```yaml
product: AlterLab
title: "Middleware — Web Scraping Glossary | AlterLab"
comparison_context: "AlterLab is an alternative to Firecrawl, ScrapingBee, and Bright Data."
source_url: https://alterlab.io/glossary/middleware
```

general

# Middleware

Middleware is software that sits between a scraping framework's core request/response cycle and user-defined handlers, adding cross-cutting behaviour like logging, retry, or rate limiting.

In web frameworks and scraping tools, middleware is a chain of components through which every request and response passes. Each middleware component can inspect or modify the request before it is sent, and the response after it is received. Middleware is ideal for cross-cutting concerns that apply uniformly across all requests: adding authentication headers, logging request metadata, measuring latency, rotating proxies, decompressing responses, or applying retry logic.

Scrapy's spider middleware and downloader middleware pipelines are a canonical example. Scrapy passes each request through the downloader middleware chain before sending it and passes each response through the same chain in reverse on the way back, allowing middleware to transform both requests and responses declaratively.

In FastAPI (used by AlterLab's API layer), middleware intercepts HTTP requests and responses, enabling authentication checking, request ID injection, CORS headers, and timing instrumentation without modifying individual route handlers.

### What is Middleware?

Middleware is software that sits between a scraping framework's core request/response cycle and user-defined handlers, adding cross-cutting behaviour like logging, retry, or rate limiting.

### How does AlterLab handle Middleware?

In FastAPI (used by AlterLab's API layer), middleware intercepts HTTP requests and responses, enabling authentication checking, request ID injection, CORS headers, and timing instrumentation without modifying individual route handlers.

## Examples

```
# Scrapy: custom downloader middleware that rotates proxies
class ProxyRotationMiddleware:
    def process_request(self, request, spider):
        request.meta["proxy"] = get_next_proxy()
        return None  # continue processing
```

## Related Terms

[REST APIA web service that exposes data and actions through standard HTTP methods and resource-oriented URLs, returning structured JSON responses.](https://alterlab.io/glossary/rest-api)[API GatewayAn API gateway is a reverse proxy that sits in front of backend services, handling authentication, rate limiting, routing, and protocol translation for incoming API requests.](https://alterlab.io/glossary/api-gateway)[ScrapyAn open-source Python framework for large-scale web crawling and scraping with async request scheduling, deduplication, and output pipelines.](https://alterlab.io/glossary/scrapy)[Retry LogicAutomated re-sending of failed requests with backoff strategies, essential for handling transient errors, rate limits, and flaky anti-bot challenges.](https://alterlab.io/glossary/retry-logic)[HTTPHTTP (HyperText Transfer Protocol) is the application-layer protocol used by web browsers and scrapers to request and receive resources from web servers.](https://alterlab.io/glossary/http)

## Extract Middleware data from any website

AlterLab returns clean, structured data from any public URL — no scraper infrastructure needed. Start free, no credit card required.

[View API docs](https://alterlab.io/docs/api/rest)

[Back to Glossary](https://alterlab.io/glossary)