name	LAYER_06_API
description	Expert knowledge for API Layer modeling in Documentation Robotics
triggers	OpenAPI, REST API, endpoint, operation, swagger, API specification, HTTP method, request, response
version	0.6.0

API Layer Skill

Layer Number: 06 Specification: Metadata Model Spec v0.6.0 Purpose: Defines REST API contracts using OpenAPI 3.0, specifying endpoints, operations, request/response schemas, and security requirements.

Layer Overview

The API Layer captures API contracts:

OPERATIONS - HTTP methods on paths (GET, POST, PUT, DELETE, PATCH)
SCHEMAS - Request/response data structures
SECURITY - Authentication and authorization schemes
DOCUMENTATION - API metadata, descriptions, examples
INTEGRATION - Links to business services, application services, data models

This layer uses OpenAPI 3.0.3 (de facto industry standard) with custom extensions for cross-layer traceability.

Central Entity: The Operation (HTTP method on a path) is the core modeling unit.

Entity Types

Core OpenAPI Entities (13 entities)

Entity Type	Description
OpenAPIDocument	Root of an OpenAPI specification file (version 3.0.3)
Info	Metadata about the API (title, description, version, contact, license)
Server	Server where the API is available (with URL and variables)
Paths	Available API endpoints and operations
PathItem	Operations available on a path
Operation	Single API operation (HTTP method on a path) - CENTRAL ENTITY
Parameter	Parameter for an operation (locations: query, header, path, cookie)
RequestBody	Request payload for an operation
Responses	Possible responses from an operation
Response	Single response definition with status code
MediaType	Media type and schema for request/response body
Components	Reusable component definitions (schemas, responses, parameters, examples, security schemes)
Schema	Data type definition (JSON Schema subset)

Metadata Entities (11 entities)

Entity Type	Description
Tag	Metadata label for grouping operations
ExternalDocumentation	Reference to external documentation
Contact	Contact information for API owner
License	Legal license for API
ServerVariable	Variable placeholder in server URL templates
Header	HTTP header parameters for requests/responses
Link	Relationship between API responses and subsequent operations (HATEOAS)
Callback	Webhook or callback URL pattern
Example	Sample values for documentation and testing
Encoding	Serialization details for multipart content
SecurityScheme	Security mechanism (types: apiKey, http, oauth2, openIdConnect)

Supporting Entities (3 entities)

Entity Type	Description
OAuthFlows	Configuration for OAuth 2.0 flows
Condition	Logical expression for policy evaluation
RetentionPolicy	Data retention policies
ValidationRule	Data validation constraints

Intra-Layer Relationships

Composition Relationships (Part cannot exist without whole)

Source	Predicate	Target	Example
OpenAPIDocument	composes	Info	Document has metadata
OpenAPIDocument	composes	Paths	Document defines endpoints
OpenAPIDocument	composes	Components	Document has reusable components
Paths	composes	PathItem	Paths contain path items
PathItem	composes	Operation	Path has HTTP methods
PathItem	composes	Parameter	Path-level parameters
Operation	composes	Parameter	Operation-specific parameters
Operation	composes	RequestBody	Request payload definition
Operation	composes	Responses	Response definitions
Responses	composes	Response	Individual status responses
RequestBody	composes	MediaType	Request content types
Response	composes	MediaType	Response content types
Response	composes	Header	Response headers
Response	composes	Link	HATEOAS links
MediaType	composes	Schema	Data structure
MediaType	composes	Example	Sample data
MediaType	composes	Encoding	Serialization details
Components	composes	Schema	Reusable schemas
Components	composes	Response	Reusable responses
Components	composes	Parameter	Reusable parameters
Components	composes	Example	Reusable examples
Components	composes	RequestBody	Reusable request bodies
Components	composes	Header	Reusable headers
Components	composes	SecurityScheme	Security definitions
Components	composes	Link	Reusable links
Components	composes	Callback	Reusable callbacks
Info	composes	Contact	API owner contact
Info	composes	License	API license
SecurityScheme	composes	OAuthFlows	OAuth2 configuration

Aggregation Relationships (Part can exist independently)

