Database.Advanced.How would you design a high-availability database system?

Designing a high-availability (HA) database system means ensuring the database remains operational with minimal downtime, even in the face of hardware failures, software bugs, or network issues. Here’s how you can approach the design, step by step:

✅ 1. Set Requirements First

Before jumping to tech choices:

• RTO (Recovery Time Objective): How quickly must it recover?
• RPO (Recovery Point Objective): How much data loss is acceptable?
• Read/Write patterns: OLTP vs OLAP, heavy reads vs writes?
• Geographic needs: Multi-region or single region?
• Budget & team expertise

🧱 2. Core Design Components

🔁 a) Replication

Maintain multiple copies of your data.

• Primary-Replica (Master-Slave):
  • One primary node handles writes, replicas handle reads.
  • Easy to set up (e.g., PostgreSQL Streaming Replication, MySQL Replication).
• Multi-Master (Active-Active):
  • All nodes accept writes.
  • Complex conflict resolution (e.g., Cassandra, Couchbase).

⚖️ b) Failover Mechanism

Automatically detect failure and switch to a backup.

• Use orchestration tools (e.g., Patroni for PostgreSQL).
• Use floating IP or DNS switching.
• Integrate with a load balancer or service mesh (e.g., HAProxy, Envoy).

📡 c) Load Balancing

Distribute traffic to healthy nodes.

• Read requests can go to replicas.
• Use read-write splitting at application or proxy level.
• Tools: HAProxy, PgBouncer, ProxySQL.

📦 d) Backups & Snapshots

Ensure point-in-time recovery.

• Automate regular backups (full + incremental).
• Store in different regions (e.g., S3, GCS).
• Test restoration procedures.

🌍 3. Geo-Redundancy

For true fault tolerance:

• Use multi-region replication.
• Ensure latency-aware routing.
• Use cloud-native solutions (e.g., AWS Aurora Global, Google Spanner).

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🧠 4. Monitoring & Alerting

• Use tools like Prometheus, Grafana, or cloud-native (CloudWatch, Stackdriver).
• Alert on replication lag, node downtime, disk I/O, CPU, etc.
• Auto-restart unhealthy nodes (via Kubernetes, systemd, or EC2 ASG).

☁️ 5. Cloud-Managed Alternatives

If you want HA without managing the complexity:

• AWS RDS/Aurora, Google Cloud SQL/Spanner, Azure SQL offer:
  • Built-in failover
  • Replication
  • Backups
  • Monitoring

🔒 6. Security Considerations

• Encrypt data at rest and in transit.
• Use IAM roles, secrets managers.
• Audit access and logs.

🛠️ Example Architecture: PostgreSQL HA Setup

               ┌────────────┐
               │ LoadBalancer│
               └────┬───────┘
                    │
          ┌─────────▼─────────┐
          │   PgBouncer (RW)  │
          └────┬───────┬──────┘
               │       │
      ┌────────▼──┐ ┌──▼────────┐
      │ Primary DB│ │ Read Replica│
      └────┬──────┘ └────┬──────┘
           │             │
      ┌────▼─────┐ ┌─────▼─────┐
      │ WAL Logs │ │ Monitoring│
      └──────────┘ └───────────┘

🧪 7. Test Everything

• Simulate failure: kill nodes, disconnect network.
• Check failover time and data consistency.
• Practice disaster recovery drills.