nostarch/chapter21.md

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[TOC]

# Final Project: Building a Multithreaded Web Server

It’s been a long journey, but we’ve reached the end of the book. In this
chapter, we’ll build one more project together to demonstrate some of the
concepts we covered in the final chapters, as well as recap some earlier
lessons.

For our final project, we’ll make a web server that says “hello” and looks like
Figure 20-1 in a web browser.

Figure 20-1: Our final shared project

Here is our plan for building the web server:

1. Learn a bit about TCP and HTTP.
1. Listen for TCP connections on a socket.
1. Parse a small number of HTTP requests.
1. Create a proper HTTP response.
1. Improve the throughput of our server with a thread pool.

Before we get started, we should mention one detail: the method we’ll use won’t
be the best way to build a web server with Rust. Community members have
published a number of production-ready crates available at *https://crates.io*
that provide more complete web server and thread pool implementations than
we’ll build. However, our intention in this chapter is to help you learn, not
to take the easy route. Because Rust is a systems programming language, we can
choose the level of abstraction we want to work with and can go to a lower
level than is possible or practical in other languages. We’ll therefore write
the basic HTTP server and thread pool manually so you can learn the general
ideas and techniques behind the crates you might use in the future.

## 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 *Hypertext Transfer
Protocol* *(HTTP)* and *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 local 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() {
  1 let listener = TcpListener::bind("127.0.0.1:7878").unwrap();

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

      4 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` [1]. 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 isn’t normally accepted on this
port, so our server is unlikely to conflict with any other web server you might
have running on your machine, 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 function is called `bind`
because, 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 it’s
possible for binding to fail. For example, connecting to port 80 requires
administrator privileges (non-administrators can listen only on ports higher
than 1023), so if we tried to connect to port 80 without being an
administrator, binding wouldn’t work. Binding also wouldn’t work, for example,
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 [2] (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, we will read from the `TcpStream` to see
what the client sent and then write our response to the stream to send data
back to the client. 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 [3]; if there aren’t any errors, the
program prints a message [4]. 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 the program by invoking the `cargo
run` command 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

```
1 use std::{
    io::{prelude::*, BufReader},
    net::{TcpListener, TcpStream},
};

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

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

      2 handle_connection(stream);
    }
}

fn handle_connection(mut stream: TcpStream) {
  3 let buf_reader = BufReader::new(&mut stream);
  4 let http_request: Vec<_> = buf_reader
      5 .lines()
      6 .map(|result| result.unwrap())
      7 .take_while(|line| !line.is_empty())
        .collect();

  8 println!("Request: {:#?}", http_request);
}
```

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

We bring `std::io::prelude` and `std::io::BufReader` into scope to get access
to traits and types that let us read from and write to the stream [1]. 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 [2].

In the `handle_connection` function, we create a new `BufReader` instance that
wraps a mutable reference to the `stream` [3]. `BufReader` adds buffering by
managing calls to the `std::io::Read` trait methods for us.

We create a variable named `http_request` to collect the lines of the request
the browser sends to our server. We indicate that we want to collect these
lines in a vector by adding the `Vec<_>` type annotation [4].

`BufReader` implements the `std::io::BufRead` trait, which provides the `lines`
method [5]. The `lines` method returns an iterator of `Result<String,
std::io::Error>` by splitting the stream of data whenever it sees a newline
byte. To get each `String`, we map and `unwrap` each `Result` [6]. The `Result`
might be an error if the data isn’t valid UTF-8 or if there was a problem
reading from the stream. Again, a production program should handle these errors
more gracefully, but we’re choosing to stop the program in the error case for
simplicity.

The browser signals the end of an HTTP request by sending two newline
characters in a row, so to get one request from the stream, we take lines until
we get a line that is the empty string [7]. Once we’ve collected the lines into
the vector, we’re printing them out using pretty debug formatting [8] so we can
take a look at the instructions the web browser is sending to our server.