Source	Predicate	Target	Example
OpenAPIDocument	aggregates	Server	API deployment servers
OpenAPIDocument	aggregates	Tag	Operation tags
OpenAPIDocument	aggregates	SecurityRequirement	Global security
Server	aggregates	ServerVariable	URL template variables
PathItem	aggregates	Parameter	Shared parameters
Operation	aggregates	Callback	Webhooks
Operation	aggregates	SecurityRequirement	Operation-level security

Reference Relationships

Source	Predicate	Target	Example
Schema	references	Schema	Schema $ref to another schema
Parameter	references	Schema	Parameter uses schema
Header	references	Schema	Header uses schema
Link	references	Operation	Link points to operation (operationId)
Callback	references	PathItem	Callback references path definition
Operation	references	Tag	Operation tagged for grouping
Tag	references	ExternalDocumentation	Tag links to external docs
OpenAPIDocument	references	ExternalDocumentation	Document links to external docs
Encoding	references	Header	Encoding uses headers

Specialization Relationships

Source	Predicate	Target	Example
Schema	specializes	Schema	Schema inheritance (allOf, oneOf, anyOf)

Behavioral Relationships

Source	Predicate	Target	Example
Operation	triggers	Callback	Operation invokes webhook
SecurityScheme	serves	Operation	Security scheme protects operation

Association Relationships

Source	Predicate	Target	Example
Contact	associated-with	OpenAPIDocument	Contact info for API
License	associated-with	OpenAPIDocument	Legal license

Cross-Layer References

Outgoing References (API → Other Layers)

OpenAPI specification includes custom extensions (x-* properties) for cross-layer traceability:

Target Layer	Extension Property	Example
Layer 1 (Motivation)	`x-supports-goals`	Operation supports business goals
Layer 1 (Motivation)	`x-fulfills-requirements`	Operation fulfills functional requirements
Layer 1 (Motivation)	`x-governed-by-principles`	Operation follows architectural principles
Layer 1 (Motivation)	`x-constrained-by`	Operation subject to constraints (GDPR, HIPAA, SOX)
Layer 2 (Business)	`x-business-service-ref`	Operation realizes business service
Layer 2 (Business)	`x-business-interface-ref`	Operation exposed via business interface
Layer 4 (Application)	`x-archimate-ref`	OpenAPI document realizes ApplicationService
Layer 7 (Data Model)	`schema.$ref`	Schema references JSON Schema definition
Layer 3 (Security)	`x-security-resource`	Operation protected by SecureResource
Layer 3 (Security)	`x-required-permissions`	Operation requires specific permissions
Layer 3 (Security)	`x-rate-limit`	Rate limiting configuration
Layer 11 (APM)	`x-apm-business-metrics`	Operation tracked by business metrics
Layer 11 (APM)	`x-apm-sla-target-latency`	Expected response time (e.g., "100ms")
Layer 11 (APM)	`x-apm-sla-target-availability`	Expected availability (e.g., "99.9%")
Layer 11 (APM)	`x-apm-trace`	Distributed tracing enabled
Layer 11 (APM)	`x-apm-criticality`	Business criticality (critical, high, medium, low)

Incoming References (Lower Layers → API)

Lower layers reference API layer to show implementation and data structure.

Codebase Detection Patterns

Pattern 1: FastAPI Python

from fastapi import FastAPI, HTTPException
from pydantic import BaseModel

app = FastAPI(
    title="User Management API",  # OpenAPIDocument: Info.title
    description="API for managing user accounts",  # Info.description
    version="1.0.0"  # Info.version
)

class UserCreateRequest(BaseModel):  # Schema (RequestBody)
    username: str
    email: str
    full_name: str

class UserResponse(BaseModel):  # Schema (Response)
    user_id: str
    username: str
    email: str
    created_at: datetime

@app.post(
    "/api/users",  # PathItem + Operation (POST)
    response_model=UserResponse,  # Response schema
    status_code=201,  # Response status
    tags=["Users"],  # Tag
    summary="Create a new user",  # Operation.summary
    description="Creates a new user account with the provided details"  # Operation.description
)
async def create_user(user: UserCreateRequest) -> UserResponse:  # RequestBody + Response
    """
    x-business-service-ref: business/service/user-management
    x-apm-sla-target-latency: 200ms
    x-required-permissions: users.write
    """
    pass

