--- name: api-patterns description: Resend API integration patterns, authentication, error handling, and rate limiting. Use when implementing API clients, handling authentication, managing rate limits, implementing retry strategies, or building resilient email service integrations. allowed-tools: Read, Write, Bash, Grep --- # API Patterns Skill Comprehensive patterns and best practices for integrating with the Resend API, covering authentication, rate limiting, error handling, and resilient retry strategies. ## Use When - Implementing Resend API clients in TypeScript/JavaScript - Building Python-based email service integrations - Handling API authentication and Bearer tokens - Managing rate limits (2 requests/second default) - Implementing exponential backoff retry logic - Processing API error responses (4xx/5xx codes) - Building resilient API integrations with error recovery - Validating API responses and handling edge cases ## Core Patterns ### 1. API Authentication **Bearer token authentication** for all Resend API requests: #### TypeScript/JavaScript ```typescript import { Resend } from 'resend'; // Initialize with API key from environment const resend = new Resend(process.env.RESEND_API_KEY); // Or with explicit initialization const apiKey = process.env.RESEND_API_KEY; if (!apiKey) { throw new Error('RESEND_API_KEY environment variable is not set'); } const resend = new Resend(apiKey); // For custom HTTP requests (if needed) async function makeAuthenticatedRequest(endpoint: string, body: any) { const response = await fetch(`https://api.resend.com/${endpoint}`, { method: 'POST', headers: { 'Authorization': `Bearer ${process.env.RESEND_API_KEY}`, 'Content-Type': 'application/json', }, body: JSON.stringify(body), }); return response.json(); } ``` #### Python ```python import os from resend import Resend # Initialize with API key from environment api_key = os.environ.get("RESEND_API_KEY") if not api_key: raise ValueError("RESEND_API_KEY environment variable is not set") client = Resend(api_key=api_key) # For custom HTTP requests (if needed) import httpx async def make_authenticated_request(endpoint: str, body: dict): async with httpx.AsyncClient() as client: response = await client.post( f"https://api.resend.com/{endpoint}", headers={ "Authorization": f"Bearer {api_key}", "Content-Type": "application/json", }, json=body, ) return response.json() ``` ### 2. Rate Limiting Handling **Resend API rate limits**: 2 requests per second per account #### TypeScript Implementation ```typescript class ResendAPIClient { private requestQueue: Array<() => Promise> = []; private isProcessing = false; private readonly requestsPerSecond = 2; private lastRequestTime = 0; async executeWithRateLimit(fn: () => Promise): Promise { return new Promise((resolve, reject) => { this.requestQueue.push(async () => { try { const result = await fn(); resolve(result); } catch (error) { reject(error); } }); this.processQueue(); }); } private async processQueue() { if (this.isProcessing || this.requestQueue.length === 0) { return; } this.isProcessing = true; while (this.requestQueue.length > 0) { const now = Date.now(); const timeSinceLastRequest = now - this.lastRequestTime; const delayNeeded = (1000 / this.requestsPerSecond) - timeSinceLastRequest; if (delayNeeded > 0) { await new Promise(resolve => setTimeout(resolve, delayNeeded)); } const request = this.requestQueue.shift(); if (request) { await request(); this.lastRequestTime = Date.now(); } } this.isProcessing = false; } async sendEmail(payload: any) { return this.executeWithRateLimit(() => resend.emails.send(payload) ); } async sendBatch(emails: any[]) { return this.executeWithRateLimit(() => resend.batch.send(emails) ); } } ``` #### Python Implementation ```python import asyncio import time from typing import TypeVar, Callable, Coroutine, Any T = TypeVar('T') class ResendAPIClient: def __init__(self, api_key: str, requests_per_second: int = 2): self.client = Resend(api_key=api_key) self.request_queue = asyncio.Queue() self.is_processing = False self.requests_per_second = requests_per_second self.last_request_time = 0 async def execute_with_rate_limit( self, fn: Callable[[], Coroutine[Any, Any, T]] ) -> T: await self.request_queue.put(fn) asyncio.create_task(self.process_queue()) # Wait for result (simplified - in production use proper queuing) return await fn() async def process_queue(self): if self.is_processing or self.request_queue.empty(): return self.is_processing = True while not self.request_queue.empty(): now = time.time() time_since_last = now - self.last_request_time delay_needed = (1.0 / self.requests_per_second) - time_since_last if delay_needed > 0: await asyncio.sleep(delay_needed) try: fn = self.request_queue.get_nowait() await fn() self.last_request_time = time.time() except asyncio.QueueEmpty: break self.is_processing = False async def send_email(self, payload: dict): async def send(): return self.client.emails.send(payload) return await self.execute_with_rate_limit(send) ``` ### 3. Error Response Codes **HTTP status codes and error handling**: | Code | Error | Handling Strategy | |------|-------|-------------------| | 200 | Success | Process response normally | | 400 | Bad Request | Validate request payload, check required fields | | 401 | Unauthorized | Verify API key is correct and valid | | 403 | Forbidden | Check API key has required permissions | | 404 | Not Found | Verify resource ID/email address exists | | 409 | Conflict | Handle duplicate resource creation attempts | | 429 | Rate Limited | Implement exponential backoff retry | | 500 | Server Error | Retry with exponential backoff | | 502 | Bad Gateway | Retry with exponential backoff | | 503 | Service Unavailable | Retry with exponential backoff | #### TypeScript Error Handler ```typescript interface APIError { code: number; message: string; statusText: string; } async function handleAPIError(error: any): Promise { if (error.response) { const status = error.response.status; const data = error.response.data; switch (status) { case 400: console.error('Bad Request:', data.message); throw new Error(`Invalid request: ${data.message}`); case 401: console.error('Unauthorized: Check API key'); throw new Error('Invalid API key. Set RESEND_API_KEY environment variable.'); case 403: console.error('Forbidden: Insufficient permissions'); throw new Error('API key lacks required permissions.'); case 404: console.error('Not Found:', data.message); throw new Error(`Resource not found: ${data.message}`); case 409: console.error('Conflict: Resource already exists'); throw new Error(`Duplicate resource: ${data.message}`); case 429: console.warn('Rate limited: Implement retry'); throw new Error('Rate limit exceeded. Retry after delay.'); case 500: case 502: case 503: console.error(`Server error (${status}): Retry recommended`); throw new Error(`Server error (${status}). Retry in progress...`); default: console.error(`Unknown error (${status}):`, data); throw new Error(`API error: ${data.message || 'Unknown error'}`); } } throw error; } ``` #### Python Error Handler ```python import logging from typing import Optional, Dict, Any logger = logging.getLogger(__name__) class ResendAPIError(Exception): """Base exception for Resend API errors""" pass class AuthenticationError(ResendAPIError): """API authentication failed""" pass class RateLimitError(ResendAPIError): """Rate limit exceeded""" pass class ServerError(ResendAPIError): """Server-side error""" pass def handle_api_error(error: Exception, status_code: Optional[int] = None) -> None: """Handle Resend API errors based on status code""" if status_code == 400: logger.error(f"Bad Request: {str(error)}") raise ResendAPIError(f"Invalid request: {str(error)}") elif status_code == 401: logger.error("Unauthorized: Check API key") raise AuthenticationError( "Invalid API key. Set RESEND_API_KEY environment variable." ) elif status_code == 403: logger.error("Forbidden: Insufficient permissions") raise AuthenticationError("API key lacks required permissions.") elif status_code == 404: logger.error(f"Not Found: {str(error)}") raise ResendAPIError(f"Resource not found: {str(error)}") elif status_code == 409: logger.error("Conflict: Resource already exists") raise ResendAPIError(f"Duplicate resource: {str(error)}") elif status_code == 429: logger.warning("Rate limited: Implement retry") raise RateLimitError("Rate limit exceeded. Retry after delay.") elif status_code in [500, 502, 503]: logger.error(f"Server error ({status_code}): Retry recommended") raise ServerError(f"Server error ({status_code}). Retry in progress...") else: logger.error(f"Unknown error: {str(error)}") raise ResendAPIError(f"API error: {str(error)}") ``` ### 4. Exponential Backoff Retry Strategy **Resilient retry logic** with exponential backoff for transient failures: #### TypeScript Implementation ```typescript interface RetryOptions { maxRetries?: number; initialDelayMs?: number; maxDelayMs?: number; backoffMultiplier?: number; retryableStatusCodes?: number[]; } const DEFAULT_RETRY_OPTIONS: Required = { maxRetries: 5, initialDelayMs: 100, maxDelayMs: 