@syncafricabs/kernspark-fastify

Fastify adapter for the SyncAfrica KernSpark ecosystem. This package provides production-ready Fastify error handlers, response utilities, and plugins that integrate the framework-independent @syncafricabs/kernspark-core with Fastify.

Table of Contents

• What is it?
• Why it exists
• Features
• Installation
• Quick Start
• Comprehensive Usage
  • Error Handling
  • Response Adapter
  • Plugin Registration
• API Reference
• Error Handling Reference
• TypeScript Support
• Compatibility Matrix
• Contributing
• License

What is it?

@syncafricabs/kernspark-fastify is a framework adapter that bridges the SyncAfrica KernSpark Core with Fastify. It provides:

• FastifyErrorHandler - Fastify error handler that maps ApplicationError instances to standardized JSON API responses
• FastifyResponseAdapter - Helper methods for sending consistent success and error responses
• FastifyPlugin - Fastify plugin that hooks into request/response lifecycle, provides correlation ID propagation, and logging

The adapter follows the adapter pattern: it depends on @syncafricabs/kernspark-core (the framework-independent layer) and Fastify only in the adapter layer. The core has zero framework dependencies.

Why it exists

Modern microservice architectures benefit from a KernSpark - a common set of contracts, types, and behaviors shared across bounded contexts. The SyncAfrica KernSpark provides:

1. Standardized API envelopes (ApiSuccess / ApiError)
2. Rich domain error hierarchy (ValidationError, BusinessError, AuthenticationError, etc.)
3. Result types (Result, Ok, Err)
4. Domain primitives (UUID, Money, Entity, ValueObject)

Without adapters, each team would reimplement framework-specific plumbing to use these core types. This adapter eliminates that duplication by providing Fastify-specific bindings.

Features

• Zero-configuration error handling - Drop-in Fastify error handler
• Standardized JSON responses - All responses follow the ApiSuccess / ApiError envelope
• Plugin-based architecture - Register once, works everywhere
• Correlation ID propagation - Automatic x-correlation-id header management
• Request/response logging - Configurable structured logging with sanitization
• Full TypeScript support - Comprehensive type definitions included
• Production-ready - Proper error handling, stack traces, cause chains
• Framework isolation - Core has no Fastify dependencies

Installation

npm install @syncafricabs/kernspark-fastify @syncafricabs/kernspark-core fastify

Or with your preferred package manager:

yarn add @syncafricabs/kernspark-fastify @syncafricabs/kernspark-core fastify

Quick Start

import Fastify from 'fastify';
import { fastifyPlugin, fastifyErrorHandler } from '@syncafricabs/kernspark-fastify';
import { ValidationError, NotFoundError } from '@syncafricabs/kernspark-core';

const app = Fastify({ logger: true });

await app.register(fastifyPlugin);

app.get('/users/:id', async (request, reply) => {
  const user = findUser(request.params.id);
  if (!user) {
    throw new NotFoundError('User not found');
  }
  return FastifyResponseAdapter.ok(reply, user, 'User retrieved successfully');
});

app.setErrorHandler(fastifyErrorHandler());

await app.listen({ port: 3000 });

Comprehensive Usage

Error Handling

The FastifyErrorHandler is a Fastify error handler that catches ApplicationError instances and converts them to standardized JSON responses.

Basic Setup

import Fastify from 'fastify';
import { fastifyPlugin, fastifyErrorHandler, FastifyResponseAdapter } from '@syncafricabs/kernspark-fastify';

const app = Fastify({ logger: true });

await app.register(fastifyPlugin);

app.get('/users/:id', async (request, reply) => {
  const user = findUser(request.params.id);
  if (!user) {
    throw new NotFoundError('User not found');
  }
  return FastifyResponseAdapter.ok(reply, user);
});

app.setErrorHandler(fastifyErrorHandler());

await app.listen({ port: 3000 });