Maps to:

OpenAPIDocument: "User Management API"
PathItem: "/api/users"
Operation: "POST /api/users"
RequestBody: MediaType (application/json) with Schema (UserCreateRequest)
Response: 201 with Schema (UserResponse)
Tag: "Users"

Pattern 2: Express.js TypeScript

import express from "express";
import { body, param, query, validationResult } from "express-validator";

const router = express.Router();

/**
 * @openapi
 * /api/orders/{orderId}:
 *   get:
 *     summary: Get order by ID
 *     description: Retrieves a single order by its unique identifier
 *     tags:
 *       - Orders
 *     parameters:
 *       - name: orderId
 *         in: path
 *         required: true
 *         schema:
 *           type: string
 *           format: uuid
 *     responses:
 *       200:
 *         description: Order found
 *         content:
 *           application/json:
 *             schema:
 *               $ref: '#/components/schemas/Order'
 *       404:
 *         description: Order not found
 *     x-business-service-ref: business/service/order-management
 *     x-apm-sla-target-latency: 100ms
 *     x-apm-criticality: high
 */
router.get("/api/orders/:orderId", param("orderId").isUUID(), async (req, res) => {
  // Implementation
});

Maps to:

PathItem: "/api/orders/{orderId}"
Operation: "GET /api/orders/{orderId}"
Parameter: "orderId" (in: path, type: string, format: uuid)
Response: 200 with schema reference
Response: 404 error response
Custom extensions: x-business-service-ref, x-apm-sla-target-latency, x-apm-criticality

Pattern 3: Spring Boot Java

@RestController
@RequestMapping("/api/products")
@Tag(name = "Products", description = "Product management operations")
public class ProductController {

    @Operation(
        summary = "List products",
        description = "Returns a paginated list of products",
        extensions = {
            @Extension(name = "x-apm-sla-target-latency", properties = @ExtensionProperty(name = "latency", value = "150ms")),
            @Extension(name = "x-required-permissions", properties = @ExtensionProperty(name = "permissions", value = "products.read"))
        }
    )
    @ApiResponses({
        @ApiResponse(
            responseCode = "200",
            description = "Products retrieved successfully",
            content = @Content(
                mediaType = "application/json",
                schema = @Schema(implementation = ProductListResponse.class)
            )
        )
    })
    @GetMapping
    public ResponseEntity<ProductListResponse> listProducts(
        @Parameter(description = "Page number", example = "1") @RequestParam(defaultValue = "1") int page,
        @Parameter(description = "Page size", example = "20") @RequestParam(defaultValue = "20") int size
    ) {
        // Implementation
    }
}

Maps to:

PathItem: "/api/products"
Operation: "GET /api/products"
Parameters: "page" (query), "size" (query)
Response: 200 with ProductListResponse schema
Tag: "Products"
Custom extensions

Pattern 4: OpenAPI YAML Definition

openapi: 3.0.3
info:
  title: Payment Processing API
  description: API for processing customer payments
  version: 2.1.0
  contact:
    name: API Support
    email: api-support@example.com
  license:
    name: MIT
    url: https://opensource.org/licenses/MIT
  x-governed-by-principles:
    - motivation/principle/api-first-design
    - motivation/principle/security-by-design

servers:
  - url: https://api.example.com/v2
    description: Production server
  - url: https://staging-api.example.com/v2
    description: Staging server

paths:
  /payments:
    post:
      summary: Process payment
      description: Processes a payment transaction
      operationId: processPayment
      tags:
        - Payments
      security:
        - oauth2: [payments.write]
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: "#/components/schemas/PaymentRequest"
            examples:
              credit-card:
                $ref: "#/components/examples/CreditCardPayment"
      responses:
        "201":
          description: Payment processed successfully
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/PaymentResponse"
        "400":
          $ref: "#/components/responses/BadRequest"
        "401":
          $ref: "#/components/responses/Unauthorized"
      x-business-service-ref: business/service/payment-processing
      x-apm-sla-target-latency: 500ms
      x-apm-sla-target-availability: 99.99%
      x-apm-criticality: critical
      x-required-permissions:
        - payments.write
      x-rate-limit:
        requests: 100
        window: 60s

components:
  schemas:
    PaymentRequest:
      type: object
      required:
        - amount
        - currency
        - payment_method
      properties:
        amount:
          type: number
          format: double
          minimum: 0.01
          example: 99.99
        currency:
          type: string
          enum: [USD, EUR, GBP]
          example: USD
        payment_method:
          $ref: "#/components/schemas/PaymentMethod"