30000, backoffMultiplier: 2, retryableStatusCodes: [408, 429, 500, 502, 503, 504], }; async function withExponentialBackoff( fn: () => Promise, options: RetryOptions = {} ): Promise { const config = { ...DEFAULT_RETRY_OPTIONS, ...options }; let lastError: Error | null = null; let delayMs = config.initialDelayMs; for (let attempt = 0; attempt <= config.maxRetries; attempt++) { try { return await fn(); } catch (error: any) { lastError = error; // Check if error is retryable const isRetryable = (error.response?.status && config.retryableStatusCodes.includes(error.response.status)) || error.code === 'ECONNREFUSED' || error.code === 'ETIMEDOUT'; if (!isRetryable || attempt === config.maxRetries) { throw error; } // Calculate delay with jitter const jitter = Math.random() * 0.1 * delayMs; const actualDelay = Math.min(delayMs + jitter, config.maxDelayMs); console.warn( `Attempt ${attempt + 1} failed. Retrying in ${actualDelay.toFixed(0)}ms...`, error.message ); await new Promise(resolve => setTimeout(resolve, actualDelay)); delayMs = Math.min(delayMs * config.backoffMultiplier, config.maxDelayMs); } } throw lastError || new Error('Retry loop exhausted'); } // Usage async function sendEmailWithRetry(payload: any) { return withExponentialBackoff( () => resend.emails.send(payload), { maxRetries: 5, initialDelayMs: 100, maxDelayMs: 10000, backoffMultiplier: 2, } ); } ``` #### Python Implementation ```python import asyncio import random import logging from typing import TypeVar, Callable, Coroutine, Optional, List from enum import Enum logger = logging.getLogger(__name__) T = TypeVar('T') class RetryStrategy(Enum): EXPONENTIAL = "exponential" LINEAR = "linear" async def with_exponential_backoff( fn: Callable[[], Coroutine], max_retries: int = 5, initial_delay_ms: float = 100, max_delay_ms: float = 30000, backoff_multiplier: float = 2, retryable_status_codes: Optional[List[int]] = None, ) -> T: """ Execute async function with exponential backoff retry Args: fn: Async function to execute max_retries: Maximum retry attempts initial_delay_ms: Initial delay in milliseconds max_delay_ms: Maximum delay in milliseconds backoff_multiplier: Multiplier for each retry retryable_status_codes: HTTP status codes to retry on Returns: Result from function call Raises: Exception: If all retries exhausted """ if retryable_status_codes is None: retryable_status_codes = [408, 429, 500, 502, 503, 504] last_error = None delay_ms = initial_delay_ms for attempt in range(max_retries + 1): try: return await fn() except Exception as error: last_error = error # Check if error is retryable is_retryable = ( (hasattr(error, 'status_code') and error.status_code in retryable_status_codes) or 'timeout' in str(error).lower() or 'connection' in str(error).lower() ) if not is_retryable or attempt == max_retries: raise error # Calculate delay with jitter jitter = random.random() * 0.1 * delay_ms actual_delay = min(delay_ms + jitter, max_delay_ms) logger.warning( f"Attempt {attempt + 1} failed. " f"Retrying in {actual_delay:.0f}ms... Error: {str(error)}" ) await asyncio.sleep(actual_delay / 1000) delay_ms = min(delay_ms * backoff_multiplier, max_delay_ms) if last_error: raise last_error raise Exception("Retry loop exhausted") ``` ### 5. Request Validation **Validate payloads before sending** to catch errors early: #### TypeScript Validator ```typescript interface EmailPayload { from: string; to: string | string[]; subject: string; html?: string; text?: string; cc?: string[]; bcc?: string[]; reply_to?: string; attachments?: Array<{ filename: string; content: Buffer | string }>; scheduled_at?: string; tags?: Array<{ name: string; value: string }>; } function validateEmailPayload(payload: any): { valid: boolean; errors: string[] } { const errors: string[] = []; // Check required fields if (!payload.from) { errors.push("'from' field is required"); } if (!payload.to) { errors.push("'to' field is required"); } if (!payload.subject) { errors.push("'subject' field is required"); } // Validate email addresses const emailRegex = /^[^\s@]+@[^\s@]+\.