## Building a Single-Threaded Web Server

We’ll start by getting a single-threaded web server working. Before we begin, let’s look at a quick overview of the protocols involved in building web servers. The details of these protocols are beyond the scope of this book, but a brief overview will give you the information you need.

The two main protocols involved in web servers are the Hypertext Transfer Protocol (HTTP) and the Transmission Control Protocol (TCP). Both protocols are request-response protocols, meaning a client initiates requests and a server listens to the requests and provides a response to the client. The contents of those requests and responses are defined by the protocols.

TCP is the lower-level protocol that describes the details of how information gets from one server to another but doesn’t specify what that information is. HTTP builds on top of TCP by defining the contents of the requests and responses. It’s technically possible to use HTTP with other protocols, but in the vast majority of cases, HTTP sends its data over TCP. We’ll work with the raw bytes of TCP and HTTP requests and responses.

### Listening to the TCP Connection

Our web server needs to listen to a TCP connection, so that’s the first part we’ll work on. The standard library offers a `std::net` module that lets us do this. Let’s make a new project in the usual fashion:

    $ cargo new hello
         Created binary (application) `hello` project
    $ cd hello

Now enter the code in Listing 20-1 in src/main.rs to start. This code will listen at the address `127.0.0.1:7878` for incoming TCP streams. When it gets an incoming stream, it will print `Connection established!`.

Filename: src/main.rs

    use std::net::TcpListener;

    fn main() {
        let listener = TcpListener::bind("127.0.0.1:7878").unwrap();

        for stream in listener.incoming() {
            let stream = stream.unwrap();

            println!("Connection established!");
        }
    }

Listing 20-1: Listening for incoming streams and printing a message when we receive a stream

Using `TcpListener`, we can listen for TCP connections at the address `127.0.0.1:7878`. In the address, the section before the colon is an IP address representing your computer (this is the same on every computer and doesn’t represent the authors’ computer specifically), and `7878` is the port. We’ve chosen this port for two reasons: HTTP is normally accepted on this port, and 7878 is rust typed on a telephone.

The `bind` function in this scenario works like the `new` function in that it will return a new `TcpListener` instance. The reason the function is called `bind` is that in networking, connecting to a port to listen to is known as “binding to a port.”

The `bind` function returns a `Result<T, E>`, which indicates that binding might fail. For example, connecting to port 80 requires administrator privileges (nonadministrators can listen only on ports higher than 1024), so if we tried to connect to port 80 without being an administrator, binding wouldn’t work. As another example, binding wouldn’t work if we ran two instances of our program and so had two programs listening to the same port. Because we’re writing a basic server just for learning purposes, we won’t worry about handling these kinds of errors; instead, we use `unwrap` to stop the program if errors happen.

The `incoming` method on `TcpListener` returns an iterator that gives us a sequence of streams (more specifically, streams of type `TcpStream`). A single stream represents an open connection between the client and the server. A connection is the name for the full request and response process in which a client connects to the server, the server generates a response, and the server closes the connection. As such, `TcpStream` will read from itself to see what the client sent and then allow us to write our response to the stream. Overall, this `for` loop will process each connection in turn and produce a series of streams for us to handle.

For now, our handling of the stream consists of calling `unwrap` to terminate our program if the stream has any errors; if there aren’t any errors, the program prints a message. We’ll add more functionality for the success case in the next listing. The reason we might receive errors from the `incoming` method when a client connects to the server is that we’re not actually iterating over connections. Instead, we’re iterating over connection attempts. The connection might not be successful for a number of reasons, many of them operating system specific. For example, many operating systems have a limit to the number of simultaneous open connections they can support; new connection attempts beyond that number will produce an error until some of the open connections are closed.

Let’s try running this code! Invoke `cargo run` in the terminal and then load 127.0.0.1:7878 in a web browser. The browser should show an error message like “Connection reset,” because the server isn’t currently sending back any data. But when you look at your terminal, you should see several messages that were printed when the browser connected to the server!

