In this section, we briefly describe the basic processing steps performed by an HTTP (Web) server. HTTP clients use the TCP transport protocol to contact Web servers and request content. The client opens a TCP connection to the server, and transmits a HTTP request header that specifies the requested content.
Static content is stored on the server in the form of disk files. Dynamic content is generated upon request by auxiliary application programs running on the server. Once the server has obtained the requested content, it transmits a HTTP response header followed by the requested data, if applicable, on the client's TCP connection.
For clarity, the following discussion focuses on serving HTTP/1.0 requests for static content on a UNIX-like operating system. However, all of the Web server architectures discussed in this paper are fully capable of handling dynamically-generated content. Likewise, the basic steps described below are similar for HTTP/1.1 requests, and for other operating systems, like Windows NT.
The basic sequential steps for serving a request for static content are illustrated in Figure 1, and consist of the following:
Accept client connection - accept an incoming connection from a client by performing an accept operation on the server's listen socket. This creates a new socket associated with the client connection.
Read request - read the HTTP request header from the client connection's socket and parse the header for the requested URL and options.
Find file - check the server filesystem to see if the requested content file exists and the client has appropriate permissions. The file's size and last modification time are obtained for inclusion in the response header.
Send response header - transmit the HTTP response header on the client connection's socket.
Read file - read the file data (or part of it, for larger files) from the filesystem.
Send data - transmit the requested content (or part of it) on the client connection's socket. For larger files, the ``Read file'' and ``Send data'' steps are repeated until all of the requested content is transmitted.
All of these steps involve operations that can potentially block. Operations that read data or accept connections from a socket may block if the expected data has not yet arrived from the client. Operations that write to a socket may block if the TCP send buffers are full due to limited network capacity. Operations that test a file's validity (using stat()) or open the file (using open()) can block until any necessary disk accesses complete. Likewise, reading a file (using read()) or accessing data from a memory-mapped file region can block while data is read from disk.
Therefore, a high-performance Web server must interleave the sequential steps associated with the serving of multiple requests in order to overlap CPU processing with disk accesses and network communication. The server's architecture determines what strategy is used to achieve this interleaving. Different server architectures are described in Section 3.
In addition to its architecture, the performance of a Web server implementation is also influenced by various optimizations, such as caching. In Section 5, we discuss specific optimizations used in the Flash Web server.
