Systematic Debugging

Overview

Random fixes waste time and create new bugs. Quick patches mask underlying issues.

Core principle: ALWAYS find root cause before attempting fixes. Symptom fixes are failure.

Violating the letter of this process is violating the spirit of debugging.

The Iron Law

NO FIXES WITHOUT ROOT CAUSE INVESTIGATION FIRST

If you haven't completed Phase 1, you cannot propose fixes.

When to Use

Use for ANY technical issue:

• Test failures
• Bugs in production
• Unexpected behavior
• Performance problems
• Build failures
• Integration issues

Use this ESPECIALLY when:

• Under time pressure (emergencies make guessing tempting)
• "Just one quick fix" seems obvious
• You've already tried multiple fixes
• Previous fix didn't work
• You don't fully understand the issue

C# .NET Specific:

• gRPC service throwing RpcException with wrong status code
• EF Core query returning unexpected results
• Async deadlocks or TaskScheduler issues
• Build failures with cryptic compiler errors
• NUnit/xUnit test failures with no clear output

The Four Phases

You MUST complete each phase before proceeding to the next.

Phase 1: Root Cause Investigation

BEFORE attempting ANY fix:

1. Read Error Messages Carefully

   • Don't skip past errors or warnings (warnings become errors in build)
   • Exception stack traces show exact line numbers
   • Read StackTrace property completely
   • Check InnerException for root cause
   • .NET error codes (CS, MSB, RPC Status codes) have specific meanings
   • For gRPC: Check StatusCode (InvalidArgument, NotFound, Internal, etc.)
   • For EF Core: Check DbUpdateException, SqlException inner exception

   C# .NET Examples:

   # Read full exception with inner exception chain
   $ dotnet test AF.ECT.Tests --filter "WorkflowTest" --no-build
   
   FAIL: GetWorkflow_WithNullId_ThrowsException
   Error: 
     System.ArgumentNullException: Value cannot be null. (Parameter 'workflowId')
       at AF.ECT.Server.Services.WorkflowServiceImpl.ValidateWorkflowId(String id)
       at AF.ECT.Server.Services.WorkflowServiceImpl.GetWorkflow(GetWorkflowRequest request, ServerCallContext context)
     InnerException: (check if significant)
   
   # Check EF Core query logs
   $ dotnet run --project AF.ECT.Server -- --debug
   # Look for: SqlException, DbUpdateException with inner message about FK constraints
   
   # Check gRPC response status codes
   # StatusCode.InvalidArgument = client sent bad request (your validation)
   # StatusCode.NotFound = resource doesn't exist (query returned null)
   # StatusCode.Internal = server error (unhandled exception)
   

2. Reproduce Consistently

   • Can you trigger it reliably with dotnet test AF.ECT.Tests or dotnet run --project AF.ECT.AppHost?
   • What are the exact steps or input?
   • Does it happen every test run?
   • If intermittent → add logging to gRPC interceptor, EF Core query logs
   • Don't guess on race conditions - instrument first

   C# .NET Examples:

   # Reproduce test failure consistently
   $ dotnet test AF.ECT.Tests --filter "GetWorkflow" --no-build -v detailed
   
   # For intermittent failures, enable EF Core SQL logging
   # In your DbContext OnConfiguring:
   optionsBuilder.LogTo(Console.WriteLine, LogLevel.Information);
   
   # For gRPC issues, check ServerCallContext logging in AuditInterceptor
   # Look for: request/response timing, connection resets, timeouts
   

3. Check Recent Changes

   • What changed that could cause this?
   • git diff main, recent commits
   • New gRPC method, EF Core query change, config values
   • Environmental differences (LocalDB vs SQL Server, network)

4. Gather Evidence in Multi-Component Systems

   WHEN system has multiple components (Blazor → gRPC → Server → EF Core → SQL Server):

   BEFORE proposing fixes, add diagnostic instrumentation:

   For EACH component boundary:
     - Log what data enters component
     - Log what data exits component
     - Verify environment/config propagation
     - Check state at each layer
   

   Example (.NET multi-layer system in ECTSystem):

   # Layer 1: Configuration & Dependency Injection
   echo "=== appsettings.Development.json values: ==="
   cat AF.ECT.Server/appsettings.Development.json | jq '.ConnectionStrings, .Logging'
   
   # Layer 2: gRPC Channel & Client Configuration
   echo "=== WorkflowClient initialization: ==="
   # Add temporary logging in AF.ECT.Shared/Services/WorkflowClient.cs constructor
   // Log: ServiceUrl, ChannelCredentials, etc.
   