         Running `target/debug/hello`
    Connection established!
    Connection established!
    Connection established!

Sometimes, you’ll see multiple messages printed for one browser request; the reason might be that the browser is making a request for the page as well as a request for other resources, like the favicon.ico icon that appears in the browser tab.

It could also be that the browser is trying to connect to the server multiple times because the server isn’t responding with any data. When `stream` goes out of scope and is dropped at the end of the loop, the connection is closed as part of the `drop` implementation. Browsers sometimes deal with closed connections by retrying, because the problem might be temporary. The important factor is that we’ve successfully gotten a handle to a TCP connection!

Remember to stop the program by pressing ctrl-c when you’re done running a particular version of the code. Then restart `cargo run` after you’ve made each set of code changes to make sure you’re running the newest code.

### Reading the Request

Let’s implement the functionality to read the request from the browser! To separate the concerns of first getting a connection and then taking some action with the connection, we’ll start a new function for processing connections. In this new `handle_connection` function, we’ll read data from the TCP stream and print it so we can see the data being sent from the browser. Change the code to look like Listing 20-2.

Filename: src/main.rs

    use std::io::prelude::*;
    use std::net::TcpStream;
    use std::net::TcpListener;

    fn main() {
        let listener = TcpListener::bind("127.0.0.1:7878").unwrap();

        for stream in listener.incoming() {
            let stream = stream.unwrap();

            handle_connection(stream);
        }
    }

    fn handle_connection(mut stream: TcpStream) {
        let mut buffer = [0; 512];

        stream.read(&mut buffer).unwrap();

        println!("Request: {}", String::from_utf8_lossy(&buffer[..]));
    }

Listing 20-2: Reading from the `TcpStream` and printing the data

We bring `std::io::prelude` into scope to get access to certain traits that let us read from and write to the stream. In the `for` loop in the `main` function, instead of printing a message that says we made a connection, we now call the new `handle_connection` function and pass the `stream` to it.

In the `handle_connection` function, we’ve made the `stream` parameter mutable. The reason is that the `TcpStream` instance keeps track of what data it returns to us internally. It might read more data than we asked for and save that data for the next time we ask for data. It therefore needs to be `mut` because its internal state might change; usually, we think of “reading” as not needing mutation, but in this case we need the `mut` keyword.

Next, we need to actually read from the stream. We do this in two steps: first, we declare a `buffer` on the stack to hold the data that is read in. We’ve made the buffer 512 bytes in size, which is big enough to hold the data of a basic request and sufficient for our purposes in this chapter. If we wanted to handle requests of an arbitrary size, buffer management would need to be more complicated; we’ll keep it simple for now. We pass the buffer to `stream.read`, which will read bytes from the `TcpStream` and put them in the buffer.

Second, we convert the bytes in the buffer to a string and print that string. The `String::from_utf8_lossy` function takes a `&[u8]` and produces a `String` from it. The “lossy” part of the name indicates the behavior of this function when it sees an invalid UTF-8 sequence: it will replace the invalid sequence with `�`, the `U+FFFD REPLACEMENT CHARACTER`. You might see replacement characters for characters in the buffer that aren’t filled by request data.

Let’s try this code! Start the program and make a request in a web browser again. Note that we’ll still get an error page in the browser, but our program’s output in the terminal will now look similar to this:

    $ cargo run
       Compiling hello v0.1.0 (file:///projects/hello)
        Finished dev [unoptimized + debuginfo] target(s) in 0.42 secs
         Running `target/debug/hello`
    Request: GET / HTTP/1.1
    Host: 127.0.0.1:7878
    User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101
    Firefox/52.0
    Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
    Accept-Language: en-US,en;q=0.5
    Accept-Encoding: gzip, deflate
    Connection: keep-alive
    Upgrade-Insecure-Requests: 1
    ������������������������������������

Depending on your browser, you might get slightly different output. Now that we’re printing the request data, we can see why we get multiple connections from one browser request by looking at the path after `Request: GET`. If the repeated connections are all requesting /, we know the browser is trying to fetch / repeatedly because it’s not getting a response from our program.