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 0.42s
     Running `target/debug/hello`
Request: [
    "GET / HTTP/1.1",
    "Host: 127.0.0.1:7878",
    "User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:99.0)
Gecko/20100101 Firefox/99.0",
    "Accept:
text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,*/*
;q=0.8",
    "Accept-Language: en-US,en;q=0.5",
    "Accept-Encoding: gzip, deflate, br",
    "DNT: 1",
    "Connection: keep-alive",
    "Upgrade-Insecure-Requests: 1",
    "Sec-Fetch-Dest: document",
    "Sec-Fetch-Mode: navigate",
    "Sec-Fetch-Site: none",
    "Sec-Fetch-User: ?1",
    "Cache-Control: max-age=0",
]
```

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 `GET` in the first line
of the request. 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, which means it is asking for
information.

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

We’re going to 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, and 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

```
fn handle_connection(mut stream: TcpStream) {
    let buf_reader = BufReader::new(&mut stream);
    let http_request: Vec<_> = buf_reader
        .lines()
        .map(|result| result.unwrap())
        .take_while(|line| !line.is_empty())
        .collect();

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

  2 stream.write_all(response.3 as_bytes()).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 [1]. Then we call `as_bytes` on our `response` to convert the
string data to bytes [3]. The `write_all` method on `stream` takes a `&[u8]`
and sends those bytes directly down the connection [2]. Because the `write_all`
operation could fail, we use `unwrap` on any error result as before. Again, in
a real application you would add error handling here.

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 handcoded receiving an HTTP
request and sending a response!

### Returning Real HTML

Let’s implement the functionality for returning more than a blank page. Create
the 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

```
use std::{
  1 fs,
    io::{prelude::*, BufReader},
    net::{TcpListener, TcpStream},
};
--snip--

fn handle_connection(mut stream: TcpStream) {
    let buf_reader = BufReader::new(&mut stream);
    let http_request: Vec<_> = buf_reader
        .lines()
        .map(|result| result.unwrap())
        .take_while(|line| !line.is_empty())
        .collect();

    let status_line = "HTTP/1.1 200 OK";
    let contents = fs::read_to_string("hello.html").unwrap();
    let length = contents.len();

  2 let response = format!(
        "{status_line}\r\n\
         Content-Length: {length}\r\n\r\n\
         {contents}"
    );

    stream.write_all(response.as_bytes()).unwrap();
}
```

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

We’ve added `fs` to the `use` statement to bring the standard library’s
filesystem module into scope [1]. The code for reading the contents of a file
to a string should look familiar; we used it 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 [2]. 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 `http_request` 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. At the moment, our server is very limited and
does not do 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

```
--snip--

fn handle_connection(mut stream: TcpStream) {
    let buf_reader = BufReader::new(&mut stream);
  1 let request_line = buf_reader
        .lines()
        .next()
        .unwrap()
        .unwrap();

  2 if request_line == "GET / HTTP/1.1" {
        let status_line = "HTTP/1.1 200 OK";
        let contents = fs::read_to_string("hello.html").unwrap();
        let length = contents.len();

        let response = format!(
            "{status_line}\r\n\
             Content-Length: {length}\r\n\r\n\
             {contents}"
        );

        stream.write_all(response.as_bytes()).unwrap();
  3 } else {
        // some other request
    }
}
```

Listing 20-6: Handling requests to */* differently from other requests

We’re only going to be looking at the first line of the HTTP request, so rather
than reading the entire request into a vector, we’re calling `next` to get the
first item from the iterator [1]. The first `unwrap` takes care of the `Option`
and stops the program if the iterator has no items. The second `unwrap` handles
the `Result` and has the same effect as the `unwrap` that was in the `map`
added in Listing 20-2.

Next, we check the `request_line` to see if it equals the request line of a GET
request to the */* path [2]. If it does, the `if` block returns the contents of
our HTML file.

If the `request_line` does *not* equal the GET request to the */* path, it
means we’ve received some other request. We’ll add code to the `else` block [3]
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

```
--snip--
} else {
  1 let status_line = "HTTP/1.1 404 NOT FOUND";
  2 let contents = fs::read_to_string("404.html").unwrap();
    let length = contents.len();

    let response = format!(
        "{status_line}\r\n\
         Content-Length: {length}\r\n\r\n
         {contents}"
    );

    stream.write_all(response.as_bytes()).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` [1]. The body of the response will be the HTML in the file
*404.html* [1]. 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 resultant code after replacing
the large `if` and `else` blocks.

Filename: src/main.rs

```
--snip--

fn handle_connection(mut stream: TcpStream) {
    --snip--

    let (status_line, filename) =
        if request_line == "GET / HTTP/1.1" {
            ("HTTP/1.1 200 OK", "hello.html")
        } else {
            ("HTTP/1.1 404 NOT FOUND", "404.html")
        };

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

    let response = format!(
        "{status_line}\r\n\
         Content-Length: {length}\r\n\r\n\
         {contents}"
    );

    stream.write_all(response.as_bytes()).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.

