Golang Web Server Example

The primary functions of a web server are to:

1. Listen for incoming HTTP requests from clients, such as web browsers or other applications.
2. Process these requests by interpreting the requested resource (like a web page or an image) and any additional data sent by the client.
3. Send the requested content or an error message back to the client in the form of an HTTP response, which typically includes headers (metadata) and a body (the actual content).

package main

import (
	"fmt"
	"net/http"
)

func hello(w http.ResponseWriter, _ *http.Request) {
	fmt.Fprintf(w, "Hello")
}

func main() {
	http.HandleFunc("/", hello)
	http.ListenAndServe("0.0.0.0:8080", nil)
}

The minimal web server you provided in Go listens for incoming HTTP requests and responds with "Hello" when it receives a request. Here's a step-by-step explanation of what happens when a request is received:

1. The web server is started by calling the main() function.
2. Inside main(), the server registers a function called hello as the handler for the root path ("/") using http.HandleFunc("/", hello). This means that when an HTTP request is received with the root path, the hello function will be called to handle it.
3. The server starts listening for incoming HTTP requests on all available network interfaces ("0.0.0.0") and port 8080 using http.ListenAndServe("0.0.0.0:8080", nil).
4. When a request is received, the server checks the path of the request. Since the root path ("/") is registered with the hello function, the server calls the hello function to handle the request.
5. The hello function takes two arguments: w http.ResponseWriter and _ *http.Request. The underscore (_) is used to ignore the second argument (the request), as it's not used in this example.
6. Inside the hello function, the server writes "Hello" to the response using fmt.Fprintf(w, "Hello"). The http.ResponseWriter (w) is an interface that allows the server to construct an HTTP response, and fmt.Fprintf writes the formatted string "Hello" to it.
7. The response is then sent back to the client, which receives the "Hello" message.
8. In summary, when the server receives a request, it calls the hello function, writes "Hello" to the response, and sends it back to the client.

To capture and explain all the system calls made by the Go web server running on Linux, you can use a tool like strace. However, providing an exhaustive list of system calls and their explanations is outside the scope of this answer. Instead, I will explain some key system calls that you would likely observe when running the provided Go web server.

1. socket(): This system call creates a new network socket for the server. It is used to create a communication endpoint for the server to listen for incoming connections.
2. bind(): This system call binds the socket created by socket() to a specific address and port (in this case, "0.0.0.0:8080"). This allows the server to listen for incoming connections on the specified address and port.
3. listen(): This system call marks the socket as a passive socket that will be used to accept incoming connection requests using the accept() system call. It also specifies the maximum length of the queue of pending connections.
4. accept(): This system call is used to accept incoming connection requests from clients. When a client connects to the server, the accept() call returns a new socket file descriptor that represents the connection to that specific client. This allows the server to handle multiple clients simultaneously.
5. epoll_create1(), epoll_ctl(), epoll_wait(): These system calls are used for efficient I/O multiplexing. They allow the server to monitor multiple file descriptors (sockets) for various I/O events (such as incoming data or readiness to write) without having to block on each one individually. This is important for the performance of the server, especially when handling a large number of clients.
6. read(), write(): These system calls are used to read incoming HTTP requests from the client and write the response back to the client. In the provided web server, these calls happen within the hello() function when the server writes "Hello" to the http.ResponseWriter.
7. close(): This system call is used to close the connection with a client when the request handling is complete.

Please note that this is not an exhaustive list of system calls that the Go web server might make, but it covers the most important ones for setting up and handling connections. The actual system calls made may vary depending on the specific implementation of the Go standard library and the underlying Linux kernel.