Let’s break down this request data to understand what the browser is asking of our program.

### A Closer Look at an HTTP Request

HTTP is a text-based protocol, and a request takes this format:

    Method Request-URI HTTP-Version CRLF
    headers CRLF
    message-body

The first line is the request line that holds information about what the client is requesting. The first part of the request line indicates the method being used, such as `GET` or `POST`, which describes how the client is making this request. Our client used a `GET` request.

The next part of the request line is /, which indicates the Uniform Resource Identifier (URI) the client is requesting: a URI is almost, but not quite, the same as a Uniform Resource Locator (URL). The difference between URIs and URLs isn’t important for our purposes in this chapter, but the HTTP spec uses the term URI, so we can just mentally substitute URL for URI here.

The last part is the HTTP version the client uses, and then the request line ends in a CRLF sequence. (CRLF stands for carriage return and line feed, which are terms from the typewriter days!) The CRLF sequence can also be written as `\r\n`, where `\r` is a carriage return and `\n` is a line feed. The CRLF sequence separates the request line from the rest of the request data. Note that when the CRLF is printed, we see a new line start rather than `\r\n`.

Looking at the request line data we received from running our program so far, we see that `GET` is the method, / is the request URI, and `HTTP/1.1` is the version.

After the request line, the remaining lines starting from `Host:` onward are headers. `GET` requests have no body.

Try making a request from a different browser or asking for a different address, such as 127.0.0.1:7878/test, to see how the request data changes.

Now that we know what the browser is asking for, let’s send back some data!

### Writing a Response

Now we’ll implement sending data in response to a client request. Responses have the following format:

    HTTP-Version Status-Code Reason-Phrase CRLF
    headers CRLF
    message-body

The first line is a status line that contains the HTTP version used in the response, a numeric status code that summarizes the result of the request, and a reason phrase that provides a text description of the status code. After the CRLF sequence are any headers, another CRLF sequence, and the body of the response.

Here is an example response that uses HTTP version 1.1, has a status code of 200, an OK reason phrase, no headers, and no body:

    HTTP/1.1 200 OK\r\n\r\n

The status code 200 is the standard success response. The text is a tiny successful HTTP response. Let’s write this to the stream as our response to a successful request! From the `handle_connection` function, remove the `println!` that was printing the request data and replace it with the code in Listing 20-3.

Filename: src/main.rs

    #![allow(unused_variables)]
    fn main() {
    use std::io::prelude::*;
    use std::net::TcpStream;
    fn handle_connection(mut stream: TcpStream) {
        let mut buffer = [0; 512];

        stream.read(&mut buffer).unwrap();

        let response = "HTTP/1.1 200 OK\r\n\r\n";

        stream.write(response.as_bytes()).unwrap();
        stream.flush().unwrap();
    }
    }

Listing 20-3: Writing a tiny successful HTTP response to the stream

The first new line defines the `response` variable that holds the success message’s data. Then we call `as_bytes` on our `response` to convert the string data to bytes. The `write` method on `stream` takes a `&[u8]` and sends those bytes directly down the connection.

Because the `write` operation could fail, we use `unwrap` on any error result as before. Again, in a real application you would add error handling here. Finally, `flush` will wait and prevent the program from continuing until all the bytes are written to the connection; `TcpStream` contains an internal buffer to minimize calls to the underlying operating system.

With these changes, let’s run our code and make a request. We’re no longer printing any data to the terminal, so we won’t see any output other than the output from Cargo. When you load 127.0.0.1:7878 in a web browser, you should get a blank page instead of an error. You’ve just hand-coded an HTTP request and response!

