Remote Debugging F# Giraffe Applications in Kubernetes with VS Code

Debugging functional F# applications running in Kubernetes is possible with the right setup. This post demonstrates remote debugging of F# Giraffe web applications using vsdbg and VS Code, allowing you to debug functional code, pipelines, and pattern matching directly in your Kubernetes environment.

Overview

This example from the k8s-vscode-remote-debug repository showcases:

• Building debug-enabled Docker images for F# with Giraffe framework

• Deploying to Kubernetes with debug configuration

• Attaching VS Code debugger to remote pods via kubectl exec

• Debugging functional programming constructs (pipelines, pattern matching, closures)

What Works:

• ✅ Breakpoints in F# code

• ✅ Variable inspection (including function closures)

• ✅ Call stack navigation (including functional composition)

• ✅ Conditional breakpoints

• ✅ Expression evaluation in Debug Console

• ✅ Watch expressions

• ✅ Pipeline debugging (F# pipe operators)

• ✅ Pattern matching inspection

Technology Stack:

• Language: F#

• Framework: Giraffe on ASP.NET Core 8.0

• Debugger: vsdbg (Visual Studio Debugger)

• Debug Method: kubectl exec with pipeTransport

How It Works

F# debugging works identically to C# since both run on the .NET runtime:

1. Docker Build: Image includes vsdbg from aka.ms/getvsdbgsh

2. VS Code Connection: Uses kubectl exec with pipeTransport

3. No Port-Forwarding: Direct connection via kubectl

4. Process Attachment: Attaches to the .NET process (PID 1)

Key Configuration

Dockerfile (Debug Mode)

The debug build installs vsdbg for remote debugging. See the complete Dockerfile for the full multi-stage build configuration.

Key Configuration

VS Code launch.json

The debugger configuration uses kubectl exec with pipeTransport:

{
  “name”: “Attach to Remote Pod”,
  “type”: “coreclr”,
  “request”: “attach”,
  “processId”: “1”,
  “pipeTransport”: {
    “pipeCwd”: “${workspaceRoot}”,
    “pipeProgram”: “kubectl”,
    “pipeArgs”: [”exec”, “-i”, “-n”, “${input:namespace}”, “${input:podName}”, “--”],
    “debuggerPath”: “/vsdbg/vsdbg”,
    “quoteArgs”: false
  },
  “sourceFileMap”: {
    “/src/FSharpGiraffe”: “${workspaceFolder}/src/FSharpGiraffe”
  }
}

This configuration prompts for namespace and pod name, providing flexibility for multi-pod debugging scenarios. See the complete launch.json with input definitions.

Quick Start

# Set your developer namespace and registry
export NAMESPACE=dev-yourname
export REGISTRY=your-registry.azurecr.io  # Or docker.io/username, gcr.io/project, etc.

# Clone the repository
git clone https://github.com/nathanfox/k8s-vscode-remote-debug.git
cd k8s-vscode-remote-debug/examples/fsharp-giraffe-dotnet8

# Build, push, and deploy
./manage.sh build
./manage.sh push
./manage.sh deploy

# Verify pod is ready
./manage.sh debug

# Open in VS Code and press F5 to attach debugger
code .

Example Debugging Session

Debugging a Functional Handler:

1. Set a breakpoint in Program.fs at line 23 (inside the debugTestHandler function)

2. Attach debugger by pressing F5 in VS Code

3. Enter namespace and pod name when prompted

4. Port-forward the application (in a separate terminal):

./manage.sh port-forward

1. Trigger the endpoint:

curl http://localhost:8080/debug-test?count=3

1. Inspect functional values:

   • Hover over count to see the parsed value

   • Inspect items list as it’s computed

   • View closure captures in lambda functions

2. Step through pipelines:

   • F10 (step over) for each pipeline stage

   • Watch transformations happen step-by-step

3. Continue execution - F5 to resume

VS Code Extensions

Required for debugging:

• Ionide-fsharp (Ionide.ionide-fsharp) - F# language support and IntelliSense

• C# (ms-dotnettools.csharp) - Required for vsdbg debugger

Recommended (helpful but not required):

• Kubernetes (ms-kubernetes-tools.vscode-kubernetes-tools) - K8s cluster management

Troubleshooting

F# IntelliSense Not Working

Ensure Ionide-fsharp extension is installed and project dependencies are restored:

dotnet restore

Example-Driven Development with AI Agents

This repository demonstrates Example-Driven Development, designed to work with AI coding assistants like Claude Code.

For more on this pattern, see Example-Driven Development Using AI Agent Claude Code.

Example AI prompt:

“Using the k8s-vscode-remote-debug repository’s F# Giraffe example, add remote debugging support to my F# web application running in Kubernetes.”

The AI can generate the appropriate configuration based on the working example.

Next Steps

For complete details including:

• Full troubleshooting guide

• Debug vs Production build configuration

• Performance considerations

• Integration patterns

See the complete README in the repository.

The repository includes examples for 8 languages/frameworks, each demonstrating the unique aspects of debugging that language in Kubernetes.