Session Storage and Leaderboard Patterns
Use ElastiCache to offload HTTP session state from your application servers and implement real-time leaderboards with Redis sorted sets.
The Problem with Server-Side Sessions
Traditional web applications store session data in memory on the server. This works with a single server but breaks when you scale horizontally — if a user's subsequent request is routed to a different EC2 instance, that instance has no knowledge of the user's session and the user is logged out. Sticky sessions (session affinity in the load balancer) solve this partially but reduce the effectiveness of load balancing. The scalable solution is to move session state to a shared, low-latency store accessible by all instances — which is exactly what ElastiCache Redis provides.
Redis for Session Storage
Storing sessions in Redis gives you: sub-millisecond session reads across all application servers, built-in TTL for automatic session expiry, atomic session updates to prevent race conditions, and the ability to immediately invalidate a session by deleting the key. The application stores the session ID in a cookie; on each request it looks up the session ID in Redis to retrieve the session data. All application servers share the same Redis, so any server can handle any user's request.
# Session storage with Redis (Python Flask example)
import redis, json, uuid
from datetime import timedelta
redis_client = redis.Redis(host='prod-redis-primary', port=6379)
SESSION_TTL = int(timedelta(hours=8).total_seconds())
def create_session(user_id):
session_id = str(uuid.uuid4())
session_data = {'user_id': user_id, 'logged_in': True}
redis_client.setex(f'session:{session_id}', SESSION_TTL, json.dumps(session_data))
return session_id
def get_session(session_id):
data = redis_client.get(f'session:{session_id}')
return json.loads(data) if data else NoneAll lessons in this course
- Redis vs Memcached: Choosing the Right Engine
- ElastiCache Redis Replication Groups and Cluster Mode
- Caching Strategies: Lazy Loading and Write-Through
- Session Storage and Leaderboard Patterns