    PaymentResponse:
      type: object
      properties:
        transaction_id:
          type: string
          format: uuid
        status:
          type: string
          enum: [pending, completed, failed]
        timestamp:
          type: string
          format: date-time

  securitySchemes:
    oauth2:
      type: oauth2
      flows:
        authorizationCode:
          authorizationUrl: https://auth.example.com/oauth/authorize
          tokenUrl: https://auth.example.com/oauth/token
          scopes:
            payments.read: Read payment information
            payments.write: Process payments

  responses:
    BadRequest:
      description: Invalid request
      content:
        application/json:
          schema:
            $ref: "#/components/schemas/Error"

  examples:
    CreditCardPayment:
      value:
        amount: 99.99
        currency: USD
        payment_method:
          type: credit_card
          card_number: "4111111111111111"

Modeling Workflow

Step 1: Create OpenAPI Document

# Create the API specification document
dr add api document "payment-api" \
  --properties version=3.0.3,title="Payment Processing API",api-version=2.1.0 \
  --description "API for processing customer payments"

# Add API metadata
dr add api info "payment-api-info" \
  --properties title="Payment Processing API",version=2.1.0,contact-email=api@example.com \
  --description "API metadata and contact information"

# Link to motivation layer
dr relationship add "api/document/payment-api" \
  governed-by "motivation/principle/api-first-design"

Step 2: Define Servers

# Production server
dr add api server "production-server" \
  --properties url=https://api.example.com/v2,description="Production server" \
  --description "Production API server"

# Staging server
dr add api server "staging-server" \
  --properties url=https://staging-api.example.com/v2,description="Staging server" \
  --description "Staging API server"

Step 3: Define Security Schemes

# OAuth2 security
dr add api security-scheme "oauth2-auth" \
  --properties type=oauth2,flow=authorizationCode,authorization-url=https://auth.example.com/oauth/authorize,token-url=https://auth.example.com/oauth/token \
  --description "OAuth2 authorization code flow"

# API Key security
dr add api security-scheme "api-key-auth" \
  --properties type=apiKey,in=header,name=X-API-Key \
  --description "API key authentication"

Step 4: Define Schemas (Data Models)

# Request schema
dr add api schema "payment-request" \
  --properties type=object,required=amount:currency:payment_method \
  --description "Payment request payload"

# Response schema
dr add api schema "payment-response" \
  --properties type=object \
  --description "Payment processing response"

# Link to data model layer
dr relationship add "api/schema/payment-request" \
  references "data-model/schema/payment-request"

Step 5: Define Operations (Core Entities)

# POST operation
dr add api operation "process-payment" \
  --properties path=/payments,method=POST,operation-id=processPayment \
  --description "Processes a payment transaction"

# Add custom extensions for cross-layer traceability
dr add api operation "process-payment" \
  --properties x-business-service-ref=business/service/payment-processing,x-apm-sla-target-latency=500ms,x-apm-sla-target-availability=99.99%,x-apm-criticality=critical,x-required-permissions=payments.write

# GET operation
dr add api operation "get-payment" \
  --properties path=/payments/{paymentId},method=GET,operation-id=getPayment \
  --description "Retrieves payment details by ID"

Step 6: Define Parameters

# Path parameter
dr add api parameter "payment-id-param" \
  --properties name=paymentId,in=path,required=true,type=string,format=uuid \
  --description "Payment transaction ID"

# Query parameter
dr add api parameter "status-filter" \
  --properties name=status,in=query,required=false,type=string,enum=pending:completed:failed \
  --description "Filter payments by status"

# Link parameter to operation
dr relationship add "api/operation/get-payment" \
  has-parameter "api/parameter/payment-id-param"

Step 7: Define Request/Response Bodies

# Request body
dr add api request-body "payment-request-body" \
  --properties required=true,content-type=application/json \
  --description "Payment request payload"