   # Layer 3: gRPC Request Interceptor
   echo "=== gRPC call audit: ==="
   # Check AF.ECT.Server/Interceptors/AuditInterceptor.cs logs
   # Look for: Request details, correlation ID, response status
   
   # Layer 4: Server Endpoint Processing
   echo "=== gRPC service method execution: ==="
   # Add logging in AF.ECT.Server/Services/WorkflowServiceImpl.cs
   // Log: Method entry, parameters, database calls
   
   # Layer 5: EF Core Data Access
   echo "=== Database query execution: ==="
   # Enable in AF.ECT.Data/EctDbContext.cs OnConfiguring:
   optionsBuilder.LogTo(Console.WriteLine, LogLevel.Debug);
   
   # Layer 6: SQL Server Execution
   echo "=== SQL tracing: ==="
   # Use SQL Server Profiler or Extended Events to trace actual queries
   

   Run once to gather evidence showing WHERE it breaks, then investigate that specific component

5. Trace Data Flow

   WHEN error is deep in call stack (gRPC → WorkflowClient → DataService → EF Core → DbContext):

   See root-cause-tracing.md in this directory for the complete backward tracing technique.

   Quick version for ECTSystem:

   Error location: WorkflowServiceImpl.GetWorkflow() returns null
   
   1. Where does null come from? 
      Answer: EF Core query returned no results (_context.Workflows.FirstOrDefaultAsync())
   
   2. Why is query returning nothing?
      Answer: Check query logic, parameters passed to query
      Look at: WorkflowDataService.GetWorkflowByIdAsync(id)
      What is 'id' value? How is it passed from client?
   
   3. Where does bad ID originate?
      Answer: Client sent invalid/empty workflow ID in gRPC request
      Look at: WorkflowClient.GetWorkflowAsync(workflowId)
      Who calls this? What ID do they pass?
   
   4. Keep tracing up until you find source:
      Answer: Component passes hardcoded ID, or user input not validated
   
   5. Fix at source:
      - If client: Add validation in WorkflowClient
      - If user input: Validate in Blazor component before calling client
      - NOT: Add defensive checks at every layer
   

   • Where does bad value originate? (null object from gRPC request, invalid Id)
   • What called this with bad value? (WorkflowClient passed wrong Id)
   • Keep tracing up call stack until you find the source
   • Fix at source, not at symptom (don't add validation everywhere, fix the caller)

Phase 2: Pattern Analysis

Find the pattern before fixing:

1. Find Working Examples

   • Locate similar working code in same codebase
   • What works that's similar to what's broken?

2. Compare Against References

   • If implementing pattern, read reference implementation COMPLETELY
   • Don't skim - read every line
   • Understand the pattern fully before applying

3. Identify Differences

   • What's different between working and broken?
   • List every difference, however small
   • Don't assume "that can't matter"

4. Understand Dependencies

   • What other components does this need?
   • What settings, config, environment?
   • What assumptions does it make?

Phase 3: Hypothesis and Testing

Scientific method:

1. Form Single Hypothesis

   • State clearly: "I think X is the root cause because Y"
   • Write it down
   • Be specific, not vague

2. Test Minimally

   • Make the SMALLEST possible change to test hypothesis
   • One variable at a time
   • Don't fix multiple things at once

3. Verify Before Continuing

   • Did it work? Yes → Phase 4
   • Didn't work? Form NEW hypothesis
   • DON'T add more fixes on top

4. When You Don't Know

   • Say "I don't understand X"
   • Don't pretend to know
   • Ask for help
   • Research more

Phase 4: Implementation

Fix the root cause, not the symptom:

1. Create Failing Test Case

   • Simplest possible reproduction
   • xUnit [Fact] test (or [Theory] with data)
   • MUST have before fixing
   • REQUIRED SUB-SKILL: Use superpowers:test-driven-development for writing proper failing tests

2. Implement Single Fix

   • Address the root cause identified
   • ONE change at a time
   • No "while I'm here" improvements
   • No bundled refactoring

3. Verify Fix

   • Test passes now?
   • No other tests broken?
   • Issue actually resolved?

