# Api Client — Patterns


# API Client Patterns

## When to Use

- Setting up HTTP clients (axios, fetch, httpx) for consuming external REST APIs
- Adding request/response interceptors for logging, auth tokens, or error transformation
- Implementing retry logic with exponential backoff for transient failures
- Generating type-safe API clients from OpenAPI or Swagger specifications
- Managing authentication tokens (Bearer injection, auto-refresh on 401)

## When NOT to Use

- Internal function calls or in-process service communication that does not cross a network boundary
- Database queries -- use an ORM or database driver instead
- WebSocket or real-time streaming connections -- use dedicated WebSocket client patterns
- GraphQL clients -- use a GraphQL-specific library such as Apollo or urql

---

## Core Patterns

### 1. HTTP Client Setup

Create a single, pre-configured client instance per external service. Never scatter raw `fetch()` or `requests.get()` calls throughout the codebase.

**Python -- httpx (async)**

```python
# BAD - creating a new client on every call, no shared config
import httpx

async def get_user(user_id: int):
    response = httpx.get(f"https://api.example.com/users/{user_id}")  # no timeout, no auth
    return response.json()

# GOOD - shared async client with base URL, timeout, and headers
import httpx

class ApiClient:
    def __init__(self, base_url: str, api_key: str):
        self._client = httpx.AsyncClient(
            base_url=base_url,
            headers={
                "Authorization": f"Bearer {api_key}",
                "Accept": "application/json",
                "User-Agent": "myapp/1.0",
            },
            timeout=httpx.Timeout(10.0, connect=5.0),
        )

    async def get_user(self, user_id: int) -> dict:
        response = await self._client.get(f"/users/{user_id}")
        response.raise_for_status()
        return response.json()

    async def close(self):
        await self._client.aclose()
```

**Python -- httpx (sync)**

```python
# GOOD - synchronous client for scripts, CLIs, or sync frameworks
import httpx

client = httpx.Client(
    base_url="https://api.example.com",
    headers={"Authorization": f"Bearer {api_key}"},
    timeout=httpx.Timeout(10.0, connect=5.0),
)

def get_user(user_id: int) -> dict:
    response = client.get(f"/users/{user_id}")
    response.raise_for_status()
    return response.json()
```

**TypeScript -- fetch wrapper**

```typescript
// BAD - raw fetch with no error handling or shared config
const res = await fetch("https://api.example.com/users/1");
const data = await res.json();

// GOOD - typed fetch wrapper with defaults
interface RequestConfig extends RequestInit {
  params?: Record<string, string>;
}

class ApiClient {
  constructor(
    private baseUrl: string,
    private defaultHeaders: Record<string, string> = {},
  ) {}

  private async request<T>(path: string, config: RequestConfig = {}): Promise<T> {
    const url = new URL(path, this.baseUrl);
    if (config.params) {
      Object.entries(config.params).forEach(([k, v]) => url.searchParams.set(k, v));
    }

    const response = await fetch(url.toString(), {
      ...config,
      headers: { ...this.defaultHeaders, ...config.headers },
    });

    if (!response.ok) {
      throw new ApiError(response.status, await response.text());
    }
    return response.json() as Promise<T>;
  }

  get<T>(path: string, config?: RequestConfig): Promise<T> {
    return this.request<T>(path, { ...config, method: "GET" });
  }

  post<T>(path: string, body: unknown, config?: RequestConfig): Promise<T> {
    return this.request<T>(path, {
      ...config,
      method: "POST",
      body: JSON.stringify(body),
      headers: { "Content-Type": "application/json", ...config?.headers },
    });
  }
}

const api = new ApiClient("https://api.example.com", {
  Authorization: `Bearer ${process.env.API_KEY}`,
  Accept: "application/json",
});
```

**TypeScript -- axios instance**

```typescript
// GOOD - axios instance with shared config
import axios from "axios";

const api = axios.create({
  baseURL: "https://api.example.com",
  timeout: 10_000,
  headers: {
    Accept: "application/json",
    "Content-Type": "application/json",
  },
});

// All requests share the same base URL, timeout, and headers
const user = await api.get<User>("/users/1");
const created = await api.post<User>("/users", { name: "Alice" });
```

### 2. Request/Response Interceptors

Interceptors centralize cross-cutting concerns so individual API calls stay clean.

