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RDS Architecture in Cloud Computing

Learn about RDS Architecture in this comprehensive Cloud Computing tutorial. Scaling databases.

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1Cost Considerations

Multi-AZ effectively doubles your database compute cost. Read Replicas also incur instance costs. Always balance performance needs with your budget.

2Step-by-Step Breakdown

Two Distinct Problems. Multi-AZ solves High Availability (Disaster Recovery). Read Replicas solve Performance (Scalability).

Multi-AZ Deployment. AWS creates a standby replica in a different Availability Zone. It uses Synchronous Replication.

Automatic Failover. If the primary database fails, AWS automatically updates the DNS record to point to the standby. No manual intervention needed.

Standby is Passive. You cannot read from or write to the Multi-AZ standby instance. It exists solely for failover.

Knowledge Check. Can you run SELECT queries against a Multi-AZ Standby instance to offload traffic?

→Yes
→No

Read Replicas. Read Replicas are active copies of your database used to offload read-heavy workloads (like analytics or reporting).

Asynchronous Replication. Unlike Multi-AZ, Read Replicas use Asynchronous Replication, meaning there may be a slight replication lag.

Active Endpoints. Each Read Replica gets its own DNS endpoint. Your application must be programmed to send reads to the replica and writes to the primary.

Cross-Region Replicas. Read Replicas can be created in different AWS Regions to serve global users with low latency.

Summary. Multi-AZ is for survival. Read Replicas are for speed.

Pascual Vila

Frontend Instructor // Code Syllabus

Common Pitfalls & Errors

The Error //

Confusing a Multi-AZ standby with a Read Replica

// Multi-AZ: automatic failover, standby not queryable
aws rds modify-db-instance --db-instance-identifier mydb --multi-az

// Read Replica: queryable, does NOT auto-failover
aws rds create-db-instance-read-replica --db-instance-identifier mydb-replica --source-db-instance-identifier mydb

The Solution //

A Multi-AZ standby exists purely for failover — it's not accessible for reads and can't be queried directly. A Read Replica is a separate, queryable instance meant for scaling read traffic. Using Multi-AZ expecting extra read capacity (or a Read Replica expecting automatic failover) leads to disappointment.

The Error //

Not accounting for replication lag when reading from a Read Replica

// Read-after-write consistency needed? Query the primary, not the replica,
// for that specific request.

The Solution //

Read Replicas use asynchronous replication, so a write to the primary may not be visible on a replica for some time. Reading immediately after a write (e.g. showing a user their own just-submitted data) from a replica can return stale results — read that specific data from the primary instead.