## Turning Our Single-Threaded Server into a Multithreaded Server

Right now, the server will process each request in turn, meaning it won’t
process a second connection until the first is finished processing. If the
server received more and more requests, this serial execution would be less and
less optimal. If the server receives a request that takes a long time to
process, subsequent requests will have to wait until the long request is
finished, even if the new requests can be processed quickly. We’ll need to fix
this, but first we’ll look at the problem in action.

### Simulating a Slow Request

We’ll look at how a slow-processing request can affect other requests made to
our current server implementation. Listing 20-10 implements handling a request
to */sleep* with a simulated slow response that will cause the server to sleep
for five seconds before responding.

Filename: src/main.rs

```
use std::{
    fs,
    io::{prelude::*, BufReader},
    net::{TcpListener, TcpStream},
    thread,
    time::Duration,
};
--snip--

fn handle_connection(mut stream: TcpStream) {
    --snip--

    let (status_line, filename) = 1 match &request_line[..] {
      2 "GET / HTTP/1.1" => ("HTTP/1.1 200 OK", "hello.html"),
      3 "GET /sleep HTTP/1.1" => {
            thread::sleep(Duration::from_secs(5));
            ("HTTP/1.1 200 OK", "hello.html")
        }
      4 _ => ("HTTP/1.1 404 NOT FOUND", "404.html"),
    };

    --snip--
}
```

Listing 20-10: Simulating a slow request by sleeping for five seconds

We switched from `if` to `match` now that we have three cases [1]. We need to
explicitly match on a slice of `request_line` to pattern-match against the
string literal values; `match` doesn’t do automatic referencing and
dereferencing, like the equality method does.

The first arm [2] is the same as the `if` block from Listing 20-9. The second
arm [3] matches a request to */sleep*. When that request is received, the
server will sleep for five seconds before rendering the successful HTML page.
The third arm [4] is the same as the `else` block from Listing 20-9.

You can see how primitive our server is: real libraries would handle the
recognition of multiple requests in a much less verbose way!

Start the server using `cargo run`. Then open two browser windows: one for
*http://127.0.0.1:7878* and the other for *http://127.0.0.1:7878/sleep*. If you
enter the */* URI a few times, as before, you’ll see it respond quickly. But if
you enter */sleep* and then load */*, you’ll see that */* waits until `sleep`
has slept for its full five seconds before loading.

There are multiple techniques we could use to avoid requests backing up behind
a slow request; the one we’ll implement is a thread pool.

### Improving Throughput with a Thread Pool

A *thread pool* is a group of spawned threads that are waiting and ready to
handle a task. When the program receives a new task, it assigns one of the
threads in the pool to the task, and that thread will process the task. The
remaining threads in the pool are available to handle any other tasks that come
in while the first thread is processing. When the first thread is done
processing its task, it’s returned to the pool of idle threads, ready to handle
a new task. A thread pool allows you to process connections concurrently,
increasing the throughput of your server.

We’ll limit the number of threads in the pool to a small number to protect us
from DoS attacks; if we had our program create a new thread for each request as
it came in, someone making 10 million requests to our server could create havoc
by using up all our server’s resources and grinding the processing of requests
to a halt.

Rather than spawning unlimited threads, then, we’ll have a fixed number of
threads waiting in the pool. Requests that come in are sent to the pool for
processing. The pool will maintain a queue of incoming requests. Each of the
threads in the pool will pop off a request from this queue, handle the request,
and then ask the queue for another request. With this design, we can process up
to N requests concurrently, where N is the number of threads. If each thread is
responding to a long-running request, subsequent requests can still back up in
the queue, but we’ve increased the number of long-running requests we can
handle before reaching that point.

This technique is just one of many ways to improve the throughput of a web
server. Other options you might explore are the fork/join model, the
single-threaded async I/O model, and the multithreaded async I/O model. If
you’re interested in this topic, you can read more about other solutions and
try to implement them; with a low-level language like Rust, all of these
options are possible.

Before we begin implementing a thread pool, let’s talk about what using the
pool should look like. When you’re trying to design code, writing the client
interface first can help guide your design. Write the API of the code so it’s
structured in the way you want to call it; then implement the functionality
within that structure rather than implementing the functionality and then
designing the public API.

