TTL and Expiration Strategies

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Key Expiration(DOCUMENTATION)

Introduction

Time-To-Live (TTL) makes keys self-cleaning — after a specified duration, Redis automatically deletes them. Mastering TTL strategies is essential for caching, session management, rate limiting, and any pattern involving temporary data.

Key Concepts

  • EXPIRE/PEXPIRE: Set timeout in seconds or milliseconds on an existing key.
  • TTL/PTTL: Check remaining time; returns -1 (no TTL) or -2 (key gone).
  • PERSIST: Remove expiration, making a key permanent again.
  • Conditional flags: NX (only if no TTL), XX (only if TTL exists), GT (only if new TTL is greater), LT (only if new TTL is less).

Real World Context

TTL drives every caching layer (5 minutes for API responses), every session store (30-minute sliding window), and every rate limiter (60-second counters). Without TTL, temporary data accumulates until Redis runs out of memory.

Deep Dive

Time-To-Live (TTL) is a powerful feature that automatically removes keys after a specified time. Essential for caching, sessions, and temporary data.

Setting Expiration

When Creating Keys

redis
# SET with expiration (seconds)
SET session:abc "user_data" EX 3600
# Expires in 1 hour

# SET with expiration (milliseconds)
SET cache:result "data" PX 30000
# Expires in 30 seconds

# SET with absolute Unix timestamp
SET event:ticket "valid" EXAT 1735689600
# Expires at specific time

# SETEX shorthand
SETEX session:xyz 3600 "user_data"

On Existing Keys

redis
# Set expiration in seconds
EXPIRE user:1001:cache 300
# Expires in 5 minutes

# Set expiration in milliseconds
PEXPIRE user:1001:cache 300000

# Set absolute expiration time
EXPIREAT user:1001:session 1735689600

# Set expiration only if key has no TTL
EXPIRE user:1001:data 3600 NX

# Set expiration only if key already has TTL
EXPIRE user:1001:data 7200 XX

# Set only if new TTL is greater than current
EXPIRE user:1001:data 7200 GT

# Set only if new TTL is less than current
EXPIRE user:1001:data 1800 LT

Checking TTL

redis
# Get remaining time in seconds
TTL user:1001:session
# Returns: 3542 (seconds remaining)
# Returns: -1 (no expiration set)
# Returns: -2 (key doesn't exist)

# Get remaining time in milliseconds
PTTL user:1001:session
# Returns: 3542000

Removing Expiration

redis
# Make key permanent
PERSIST user:1001:session
# Returns: 1 (removed TTL)
# Returns: 0 (no TTL existed)

Expiration Strategies

Cache with TTL

redis
# Cache API response for 5 minutes
SET cache:api:users:page:1 "[{...}]" EX 300

# Check if cached, otherwise fetch and cache
GET cache:api:users:page:1
# If nil, fetch from DB and SET with EX

Session Management

redis
# Create session with 30-minute expiration
HSET session:abc123 user_id 1001 created_at 1704067200
EXPIRE session:abc123 1800

# Refresh session on activity (sliding expiration)
EXPIRE session:abc123 1800

Rate Limiting with TTL

redis
# Simple rate limit: 100 requests per minute
INCR rate:user:1001:minute
EXPIRE rate:user:1001:minute 60 NX   # Only set if no TTL

# Check count
GET rate:user:1001:minute
# If > 100, reject request

Temporary Locks

redis
# Acquire lock for 30 seconds
SET lock:resource:123 "owner:abc" NX EX 30
# NX = only if not exists
# EX 30 = auto-expire (prevents deadlocks)

# Release lock (only if we own it)
# Use Lua script in production for atomicity

Hash Field Expiration (Redis 8+)

redis
# Set TTL on individual hash fields
HSET user:1001 name "Alice" temp_token "xyz123"
HEXPIRE user:1001 3600 FIELDS 1 temp_token

# Check field TTL
HTTL user:1001 FIELDS 1 temp_token

# Persist a field (remove TTL)
HPERSIST user:1001 FIELDS 1 temp_token

📖 EXPIRE Command

Common Pitfalls

  1. Separate SET and EXPIRE calls — If your app crashes between SET and EXPIRE, the key lives forever. Use SET with EX in a single command.
  2. Forgetting EXPIRE NX for rate limiters — Without NX, each request resets the TTL window. Use EXPIRE key 60 NX so only the first request sets the timer.

Best Practices

  1. Use sliding TTL for sessions — Call EXPIRE on every request to reset the timeout, creating an activity-based session window.
  2. Use SET with EX or PX — Atomic set-with-expiration prevents orphaned keys from a crash between SET and EXPIRE.

Summary

  • EXPIRE sets automatic key deletion after N seconds.
  • TTL returns remaining time (-1 = permanent, -2 = deleted).
  • Use SET with EX for atomic set-and-expire.
  • Use EXPIRE NX for rate limiters (only set TTL on first request).
  • Redis 8 adds hash field expiration with HEXPIRE and HGETEX.

Code Examples

bash
# Set expiration when creating a key
SET cache:data "value" EX 300

# Add expiration to an existing key
EXPIRE session:abc123 1800

# Check remaining TTL
TTL session:abc123
# (integer) 1798

# Remove expiration (make permanent)
PERSIST session:abc123
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