Elasticity & Scalability

Scale: Ability to meet demands over the long term (think: long term, months / years)
Elasticity: Stretch and retract infrastructure based on demand (think: hours or days)

Worth calling out on it’s own. Read replica’s are to help with additional read load whereas multiple AZ is to help with disaster recover.

Backups are taken from a secondary if you’ve got multiple AZ turned on.
You can force a failover by re-booting (for chaos eng) your instance.
Connection string points to the DB instance and managed privately by AWS.
RDS will detect if a database is down and point the DNS to a different AZ
Multi AZ has an exact copy of your data in the other AZ (sometimes physical replication and sometimes - logical replication)
Failover priority (set the instance you want to become the master in failover)

Helps with easing read-heavy workloads (such as wordpress)
AWS takes a snapshot of the primary DB (with no multi AZ it causes I/O suspension)
Easily have multiple instances of your RDS (through the interface)
Data is automatically synced between databases
You can know which RDS you’re running (by viewing the “engine” field)
Can direct traffic to reads if the write replica is under scale
Useful for reporting such as data warehousing (or could use S3 and redshift)
Creates a new endpoint DNS record to connect to
You can promote a read replica to become a primary DB if you want
Key metric: Replication lag (the time taken to replicate between databases)

ElastiCache

A web service that makes it easy to deploy, operate and scale an in-memory cache in the cloud. Much - faster than a disk-based database.
Can significantly improve latency and throughput for many read-heavy application workloads (social - networking, gaming, media sharing, Q&A portals)
Improves performance by storing critical pieces of data in memory, for low latency access.

Memcache does not support multi AZ, Redis does support multiple AZ.

Serverless database solution (can be provisioned if you want)

Below are some reasons that instances might not be able to autoscale...