Custom Error Handler Configuration

import { createFastifyErrorHandler, FastifyErrorHandlerOptions } from '@syncafricabs/kernspark-fastify';

const options: FastifyErrorHandlerOptions = {
  logErrors: true,
  logStack: true,
  includeCause: true,
  includeStack: process.env.NODE_ENV === 'development',
  defaultStatusCode: 500,
  logger: (error, request) => {
    console.error({
      correlationId: request.correlationId,
      method: request.method,
      url: request.url,
      error: error.message,
      stack: error.stack,
    });
  },
};

app.setErrorHandler(createFastifyErrorHandler(options));

Throwing Errors from Route Handlers

import {
  ValidationError,
  MissingFieldsError,
  BusinessError,
  ConflictError,
  NotFoundError,
  AuthenticationError,
  InvalidTokenError,
  AuthorizationError,
  PermissionDeniedError,
  InfrastructureError,
  ExternalServiceError,
} from '@syncafricabs/kernspark-core';

app.post('/users', async (request, reply) => {
  const { name, email } = request.body as any;

  if (!name || !email) {
    throw new MissingFieldsError('Name and email are required');
  }

  if (!isValidEmail(email)) {
    throw new ValidationError('INVALID_EMAIL', 'Email format is invalid');
  }

  if (userExists(email)) {
    throw new ConflictError('User with this email already exists');
  }

  const user = createUser(request.body);
  return FastifyResponseAdapter.created(reply, user, 'User created successfully');
});

app.get('/users/:id', async (request, reply) => {
  const user = findUser(request.params.id);
  if (!user) {
    throw new NotFoundError('User not found');
  }
  return FastifyResponseAdapter.ok(reply, user);
});

app.delete('/users/:id', async (request, reply) => {
  if (!request.user) {
    throw new InvalidTokenError('Invalid or expired token');
  }

  if (!request.user.canDeleteUsers) {
    throw new PermissionDeniedError('You do not have permission to delete users');
  }

  deleteUser(request.params.id);
  return FastifyResponseAdapter.noContent(reply);
});

Response Adapter

The FastifyResponseAdapter provides static helper methods for sending responses.

Success Responses

import { FastifyResponseAdapter } from '@syncafricabs/kernspark-fastify';

// 200 OK with data
app.get('/users/:id', async (request, reply) => {
  const user = findUser(request.params.id);
  FastifyResponseAdapter.ok(reply, user, 'User retrieved');
});

// 201 Created with data
app.post('/users', async (request, reply) => {
  const user = createUser(request.body);
  FastifyResponseAdapter.created(reply, user, 'User created');
});

// 204 No Content
app.delete('/users/:id', async (request, reply) => {
  deleteUser(request.params.id);
  FastifyResponseAdapter.noContent(reply);
});

// Send raw ApiSuccess
app.get('/users', async (request, reply) => {
  const users = getAllUsers();
  const success = {
    status: 200,
    success: true,
    data: users,
    message: 'Users retrieved',
  };
  FastifyResponseAdapter.sendSuccess(reply, success);
});

Error Responses

import { FastifyResponseAdapter } from '@syncafricabs/kernspark-fastify';

app.post('/login', async (request, reply) => {
  const { email, password } = request.body as any;

  if (!email || !password) {
    return FastifyResponseAdapter.badRequest(reply, 'MISSING_FIELDS', 'Email and password are required');
  }

  const token = authenticate(email, password);
  if (!token) {
    return FastifyResponseAdapter.unauthorized(reply, 'INVALID_CREDENTIALS', 'Invalid email or password');
  }

  return FastifyResponseAdapter.ok(reply, { token }, 'Login successful');
});

Plugin Registration

The fastifyPlugin is a Fastify plugin that registers hooks and decorators for correlation ID, request/response logging, and global error handling.

