Database Selection Matrix#

The most common question format in Domain 3: “the application needs X - which database?” There are ~10 AWS database services and exactly one right answer per workload. Learn the workload → service mapping.

The decision table#

Workload	Service	Why
Relational OLTP (online transaction processing), joins, transactions, known schema	RDS (Relational Database Service) (or Aurora for cloud-native scale)	Managed SQL, Multi-AZ, replicas
Massive-scale key-value, any traffic shape, single-digit ms	DynamoDB	Serverless NoSQL, unlimited scale
Hot data, microsecond reads, session store	ElastiCache (Redis/Memcached)	In-memory
Data warehouse, OLAP (online analytical processing), columnar analytics	Redshift	Petabyte analytics
Interactive SQL on S3 (Simple Storage Service) data lake	Athena	Serverless, pay per query
Search, autocomplete, log analytics	OpenSearch	Full-text search
Document workloads, MongoDB-compatible	DocumentDB	Mongo API without Mongo ops
Graph relationships (social, fraud, routes)	Neptune	Graph DB
Time series (IoT, metrics)	Timestream	Built for timestamps
Immutable ledger / audit	Aurora PostgreSQL / S3 Object Lock (Note: AWS deprecated QLDB* on July 31, 2025)*	Cryptographic verifiability / WORM
In-memory cache with SQL on RDS-adjacent	Redis	Microsecond latency
Query exabytes in S3 directly from a data warehouse	Redshift Spectrum	Joins Redshift tables with S3 Parquet files without loading

RDS vs DynamoDB - the heavyweight bout#

Pick RDS/Aurora when:

The data is relational (joins, foreign keys, complex queries)
You need stored procedures, or the app is already SQL-based
Schema is stable and the team knows SQL
Strong secondary-index query flexibility

Pick DynamoDB when:

Massive scale, unpredictable traffic (“internet scale”)
Simple access patterns: key lookup, few queries
You want zero server management and per-request pricing
Need single-digit ms latency at any scale

Hybrid is fine and common: DynamoDB for hot user data, RDS for orders/invoices, ElastiCache in front.

flowchart TB
    q{What kind of data?}
    q -->|relational, joins| rds[RDS / Aurora]
    q -->|key-value, internet scale| ddb[DynamoDB]
    q -->|hot cache| ec[ElastiCache]
    q -->|analytics| rs[Redshift / Athena]
    q -->|relationships| np[Neptune]
    q -->|full-text search| os[OpenSearch]

OLTP vs OLAP (favorite definitions)#

OLTP (online transaction processing) - many small fast transactions: RDS, Aurora, DynamoDB
OLAP (online analytical processing) - big scans and aggregations: Redshift (warehouse), Athena (serverless on S3), EMR (Elastic MapReduce) - Hadoop/Spark

“Run complex analytical queries over years of data without affecting the transactional database” → ETL into Redshift (Glue for the extract/transform part).

Not: sinavda “is yukunu acikla, sen dogru veritabanini sec” tipi soru gelir. Anahtar kelimeler: “join/transaction” = RDS, “internet olcegi, anahtar-deger” = DynamoDB, “analiz/data warehouse” = Redshift, “arama” = OpenSearch, “graf” = Neptune.

Exam check#

Q1: A startup expects unpredictable traffic from 0 to 10 million users and stores simple user profile records accessed by user ID. Which database?

A) RDS MySQL
B) DynamoDB
C) Redshift
D) ElastiCache

Cevabı göster

Answer: B. Key-value access + internet scale + unpredictable = DynamoDB.

Q2: A data team needs to run complex analytical joins over 3 years of sales data without slowing down the order-processing database. What should be used?

A) Add read replicas to RDS
B) Move analytics to Redshift and load with Glue
C) Run the queries on the RDS primary
D) Enable DynamoDB DAX (DynamoDB Accelerator)

Cevabı göster

Answer: B. OLAP workload = warehouse (Redshift), ETL via Glue.

Ozet#

RDS/Aurora = iliskisel, join, transaction, SQL
DynamoDB = anahtar-deger, sinirsiz olcek, tek haneli ms
ElastiCache = hafiza ici cache / session
Redshift = data warehouse (OLAP), Athena = S3 uzerinde sunucusuz SQL
OpenSearch = tam metin arama, Neptune = graf, Timestream = zaman serisi
OLTP = RDS/DynamoDB, OLAP = Redshift/Athena/EMR
Genelde hibrit: sicak veri DynamoDB, faturalar RDS, on cache Redis