**Axios interceptors (TypeScript)**

```typescript
// GOOD - auth token injection
api.interceptors.request.use((config) => {
  const token = tokenStore.getAccessToken();
  if (token) {
    config.headers.Authorization = `Bearer ${token}`;
  }
  return config;
});

// GOOD - request/response logging
api.interceptors.request.use((config) => {
  console.debug(`[API] ${config.method?.toUpperCase()} ${config.url}`);
  return config;
});

api.interceptors.response.use(
  (response) => {
    console.debug(`[API] ${response.status} ${response.config.url}`);
    return response;
  },
  (error) => {
    console.error(`[API] Error ${error.response?.status} ${error.config?.url}`);
    return Promise.reject(error);
  },
);

// GOOD - error transformation to application-specific errors
api.interceptors.response.use(
  (response) => response,
  (error) => {
    if (error.response) {
      const { status, data } = error.response;
      throw new ApiError(status, data?.message ?? "Unknown API error", data?.code);
    }
    if (error.code === "ECONNABORTED") {
      throw new TimeoutError(`Request timed out: ${error.config?.url}`);
    }
    throw new NetworkError("Network connection failed");
  },
);
```

**httpx event hooks (Python)**

```python
# GOOD - logging and error hooks on httpx client
import httpx
import logging

logger = logging.getLogger("api_client")

async def log_request(request: httpx.Request):
    logger.debug(f"Request: {request.method} {request.url}")

async def log_response(response: httpx.Response):
    logger.debug(f"Response: {response.status_code} {response.url}")

async def raise_on_error(response: httpx.Response):
    if response.status_code >= 400:
        await response.aread()
        logger.error(f"API error {response.status_code}: {response.text[:200]}")

client = httpx.AsyncClient(
    base_url="https://api.example.com",
    event_hooks={
        "request": [log_request],
        "response": [log_response, raise_on_error],
    },
)
```

### 3. Retry Logic

Retry transient failures with exponential backoff. Never retry non-idempotent requests blindly.

**Python -- tenacity**

```python
# GOOD - retry with exponential backoff for specific status codes
from tenacity import retry, stop_after_attempt, wait_exponential, retry_if_exception
import httpx

def is_retryable(exc: BaseException) -> bool:
    if isinstance(exc, httpx.HTTPStatusError):
        return exc.response.status_code in (429, 500, 502, 503, 504)
    if isinstance(exc, (httpx.ConnectTimeout, httpx.ReadTimeout)):
        return True
    return False

@retry(
    retry=retry_if_exception(is_retryable),
    stop=stop_after_attempt(3),
    wait=wait_exponential(multiplier=1, min=1, max=10),
    reraise=True,
)
async def fetch_with_retry(client: httpx.AsyncClient, url: str) -> dict:
    response = await client.get(url)
    response.raise_for_status()
    return response.json()
```

**Python -- manual retry with Retry-After**

```python
# GOOD - respects Retry-After header from rate-limited responses
import asyncio
import httpx

async def fetch_respecting_rate_limit(
    client: httpx.AsyncClient,
    url: str,
    max_retries: int = 3,
) -> httpx.Response:
    for attempt in range(max_retries):
        response = await client.get(url)
        if response.status_code == 429:
            retry_after = int(response.headers.get("Retry-After", 2 ** attempt))
            await asyncio.sleep(min(retry_after, 60))
            continue
        response.raise_for_status()
        return response
    raise httpx.HTTPStatusError(
        "Max retries exceeded", request=response.request, response=response
    )
```

**TypeScript -- custom retry wrapper**

```typescript
// GOOD - generic retry wrapper with exponential backoff
interface RetryOptions {
  maxRetries: number;
  baseDelay: number;
  maxDelay: number;
  retryableStatuses: number[];
}

const DEFAULT_RETRY: RetryOptions = {
  maxRetries: 3,
  baseDelay: 1000,
  maxDelay: 10000,
  retryableStatuses: [429, 500, 502, 503, 504],
};

async function withRetry<T>(
  fn: () => Promise<T>,
  options: Partial<RetryOptions> = {},
): Promise<T> {
  const opts = { ...DEFAULT_RETRY, ...options };

  for (let attempt = 0; attempt <= opts.maxRetries; attempt++) {
    try {
      return await fn();
    } catch (error) {
      const status = error instanceof ApiError ? error.status : 0;
      const isRetryable = opts.retryableStatuses.includes(status);
      const isLastAttempt = attempt === opts.maxRetries;

      if (!isRetryable || isLastAttempt) throw error;

      const delay = Math.min(opts.baseDelay * 2 ** attempt, opts.maxDelay);
      const jitter = delay * (0.5 + Math.random() * 0.5);
      await new Promise((resolve) => setTimeout(resolve, jitter));
    }
  }
  throw new Error("Unreachable");
}

// Usage
const user = await withRetry(() => api.get<User>("/users/1"));
```

### 4. Type-Safe Clients from OpenAPI

Generate clients from OpenAPI specs to eliminate hand-written API types and reduce drift between backend and frontend.

