Elasticache

is a web service
makes it easy to
- deploy
- operate
- scale an in-memory data store or cache in the cloud.
Can be used for DB caching in conjunction with services like RDS
Web service that makes it easy to deploy, operate, and scale in memory cache in the cloud
Improves the performance of web applications by allowing you to retrieve information from fast, managed in-memory caches, instead of relying entirely on slower disk based databases
Improves application performance by storing critical pieces of data in memory for low-latency access
Cached information may include the results of I/O intensive database queries or the results of computationally intensive calculations
Supports 2 open-source in-memory caching engines:
- Memcached:
  - Widely adopted memory object caching system
  - Elasticache is protocol complaint with memcached, so popular tools that you use today with existing memcached environments will work seamlessly with the service
  - No Multi AZ support
- Redis:
  - Popular open-source in-memory key-value store that supports data structures such as sorted sets and lists
  - Elasticache supports Master/Slave replication and Multi-AZ which can be used to achieve cross AZ redundancy
  - Good choice if your db is read heavy and not prone to frequent changing

All Region Resource or Operation	Default Limit	Description
Nodes per region:	50	The maximum number of nodes across all clusters in a region.
Nodes per cluster (Memcached):	20	The maximum number of nodes in an individual Memcached cluster.
Nodes per cluster (Redis):	1	The maximum number of nodes in an individual Redis cluster.
Clusters per replication group (Redis):	6	The maximum number of clusters in a Redis replication group. One is the read/write primary. All others are read-only replicas.
Parameter groups per region:	20	The maximum number of parameters groups you can create in a region.
Security groups per region:	50	The maximum number of security groups you can create in a region.
Subnet groups per region:	50	The maximum number of subnet groups you can create in a region.
Subnets per subnet group:	20	The maximum number of subnets you can define for a subnet group.