Similar to how we used test-driven development in the project in Chapter 12,
we’ll use compiler-driven development here. We’ll write the code that calls the
functions we want, and then we’ll look at errors from the compiler to determine
what we should change next to get the code to work. Before we do that, however,
we’ll explore the technique we’re not going to use as a starting point.

#### Spawning a Thread for Each Request

First, let’s explore how our code might look if it did create a new thread for
every connection. As mentioned earlier, this isn’t our final plan due to the
problems with potentially spawning an unlimited number of threads, but it is a
starting point to get a working multithreaded server first. Then we’ll add the
thread pool as an improvement, and contrasting the two solutions will be easier.

Listing 20-11 shows the changes to make to `main` to spawn a new thread to
handle each stream within the `for` loop.

Filename: src/main.rs

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

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

        thread::spawn(|| {
            handle_connection(stream);
        });
    }
}
```

Listing 20-11: Spawning a new thread for each stream

As you learned in Chapter 16, `thread::spawn` will create a new thread and then
run the code in the closure in the new thread. If you run this code and load
*/sleep* in your browser, then */* in two more browser tabs, you’ll indeed see
that the requests to */* don’t have to wait for */sleep* to finish. However, as
we mentioned, this will eventually overwhelm the system because you’d be making
new threads without any limit.

#### Creating a Finite Number of Threads

We want our thread pool to work in a similar, familiar way so that switching
from threads to a thread pool doesn’t require large changes to the code that
uses our API. Listing 20-12 shows the hypothetical interface for a `ThreadPool`
struct we want to use instead of `thread::spawn`.

Filename: src/main.rs

```
fn main() {
    let listener = TcpListener::bind("127.0.0.1:7878").unwrap();
  1 let pool = ThreadPool::new(4);

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

      2 pool.execute(|| {
            handle_connection(stream);
        });
    }
}
```

Listing 20-12: Our ideal `ThreadPool` interface

We use `ThreadPool::new` to create a new thread pool with a configurable number
of threads, in this case four [1]. Then, in the `for` loop, `pool.execute` has
a similar interface as `thread::spawn` in that it takes a closure the pool
should run for each stream [2]. We need to implement `pool.execute` so it takes
the closure and gives it to a thread in the pool to run. This code won’t yet
compile, but we’ll try so that the compiler can guide us in how to fix it.

#### Building ThreadPool Using Compiler-Driven Development

Make the changes in Listing 20-12 to *src/main.rs*, and then let’s use the
compiler errors from `cargo check` to drive our development. Here is the first
error we get:

```
$ cargo check
    Checking hello v0.1.0 (file:///projects/hello)
error[E0433]: failed to resolve: use of undeclared type `ThreadPool`
  --> src/main.rs:11:16
   |
11 |     let pool = ThreadPool::new(4);
   |                ^^^^^^^^^^ use of undeclared type `ThreadPool`
```

Great! This error tells us we need a `ThreadPool` type or module, so we’ll
build one now. Our `ThreadPool` implementation will be independent of the kind
of work our web server is doing. So let’s switch the `hello` crate from a
binary crate to a library crate to hold our `ThreadPool` implementation. After
we change to a library crate, we could also use the separate thread pool
library for any work we want to do using a thread pool, not just for serving
web requests.

Create a *src/lib.rs* file that contains the following, which is the simplest
definition of a `ThreadPool` struct that we can have for now:

Filename: src/lib.rs

```
pub struct ThreadPool;
```

Then edit the *main.rs* file to bring `ThreadPool` into scope from the library
crate by adding the following code to the top of *src/main.rs*:

Filename: src/main.rs

```
use hello::ThreadPool;
```

This code still won’t work, but let’s check it again to get the next error that
we need to address:

```
$ cargo check
    Checking hello v0.1.0 (file:///projects/hello)
error[E0599]: no function or associated item named `new` found for struct
`ThreadPool` in the current scope
  --> src/main.rs:12:28
   |
12 |     let pool = ThreadPool::new(4);
   |                            ^^^ function or associated item not found in
`ThreadPool`
```

This error indicates that next we need to create an associated function named
`new` for `ThreadPool`. We also know that `new` needs to have one parameter
that can accept `4` as an argument and should return a `ThreadPool` instance.
Let’s implement the simplest `new` function that will have those
characteristics:

Filename: src/lib.rs