**TypeScript -- openapi-typescript + openapi-fetch**

```bash
# Generate types from an OpenAPI spec
npx openapi-typescript https://api.example.com/openapi.json -o src/api/schema.d.ts
```

```typescript
// GOOD - fully typed client from generated schema
import createClient from "openapi-fetch";
import type { paths } from "./schema";

const api = createClient<paths>({
  baseUrl: "https://api.example.com",
  headers: { Authorization: `Bearer ${token}` },
});

// Paths, methods, params, and response types are all inferred
const { data, error } = await api.GET("/users/{id}", {
  params: { path: { id: 42 } },
});
// data is typed as the 200 response schema
// error is typed as the error response schema
```

**TypeScript -- zodios (Zod + axios)**

```typescript
// GOOD - runtime-validated API client with Zod schemas
import { makeApi, Zodios } from "@zodios/core";
import { z } from "zod";

const userSchema = z.object({
  id: z.number(),
  name: z.string(),
  email: z.string().email(),
});

const api = makeApi([
  {
    method: "get",
    path: "/users/:id",
    alias: "getUser",
    response: userSchema,
  },
  {
    method: "post",
    path: "/users",
    alias: "createUser",
    parameters: [{ name: "body", type: "Body", schema: userSchema.omit({ id: true }) }],
    response: userSchema,
  },
]);

const client = new Zodios("https://api.example.com", api);

// Fully typed and runtime validated
const user = await client.getUser({ params: { id: 42 } });
```

**Python -- datamodel-code-generator**

```bash
# Generate Pydantic models from an OpenAPI spec
pip install datamodel-code-generator
datamodel-codegen --input openapi.json --output src/api/models.py --input-file-type openapi
```

```python
# Generated models are Pydantic BaseModel classes
from api.models import User, CreateUserRequest

# Use them with httpx for typed requests
async def create_user(client: httpx.AsyncClient, payload: CreateUserRequest) -> User:
    response = await client.post("/users", json=payload.model_dump())
    response.raise_for_status()
    return User.model_validate(response.json())
```

### 5. Authentication

Centralize auth token management so every request gets the right credentials without per-call boilerplate.

**Bearer token injection (axios)**

```typescript
// GOOD - automatic token refresh on 401
let isRefreshing = false;
let failedQueue: Array<{
  resolve: (token: string) => void;
  reject: (error: unknown) => void;
}> = [];

function processQueue(error: unknown, token: string | null) {
  failedQueue.forEach(({ resolve, reject }) => {
    if (error) reject(error);
    else resolve(token!);
  });
  failedQueue = [];
}

api.interceptors.response.use(
  (response) => response,
  async (error) => {
    const originalRequest = error.config;

    if (error.response?.status !== 401 || originalRequest._retry) {
      return Promise.reject(error);
    }

    if (isRefreshing) {
      return new Promise((resolve, reject) => {
        failedQueue.push({ resolve, reject });
      }).then((token) => {
        originalRequest.headers.Authorization = `Bearer ${token}`;
        return api(originalRequest);
      });
    }

    originalRequest._retry = true;
    isRefreshing = true;

    try {
      const { data } = await axios.post("/auth/refresh", {
        refreshToken: tokenStore.getRefreshToken(),
      });
      tokenStore.setAccessToken(data.accessToken);
      processQueue(null, data.accessToken);
      originalRequest.headers.Authorization = `Bearer ${data.accessToken}`;
      return api(originalRequest);
    } catch (refreshError) {
      processQueue(refreshError, null);
      tokenStore.clear();
      window.location.href = "/login";
      return Promise.reject(refreshError);
    } finally {
      isRefreshing = false;
    }
  },
);
```

