Exam Tips - AWS Database Specialty Overview Quick-reference guide for the DBS-C01 exam with decision trees, key numbers, and common patterns.
Database Selection Decision Tree What is the primary workload?
├── Relational (SQL, ACID, joins)
│ ├── Need > 64 TB or auto-scaling storage? -> Aurora
│ ├── Need specific engine (Oracle, SQL Server)? -> RDS
│ ├── Variable/unpredictable workload? -> Aurora Serverless v2
│ ├── Global DR with < 1 sec RPO? -> Aurora Global Database
│ └── Standard OLTP? -> RDS or Aurora
├── Key-value / Document
│ ├── Need single-digit ms latency at scale? -> DynamoDB
│ ├── Need microsecond reads? -> DynamoDB + DAX
│ ├── Need MongoDB compatibility? -> DocumentDB
│ └── Need Cassandra compatibility? -> Keyspaces
├── In-memory
│ ├── Need data structures (sorted sets, pub/sub)? -> ElastiCache Redis
│ ├── Need multi-threaded simple caching? -> ElastiCache Memcached
│ └── Need session store with persistence? -> ElastiCache Redis
├── Graph -> Neptune
├── Ledger / Immutable audit -> QLDB
├── Time-series -> Timestream
└── Full-text search -> OpenSearch
Migration Decision Tree Migration type?
├── Same engine (homogeneous)
│ ├── Small (< 5 GB)? -> Native dump/restore
│ ├── Medium? -> DMS full load
│ └── Large with zero downtime? -> DMS full load + CDC
└── Different engine (heterogeneous)
├── Step 1: Run SCT assessment
├── Step 2: Convert schema with SCT
├── Step 3: Manual conversion for red items
└── Step 4: DMS for data migration
High Availability Decision Tree HA requirement?
├── Automatic failover (same region)
│ ├── RDS: Multi-AZ (< 60 sec failover)
│ ├── Aurora: Multi-AZ replicas (< 30 sec failover)
│ └── DynamoDB: Automatic (built-in, 3 AZs)
├── Cross-region DR
│ ├── Aurora Global Database (< 1 sec RPO)
│ ├── RDS cross-region read replica (minutes RPO)
│ └── DynamoDB Global Tables (active-active)
└── Zero-downtime upgrades -> Blue-green deployment
Key Numbers to Remember RDS/Aurora Metric Value RDS max storage 64 TB (16 TB for SQL Server) Aurora max storage 128 TB RDS read replicas (MySQL/PostgreSQL) Up to 15 RDS read replicas (Oracle/SQL Server) Up to 5 Aurora read replicas Up to 15 Multi-AZ failover time < 60 sec (RDS), < 30 sec (Aurora) Automated backup retention 0-35 days Aurora Global Database secondary regions Up to 5 Aurora Global Database replication lag < 1 second IAM auth token validity 15 minutes RDS Proxy idle connection timeout 1,800 sec default Blue-green switchover downtime < 1 minute
DynamoDB Metric Value Max item size 400 KB Max GSI per table 20 Max LSI per table 5 LSI creation At table creation only Partition key max size 2,048 bytes Sort key max size 1,024 bytes DynamoDB Streams retention 24 hours On-demand to provisioned switch Once per 24 hours DAX read latency Microseconds Global Tables replication < 1 second (typical) PITR retention 35 days
ElastiCache Metric Value Redis max nodes per cluster 500 (250 shards x 2 replicas) Redis max item size 512 MB Memcached max nodes 40 Memcached max item size 1 MB
Common Traps and Distractors "Encrypt existing RDS instance" - Cannot enable encryption on existing instance. Must snapshot, copy with encryption, restore."DynamoDB LSI after table creation" - Cannot add LSI after creation. Must recreate table."Aurora Multi-Master" - Discontinued. Use Aurora Global Database with write forwarding instead."RDS Proxy for Oracle" - RDS Proxy supports MySQL and PostgreSQL only."IAM auth for SQL Server" - Not supported. Use native SQL Server authentication."DynamoDB strong consistency on GSI" - Not possible. GSIs are eventually consistent only."Restore to same instance" - PITR and snapshot restore always create new instances."Read replica as Multi-AZ standby" - Different features. Read replicas are readable and async. Multi-AZ standby is not readable and sync."ElastiCache Memcached persistence" - Memcached has no persistence. Use Redis for durability."DynamoDB Global Tables without Streams" - Streams must be enabled for Global Tables. Scenario Quick-Match Scenario Keyword Answer Connection spike from Lambda RDS Proxy Microsecond reads DAX MongoDB migration DocumentDB Cassandra migration Keyspaces Immutable audit trail QLDB IoT time-series Timestream Social network graphs Neptune Cross-region active-active NoSQL DynamoDB Global Tables Cross-region DR for Aurora Aurora Global Database Zero-downtime engine upgrade Blue-green deployment Credential rotation Secrets Manager Fine-grained Redis access Redis RBAC (ACLs) Tamper-proof database audit Database Activity Streams
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