Staff / Principal SRE Cloud Migration Enterprise Cloud Adoption Enterprise Migration

Q: Scenario: Your organization needs to migrate a large production workload from on-premises infrastructure to AWS/Azure with minimal downtime and no major service disruption. Explain your migration strategy, dependency mapping, networking, data replication, security, IaC, testing, cutover, rollback, and post-migration optimization.

Production playbook for migrating mission-critical enterprise workloads from on-premises to AWS with near-zero downtime: discovery, Direct Connect hybrid networking, DMS continuous CDC sync, cutover, and rollback.

#Cloud Migration #AWS #Direct Connect #DMS #CDC #Database #Networking #Cutover

🎙️ Candidate Opening & Architectural Context

"Migrating enterprise workloads with near-zero downtime requires decoupling the migration into distinct phases: hybrid network foundation, continuous data synchronization with Change Data Capture (CDC), and a low-risk DNS/router cutover with an instant rollback mechanism."

🛠️ Production Runbook & Step-by-Step Resolution

1️⃣

Discovery, Dependency Mapping & Classification (The 7 Rs)

Audit and assess every service before touching infrastructure:

Automated Dependency Mapping: Deploy agents (AWS Application Discovery Service / Cloudamize) to discover server inventory, inter-service network dependencies, throughput, and database topologies.
Classify by Migration Strategy (7 Rs): Rehost (lift-and-shift via AWS Application Migration Service/MGN), Replatform (move databases to managed AWS Aurora, apps to EKS), or Refactor.
Identify hard constraints: latency-sensitive database dependencies and regulatory compliance boundaries.

2️⃣

Hybrid Networking & Landing Zone Foundation

Establishing high-throughput, secure communication:

AWS Control Tower Landing Zone: Multi-account structure with Core Services, Security/Audit, and Environment accounts.
Dedicated Hybrid Connectivity: AWS Direct Connect (DX) with 10Gbps dedicated connection and IPSec VPN backup over internet.
AWS Transit Gateway (TGW): Interconnects on-premises data centers with multiple AWS VPCs.
Hybrid DNS: Route 53 Resolver Inbound and Outbound Endpoints enabling bi-directional domain resolution between on-prem Active Directory and AWS.

3️⃣

Continuous Data Replication (Change Data Capture - CDC)

Eliminating data transfer downtime during cutover:

Database Replication (AWS DMS): Full load migration followed by continuous Change Data Capture (CDC) from on-prem Oracle/PostgreSQL to Amazon Aurora. Transactions stream in real-time with sub-second replication lag.
Storage & Files (AWS DataSync): Continuously synchronizes on-premises NAS/SAN file shares to Amazon EFS / S3.
Target IaC: Recreate the entire target architecture declaratively in Terraform (EKS, RDS, SQS, Security Groups).

4️⃣

Dry-Run Testing, Cutover & Reverse Rollback

Executing the cutover with minimum downtime (<5 minutes):

Pre-Cutover Testing: Deploy application on AWS EKS; run load testing and security scans against the Aurora read replica.
DNS Preparation: Reduce Route 53 DNS TTL to 60 seconds one week in advance.
Cutover Window (Scheduled off-peak):
1. Put on-premises application in read-only mode.
2. Wait for final AWS DMS replication lag to reach zero (typically <30 seconds).
3. Promote AWS Aurora to primary writer.
4. Switch Route 53 DNS / CloudFront origin to point to AWS ALB.
5. Validate live traffic.
The Safety Net (Reverse CDC Rollback): Configure reverse DMS replication from AWS Aurora back to on-premises for the first 72 hours. If a catastrophic unforeseen bug occurs in cloud, traffic can flip back to on-prem with ZERO data loss!

5️⃣

Post-Migration Optimization

Cost reduction and cloud-native refinement:

Right-size EC2/EKS compute using AWS Compute Optimizer.
Decommission Direct Connect and legacy on-premises hardware.
Purchase AWS Savings Plans / Reserved Instances to cut compute costs by 40%+.

💡 The Senior SRE Gold Nugget (Key Architectural Takeaway)

"Near-zero downtime migration is achieved by pre-syncing data with AWS DMS CDC (Change Data Capture) over Direct Connect, lowering DNS TTL to 60s, and configuring reverse CDC back to on-prem as an instant safety rollback net."

⚡ 60-Second Elevator Pitch Talking Points

Phase 1 (Discovery): Map dependencies via AWS Discovery Service; classify workloads using 7 Rs framework.
Phase 2 (Hybrid Network): 10G AWS Direct Connect + Transit Gateway + Route 53 Resolver endpoints for hybrid DNS.
Phase 3 (Data Sync): AWS DMS with continuous Change Data Capture (CDC) to keep Aurora in real-time sync with on-prem DB.
Phase 4 (Testing): Terraform deploys target EKS/Aurora; execute load and security testing against cloud replica.
Phase 5 (Cutover & Rollback): Set DNS TTL to 60s, set on-prem read-only, promote Aurora, flip DNS. Keep reverse CDC active for 72h rollback safety.