**Python -- httpx auth class**

```python
# GOOD - custom auth flow with automatic refresh
import httpx
import time

class BearerAuth(httpx.Auth):
    def __init__(self, token_url: str, client_id: str, client_secret: str):
        self.token_url = token_url
        self.client_id = client_id
        self.client_secret = client_secret
        self._access_token: str | None = None
        self._expires_at: float = 0

    def auth_flow(self, request: httpx.Request):
        if self._is_expired():
            token_response = yield self._build_token_request()
            token_response.raise_for_status()
            data = token_response.json()
            self._access_token = data["access_token"]
            self._expires_at = time.time() + data["expires_in"] - 30  # 30s buffer

        request.headers["Authorization"] = f"Bearer {self._access_token}"
        yield request

    def _is_expired(self) -> bool:
        return self._access_token is None or time.time() >= self._expires_at

    def _build_token_request(self) -> httpx.Request:
        return httpx.Request(
            "POST",
            self.token_url,
            data={
                "grant_type": "client_credentials",
                "client_id": self.client_id,
                "client_secret": self.client_secret,
            },
        )

# Usage
auth = BearerAuth(
    token_url="https://auth.example.com/token",
    client_id=os.environ["CLIENT_ID"],
    client_secret=os.environ["CLIENT_SECRET"],
)
client = httpx.AsyncClient(base_url="https://api.example.com", auth=auth)
```

**API key via custom header**

```python
# GOOD - API key as header, loaded from environment
import os
import httpx

client = httpx.AsyncClient(
    base_url="https://api.example.com",
    headers={"X-API-Key": os.environ["EXAMPLE_API_KEY"]},
)
```

### 6. Error Handling

Distinguish between network errors, timeout errors, and API-level errors. Never swallow exceptions silently.

**TypeScript -- structured error classes**

```typescript
// GOOD - error hierarchy for API calls
class ApiError extends Error {
  constructor(
    public readonly status: number,
    public readonly body: string,
    public readonly code?: string,
  ) {
    super(`API error ${status}: ${body.slice(0, 200)}`);
    this.name = "ApiError";
  }

  get isClientError(): boolean {
    return this.status >= 400 && this.status < 500;
  }

  get isServerError(): boolean {
    return this.status >= 500;
  }
}

class NetworkError extends Error {
  constructor(message: string, public readonly cause?: Error) {
    super(message);
    this.name = "NetworkError";
  }
}

class TimeoutError extends Error {
  constructor(message: string) {
    super(message);
    this.name = "TimeoutError";
  }
}
```

**Timeout and cancellation with AbortController**

```typescript
// GOOD - cancel requests that take too long or on component unmount
async function fetchWithTimeout<T>(url: string, timeoutMs: number = 5000): Promise<T> {
  const controller = new AbortController();
  const timeoutId = setTimeout(() => controller.abort(), timeoutMs);

  try {
    const response = await fetch(url, { signal: controller.signal });
    if (!response.ok) throw new ApiError(response.status, await response.text());
    return response.json() as Promise<T>;
  } catch (error) {
    if (error instanceof DOMException && error.name === "AbortError") {
      throw new TimeoutError(`Request to ${url} timed out after ${timeoutMs}ms`);
    }
    throw error;
  } finally {
    clearTimeout(timeoutId);
  }
}

// React -- cancel on unmount
function useApiData(url: string) {
  const [data, setData] = useState(null);

  useEffect(() => {
    const controller = new AbortController();
    fetch(url, { signal: controller.signal })
      .then((res) => res.json())
      .then(setData)
      .catch((err) => {
        if (err.name !== "AbortError") console.error(err);
      });
    return () => controller.abort();
  }, [url]);

  return data;
}
```

**Python -- structured error handling**

```python
# GOOD - catch specific httpx exceptions
import httpx

async def safe_api_call(client: httpx.AsyncClient, path: str) -> dict | None:
    try:
        response = await client.get(path)
        response.raise_for_status()
        return response.json()
    except httpx.ConnectTimeout:
        logger.error(f"Connection timeout: {path}")
        raise
    except httpx.ReadTimeout:
        logger.error(f"Read timeout: {path}")
        raise
    except httpx.HTTPStatusError as exc:
        logger.error(f"HTTP {exc.response.status_code} from {path}: {exc.response.text[:200]}")
        if exc.response.status_code == 404:
            return None
        raise
    except httpx.ConnectError:
        logger.error(f"Connection failed: {path}")
        raise
```

### 7. Rate Limiting (Client-Side)

Respect API rate limits to avoid being throttled or banned.

