Asynchronous Web Server

Objectives

• Deepening the concepts related to working with sockets.
• Developing skills in implementing and designing applications that use asynchronous operations and other advanced I/O operations.
• Deepening the use of the API for advanced I/O operations in the Linux operating system.

Statement

Implement a web server that uses the following advanced I/O operations:

• Asynchronous operations on files
• Non-blocking operations on sockets
• Zero-copying
• Multiplexing I/O operations

The server implements a limited functionality of the HTTP protocol: passing files to clients.

The web server will use the multiplexing API to wait for connections from clients - epoll. On the established connections, requests from clients will be received and then responses will be distributed to them.

The server will serve files from the AWS_DOCUMENT_ROOT directory, defined within the assignments' header. Files are only found in subdirectories AWS_DOCUMENT_ROOT/static/ and AWS_DOCUMENT_ROOT/dynamic/. The corresponding request paths will be, for example, AWS_DOCUMENT_ROOT/static/test.dat and AWS_DOCUMENT_ROOT/dynamic/test.dat. The file processing will be:

• The files in the AWS_DOCUMENT_ROOT/static/ directory are static files that will be transmitted to clients using the zero-copying API - sendfile
• Files in the AWS_DOCUMENT_ROOT/dynamic/ directory are files that are supposed to require a server-side post-processing phase. These files will be read from disk using the asynchronous API and then pushed to the clients. Streaming will use non-blocking sockets (Linux)
• An HTTP 404 message will be sent for invalid request paths

After transmitting a file, according to the HTTP protocol, the connection is closed.

Details and recommendations for the implementation

• Implementing the assignment requires having a state machine for each connection, which you periodically query and update as the transfer proceeds. Check the connection_state data structure defined in the assignment header.
• Find the connection data structure defined in the assignment header. This can be used to keep track of an open connection.
• Definitions of other useful macros and data structures can be found in the assignment header.
• HTTP responses will have the code 200 for existing files and 404 for not existing files.
  • A valid response consists of the HTTP header, containing the related directives, two newlines (\r\n\r\n), followed by the actual content (the file).
  • Sample answers can be found in the parser test file or in the provided sample.
  • You can use predefined request directives such as Date, Last-Modified, etc.
    • The Content-Length directive must specify the size of the HTTP content (actual data) in bytes.
    • The Connection directive must be initialized to close.
• The port on which the web server listens for connections is defined within the assignment header: the AWS_LISTEN_PORT macro.
• The root directory relative to which the resources/files are searched is defined within the assignment header as the AWS_DOCUMENT_ROOT macro.

Support Code

HTTP Parser

The clients and server will communicate using the HTTP protocol. For parsing HTTP requests from clients we recommend using this HTTP parser, also available in the assignments' http-parser. You will need to use a callback to get the path to the local resource requested by the client. Find a simplified example of using the parser in the samples directory.

Debugging

Logs are collected in test.log and wget.log files.

Resources

• sendfile

• io_setup & friends

• epoll