Basic Plugin Registration

import Fastify from 'fastify';
import { fastifyPlugin } from '@syncafricabs/kernspark-fastify';

const app = Fastify({ logger: true });

await app.register(fastifyPlugin);

// Now correlationId is available on all requests
app.get('/health', async (request, reply) => {
  console.log(`Health check with correlation ID: ${request.correlationId}`);
  return { status: 'healthy' };
});

Custom Plugin Options

import { fastifyPlugin, FastifyPluginOptions } from '@syncafricabs/kernspark-fastify';

const options: FastifyPluginOptions = {
  correlationIdHeader: 'x-correlation-id',
  logRequests: true,
  logResponses: true,
  logErrors: true,
  sensitiveFields: ['password', 'token', 'secret', 'apiKey', 'creditCard', 'cvv'],
  logger: (meta) => {
    console.log(JSON.stringify(meta));
  },
};

await app.register(fastifyPlugin, options);

Plugin with Error Handler

import Fastify from 'fastify';
import { fastifyPlugin, fastifyErrorHandler, FastifyResponseAdapter } from '@syncafricabs/kernspark-fastify';

const app = Fastify({ logger: true });

await app.register(fastifyPlugin, {
  correlationIdHeader: 'x-request-id',
  logRequests: true,
  logResponses: true,
  sensitiveFields: ['password', 'secret'],
});

app.get('/users/:id', async (request, reply) => {
  const user = findUser(request.params.id);
  if (!user) {
    throw new NotFoundError('User not found');
  }
  return FastifyResponseAdapter.ok(reply, user);
});

app.setErrorHandler(fastifyErrorHandler({
  logErrors: true,
  includeStack: process.env.NODE_ENV === 'development',
}));

await app.listen({ port: 3000 });

API Reference

fastifyErrorHandler

Fastify error handler function. Catches all errors and maps them to standardized JSON responses.

function fastifyErrorHandler(options?: FastifyErrorHandlerOptions): (error: Error, request: FastifyRequest, reply: FastifyReply) => Promise<void>;

createFastifyErrorHandler(options?: FastifyErrorHandlerOptions)

Creates a customized error handler.

| Option | Type | Default | Description | |--------|------|---------|-------------| | logErrors | boolean | true | Enable/disable error logging | | logStack | boolean | false | Include stack traces in logs | | includeCause | boolean | true | Include error cause in response | | includeStack | boolean | false | Include stack traces in response (dev only) | | defaultStatusCode | number | 500 | Default status for unknown errors | | logger | function | console.error | Custom logger function |

FastifyResponseAdapter

Static utility class for sending responses.

| Method | HTTP Status | Description | |--------|-------------|-------------| | ok(reply, data?, message?) | 200 | Send success response with data | | created(reply, data?, message?) | 201 | Send created response with data | | noContent(reply, message?) | 204 | Send no content response | | sendSuccess(reply, apiSuccess) | varies | Send raw ApiSuccess | | sendError(reply, apiError) | varies | Send raw ApiError | | badRequest(reply, errorCode, message) | 400 | Send bad request error | | unauthorized(reply, errorCode, message) | 401 | Send unauthorized error | | forbidden(reply, errorCode, message) | 403 | Send forbidden error | | notFound(reply, errorCode, message) | 404 | Send not found error | | conflict(reply, errorCode, message) | 409 | Send conflict error | | unprocessableEntity(reply, errorCode, message) | 422 | Send unprocessable entity error | | tooManyRequests(reply, errorCode, message) | 429 | Send too many requests error | | internalServerError(reply, errorCode, message) | 500 | Send internal server error | | badGateway(reply, errorCode, message) | 502 | Send bad gateway error | | serviceUnavailable(reply, errorCode, message) | 503 | Send service unavailable error | | gatewayTimeout(reply, errorCode, message) | 504 | Send gateway timeout error |

fastifyPlugin

Fastify plugin for KernSpark functionality.

| Option | Type | Default | Description | |--------|------|---------|-------------| | correlationIdHeader | string | 'x-correlation-id' | Header name for correlation ID | | logRequests | boolean | true | Log incoming requests | | logResponses | boolean | true | Log outgoing responses | | logErrors | boolean | true | Log errors | | sensitiveFields | string[] | ['password', 'token', ...] | Body fields to redact | | logger | function | console.info | Custom logger function |

Decorators added:

• request.correlationId - Correlation ID for the request
• request.startTime - Request start timestamp

