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+# ElastiCache
+
+## Overview
+
+ElastiCache is Amazon's offering for managed Redis or Memcached.
+
+Caches are in-memory dbs with high perf and low latency. They allow quick access to data without having to reach the DB disk.
+
+This helps:
+
+- Latency
+- Reduce load on DB resources
+- Makes application stateless
+
+> AWS will take care of maintenance, patching, optimisations, monitoring, failure etc for you
+
+## Architecture
+
+Apps query the cache, but if it misses, then it will read from DB.
+
+Caches should have an invalidation strategy
+
+![cache architecture](assets/elasticache.png)
+
+## Use session store
+
+Another good use case is for a session store for a user's session:
+
+![cache architecture](assets/session-store.png)
+
+## Elasticcache security
+
+- ElastiCache support IAM Auth for Redis
+- IAM is only used for API level access, though
+- There is also Redis auth, which allows you to create a user/pw combination for Redis cluster and is an extra layer of security
+- Memcached supports SASL based auth
+
+## ElastiCache patterns
+
+1. **LazyLoading**: this involves writing all data to cache for quick access. Data can quickly become stale. 
+1. **Write Throgh**: When data is written to the the db, also write to cache  
+1. **Session store**: Store temporary data in cache (such as session info)
+
+## Redis use case
+
+> Important for the exam
+
+- Gaming leaderboards are computationally complex
+- **Redis sorted sets** guarantees uniqueness and element ordering
+- Each time an element is added, it's ranked in realtime, then added in correct order:
+
+
+![](assets/redis-sorted-sets.png)