# Link request body to schema
dr relationship add "api/request-body/payment-request-body" \
  uses-schema "api/schema/payment-request"

# Response
dr add api response "payment-success-response" \
  --properties status-code=201,description="Payment processed successfully" \
  --description "Successful payment response"

# Link response to schema
dr relationship add "api/response/payment-success-response" \
  uses-schema "api/schema/payment-response"

Step 8: Define Tags for Organization

# Add tags
dr add api tag "payments" \
  --properties name=Payments \
  --description "Payment processing operations"

dr add api tag "refunds" \
  --properties name=Refunds \
  --description "Refund operations"

# Tag operations
dr relationship add "api/operation/process-payment" \
  tagged-with "api/tag/payments"

Step 9: Cross-Layer Integration

# Link to business layer
dr relationship add "api/operation/process-payment" \
  realizes "business/service/payment-processing"

# Link to application layer
dr relationship add "api/document/payment-api" \
  realizes "application/service/payment-api"

# Link to security layer
dr relationship add "api/operation/process-payment" \
  protected-by "security/secure-resource/payment-api"

# Link to motivation layer
dr relationship add "api/operation/process-payment" \
  supports "motivation/goal/reduce-checkout-time"

# Link to APM layer
dr relationship add "api/operation/process-payment" \
  tracked-by "apm/metric/payment-processing-latency"

Step 10: Validate and Export

# Validate API layer
dr validate --layer api

# Export to OpenAPI YAML
dr export openapi --output payment-api.yaml

# Validate exported OpenAPI with external tools
spectral lint payment-api.yaml

Operation-Level SLA Patterns

Different operations have different SLA targets:

# Search operation: fast response
x-apm-sla-target-latency: "50ms"
x-apm-sla-target-availability: "99.9%"
x-apm-criticality: "high"

# Write operation: moderate latency
x-apm-sla-target-latency: "200ms"
x-apm-sla-target-availability: "99.95%"
x-apm-criticality: "critical"

# Batch operation: longer latency acceptable
x-apm-sla-target-latency: "10s"
x-apm-sla-target-availability: "99.5%"
x-apm-criticality: "medium"

# Reporting: can be slower
x-apm-sla-target-latency: "5s"
x-apm-sla-target-availability: "99%"
x-apm-criticality: "low"

Best Practices

Operation is the Central Entity - Model operations, not just paths
Use OpenAPI 3.0.3 - Latest stable version with broad tooling support
Tag Operations - Organize operations by domain or resource
Define Reusable Components - Schemas, responses, parameters in Components section
Document Examples - Include request/response examples for testing and documentation
Security at Operation Level - Different operations can have different security requirements
Link to Business Services - Use x-business-service-ref for traceability
Define SLA Targets - Use x-apm-sla-target-* for monitoring
Version APIs Properly - Use semantic versioning (major.minor.patch)
Generate from Code - Use FastAPI, NestJS decorators, or Springdoc to auto-generate OpenAPI specs

OpenAPI Tooling Ecosystem

Generation Tools

FastAPI (Python) - Auto-generates OpenAPI from Python decorators
NestJS (TypeScript) - Swagger module for OpenAPI generation
Springdoc (Java) - OpenAPI 3 for Spring Boot applications
Express + swagger-jsdoc (Node.js) - Generate from JSDoc comments

Validation Tools

Spectral - OpenAPI linter and validator
openapi-validator - IBM's OpenAPI validator
swagger-cli - Validate and bundle OpenAPI specs

Documentation Tools

Swagger UI - Interactive API documentation
Redoc - Clean, customizable API documentation
Postman - Import OpenAPI for API testing

Code Generation Tools

OpenAPI Generator - Generate client SDKs and server stubs
swagger-codegen - Legacy code generation tool
oapi-codegen (Go) - Generate Go server/client from OpenAPI

Validation Tips

Issue	Cause	Fix
Missing Operations	Paths defined but no operations	Add HTTP methods (GET, POST, etc.)
Unlinked Schemas	Schemas not referenced by operations	Link schemas to request/response bodies
Missing Security	Operations lack security requirements	Add securitySchemes and apply to operations
No Cross-Layer Links	API not linked to business/application	Add x-business-service-ref and x-archimate-ref
Missing SLA Targets	Operations lack performance targets	Add x-apm-sla-target-* extensions
Untagged Operations	Operations not organized by tags	Add tags for grouping
No Examples	Schemas lack examples	Add example values for documentation
Invalid OpenAPI	Spec doesn't validate	Use Spectral or openapi-validator

