AWS ElastiCache

• ElastiCache is a managed cache offering which supports Redis and Memcached
• Caches are in-memory databases with high performance and low latency
• The role of a cache is to reduce load from a database by caching query results
• It also can help make an application stateless by storing the application state in memory
• Provides:
  • Write scaling using sharding
  • Read scaling using Read Replicas
  • Multi AZ with Failover Capability
• Since it is a managed solution, AWS takes care of OS maintenance, patching, optimization, setup, monitoring, failure recovery and backups

ElastiCache Solution Architecture

DB Cache

• Application queries the ElastiCache first. If no data is available for the query (cache miss), the application gets the data from RDS and stores it into the cache
• Caching helps relieve the load from the database
• Cache must have an invalidation strategy to make sure only the most current data is stored in the cache

User Session Store

• Uses logs into the application
• The application writes the session data to the cache
• The session data can be reused by other instance of he back-end

Redis vs Memcached

Redis	Memcached
Multi AZ with auto-failover	Multi-node for partitioning data (sharding)
Read replicas to scale reads and have high availability	Non persistent
Data durability and AOF persistance	No backup and restore
Backup and restore features	Multi-threaded architecture

ElastiCache Security

• All caches in ElastiCache:
  • Support SSL in flight encryption
  • Do not support IAM authentication
  • IAM policies on ElastiCache are only used for AWS API-level security
• Redis AUTH
  • We can set a password/token when we create a Redis cluster
  • This is an extra layer of security on top of the security groups
• Memcached
  • Supports SASL-based authentication

Caching Patterns

• Lazy loading: all the reads are cached, data can become stale in cache
• Write Through: adds/updates of data are cached when written to the database
• Session Store: store temporary session data in cache using TTL features