### Returning Real HTML

Let’s implement the functionality for returning more than a blank page. Create a new file, hello.html, in the root of your project directory, not in the src directory. You can input any HTML you want; Listing 20-4 shows one possibility.

Filename: hello.html

    <!DOCTYPE html>
    <html lang="en">
      <head>
        <meta charset="utf-8">
        <title>Hello!</title>
      </head>
      <body>
        <h1>Hello!</h1>
        <p>Hi from Rust</p>
      </body>
    </html>

Listing 20-4: A sample HTML file to return in a response

This is a minimal HTML5 document with a heading and some text. To return this from the server when a request is received, we’ll modify `handle_connection` as shown in Listing 20-5 to read the HTML file, add it to the response as a body, and send it.

Filename: src/main.rs

    #![allow(unused_variables)]
    fn main() {
    use std::io::prelude::*;
    use std::net::TcpStream;
    use std::fs;
    // --snip--

    fn handle_connection(mut stream: TcpStream) {
        let mut buffer = [0; 512];
        stream.read(&mut buffer).unwrap();

        let contents = fs::read_to_string("hello.html").unwrap();

        let response = format!("HTTP/1.1 200 OK\r\n\r\n{}", contents);

        stream.write(response.as_bytes()).unwrap();
        stream.flush().unwrap();
    }
    }

Listing 20-5: Sending the contents of hello.html as the body of the response

We’ve added a line at the top to bring the standard library’s filesystem module into scope. The code for reading the contents of a file to a string should look familiar; we used it in Chapter 12 when we read the contents of a file for our I/O project in Listing 12-4.

Next, we use `format!` to add the file’s contents as the body of the success response.

Run this code with `cargo run` and load 127.0.0.1:7878 in your browser; you should see your HTML rendered!

Currently, we’re ignoring the request data in `buffer` and just sending back the contents of the HTML file unconditionally. That means if you try requesting 127.0.0.1:7878/something-else in your browser, you’ll still get back this same HTML response. Our server is very limited and is not what most web servers do. We want to customize our responses depending on the request and only send back the HTML file for a well-formed request to /.

### Validating the Request and Selectively Responding

Right now, our web server will return the HTML in the file no matter what the client requested. Let’s add functionality to check that the browser is requesting / before returning the HTML file and return an error if the browser requests anything else. For this we need to modify `handle_connection`, as shown in Listing 20-6. This new code checks the content of the request received against what we know a request for / looks like and adds `if` and `else` blocks to treat requests differently.

Filename: src/main.rs

    #![allow(unused_variables)]
    fn main() {
    use std::io::prelude::*;
    use std::net::TcpStream;
    use std::fs;
    // --snip--

    fn handle_connection(mut stream: TcpStream) {
        let mut buffer = [0; 512];
        stream.read(&mut buffer).unwrap();

        let get = b"GET / HTTP/1.1\r\n";

        if buffer.starts_with(get) {
            let contents = fs::read_to_string("hello.html").unwrap();

            let response = format!("HTTP/1.1 200 OK\r\n\r\n{}", contents);

            stream.write(response.as_bytes()).unwrap();
            stream.flush().unwrap();
        } else {
            // some other request
        }
    }
    }

Listing 20-6: Matching the request and handling requests to / differently from other requests

First, we hardcode the data corresponding to the / request into the `get` variable. Because we’re reading raw bytes into the buffer, we transform `get` into a byte string by adding the `b""` byte string syntax at the start of the content data. Then we check whether `buffer` starts with the bytes in `get`. If it does, it means we’ve received a well-formed request to /, which is the success case we’ll handle in the `if` block that returns the contents of our HTML file.

If `buffer` does not start with the bytes in `get`, it means we’ve received some other request. We’ll add code to the `else` block in a moment to respond to all other requests.

Run this code now and request 127.0.0.1:7878; you should get the HTML in hello.html. If you make any other request, such as 127.0.0.1:7878/something-else, you’ll get a connection error like those you saw when running the code in Listing 20-1 and Listing 20-2.

