Recovering from a failed migration that deleted critical application code

During a framework migration from an older version (e.g., Fast-Agent 0.3.15) to a new system (e.g., FastMCP 2.0), the entire source code directory was unexpectedly deleted without functional replacements. This resulted in a completely broken build pipeline, preventing further development and deployment. The incident highlights the importance of safeguards during migrations, especially in Kubernetes-based deployments.

Issue

The migration process accidentally deleted the entire source code directory (k3s/api/) containing critical files, including:

• Docker configuration (Dockerfile)
• Dependency management (requirements.txt)
• Core application logic (main.py, worker scripts)
• Configuration files (e.g., fastagent.config.yaml)

This deletion led to:

• A broken build pipeline due to missing Dockerfile and dependencies.
• An empty source directory, preventing any deployment.
• Broken Kubernetes manifests with unresolved environment variable references.
• Failed deployment scripts due to missing context configurations.

Culprit

The root cause was a flawed migration strategy that:

1. Deleted before creating: Removed all source files without ensuring replacements were ready.
2. Committed non-functional state: Migration commits contained only documentation, TODO lists, and architecture plans—no working code.
3. Lack of parallel implementation: No feature flags or gradual rollout strategy was implemented.
4. Missing pre-deletion validation: No verification that new code existed or that the build pipeline would pass.

Solution

Codebase Restoration

1. Reverted to a known working state using Git history to restore the deleted directory:

   git checkout HEAD -- k3s/api/

   This recovered all critical components, including the Dockerfile, dependencies, and application logic.

Deployment Fixes

1. Fixed Kubernetes Context Issues: Added explicit context flags to deployment scripts to ensure correct cluster targeting.
2. Simplified Configuration: Consolidated environment variables using Kubernetes ConfigMaps to reduce complexity and errors.

Migration Safeguards

To prevent future incidents, the following measures were implemented:

1. Parallel Implementation: New code must coexist with legacy code during transitions.
2. Feature Flagging: Runtime switching between implementations to allow gradual testing.
3. Traffic Splitting: Domain-based or percentage-based rollout to compare metrics.

Verification Process

A pre-deletion checklist was established to ensure:

• The build pipeline passes.
• All tests execute successfully.
• The cluster remains healthy before any file deletions.

This solution restored functionality while providing a safer framework for future migrations, including:

• Automated verification gates.
• Backup systems for critical components.
• Gradual rollout mechanisms to minimize risk.