PAS-C01 Synchronous Replication Practice Question
An SAP system on AWS uses a Multi-AZ deployment for the database layer with synchronous replication. The application servers are in the same region but different Availability Zones. During a recent failure of the primary database instance, the automatic failover to the standby instance took longer than expected, exceeding the RTO of 5 minutes. Upon investigation, the team finds that the standby instance was not fully synchronized at the time of failure. What could be the cause?
⚠ Common exam trap
Candidates often assume that replication lag is caused by storage performance or network configuration errors, but in a synchronous replication setup, network latency is a primary factor affecting synchronization.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The network latency between the Availability Zones is high.
In a Multi-AZ deployment with synchronous replication, high network latency between Availability Zones can cause the primary database to wait for acknowledgment from the standby. This increases transaction commit time and can lead to the standby falling behind, especially if the primary continues processing transactions while waiting. During failover, the standby may not be fully synchronized, causing longer failover time and exceeding RTO. Option A is unlikely because EBS IOPS affect storage performance but not replication lag directly; Option B is incorrect because Multi-AZ uses multiple network interfaces; Option C is incorrect because blocked replication traffic would cause complete failure, not just lag.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The EBS volumes on the standby instance have lower IOPS than the primary.
Why it's wrong here
Standby is not under write load; IOPS mismatch doesn't cause lag.
- ✗
The Multi-AZ configuration uses a single network interface.
Why it's wrong here
This does not cause replication lag.
- ✗
The security groups on the standby instance block replication traffic.
Why it's wrong here
If blocked, replication would fail completely, not just lag.
- ✓
The network latency between the Availability Zones is high.
Why this is correct
High latency can cause synchronous replication to slow down.
Visual reference
Quick reference
AWS S3 Storage Class Comparison
| Storage Class | Min Duration | Retrieval | Use Case |
|---|---|---|---|
| S3 Standard | None | Immediate | Frequently accessed data |
| S3 Standard-IA | 30 days | Immediate | Infrequent access, rapid retrieval |
| S3 One Zone-IA | 30 days | Immediate | Non-critical infrequent data |
| S3 Intelligent-Tiering | None | Immediate–hours | Unknown or changing access patterns |
| S3 Glacier Instant | 90 days | Milliseconds | Archive with instant retrieval |
| S3 Glacier Flexible | 90 days | Minutes–hours | Archive, flexible retrieval |
| S3 Glacier Deep Archive | 180 days | Hours | Long-term compliance archive |
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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