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## 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:

```text
$ 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!`.

<span class="filename">Filename: src/main.rs</span>

```rust,no_run
{{#rustdoc_include ../listings/ch20-web-server/listing-20-01/src/main.rs}}
```

<span class="caption">Listing 20-1: Listening for incoming streams and printing
a message when we receive a stream</span>

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!

```text
     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 <span class="keystroke">ctrl-c</span>
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.

<span class="filename">Filename: src/main.rs</span>

```rust,no_run
{{#rustdoc_include ../listings/ch20-web-server/listing-20-02/src/main.rs}}
```

<span class="caption">Listing 20-2: Reading from the `TcpStream` and printing
the data</span>

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 1024 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:

```text
$ cargo run
   Compiling hello v0.1.0 (file:///projects/hello)
    Finished dev [unoptimized + debuginfo] target(s) in 0.42s
     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:

```text
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:

```text
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:

```text
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.

<span class="filename">Filename: src/main.rs</span>

```rust,no_run
{{#rustdoc_include ../listings/ch20-web-server/listing-20-03/src/main.rs:here}}
```

<span class="caption">Listing 20-3: Writing a tiny successful HTTP response to
the stream</span>

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.

<span class="filename">Filename: hello.html</span>

```html
{{#include ../listings/ch20-web-server/listing-20-04/hello.html}}
```

<span class="caption">Listing 20-4: A sample HTML file to return in a
response</span>

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.

<span class="filename">Filename: src/main.rs</span>

```rust,no_run
{{#rustdoc_include ../listings/ch20-web-server/listing-20-05/src/main.rs:here}}
```

<span class="caption">Listing 20-5: Sending the contents of *hello.html* as the
body of the response</span>

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. To ensure a valid HTTP response, we add the `Content-Length` header
which is set to the size of our response body, in this case the size of `hello.html`.

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.

<span class="filename">Filename: src/main.rs</span>

```rust,no_run
{{#rustdoc_include ../listings/ch20-web-server/listing-20-06/src/main.rs:here}}
```

<span class="caption">Listing 20-6: Matching the request and handling requests
to */* differently from other requests</span>

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.

<span class="filename">Filename: src/main.rs</span>

```rust,no_run
{{#rustdoc_include ../listings/ch20-web-server/listing-20-07/src/main.rs:here}}
```

<span class="caption">Listing 20-7: Responding with status code 404 and an
error page if anything other than */* was requested</span>

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.

<span class="filename">Filename: 404.html</span>

```html
{{#include ../listings/ch20-web-server/listing-20-08/404.html}}
```

<span class="caption">Listing 20-8: Sample content for the page to send back
with any 404 response</span>

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.

<span class="filename">Filename: src/main.rs</span>

```rust,no_run
{{#rustdoc_include ../listings/ch20-web-server/listing-20-09/src/main.rs:here}}
```

<span class="caption">Listing 20-9: Refactoring the `if` and `else` blocks to
contain only the code that differs between the two cases</span>

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.