[^\s@]+$/; if (payload.from && !emailRegex.test(payload.from)) { errors.push(`Invalid 'from' email: ${payload.from}`); } const recipients = Array.isArray(payload.to) ? payload.to : [payload.to]; recipients.forEach((email: string) => { if (!emailRegex.test(email)) { errors.push(`Invalid recipient email: ${email}`); } }); // Validate optional email fields if (payload.reply_to && !emailRegex.test(payload.reply_to)) { errors.push(`Invalid 'reply_to' email: ${payload.reply_to}`); } if (payload.cc) { payload.cc.forEach((email: string) => { if (!emailRegex.test(email)) { errors.push(`Invalid 'cc' email: ${email}`); } }); } if (payload.bcc) { payload.bcc.forEach((email: string) => { if (!emailRegex.test(email)) { errors.push(`Invalid 'bcc' email: ${email}`); } }); } // Validate attachment content if (payload.attachments) { payload.attachments.forEach((att: any, index: number) => { if (!att.filename) { errors.push(`Attachment ${index} missing 'filename'`); } if (!att.content) { errors.push(`Attachment ${index} missing 'content'`); } }); } // Validate scheduled_at format if present if (payload.scheduled_at) { try { new Date(payload.scheduled_at); } catch { errors.push(`Invalid 'scheduled_at' format: ${payload.scheduled_at}`); } } return { valid: errors.length === 0, errors, }; } // Usage async function sendEmailSafely(payload: EmailPayload) { const validation = validateEmailPayload(payload); if (!validation.valid) { console.error('Validation errors:', validation.errors); throw new Error(`Payload validation failed: ${validation.errors.join(', ')}`); } return resend.emails.send(payload); } ``` #### Python Validator ```python import re from typing import Dict, List, Any, Tuple from datetime import datetime class EmailPayloadValidator: EMAIL_REGEX = r'^[^\s@]+@[^\s@]+\.[^\s@]+$' @staticmethod def validate_email(email: str) -> bool: """Validate email address format""" return re.match(EmailPayloadValidator.EMAIL_REGEX, email) is not None @staticmethod def validate_payload(payload: Dict[str, Any]) -> Tuple[bool, List[str]]: """ Validate email payload before sending Returns: Tuple of (is_valid, error_list) """ errors = [] # Check required fields if not payload.get('from'): errors.append("'from' field is required") if not payload.get('to'): errors.append("'to' field is required") if not payload.get('subject'): errors.append("'subject' field is required") # Validate email addresses if payload.get('from'): if not EmailPayloadValidator.validate_email(payload['from']): errors.append(f"Invalid 'from' email: {payload['from']}") # Validate recipients recipients = payload.get('to', []) if isinstance(recipients, str): recipients = [recipients] for email in recipients: if not EmailPayloadValidator.validate_email(email): errors.append(f"Invalid recipient email: {email}") # Validate optional email fields if payload.get('reply_to'): if not EmailPayloadValidator.validate_email(payload['reply_to']): errors.append(f"Invalid 'reply_to' email: {payload['reply_to']}") for cc_email in payload.get('cc', []): if not EmailPayloadValidator.validate_email(cc_email): errors.append(f"Invalid 'cc' email: {cc_email}") for bcc_email in payload.get('bcc', []): if not EmailPayloadValidator.validate_email(bcc_email): errors.append(f"Invalid 'bcc' email: {bcc_email}") # Validate attachments for i, attachment in enumerate(payload.get('attachments', [])): if not attachment.get('filename'): errors.append(f"Attachment {i} missing 'filename'") if not attachment.get('content'): errors.append(f"Attachment {i} missing 'content'") # Validate scheduled_at format if payload.get('scheduled_at'): try: datetime.fromisoformat( payload['scheduled_at'].replace('Z', '+00:00') ) except ValueError: errors.append(f"Invalid 'scheduled_at' format: {payload['scheduled_at']}") return len(errors) == 0, errors def send_email_safely(client, payload: Dict[str, Any]): """Send email with validation""" is_valid, errors = EmailPayloadValidator.validate_payload(payload) if not is_valid: raise ValueError(f"Payload validation failed: {', '.join(errors)}") return client.emails.send(payload) ``` ## Environment Setup ### Required Environment Variables ```bash # .env RESEND_API_KEY=your_resend_api_key_here ``` ### .env.example (Safe to Commit) ```bash # Resend API Configuration RESEND_API_KEY=your_resend_api_key_here # Optional: Custom request timeout (ms) RESEND_REQUEST_TIMEOUT=30000 # Optional: Max retries for transient failures RESEND_MAX_RETRIES=5 # Optional: Rate limit requests per second RESEND_RATE_LIMIT=2 ``` ### Installation #### TypeScript/JavaScript ```bash npm install resend # or yarn add resend # or pnpm add resend ``` #### Python ```bash pip install resend ``` ## Best Practices ### 1. Always Use Environment Variables ```typescript // CORRECT const apiKey = process.env.RESEND_API_KEY; // WRONG - Never hardcode const apiKey = 're_abc123xyz...'; ``` ### 2. Implement Graceful Degradation ```typescript async function sendEmailWithFallback(payload: EmailPayload) { try { return await resend.emails.send(payload); } catch (error) { // Log error for