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High Availability for Stateful Database with Regional Persistent Disk
A company runs a critical application on Compute Engine with a stateful database. They need to achieve 99.99% availability for the database tier. Which architecture should they implement?
Quick Answer
The answer is a Compute Engine instance group with managed instance groups and a regional persistent disk configured for synchronous replication. This architecture achieves 99.99% availability for a stateful database by running the database across two zones while the regional persistent disk synchronously writes every data block to both zones, ensuring zero data loss during a zone failure. On the Google Professional Cloud Developer exam, this scenario tests your understanding of how to combine zonal redundancy with stateful workloads, often appearing as a trap where candidates mistakenly choose a snapshot-based or backup-only solution that cannot meet the 99.99% uptime requirement. The key distinction is that regional persistent disks provide synchronous replication at the block level, unlike asynchronous replication or manual failover. Remember the mnemonic “MIG with RPD” — Managed Instance Group plus Regional Persistent Disk equals synchronous, zone-fault-tolerant high availability.
⚠ Common exam trap
The PCD exam often tests the misconception that a shared zonal persistent disk across two instances provides high availability, but the trap is that a zonal disk is still tied to a single zone and fails if that zone goes down, whereas a regional persistent disk is required for true multi-zone resilience.
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
✓
A Compute Engine instance group with managed instance groups and a regional persistent disk configured for synchronous replication.
A managed instance group with a regional persistent disk configured for synchronous replication provides the necessary 99.99% availability by ensuring the database runs across two zones with synchronous writes to both replicas. This architecture allows automatic failover within seconds if one zone fails, meeting the high-availability requirement without data loss.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
A Compute Engine instance group with managed instance groups and a regional persistent disk configured for synchronous replication.
Why this is correct
Regional persistent disks replicate data synchronously across zones, and the managed instance group can automatically fail over to a new instance in another zone on failure, achieving high availability.
- ✗
Use Cloud SQL with automatic failover and read replicas.
Why it's wrong here
Cloud SQL is a managed service; the question specifies running the database on Compute Engine, implying the team wants to manage the database themselves.
- ✗
Two Compute Engine instances in different zones with a shared Zonal persistent disk.
Why it's wrong here
Zonal persistent disks cannot be attached to instances in different zones simultaneously. This configuration is not supported.
- ✗
Single Compute Engine instance with a persistent disk snapshot scheduled every hour.
Why it's wrong here
Snapshots provide point-in-time recovery but do not ensure high availability. Recovery time can be hours, failing the 99.99% availability target.
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Same concept, more angles
2 more ways this is tested on PCD
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company runs a stateful application on Compute Engine instances with persistent disks. The application must be highly available and be able to recover from a zonal failure with minimal data loss. The current architecture uses a single instance in one zone. Which design should the team implement?
medium- A.Use a standard persistent disk and configure a global load balancer to failover.
- B.Create a snapshot schedule and restore the snapshot to a new instance in another zone on failure.
- ✓ C.Use a regional persistent disk attached to a managed instance group across two zones.
- D.Migrate to Cloud Filestore for shared file storage across zones.
Why C: A regional persistent disk synchronously replicates data across two zones, and when attached to a managed instance group (MIG) spanning those zones, it provides automatic failover with minimal data loss. This design ensures that if one zone fails, the MIG can detach the disk from the failed instance and attach it to a healthy instance in the surviving zone, preserving state with near-zero RPO.
Variation 2. A company runs a stateful application on Compute Engine with regional persistent disks. They want to achieve high availability with automatic failover in case of a zone failure. Which architecture meets these requirements?
hard- A.Use Cloud Storage FUSE to mount a multi-regional bucket as a filesystem
- B.Use standard persistent disks with scheduled snapshots to a multi-region bucket
- C.Use zonal persistent disks with a managed instance group in a single zone
- ✓ D.Use regional persistent disks with a managed instance group spanning two zones
Why D: Regional persistent disks synchronously replicate data across two zones, and when combined with a managed instance group (MIG) spanning those same two zones, the application can automatically fail over to the healthy zone if one zone fails. The MIG's autohealing and health-check mechanisms detect the failure and recreate instances in the surviving zone, while the regional PD remains accessible from either zone, ensuring high availability without manual intervention.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This PCD practice question is part of Courseiva's free Google Cloud certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PCD exam.