Complex State Flows

Problem Statement

Multi-step operations with dependencies between steps are prone to ordering bugs, missing preconditions, and untested edge cases. Even without a formal state machine library, thinking in states and transitions prevents bugs.

Pattern: State Machine Thinking

Problem: Complex flows have implicit states that aren't modeled, leading to invalid transitions.

Example - Retake flow states:

IDLE → LOADING_COMPLETED → ENABLING_RETAKE → CLEARING_ANSWERS → READY → ANSWERING → MERGING → SUBMITTING → COMPLETE
                                                                                                              ↓
                                                                                                           ERROR

Each transition should have:

1. Preconditions - What must be true before this step
2. Action - What happens during this step
3. Postconditions - What must be true after this step
4. Error handling - What to do if this step fails

// Document the flow explicitly
/*
 * RETAKE FLOW
 * 
 * State: IDLE
 * Precondition: assessment exists
 * Action: loadCompletedAssessmentAnswers
 * Postcondition: completedAssessmentAnswers populated
 * 
 * State: LOADING_COMPLETED
 * Precondition: completedAssessmentAnswers loaded
 * Action: enableSkillAreaRetake
 * Postcondition: skillArea in retakeAreas set
 * 
 * State: ENABLING_RETAKE
 * Precondition: skillArea in retakeAreas
 * Action: clearSkillAreaAnswers
 * Postcondition: existing answers for skillArea removed
 * 
 * ... continue for each state
 */

Pattern: Explicit Flow Implementation

Problem: Flow logic scattered across multiple functions, hard to verify ordering.

// WRONG - implicit flow, easy to miss steps or misordering
async function startRetake(assessmentId: string, skillArea: string) {
  loadCompletedAssessmentAnswers(assessmentId); // Missing await!
  await enableSkillAreaRetake(skillArea);
  await clearSkillAreaAnswers(skillArea);
}

// CORRECT - explicit flow with validation
async function startRetake(assessmentId: string, skillArea: string) {
  const flowId = `retake-${Date.now()}`;
  logger.info(`[${flowId}] Starting retake flow`, { assessmentId, skillArea });
  
  // Step 1: Load completed answers
  await loadCompletedAssessmentAnswers(assessmentId);
  const completedAnswers = useStore.getState().completedAssessmentAnswers;
  if (Object.keys(completedAnswers).length === 0) {
    throw new Error(`[${flowId}] Failed to load completed answers`);
  }
  logger.debug(`[${flowId}] Loaded ${Object.keys(completedAnswers).length} answers`);
  
  // Step 2: Enable retake for skill area
  await enableSkillAreaRetake(skillArea);
  const retakeAreas = useStore.getState().retakeAreas;
  if (!retakeAreas.has(skillArea)) {
    throw new Error(`[${flowId}] Failed to enable retake for ${skillArea}`);
  }
  logger.debug(`[${flowId}] Enabled retake for ${skillArea}`);
  
  // Step 3: Clear existing answers
  await clearSkillAreaAnswers(skillArea);
  logger.debug(`[${flowId}] Cleared answers for ${skillArea}`);
  
  logger.info(`[${flowId}] Retake flow completed`);
}

Pattern: Flow Object

Problem: Long async functions with many steps become unwieldy.

interface FlowStep<TContext> {
  name: string;
  execute: (context: TContext) => Promise<void>;
  validate?: (context: TContext) => void;  // Postcondition check
}

interface RetakeContext {
  assessmentId: string;
  skillArea: string;
  flowId: string;
}

const retakeSteps: FlowStep<RetakeContext>[] = [
  {
    name: 'loadCompletedAnswers',
    execute: async (ctx) => {
      await loadCompletedAssessmentAnswers(ctx.assessmentId);
    },
    validate: (ctx) => {
      const answers = useStore.getState().completedAssessmentAnswers;
      if (Object.keys(answers).length === 0) {
        throw new Error(`[${ctx.flowId}] No completed answers loaded`);
      }
    },
  },
  {
    name: 'enableRetake',
    execute: async (ctx) => {
      await enableSkillAreaRetake(ctx.skillArea);
    },
    validate: (ctx) => {
      const retakeAreas = useStore.getState().retakeAreas;
      if (!retakeAreas.has(ctx.skillArea)) {
        throw new Error(`[${ctx.flowId}] Retake not enabled for ${ctx.skillArea}`);
      }
    },
  },
  {
    name: 'clearAnswers',
    execute: async (ctx) => {
      await clearSkillAreaAnswers(ctx.skillArea);
    },
  },
];

async function executeFlow<TContext>(
  steps: FlowStep<TContext>[],
  context: TContext,
  flowName: string
) {
  const flowId = `${flowName}-${Date.now()}`;
  logger.info(`[${flowId}] Starting flow`, context);
  
  for (const step of steps) {
    logger.debug(`[${flowId}] Executing: ${step.name}`);
    try {
      await step.execute(context);
      if (step.validate) {
        step.validate(context);
      }
      logger.debug(`[${flowId}] Completed: ${step.name}`);
    } catch (error) {
      logger.error(`[${flowId}] Failed at: ${step.name}`, { error: error.message });
      throw error;
    }
  }
  
  logger.info(`[${flowId}] Flow completed`);
}

// Usage
await executeFlow(retakeSteps, { assessmentId, skillArea, flowId }, 'retake');