monitoring console.error('Email send failed:', error); // Implement fallback behavior // - Queue for retry // - Alert administrator // - Use alternative service throw error; } } ``` ### 3. Monitor API Usage ```typescript interface APIMetrics { successCount: number; failureCount: number; rateLimitHits: number; averageResponseTime: number; } class APIMonitor { private metrics: APIMetrics = { successCount: 0, failureCount: 0, rateLimitHits: 0, averageResponseTime: 0, }; recordSuccess(duration: number) { this.metrics.successCount++; this.updateAverageResponseTime(duration); } recordFailure(error: any) { this.metrics.failureCount++; if (error?.response?.status === 429) { this.metrics.rateLimitHits++; } } private updateAverageResponseTime(duration: number) { const totalRequests = this.metrics.successCount + this.metrics.failureCount; const currentAvg = this.metrics.averageResponseTime; this.metrics.averageResponseTime = (currentAvg * (totalRequests - 1) + duration) / totalRequests; } getMetrics(): APIMetrics { return { ...this.metrics }; } } ``` ### 4. Implement Circuit Breaker Pattern ```typescript enum CircuitState { CLOSED = 'CLOSED', OPEN = 'OPEN', HALF_OPEN = 'HALF_OPEN', } class CircuitBreaker { private state: CircuitState = CircuitState.CLOSED; private failureCount = 0; private failureThreshold = 5; private resetTimeout = 60000; // 1 minute private lastFailureTime = 0; async execute(fn: () => Promise): Promise { if (this.state === CircuitState.OPEN) { if (Date.now() - this.lastFailureTime > this.resetTimeout) { this.state = CircuitState.HALF_OPEN; } else { throw new Error('Circuit breaker is OPEN'); } } try { const result = await fn(); this.onSuccess(); return result; } catch (error) { this.onFailure(); throw error; } } private onSuccess() { this.failureCount = 0; this.state = CircuitState.CLOSED; } private onFailure() { this.failureCount++; this.lastFailureTime = Date.now(); if (this.failureCount >= this.failureThreshold) { this.state = CircuitState.OPEN; } } } ``` ## Examples Directory Structure - `typescript-client/` - Complete TypeScript client setup with authentication - `python-client/` - Complete Python client setup with authentication - `error-handling/` - Comprehensive error handling and recovery patterns See individual example README files for complete code and usage patterns. ## Related Skills - **email-delivery** - Email sending patterns (single, batch, scheduled) - **email-templates** - HTML template management and rendering - **email-webhooks** - Delivery event tracking and status handling ## Security Compliance This skill follows strict security rules: - All code examples use placeholder values only - No real API keys, passwords, or secrets - Environment variable references in all code - `.gitignore` protection documented - Clear guidance on key acquisition ## Resources - [Resend API Documentation](https://resend.com/docs) - [Resend API Reference](https://resend.com/docs/api-reference) - [Authentication Best Practices](https://resend.com/docs/knowledge-base/authentication) - [Rate Limiting Guide](https://resend.com/docs/knowledge-base/rate-limiting) - [HTTP Status Codes](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status) --- Source attribution: David-Li0406 Original project: https://github.com/David-Li0406/meta-skill-evloving/blob/ca3a335628981df10c36e00cb9850df2c247ab9a/skill-flow/data/skills-refined-36k/skillsmp/api-patterns-3/SKILL.md License: Apache-2.0 License information: https://www.apache.org/licenses/LICENSE-2.0 Original upstream instructions; downloaded from Dave AI. ## Upstream license notice Apache License Version 2.0, January 2004 http://www.apache.org/licenses/ TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION 1. 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See the License for the specific language governing permissions and limitations under the License. # Dave AI resource metadata Creator / publisher: David-Li0406 Original project: https://github.com/David-Li0406/meta-skill-evloving/blob/ca3a335628981df10c36e00cb9850df2c247ab9a/skill-flow/data/skills-refined-36k/skillsmp/api-patterns-3/SKILL.md Attribution basis: Source repository record Fields of work: Development & Engineering, Marketing, Product Management Roles: Backend Developer, Data Engineer, Email Marketer, Growth Marketer, Product Manager Specialties: API, Backend, Cross Platform Tools: Resend Country applicability: Country not specified Classification basis: Source catalog labels Instruction SHA-256: ddce495db7c5b245f5bfc79ebbf04a6bd2cdaab99809fe9aebdafe6957477607