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[ Databases in AWS : ElastiCache ]

1. Managed Redis/Memcached (similar offering as RDS, but for caches)

2. In-memory data store, sub-milisecond latency

3. Must provision an EC2 instance type

4. Support for Clustering (Redis) and Multi AZ, Read Replicas (sharding)

5. Security through IAM, Security Groups, KMS, Redis Auth

6. Backup / Snapshot / Point in time restore feature

7. Managed and Scheduled maintenance

8. Monitoring through CloudWatch

※Use Case : Key/Value store, Frequent reads, less writes, cache results for DB queries, store session data for websites, cannot use SQL. 

[ ElastiCache for Solutions Architect ]

Operations : same as RDS

Security : AWS responsible for OS security, we are responsible for setting up KMS, security groups, IAM policies, users (Redis Auth), using SSL

Reliability : Clustering, Multi AZ

Performance : Sub-millisecond performance, in memory, read replicas for sharding, very popular cache option

Cost : Pay per hour based on EC2 and storage usage

[ AWS ElastiCache ]

- The same way RDS is to get managed Relational Databases

- ElastiCache is to get managed Redis or Memcached

- Caches are in-memory databases with really high performance, low latency

- Helps reduce load off of databases for read intensive workloads

- Helps make your application stateless

- Write Scaling using sharding (파편화)

- Read Scaling using Read Replicas

- Multi AZ with Failover Capability

- AWS takes care of OS maintenance/patching, optimizations, setup, configuration, monitoring, failure recovery and backups

[ ElastiCache Solution Architecture - DB Cache ]

app 은 elasticache 에 우선적으로 쿼리한 후 존재하지 않을 경우(miss) RDS 에서 SELECT, cache 에 write

다음번 동일한 데이터를 읽을 땐 캐시에 존재 (hit)]

[ ElastiCache Solution Architecture - User Session Store ]

앱에 로그인을 한 후 session data 를 Elasticache 에 저장.

유저가 다른 인스턴스에서 접속 할 경우 elasticache 에서 세션정보를 가져와 로그인 유지상태로 만듬. 

매번 인증이 필요없음.

[ Redis vs Memcached ]

* Redis (RDS와 비슷) 

 - Multi AZ with Auto-Failover

 - Read Replicas to scale reads and have high availability

 - Data Durability using AOF persistence

 - Backup and restore features

* Memcached

 - Multi-node for partitioning of data (sharding)

 - Non persistent

 - No backup and restore

 - Multi-threaded architecture

[ ElastiCache - Cache Security ]

1. 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

2. Redis AUTH

  - You can set a pw/token when you create a Redis cluster

  - This is an extra level of security for your cache (on top of security groups)

3. Memcached

  - Supports SASL-based authentication (advanced)

[ # ElastiCache for Solutions Architects ] 

캐시데이터를 읽는 경우 캐시에 저장된 데이터는 방금 꺼내온 데이터가 아니므로 stale 함. (Lazy Loading)

DB에서 데이터를 쓸 경우 cache 에도 추가 및 수정을 한다 (Write Through)

Patterns for ElastiCache

- Lazy Loading : all the read data is cached, data can become stale(오래된) in cache

- Write Through : Adds or update data in the cache when written to a DB (no stale data)

- Session Store : store temporary session data in a cache (using TTL features)