remove expiry)
PERSIST api:response:home

# Set with exact Unix timestamp expiry
EXPIREAT notification:123 1800000000

Real-World Examples

Example 1: OTP (One-Time Password)

# Store OTP valid for 10 minutes
SET otp:user:42 "293847" EX 600

# Verify OTP
GET otp:user:42
# Returns nil if expired, value if valid

# After verification, delete immediately
DEL otp:user:42

Example 2: API Response Caching

# Cache API response for 5 minutes
SET cache:api:products '{"items": [...]}' EX 300

# Check how fresh the cache is
TTL cache:api:products

Example 3: Session Management

# Create session with 24-hour expiry
SET session:abc123 '{"userId": 42, "role": "admin"}' EX 86400

# Refresh session on user activity (extend TTL)
EXPIRE session:abc123 86400

Warning: Always set TTL on cache keys. Keys without TTL will accumulate in memory indefinitely and eventually cause Redis to run out of RAM.