Pattern: Flow State Tracking

Problem: Components need to know current flow state for UI feedback.

type RetakeFlowState =
  | { status: 'idle' }
  | { status: 'loading'; step: string }
  | { status: 'ready' }
  | { status: 'answering'; answeredCount: number }
  | { status: 'submitting' }
  | { status: 'complete' }
  | { status: 'error'; message: string; step: string };

const useRetakeStore = create<{
  flowState: RetakeFlowState;
  setFlowState: (state: RetakeFlowState) => void;
}>((set) => ({
  flowState: { status: 'idle' },
  setFlowState: (flowState) => set({ flowState }),
}));

async function startRetake(assessmentId: string, skillArea: string) {
  const { setFlowState } = useRetakeStore.getState();
  
  try {
    setFlowState({ status: 'loading', step: 'loadingAnswers' });
    await loadCompletedAssessmentAnswers(assessmentId);
    
    setFlowState({ status: 'loading', step: 'enablingRetake' });
    await enableSkillAreaRetake(skillArea);
    
    setFlowState({ status: 'loading', step: 'clearingAnswers' });
    await clearSkillAreaAnswers(skillArea);
    
    setFlowState({ status: 'ready' });
  } catch (error) {
    setFlowState({ 
      status: 'error', 
      message: error.message,
      step: useRetakeStore.getState().flowState.step,
    });
  }
}

// Component usage
function RetakeScreen() {
  const flowState = useRetakeStore((s) => s.flowState);
  
  if (flowState.status === 'loading') {
    return <Loading step={flowState.step} />;
  }
  
  if (flowState.status === 'error') {
    return <Error message={flowState.message} step={flowState.step} />;
  }
  
  // ... render based on state
}

Pattern: Integration Testing Flows

Problem: Unit tests for individual functions don't catch flow-level bugs.

describe('Retake Flow', () => {
  beforeEach(() => {
    useAssessmentStore.getState()._reset();
  });

  it('persists answers through complete retake flow', async () => {
    const assessmentId = 'test-assessment';
    const skillArea = 'fundamentals';
    const store = useAssessmentStore;
    
    // Setup: Simulate existing completed assessment
    store.getState().setCompletedAnswers(assessmentId, mockCompletedAnswers);
    
    // Execute full flow
    await store.getState().loadCompletedAssessmentAnswers(assessmentId);
    
    // Verify postcondition
    expect(Object.keys(store.getState().completedAssessmentAnswers).length)
      .toBeGreaterThan(0);
    
    await store.getState().enableSkillAreaRetake(skillArea);
    
    // Verify postcondition
    expect(store.getState().retakeAreas.has(skillArea)).toBe(true);
    
    await store.getState().clearSkillAreaAnswers(skillArea);
    
    // Simulate user answering
    await store.getState().saveAnswer('q1', 4);
    
    // THE CRITICAL CHECK - does the answer persist?
    expect(store.getState().userAnswers['q1']).toBe(4);
    
    // Complete flow
    await store.getState().submitRetake(assessmentId);
    
    // Verify final state
    expect(store.getState().flowState.status).toBe('complete');
  });

  it('handles error at each step', async () => {
    // Test error handling at step 1
    mockApi.loadAnswers.mockRejectedValueOnce(new Error('Network error'));
    
    await expect(
      store.getState().startRetake(assessmentId, skillArea)
    ).rejects.toThrow('Network error');
    
    expect(store.getState().flowState.status).toBe('error');
    expect(store.getState().flowState.step).toBe('loadingAnswers');
  });
});

Pattern: Flow Documentation

Document complex flows with diagrams for team understanding:

## Retake Flow

### Happy Path

┌─────────┐ ┌──────────────┐ ┌───────────────┐ ┌───────────────┐ │ Start │────▶│ Load Answers │────▶│ Enable Retake │────▶│ Clear Answers │ └─────────┘ └──────────────┘ └───────────────┘ └───────────────┘ │ │ │ ▼ ▼ ▼ Postcondition: Postcondition: Postcondition: answers.length > 0 retakeAreas.has(x) cleared for area

                                                               │
                                                               ▼

┌──────────┐ ┌─────────┐ ┌───────────────┐ ┌──────────────────┐ │ Complete │◀────│ Merge │◀────│ User Answers │◀────│ Ready for Input │ └──────────┘ └─────────┘ └───────────────┘ └──────────────────┘

### Error States

Any step can fail → transition to ERROR state with step context.
From ERROR: user can retry (back to IDLE) or exit.

Checklist: Designing Complex Flows

Before implementing:

• Sketch state diagram (even on paper)
• Identify all states, including error states
• Document preconditions for each transition
• Document postconditions to verify
• Plan how to surface state to UI

During implementation:

• Verify preconditions before each step
• Validate postconditions after each step
• Log state transitions with flow ID
• Handle errors at each step with context
• Surface flow state for UI feedback

After implementation:

• Integration test for happy path
• Integration test for error at each step
• Verify logs are sufficient for debugging
• Document flow for team

When to Use XState

Consider XState when:

• Flow has > 6 states
• Complex branching/parallel states
• Need visualization/debugging tools
• State machine is shared across team

For simpler flows, explicit steps with validation (as shown above) are often sufficient and more readable.