4. If Fix Doesn't Work

   • STOP
   • Count: How many fixes have you tried?
   • If < 3: Return to Phase 1, re-analyze with new information
   • If ≥ 3: STOP and question the architecture (step 5 below)
   • DON'T attempt Fix #4 without architectural discussion

5. If 3+ Fixes Failed: Question Architecture

   Pattern indicating architectural problem:

   • Each fix reveals new shared state/coupling/problem in different place
   • Fixes require "massive refactoring" to implement
   • Each fix creates new symptoms elsewhere

   STOP and question fundamentals:

   • Is this pattern fundamentally sound?
   • Are we "sticking with it through sheer inertia"?
   • Should we refactor architecture vs. continue fixing symptoms?

   Discuss with your human partner before attempting more fixes

   This is NOT a failed hypothesis - this is a wrong architecture.

Red Flags - STOP and Follow Process

If you catch yourself thinking:

• "Quick fix for now, investigate later"
• "Just try changing X and see if it works"
• "Add multiple changes, run tests"
• "Skip the test, I'll manually verify"
• "It's probably X, let me fix that"
• "I don't fully understand but this might work"
• "Pattern says X but I'll adapt it differently"
• "Here are the main problems: [lists fixes without investigation]"
• Proposing solutions before tracing data flow
• "One more fix attempt" (when already tried 2+)
• Each fix reveals new problem in different place

ALL of these mean: STOP. Return to Phase 1.

If 3+ fixes failed: Question the architecture (see Phase 4.5)

your human partner's Signals You're Doing It Wrong

Watch for these redirections:

• "Is that not happening?" - You assumed without verifying
• "Will it show us...?" - You should have added evidence gathering
• "Stop guessing" - You're proposing fixes without understanding
• "Ultrathink this" - Question fundamentals, not just symptoms
• "We're stuck?" (frustrated) - Your approach isn't working

When you see these: STOP. Return to Phase 1.

Common Rationalizations

Excuse	Reality
"Issue is simple, don't need process"	Simple issues have root causes too. Process is fast for simple bugs.
"Emergency, no time for process"	Systematic debugging is FASTER than guess-and-check thrashing.
"Just try this first, then investigate"	First fix sets the pattern. Do it right from the start.
"I'll write test after confirming fix works"	Untested fixes don't stick. Test first proves it.
"Multiple fixes at once saves time"	Can't isolate what worked. Causes new bugs.
"Reference too long, I'll adapt the pattern"	Partial understanding guarantees bugs. Read it completely.
"I see the problem, let me fix it"	Seeing symptoms ≠ understanding root cause.
"One more fix attempt" (after 2+ failures)	3+ failures = architectural problem. Question pattern, don't fix again.

Quick Reference

Phase	Key Activities	Success Criteria
1. Root Cause	Read errors, reproduce, check changes, gather evidence	Understand WHAT and WHY
2. Pattern	Find working examples, compare	Identify differences
3. Hypothesis	Form theory, test minimally	Confirmed or new hypothesis
4. Implementation	Create test, fix, verify	Bug resolved, tests pass

When Process Reveals "No Root Cause"

If systematic investigation reveals issue is truly environmental, timing-dependent, or external:

1. You've completed the process
2. Document what you investigated
3. Implement appropriate handling (retry, timeout, error message)
4. Add monitoring/logging for future investigation

But: 95% of "no root cause" cases are incomplete investigation.

Supporting Techniques

These techniques are part of systematic debugging and available in this directory:

• root-cause-tracing.md - Trace bugs backward through call stack to find original trigger
• defense-in-depth.md - Add validation at multiple layers after finding root cause
• condition-based-waiting.md - Replace arbitrary timeouts with condition polling

Related skills:

• superpowers:test-driven-development - For creating failing test case (Phase 4, Step 1)
• superpowers:verification-before-completion - Verify fix worked before claiming success

Real-World Impact

From debugging sessions:

• Systematic approach: 15-30 minutes to fix
• Random fixes approach: 2-3 hours of thrashing
• First-time fix rate: 95% vs 40%
• New bugs introduced: Near zero vs common