Chain Design Principles

Well-designed chains are maintainable, debuggable, and reliable. This lesson covers the core principles that separate fragile chains from robust ones.

The SOLID Principles for Chains

Traditional software engineering principles adapt well to chain design:

Single Responsibility

Each step should do one thing well.

Bad: A step that extracts data, validates it, transforms it, and generates output

Good: Separate steps for extraction, validation, transformation, and generation

Open for Extension

Design chains so new steps can be added without rewriting existing ones.

Original: Extract → Analyze → Generate

Extended: Extract → Translate → Analyze → Generate
                   └─ New step slots in easily

Interface Segregation

Don't pass more data than a step needs.

Bad: Passing the entire chain state to every step

Good: Each step receives only the specific fields it requires

Principle 1: Clear Boundaries

Define Entry and Exit Points

Every chain should have:

• Entry validation: What input format is acceptable?
• Exit contract: What does success look like?

Mark Step Boundaries Clearly

In your chain definition, make boundaries explicit:

const chain = {
  name: "document_processor",
  version: "1.2.0",
  steps: [
    {
      id: "validate",
      input_schema: { /* what it accepts */ },
      output_schema: { /* what it produces */ }
    },
    {
      id: "extract",
      input_schema: { /* matches validate's output */ },
      output_schema: { /* what it produces */ }
    }
  ]
};

Principle 2: Fail Fast

Validate Early

Check inputs before expensive processing:

┌──────────┐   ┌──────────┐   ┌──────────┐   ┌──────────┐
│ Validate │ → │ Process  │ → │ Refine   │ → │ Format   │
│ (cheap)  │   │ (costly) │   │ (costly) │   │ (cheap)  │
└──────────┘   └──────────┘   └──────────┘   └──────────┘
     ↓
   Fail here if invalid
   (save time and cost)

Validate Between Steps

Don't assume a step succeeded—verify:

const step1Output = await runStep1(input);

// Validate before proceeding
if (!step1Output.required_field) {
  return { error: "Step 1 did not produce required_field" };
}

const step2Output = await runStep2(step1Output);

Principle 3: Idempotency

Steps should produce the same output given the same input, when possible.

Why Idempotency Matters

• Debugging: You can reproduce issues
• Retries: Safe to retry failed steps
• Testing: Predictable test results

Achieving Idempotency

Note how explicit rules create deterministic behavior.

Principle 4: Observability

Log Everything Meaningful

Each step should produce logs that help debug issues:

{
  step_id: "extract_entities",
  timestamp: "2024-01-15T10:30:00Z",
  duration_ms: 1250,
  input_size: 2500,
  output_size: 450,
  status: "success",
  entities_found: 12,
  warnings: ["Low confidence on entity 'XYZ Corp'"]
}

Include Step Metadata in Outputs

{
  "result": { /* actual output */ },
  "metadata": {
    "step_id": "analyze",
    "model": "gpt-4",
    "tokens_used": 850,
    "processing_time_ms": 2100
  }
}

Principle 5: Graceful Degradation

Plan for Partial Success

Not every step needs to fully succeed for the chain to be useful:

const results = {
  extraction: { status: "success", data: {...} },
  enrichment: { status: "partial", data: {...}, missing: ["company_info"] },
  analysis: { status: "success", data: {...} }  // Works with partial enrichment
};

Provide Fallbacks

Principle 6: Modularity

Design for Reuse

Steps that might be useful elsewhere should be self-contained:

// Reusable step - no external dependencies
const summarizeStep = {
  name: "summarize",
  input: "text to summarize",
  output: "summary",
  config: {
    maxLength: 100,
    style: "bullet_points"
  }
};

// Can be used in multiple chains
const researchChain = [fetchStep, summarizeStep, compileStep];
const newsChain = [scrapeStep, summarizeStep, publishStep];

Keep Steps Stateless

Steps should not depend on external state:

Bad: Step relies on global variable or external database Good: Step receives all needed data as input

Design Patterns Comparison

Pattern	Use When	Avoid When
Linear Chain	Steps naturally sequential	High parallelism possible
Parallel Fan-out	Independent sub-tasks	Order matters
Conditional Branch	Different paths needed	Always same path
Loop with Exit	Iterative refinement	Fixed number of iterations

Exercise: Apply Design Principles

Review this chain design and identify violations of the principles:

Expected issues:

1. Step 1 violates Single Responsibility (does 6 things)
2. No validation between steps
3. No logging/metadata mentioned
4. Database and email operations make it stateful

Key Takeaways

• Single Responsibility: Each step does one thing well
• Fail Fast: Validate early and between steps
• Idempotency: Same input → same output
• Observability: Log meaningful data at each step
• Graceful Degradation: Plan for partial success
• Modularity: Design reusable, stateless steps

Next, we'll look at how to define clear interfaces between your chain steps.