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R&D

RDS vs Aurora vs DynamoDB vs ElastiCache

The four databases

Section titled “The four databases”
RDS Aurora DynamoDB ElastiCache
Type Managed traditional SQL AWS-rewritten MySQL/Postgres NoSQL key-value/doc Managed Redis/Memcached
Engine MySQL, Postgres, MariaDB, Oracle, SQL Server MySQL/Postgres compatible (AWS engine) Proprietary NoSQL Redis or Memcached
Scale Vertical, ~5 read replicas Vertical + 15 replicas, auto-scale, 128TB Horizontal, infinite Cluster / shards
Durability Disk (EBS), Multi-AZ (2 copies) 6 copies across 3 AZs SSD 3 AZs Memory only (volatile)
Latency ms ms single-digit ms microseconds
Cost Instance + storage ~20% more/instance, less/IO (cheaper at scale) Pay per request / provisioned Node-hours

One-liners

Section titled “One-liners”
RDS        = managed traditional SQL — same MySQL/Postgres you know, AWS runs ops
Aurora     = AWS REWROTE MySQL/Postgres engine — same SQL, 5× faster, 15 replicas, auto-grow
DynamoDB   = NoSQL key-value/document — schemaless, serverless, infinite horizontal scale
ElastiCache = managed REDIS (or Memcached) — not "similar to Redis", it IS Redis

The naming surprise

Section titled “The naming surprise”

AWS rarely names managed open-source tools by their community name:

Open source AWS managed Same software?
Redis / Memcached ElastiCache Yes — standard Redis client + commands
MySQL / PostgreSQL RDS Yes (pick engine)
Kubernetes EKS Yes
Kafka MSK Yes
Elasticsearch OpenSearch Yes (fork)
MongoDB DocumentDB API-compatible, AWS-rewritten engine
RabbitMQ / ActiveMQ Amazon MQ Yes

Connect to ElastiCache with a normal Redis client pointed at the AWS endpoint — same commands, same protocol.

Decision tree

Section titled “Decision tree”
Need relational (joins, SQL)?
├── Yes, standard needs → RDS
├── Yes, needs scale/HA/performance → Aurora
├── No — key-value, huge scale, serverless → DynamoDB
└── Need sub-ms caching / sessions → ElastiCache (Redis)

Common combos: RDS/Aurora (truth) + ElastiCache (hot reads); DynamoDB + DAX; Aurora → Redshift (analytics).

Exam facts

Section titled “Exam facts”
  • “managed MySQL” → RDS
  • “MySQL-compatible, 5× faster” → Aurora
  • “NoSQL web-scale” → DynamoDB
  • “caching layer” → ElastiCache
  • “in-memory, sub-ms, sessions, pub/sub” → ElastiCache (Redis)
Section titled “Related”
  • [[aws/index]]
  • [[aws/dynamodb-deep-dive/index]]
  • [[aws/storage-types/index]]