Understanding the difference between Read Replicas and Multi AZ is essential for the exam. Many questions test this.
Read Replicas — Scaling Reads
┌──────────────┐
│ Application │
└──────┬───────┘
reads & writes
│
┌──────▼───────┐
│ Main RDS DB │
└──┬───────┬───┘
async │ │ async
replication │ │ replication
┌──────▼──┐ ┌──▼──────┐
│ Replica │ │ Replica │
│ #1 │ │ #2 │
└─────────┘ └─────────┘
(reads) (reads)
| Feature | Detail |
|---|---|
| Max replicas | Up to 15 |
| Replication | Asynchronous (eventually consistent) |
| Placement | Same AZ, Cross AZ, or Cross Region |
| Promotion | Can be promoted to independent database |
| Operations | SELECT only (no INSERT, UPDATE, DELETE) |
| Connection | App must update connection string to use replicas |
A reporting/analytics team needs to run queries on production data. Instead of overloading the main DB, create a Read Replica and point the reporting app to it. Production is unaffected.
Read Replicas scale reads
Read Replica Network Cost
| Scenario | Cost |
|---|---|
| Same Region, different AZ | Free (managed service exception) |
| Cross Region | Paid (network replication fee) |
Same Region (FREE): Cross Region (PAID):
┌──────────┐ ┌──────────┐ ┌──────────┐ ┌──────────┐
│us-east-1a│───▶│us-east-1b│ │us-east-1 │───▶│eu-west-1 │
│ Main DB │ │ Replica │ │ Main DB │ │ Replica │
└──────────┘ └──────────┘ └──────────┘ └──────────┘
FREE replication PAID replication
Multi AZ — Disaster Recovery
┌──────────────┐
│ Application │
└──────┬───────┘
One DNS name
│
┌──────▼───────┐ SYNC ┌──────────────┐
│ Master │────────▶│ Standby │
│ (AZ A) │ │ (AZ B) │
└──────────────┘ └──────────────┘
reads & writes NO reads/writes
(failover only)
| Feature | Detail |
|---|---|
| Replication | Synchronous |
| Purpose | Disaster Recovery only (not scaling) |
| DNS | One DNS name — automatic failover |
| Standby | Not readable/writable — only for failover |
| Failover triggers | AZ loss, network failure, instance/storage failure |
| Manual intervention | None required — automatic |
The standby instance is not used for reads. It's purely a failover target.
Multi AZ failover target
Read Replicas as Multi AZ?
You can set up Read Replicas with Multi AZ for disaster recovery. This is a common exam question.
Single AZ → Multi AZ Conversion
| Aspect | Detail |
|---|---|
| Downtime | Zero downtime operation |
| Action | Click "Modify" → Enable Multi AZ |
| Internal process | Snapshot taken → Restored to standby → Sync established |
Step 1: Snapshot Step 2: Restore Step 3: Sync
┌──────────┐ ┌──────────┐ ┌──────────┐
│ Main DB │──snapshot──▶│ Standby │──sync────────│ Main DB │
│ │ │ (new) │ │ │
└──────────┘ └──────────┘ └──────────┘
Comparison Table
| Feature | Read Replicas | Multi AZ |
|---|---|---|
| Purpose | Read scaling | Disaster Recovery |
| Replication | Async | Sync |
| Readable | Yes (SELECT only) | No (standby) |
| Max count | 15 | 1 standby |
| Cross Region | Yes | No |
| DNS | Separate endpoints | Single DNS (auto failover) |
| Promotion | Can become independent DB | Standby becomes master on failure |
| Network cost | Free (same region) | N/A |
Questions & Answers
Q1: What is the maximum number of RDS Read Replicas?
Answer:
Up to 15 Read Replicas. They can be within the same AZ, cross AZ, or cross region.
Q2: What type of replication do Read Replicas use?
Answer:
Asynchronous replication. This means reads are eventually consistent — you may get stale data if the replica hasn't caught up yet.
Q3: Can you run INSERT/UPDATE/DELETE on a Read Replica?
Answer:
No. Read Replicas only support SELECT statements (read-only).
Q4: Is there a network cost for Read Replicas in the same region?
Answer:
No. Cross-AZ replication within the same region is free for RDS (managed service exception). Cross-region replication does incur a fee.
Q5: What is the purpose of Multi AZ?
Answer:
Disaster Recovery. It provides a synchronous standby in another AZ with automatic failover via a single DNS name. It is not used for scaling reads.
Q6: Can the Multi AZ standby be used for reads?
Answer:
No. The standby instance is purely for failover. No one can read from or write to it.
Q7: Is there downtime when converting from Single AZ to Multi AZ?
Answer:
No. It's a zero downtime operation. Just click "Modify" and enable Multi AZ. Internally, a snapshot is taken, restored to a new standby, and sync is established.
Q8: Can Read Replicas be set up as Multi AZ?
Answer:
Yes. You can configure Read Replicas with Multi AZ for disaster recovery. This is a common exam question.
Q9: What happens when the master fails in a Multi AZ setup?
Answer:
The standby is automatically promoted to master. The DNS name stays the same, so no application changes are needed.
Q10: What is the difference between async and sync replication?
Answer:
- Async (Read Replicas): Write accepted on master first, then replicated. Eventually consistent.
- Sync (Multi AZ): Write must be replicated to standby before being accepted. Strong consistency.
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