In the study of web crawlers, it is very important to understand the working process of the HTTP protocol. HTTP (Hypertext Transfer Protocol) is a protocol used to transfer data between a web browser and a server. It is responsible for communication between client requests and server responses. This article will introduce the working process of the HTTP protocol in detail and help you deeply understand the basic knowledge of web crawling. Let’s explore together!
1. Introduction to HTTP protocol
- Definition: HTTP is a stateless, connectionless protocol based on the request-response model and uses URLs to locate resources.
- Request method: HTTP defines a variety of request methods, including GET, POST, PUT, DELETE, etc., which are used to specify the client's operation type on resources.
- Response status code: HTTP uses status codes to indicate the server's processing results of requests. Common status codes include 200 (success), 404 (resource not found), 500 (server error), etc.
2. Working process of HTTP protocol - Establishing a connection: The client establishes a connection with the server through the TCP/IP protocol, using the default HTTP port (80) or the encrypted HTTPS port (443).
- Send request: The client sends an HTTP request, including request line, request header and request body. The request line contains the request method, URL, and HTTP protocol version.
- Server processing: After the server receives the request, it processes it according to the URL and request method in the request line. The server may need to read the database, perform business logic, etc.
- Send response: The server generates an HTTP response, including response lines, response headers, and response bodies. The response line contains the HTTP protocol version, status code, and status description.
- Receive response: The client receives the HTTP response and determines whether the request is successful based on the response code. If successful, the client will continue processing the data in the response body.
- Close the connection: After completing the request and response, both the client and the server can choose to close the connection and release resources.
3. Request methods and common uses - GET: Obtain resources from the server, suitable for obtaining static resources such as web pages and pictures.
- POST: Submit data to the server, suitable for operations that require data transfer such as logging in and submitting forms.
- PUT: Upload files or create resources to the server.
- DELETE: Delete resources on the server.
4. Request headers and common fields - User-Agent: The browser identifier of the client, used to inform the server what type of client is used.
- Referer: Indicates the source page URL of the current request.
- Cookie: A key-value pair stored on the client side and used to maintain session state across multiple requests.
- Authorization: Credential information used for authentication.
- Content-Type: Specify the data type in the request or response, such as application/json, application/x-www-form-urlencoded, etc.
5. Status codes and common meanings - 200: Request successful.
- 404: Resource not found.
- 500: Server internal error.
- 302: Temporary redirect.
6. Advanced topics and precautions - HTTPS: Introduces the difference between HTTP and HTTPS and how to carry out encrypted communication.
- HTTP header extensions: Learn more about the meanings and uses of HTTP header fields.
- Prevent crawler blocking: Learn how to set appropriate request headers to avoid being blocked by websites.
Through the introduction of this article, you have already understood the working process of the HTTP protocol as well as key concepts such as common request methods and response status codes. A deep understanding of the HTTP protocol is crucial for web scraping operations. In practical applications, we need to choose the appropriate request method and set the appropriate request header according to the specific situation, while complying with the rules of the website and crawler ethics. I hope this article can be helpful to your web crawling learning journey. If you have any questions or need further information, please feel free to chat with me. Wishing you a wealth of data and knowledge in the world of web scraping.