Asynchronous Web Server - SO Homework 3

Copyright 2023 Toaca Alexandra Simona

Implementation

• I started from the epoll echo server example
• A server_t struct is used to store the server-specific data
• The server reads from a socket until the header is read completely (to make sure the request path and other info is received before sending any message back). This is implemented by using the server flag can_send, which becomes 1 in the on_headers_complete callback of the http_parser.
• the handle_client_request method reads the message from the socket and only when the can_send flag is 1, it parses the request path and calls the set_response method.
• the set_response method writes the response headers (404 Not Found if the requested file does not exist, 200 OK, otherwise) and returns the requested file (if it does not exist, the file_type field of the structure is of type NO_FILE).
• If the file exists, the file_type indicates whether it is STATIC or DYNAMIC.
• After the request is handled, the socket is marked as an out-only socket and send_message is called until everything is sent.
• The send_message method first sends the headers of the response in their entirety (the method is called multiple times until this is accomplished).
• After the headers, the file is sent, depending on its type
• The STATIC files are sent by using the mechanism of zero-copy, implemented by the sendfile method.
• The DYNAMIC files are sent by using libaio async I/O methods, used in the send_dynamic method.
• send_dynamic works by pairing a read from the server file with a write to the socket. First, a write is done, then a read. Why? The first write will write 0 bytes, and the read will return how many bytes the next write will send. By putting that in a loop, the write will send exactly the number of bytes that were read from the file. When everything is read, the loop terminates.
• After sending the whole file, the connection is closed.

What I have learned

• Working with sockets and how sometimes they don't receive the whole buffer sent
• Event-driven programming (EPOLL)
• The basic idea of what goes on when a HTTP request is made
• HTTP requests and their headers
• Working with the libaio library

Comments

• This has been a fun and interesting homework. It was very satisfying to send HTTP requests from my terminal and see how the server logged every action and then sent back the response.
• I was thinking of trying to also implement the POST and HEAD requests
• Also the server could be looking at the file extension and setting the Content-Type header according to that

How to use

• The Makefile builds the executable aws, which is the server, ready to take requests.