Now let’s add the code in Listing 20-7 to the `else` block to return a response with the status code 404, which signals that the content for the request was not found. We’ll also return some HTML for a page to render in the browser indicating the response to the end user.

Filename: src/main.rs

    #![allow(unused_variables)]
    fn main() {
    use std::io::prelude::*;
    use std::net::TcpStream;
    use std::fs;
    fn handle_connection(mut stream: TcpStream) {
    if true {
    // --snip--

    } else {
        let status_line = "HTTP/1.1 404 NOT FOUND\r\n\r\n";
        let contents = fs::read_to_string("404.html").unwrap();

        let response = format!("{}{}", status_line, contents);

        stream.write(response.as_bytes()).unwrap();
        stream.flush().unwrap();
    }
    }
    }

Listing 20-7: Responding with status code 404 and an error page if anything other than / was requested

Here, our response has a status line with status code 404 and the reason phrase `NOT FOUND`. We’re still not returning headers, and the body of the response will be the HTML in the file 404.html. You’ll need to create a 404.html file next to hello.html for the error page; again feel free to use any HTML you want or use the example HTML in Listing 20-8.

Filename: 404.html

    <!DOCTYPE html>
    <html lang="en">
      <head>
        <meta charset="utf-8">
        <title>Hello!</title>
      </head>
      <body>
        <h1>Oops!</h1>
        <p>Sorry, I don't know what you're asking for.</p>
      </body>
    </html>

Listing 20-8: Sample content for the page to send back with any 404 response

With these changes, run your server again. Requesting 127.0.0.1:7878 should return the contents of hello.html, and any other request, like 127.0.0.1:7878/foo, should return the error HTML from 404.html.

### A Touch of Refactoring

At the moment the `if` and `else` blocks have a lot of repetition: they’re both reading files and writing the contents of the files to the stream. The only differences are the status line and the filename. Let’s make the code more concise by pulling out those differences into separate `if` and `else` lines that will assign the values of the status line and the filename to variables; we can then use those variables unconditionally in the code to read the file and write the response. Listing 20-9 shows the resulting code after replacing the large `if` and `else` blocks.

Filename: src/main.rs

    #![allow(unused_variables)]
    fn main() {
    use std::io::prelude::*;
    use std::net::TcpStream;
    use std::fs;
    // --snip--

    fn handle_connection(mut stream: TcpStream) {
        let mut buffer = [0; 512];
        stream.read(&mut buffer).unwrap();

        let get = b"GET / HTTP/1.1\r\n";
        // --snip--

        let (status_line, filename) = if buffer.starts_with(get) {
            ("HTTP/1.1 200 OK\r\n\r\n", "hello.html")
        } else {
            ("HTTP/1.1 404 NOT FOUND\r\n\r\n", "404.html")
        };

        let contents = fs::read_to_string(filename).unwrap();

        let response = format!("{}{}", status_line, contents);

        stream.write(response.as_bytes()).unwrap();
        stream.flush().unwrap();
    }
    }

Listing 20-9: Refactoring the `if` and `else` blocks to contain only the code that differs between the two cases

Now the `if` and `else` blocks only return the appropriate values for the status line and filename in a tuple; we then use destructuring to assign these two values to `status_line` and `filename` using a pattern in the `let` statement, as discussed in Chapter 18.

The previously duplicated code is now outside the `if` and `else` blocks and uses the `status_line` and `filename` variables. This makes it easier to see the difference between the two cases, and it means we have only one place to update the code if we want to change how the file reading and response writing work. The behavior of the code in Listing 20-9 will be the same as that in Listing 20-8.

Awesome! We now have a simple web server in approximately 40 lines of Rust code that responds to one request with a page of content and responds to all other requests with a 404 response.

Currently, our server runs in a single thread, meaning it can only serve one request at a time. Let’s examine how that can be a problem by simulating some slow requests. Then we’ll fix it so our server can handle multiple requests at once.