Quick Reference

Add Commands:

dr add api document <name> --properties version=3.0.3,title=<title>
dr add api operation <name> --properties path=<path>,method=<method>
dr add api schema <name> --properties type=<type>
dr add api parameter <name> --properties name=<name>,in=<location>
dr add api security-scheme <name> --properties type=<type>
dr add api tag <name> --properties name=<display-name>

Relationship Commands:

dr relationship add <operation> uses-schema <schema>
dr relationship add <operation> has-parameter <parameter>
dr relationship add <operation> tagged-with <tag>
dr relationship add <schema> references <schema>

Cross-Layer Commands:

dr relationship add api/<operation> realizes business/<service>
dr relationship add api/<document> realizes application/<service>
dr relationship add api/<schema> references data-model/<schema>
dr relationship add api/<operation> protected-by security/<resource>
dr relationship add api/<operation> supports motivation/<goal>

Export Commands:

dr export openapi --output api-spec.yaml
dr export openapi --layer api --format json --output api-spec.json

Validation Commands:

dr validate --layer api
spectral lint api-spec.yaml
swagger-cli validate api-spec.yaml

Custom Extension Reference

Documentation Robotics defines custom OpenAPI extensions for cross-layer traceability:

# Motivation Layer Links
x-supports-goals: [motivation/goal/id1, motivation/goal/id2]
x-fulfills-requirements: [motivation/requirement/id1]
x-governed-by-principles: [motivation/principle/id1]
x-constrained-by: [motivation/constraint/id1]

# Business Layer Links
x-business-service-ref: business/service/id
x-business-interface-ref: business/interface/id

# Application Layer Links
x-archimate-ref: application/service/id

# Security Layer Links
x-security-resource: security/secure-resource/id
x-required-permissions: [users.read, users.write]
x-rate-limit:
  requests: 100
  window: 60s

# APM Layer Links
x-apm-business-metrics: [apm/metric/id1]
x-apm-sla-target-latency: "100ms"
x-apm-sla-target-availability: "99.9%"
x-apm-trace: true
x-apm-criticality: "high" # critical, high, medium, low

These extensions enable full traceability from API operations to business goals, requirements, security controls, and monitoring metrics.

Summary

The API Layer is the contract layer - it defines HOW external consumers interact with your system. Every operation should:

Be linked to a business service (traceability)
Have clear request/response schemas
Define security requirements
Specify SLA targets for monitoring
Include examples for documentation and testing

Use OpenAPI 3.0.3 as the foundation, leverage auto-generation tools from your framework, and extend with custom properties for full cross-layer integration.

Install Skill

SKILL.md

API Layer Skill

Layer Overview

Entity Types

Core OpenAPI Entities (13 entities)

Metadata Entities (11 entities)

Supporting Entities (3 entities)

Intra-Layer Relationships

Composition Relationships (Part cannot exist without whole)

Aggregation Relationships (Part can exist independently)

Reference Relationships

Specialization Relationships

Behavioral Relationships

Association Relationships

Cross-Layer References

Outgoing References (API → Other Layers)

Incoming References (Lower Layers → API)

Codebase Detection Patterns

Pattern 1: FastAPI Python

Pattern 2: Express.js TypeScript

Pattern 3: Spring Boot Java

Pattern 4: OpenAPI YAML Definition

Modeling Workflow

Step 1: Create OpenAPI Document

Step 2: Define Servers

Step 3: Define Security Schemes

Step 4: Define Schemas (Data Models)

Step 5: Define Operations (Core Entities)

Step 6: Define Parameters

Step 7: Define Request/Response Bodies

Step 8: Define Tags for Organization

Step 9: Cross-Layer Integration

Step 10: Validate and Export

Operation-Level SLA Patterns

Best Practices

OpenAPI Tooling Ecosystem

Generation Tools

Validation Tools

Documentation Tools

Code Generation Tools

Validation Tips

Quick Reference

Custom Extension Reference

Summary