What Is an HTTP Server?

http is a simple request-response protocol, which usually runs on top of TCP. It specifies what messages the client might send to the server and what kind of response it gets. The request and response message headers are given in ASCII; the message content has a MIME-like format. This simple model contributed to the success of the early Web because it made development and deployment so straightforward. ^[1]

At the beginning of the birth of HTTP, it was mainly used to obtain content on the WEB side. At that time, the content was not as rich as it is now, the layout was not so exquisite, and there were almost no user interaction scenarios. For this simple scenario of getting web content, HTTP performed reasonably well. But with the development of the Internet and the birth of WEB2.0, more content began to be displayed (more image files), typography became more exquisite (more CSS), and more complex interactions were also introduced (more JS). The total amount of data and requests that users load when opening the homepage of a website is also increasing.

Today, the homepage of most portals will exceed 2M in size, and the number of requests can be as high as 100. Another widely used application is the client APP on the mobile Internet. Different types of apps use HTTP very differently. For e-commerce apps, there may be more than 10 requests to load the homepage. For IM such as WeChat, HTTP requests may be limited to the download of voice and image files, and the frequency of requests is not high.

HTTP is based on the client / server model and is connection-oriented. A typical HTTP transaction is processed as follows: ^[8]

In the WWW, "client" and "server" are relative concepts that exist only during a specific connection, that is, a client in one connection may serve as a server in another connection. The client / server mode information exchange process based on the HTTP protocol is divided into four processes: establishing a connection, sending request information, sending response information, and closing the connection. ^[7]

HTTP messages consist of a request from the client to the server and a response from the server to the client. The request message format is as follows: ^[7]

1xx: information

News	description
100 Continue	The server received only part of the request, but once the server did not reject the request, the client should continue to send the rest of the request.
101 Switching Protocols	Server conversion protocol: The server will switch to another protocol in accordance with the client's request.

2xx: success

News	description
200 OK	Request succeeded (followed by response documents to GET and POST requests.)
201 Created	The request is created and a new resource is created.
202 Accepted	The request for processing was accepted, but processing was not completed.
203 Non-authoritative Information	The document has returned normally, but some response headers may be incorrect because a copy of the document is used.
204 No Content	No new documents. The browser should continue to display the original document. This status code is useful if the user refreshes the page periodically and the servlet can determine that the user documentation is sufficiently new.
205 Reset Content	No new documents. But the browser should reset what it displays. Used to force the browser to clear form input.
206 Partial Content	The client sends a GET request with a Range header and the server completes it.

3xx: redirect

News	description
300 Multiple Choices	Multiple selection. List of links. The user can select a link to reach the destination. A maximum of five addresses is allowed.
301 Moved Permanently	The requested page has been transferred to a new url.
302 Found	The requested page has been temporarily transferred to a new url.
303 See Other	The requested page can be found under another URL.
304 Not Modified	The document was not modified as expected. The client has a buffered document and sends a conditional request (usually providing an If-Modified-Since header indicating that the client only wants documents that are newer than the specified date). The server tells the client that the original buffered document can still be used.
305 Use Proxy	The document requested by the client should be fetched through the proxy server indicated by the Location header.
306 Unused	This code was used in the previous version. It is no longer used, but the code is still retained.
307 Temporary Redirect	The requested page has been temporarily moved to a new url.

4xx: client error

News	description
400 Bad Request	The server failed to understand the request.
401 Unauthorized	The requested page requires a username and password.
401.1	Login failed.
401.2	Server configuration caused login failure.
401.3	Unauthorized due to ACL resource restrictions.
401.4	Filter authorization failed.
401.5	ISAPI / CGI application authorization failed.
401.7	Access is denied by the URL authorization policy on the web server. This error code is specific to IIS 6.0.
402 Payment Required	This code is not yet available.
403 Forbidden	Access to the requested page is prohibited.
403.1	Executive access is prohibited.
403.2	Read access is prohibited.
403.3	Write access is prohibited.
403.4	Requires SSL.
403.5	Requires SSL 128.
403.6	The IP address was rejected.
403.7	Requires client certificate.
403.8	Site access is denied.
403.9	Too many users.
403.10	The configuration is invalid.
403.11	Password change.
403.12	Access denied to the mapping table.
403.13	The client certificate is revoked.
403.14	Reject directory listing.
403.15	Client access exceeded.
403.16	The client certificate is untrusted or invalid.
403.17	The client certificate has expired or is not yet valid.
403.18	The requested URL cannot be executed in the current application pool. This error code is specific to IIS 6.0.
403.19	CGI cannot be performed for clients in this application pool. This error code is specific to IIS 6.0.
403.20	Passport login failed. This error code is specific to IIS 6.0.
404 Not Found	The server cannot find the requested page.
404.0	(None)-No files or directories were found.
404.1	The Web site cannot be accessed on the requested port.
404.2	The web service extension locking policy is blocking this request.
404.3	The MIME mapping policy is blocking this request.
405 Method Not Allowed	The method specified in the request is not allowed.
406 Not Acceptable	The server-generated response cannot be accepted by the client.
407 Proxy Authentication Required	The user must first authenticate with a proxy server before the request can be processed.
408 Request Timeout	The request exceeded the server's waiting time.
409 Conflict	The request could not be completed due to a conflict.
410 Gone	The requested page is not available.
411 Length Required	"Content-Length" is undefined. Without this content, the server will not accept the request.
412 Precondition Failed	The precondition in the request was evaluated as failed by the server.
413 Request Entity Too Large	The server will not accept the request because the requested entity is too large.
414 Request-url Too Long	Because the URL is too long, the server will not accept the request. This happens when a post request is converted into a get request with a long query.
415 Unsupported Media Type	The server will not accept the request because the media type is not supported.
416 Requested Range Not Satisfiable	The server cannot satisfy the Range header specified by the client in the request.
417 Expectation Failed	Execution failed.
423	Locking error.

5xx: server error

News	description
500 Internal Server Error	The request was not completed. The server encountered an unpredictable situation.
500.12	The application is busy restarting on the web server.
500.13	The web server is too busy.
500.15	Direct requests to Global.asa are not allowed.
500.16	UNC authorization credentials are incorrect. This error code is specific to IIS 6.0.
500.18	The URL authorization store cannot be opened. This error code is specific to IIS 6.0.
500.100	Internal ASP error.
501 Not Implemented	The request was not completed. The server does not support the requested function.
502 Bad Gateway	The request was not completed. The server received an invalid response from the upstream server.
502.1	CGI application timed out. ·
502.2	CGI application error.
503 Service Unavailable	The request was not completed. The server is temporarily overloaded or down.
504 Gateway Timeout	Gateway timed out.
505 HTTP Version Not Supported	The server does not support the HTTP protocol version specified in the request.

The HTTP request and response messages include the HTTP protocol version number. There is some confusion about the correct use and interpretation of the HTTP version number and the interoperability of the HTTP implementation of different protocol conversions. Refer to RFC 2145 for the use and interpretation of HTTP version numbers. ^[9]