**TypeScript -- request queue with concurrency control**

```typescript
// GOOD - throttle outgoing requests to stay under rate limits
class RequestQueue {
  private queue: Array<() => Promise<void>> = [];
  private running = 0;

  constructor(
    private maxConcurrent: number = 5,
    private minDelay: number = 100,
  ) {}

  async add<T>(fn: () => Promise<T>): Promise<T> {
    return new Promise((resolve, reject) => {
      this.queue.push(async () => {
        try {
          resolve(await fn());
        } catch (error) {
          reject(error);
        }
      });
      this.process();
    });
  }

  private async process(): Promise<void> {
    if (this.running >= this.maxConcurrent || this.queue.length === 0) return;

    this.running++;
    const task = this.queue.shift()!;
    await task();
    await new Promise((resolve) => setTimeout(resolve, this.minDelay));
    this.running--;
    this.process();
  }
}

// Usage -- at most 5 concurrent requests, 100ms between each
const queue = new RequestQueue(5, 100);
const users = await Promise.all(
  userIds.map((id) => queue.add(() => api.get<User>(`/users/${id}`))),
);
```

**Python -- asyncio semaphore throttle**

```python
# GOOD - limit concurrent requests with a semaphore
import asyncio
import httpx

class ThrottledClient:
    def __init__(self, client: httpx.AsyncClient, max_concurrent: int = 5):
        self._client = client
        self._semaphore = asyncio.Semaphore(max_concurrent)

    async def get(self, url: str, **kwargs) -> httpx.Response:
        async with self._semaphore:
            response = await self._client.get(url, **kwargs)
            # Respect Retry-After if rate limited
            if response.status_code == 429:
                retry_after = int(response.headers.get("Retry-After", "5"))
                await asyncio.sleep(retry_after)
                response = await self._client.get(url, **kwargs)
            return response
```

---

## Best Practices

1. **Create one client instance per external service.** Share it across your application. Instantiating new clients on every call wastes connections and prevents connection pooling.

2. **Always set explicit timeouts.** A missing timeout means a stuck request can hang your entire application. Set both connect and read timeouts. Five to ten seconds is a sensible default for most APIs.

3. **Centralize error handling in interceptors or middleware.** Do not scatter try/catch blocks around every individual API call. Use interceptors to transform HTTP errors into typed application errors.

4. **Add jitter to retry backoff.** Pure exponential backoff causes thundering herd problems when many clients retry simultaneously. Add random jitter to spread retries across time.

5. **Never retry non-idempotent requests automatically.** POST requests that create resources can cause duplicates if retried blindly. Only retry GET, HEAD, and PUT (idempotent methods) by default.

6. **Generate types from OpenAPI specs instead of writing them by hand.** This eliminates drift between backend and frontend types and reduces maintenance effort.

7. **Log request and response metadata, not bodies.** Log method, URL, status code, and duration. Avoid logging request or response bodies by default -- they may contain sensitive data like tokens or PII.

8. **Close clients when the application shuts down.** In Python, use `async with` or call `aclose()`. In Node.js, use AbortController or connection pool shutdown. Leaked connections cause resource exhaustion.

---

## Common Pitfalls

1. **Not closing httpx clients.** Failing to call `aclose()` leaks connections and file descriptors. Use `async with httpx.AsyncClient() as client:` or register a shutdown handler.

2. **Storing API keys in source code.** Always load secrets from environment variables or a secret manager. Never commit API keys, tokens, or credentials to version control.

3. **Ignoring response status codes.** `fetch()` does not throw on 4xx/5xx -- you must check `response.ok` or call `.raise_for_status()`. This is the most common fetch mistake.

4. **Retrying 400-level errors.** Client errors (400, 401, 403, 404, 422) are not transient. Retrying them wastes time and load. Only retry on 429 (rate limit) and 5xx (server errors).

5. **Building URLs with string concatenation.** Concatenating user input into URLs creates injection risks and encoding bugs. Use `URL` constructor (JS) or `httpx.URL` (Python) for safe URL building.

6. **Not cancelling requests on component unmount.** In React, fetch requests that complete after unmount cause state-update-on-unmounted-component warnings and potential memory leaks. Always use AbortController with a cleanup function.

---

## Related Skills

- `openapi` - OpenAPI spec design and documentation
- `error-handling` - Structured error handling patterns across the stack
- `authentication` - Authentication token management and OAuth2 flows
- `caching` - HTTP caching, conditional requests, and cache invalidation
- `logging` - Logging HTTP requests and responses