```
pub struct ThreadPool;

impl ThreadPool {
    pub fn new(size: usize) -> ThreadPool {
        ThreadPool
    }
}
```

We chose `usize` as the type of the `size` parameter because we know that a
negative number of threads doesn’t make any sense. We also know we’ll use this
`4` as the number of elements in a collection of threads, which is what the
`usize` type is for, as discussed in “Integer Types” on page XX.

Let’s check the code again:

```
$ cargo check
    Checking hello v0.1.0 (file:///projects/hello)
error[E0599]: no method named `execute` found for struct `ThreadPool` in the
current scope
  --> src/main.rs:17:14
   |
17 |         pool.execute(|| {
   |              ^^^^^^^ method not found in `ThreadPool`
```

Now the error occurs because we don’t have an `execute` method on `ThreadPool`.
Recall from “Creating a Finite Number of Threads” on page XX that we decided
our thread pool should have an interface similar to `thread::spawn`. In
addition, we’ll implement the `execute` function so it takes the closure it’s
given and gives it to an idle thread in the pool to run.

We’ll define the `execute` method on `ThreadPool` to take a closure as a
parameter. Recall from “Moving Captured Values Out of Closures and the Fn
Traits” on page XX that we can take closures as parameters with three different
traits: `Fn`, `FnMut`, and `FnOnce`. We need to decide which kind of closure to
use here. We know we’ll end up doing something similar to the standard library
`thread::spawn` implementation, so we can look at what bounds the signature of
`thread::spawn` has on its parameter. The documentation shows us the following:

```
pub fn spawn<F, T>(f: F) -> JoinHandle<T>
    where
        F: FnOnce() -> T,
        F: Send + 'static,
        T: Send + 'static,
```

The `F` type parameter is the one we’re concerned with here; the `T` type
parameter is related to the return value, and we’re not concerned with that. We
can see that `spawn` uses `FnOnce` as the trait bound on `F`. This is probably
what we want as well, because we’ll eventually pass the argument we get in
`execute` to `spawn`. We can be further confident that `FnOnce` is the trait we
want to use because the thread for running a request will only execute that
request’s closure one time, which matches the `Once` in `FnOnce`.

The `F` type parameter also has the trait bound `Send` and the lifetime bound
`'static`, which are useful in our situation: we need `Send` to transfer the
closure from one thread to another and `'static` because we don’t know how long
the thread will take to execute. Let’s create an `execute` method on
`ThreadPool` that will take a generic parameter of type `F` with these bounds:

Filename: src/lib.rs

```
impl ThreadPool {
    --snip--
    pub fn execute<F>(&self, f: F)
    where
        F: FnOnce() 1 + Send + 'static,
    {
    }
}
```

We still use the `()` after `FnOnce` [1] because this `FnOnce` represents a
closure that takes no parameters and returns the unit type `()`. Just like
function definitions, the return type can be omitted from the signature, but
even if we have no parameters, we still need the parentheses.

Again, this is the simplest implementation of the `execute` method: it does
nothing, but we’re only trying to make our code compile. Let’s check it again:

```
$ cargo check
    Checking hello v0.1.0 (file:///projects/hello)
    Finished dev [unoptimized + debuginfo] target(s) in 0.24s
```

It compiles! But note that if you try `cargo run` and make a request in the
browser, you’ll see the errors in the browser that we saw at the beginning of
the chapter. Our library isn’t actually calling the closure passed to `execute`
yet!

> Note: A saying you might hear about languages with strict compilers, such as
Haskell and Rust, is “if the code compiles, it works.” But this saying is not
universally true. Our project compiles, but it does absolutely nothing! If we
were building a real, complete project, this would be a good time to start
writing unit tests to check that the code compiles *and* has the behavior we
want.

#### Validating the Number of Threads in new

We aren’t doing anything with the parameters to `new` and `execute`. Let’s
implement the bodies of these functions with the behavior we want. To start,
let’s think about `new`. Earlier we chose an unsigned type for the `size`
parameter because a pool with a negative number of threads makes no sense.
However, a pool with zero threads also makes no sense, yet zero is a perfectly
valid `usize`. We’ll add code to check that `size` is greater than zero before
we return a `ThreadPool` instance and have the program panic if it receives a
zero by using the `assert!` macro, as shown in Listing 20-13.

Filename: src/lib.rs