Error Handling Reference

The error handler maps the following ApplicationError subclasses:

| Error Class | HTTP Status | Error Code | |-------------|-------------|------------| | ValidationError | 400 | Custom (e.g., INVALID, MISSING_FIELDS) | | MissingFieldsError | 400 | MISSING_FIELDS | | BusinessError | Varies | Custom | | ConflictError | 409 | CONFLICT | | InsufficientFundsError | 400 | INSUFFICIENT_FUNDS | | QuotaExceededError | 429 | QUOTA_EXCEEDED | | NotFoundError | 404 | NOT_FOUND | | ExpiredError | 410 | EXPIRED | | TooManyRequestsError | 429 | TOO_MANY_REQUESTS | | PaymentRequiredError | 402 | PAYMENT_REQUIRED | | LockedError | 423 | LOCKED | | AccountSuspendedError | 403 | ACCOUNT_SUSPENDED | | FeatureNotAvailableError | 501 | FEATURE_NOT_AVAILABLE | | DataIntegrityError | 422 | DATA_INTEGRITY_ERROR | | AuthenticationError | 401 | Custom (e.g., INVALID_TOKEN) | | InvalidTokenError | 401 | INVALID_TOKEN | | TokenExpiredError | 401 | TOKEN_EXPIRED | | SessionExpiredError | 401 | SESSION_EXPIRED | | AuthorizationError | Varies | Custom | | PermissionDeniedError | 403 | PERMISSION_DENIED | | NotAllowedError | 403 | NOT_ALLOWED | | InfrastructureError | Varies | Custom | | ExternalServiceError | 502 | EXTERNAL_SERVICE_ERROR | | BadGatewayError | 502 | BAD_GATEWAY | | GatewayTimeoutError | 504 | GATEWAY_TIMEOUT | | ServiceUnavailableError | 503 | SERVICE_UNAVAILABLE | | RequestFailedError | 500 | REQUEST_FAILED | | NotImplementedError | 501 | NOT_IMPLEMENTED | | MaintenanceModeError | 503 | MAINTENANCE_MODE |

Unknown ApplicationError instances use their embedded statusCode. Non-ApplicationError errors return HTTP 500 with INTERNAL_SERVER_ERROR code.

TypeScript Support

Full TypeScript support with strict mode enabled. The adapter provides type augmentations for Fastify:

import 'fastify';
import { FastifyRequest, FastifyReply } from 'fastify';

// With the plugin registered, request.correlationId is typed
const handler = async (request: FastifyRequest, reply: FastifyReply) => {
  const correlationId = request.correlationId; // string
  console.log(`Request with correlation ID: ${correlationId}`);
};

Strict Mode

This package is compiled with strict: true. Enable strict mode in your project for full type safety:

{
  "compilerOptions": {
    "strict": true,
    "noImplicitAny": true,
    "strictNullChecks": true
  }
}

Compatibility Matrix

| Package Version | Node.js | Fastify | @syncafricabs/kernspark-core | |----------------|---------|---------|----------------------------------| | 1.0.0 | >=18.0.0 | ^4.0.0 | ^1.0.0 |

| Feature | Fastify 4.x | |---------|-------------| | Error handler | Supported | | Plugin registration | Supported | | Request/response hooks | Supported | | Decorators | Supported | | Correlation ID propagation | Supported | | TypeScript 5.0+ | Supported |

Contributing

1. Fork the repository
2. Create your feature branch: git checkout -b feature/my-feature
3. Commit your changes: git commit -am 'Add my feature'
4. Push to the branch: git push origin feature/my-feature
5. Submit a pull request

Development Setup

# Clone the repository
git clone https://github.com/iamprovy-dev/kernspark-js.git
cd kernspark/packages/kernspark-fastify

# Install dependencies
npm install

# Build
npm run build

# Run lint
npm run lint

# Run tests
npm test

Code Standards

• Use TypeScript with strict mode
• Follow the existing code style (2-space indentation, semicolons)
• Add JSDoc comments for public APIs
• Ensure all tests pass before submitting PR

License

Apache-2.0

Author

Providence Chikukwa

• Email: [email protected]
• GitHub: https://github.com/iamprovy-dev
• LinkedIn: https://www.linkedin.com/in/provychikukwa
• Organization: SyncAfrica Business Solutions