```
impl ThreadPool {
    /// Create a new ThreadPool.
    ///
    /// The size is the number of threads in the pool.
    ///
  1 /// # Panics
    ///
    /// The `new` function will panic if the size is zero.
    pub fn new(size: usize) -> ThreadPool {
      2 assert!(size > 0);

        ThreadPool
    }

    --snip--
}
```

Listing 20-13: Implementing `ThreadPool::new` to panic if `size` is zero

We’ve also added some documentation for our `ThreadPool` with doc comments.
Note that we followed good documentation practices by adding a section that
calls out the situations in which our function can panic [1], as discussed in
Chapter 14. Try running `cargo doc --open` and clicking the `ThreadPool` struct
to see what the generated docs for `new` look like!

Instead of adding the `assert!` macro as we’ve done here [2], we could change
`new` into `build` and return a `Result` like we did with `Config::build` in
the I/O project in Listing 12-9. But we’ve decided in this case that trying to
create a thread pool without any threads should be an unrecoverable error. If
you’re feeling ambitious, try to write a function named `build` with the
following signature to compare with the `new` function:

```
pub fn build(
    size: usize
) -> Result<ThreadPool, PoolCreationError> {
```

#### Creating Space to Store the Threads

Now that we have a way to know we have a valid number of threads to store in
the pool, we can create those threads and store them in the `ThreadPool` struct
before returning the struct. But how do we “store” a thread? Let’s take another
look at the `thread::spawn` signature:

```
pub fn spawn<F, T>(f: F) -> JoinHandle<T>
    where
        F: FnOnce() -> T,
        F: Send + 'static,
        T: Send + 'static,
```

The `spawn` function returns a `JoinHandle<T>`, where `T` is the type that the
closure returns. Let’s try using `JoinHandle` too and see what happens. In our
case, the closures we’re passing to the thread pool will handle the connection
and not return anything, so `T` will be the unit type `()`.

The code in Listing 20-14 will compile but doesn’t create any threads yet.
We’ve changed the definition of `ThreadPool` to hold a vector of
`thread::JoinHandle<()>` instances, initialized the vector with a capacity of
`size`, set up a `for` loop that will run some code to create the threads, and
returned a `ThreadPool` instance containing them.

Filename: src/lib.rs

```
1 use std::thread;

pub struct ThreadPool {
  2 threads: Vec<thread::JoinHandle<()>>,
}

impl ThreadPool {
    --snip--
    pub fn new(size: usize) -> ThreadPool {
        assert!(size > 0);

      3 let mut threads = Vec::with_capacity(size);

        for _ in 0..size {
            // create some threads and store them in the vector
        }

        ThreadPool { threads }
    }
    --snip--
}
```

Listing 20-14: Creating a vector for `ThreadPool` to hold the threads

We’ve brought `std::thread` into scope in the library crate [1] because we’re
using `thread::JoinHandle` as the type of the items in the vector in
`ThreadPool` [2].

Once a valid size is received, our `ThreadPool` creates a new vector that can
hold `size` items [3]. The `with_capacity` function performs the same task as
`Vec::new` but with an important difference: it pre-allocates space in the
vector. Because we know we need to store `size` elements in the vector, doing
this allocation up front is slightly more efficient than using `Vec::new`,
which resizes itself as elements are inserted.

When you run `cargo check` again, it should succeed.

#### Sending Code from the ThreadPool to a Thread

We left a comment in the `for` loop in Listing 20-14 regarding the creation of
threads. Here, we’ll look at how we actually create threads. The standard
library provides `thread::spawn` as a way to create threads, and
`thread::spawn` expects to get some code the thread should run as soon as the
thread is created. However, in our case, we want to create the threads and have
them *wait* for code that we’ll send later. The standard library’s
implementation of threads doesn’t include any way to do that; we have to
implement it manually.

We’ll implement this behavior by introducing a new data structure between the
`ThreadPool` and the threads that will manage this new behavior. We’ll call
this data structure *Worker*, which is a common term in pooling
implementations. The `Worker` picks up code that needs to be run and runs the
code in its thread.

Think of people working in the kitchen at a restaurant: the workers wait until
orders come in from customers, and then they’re responsible for taking those
orders and filling them.

Instead of storing a vector of `JoinHandle<()>` instances in the thread pool,
we’ll store instances of the `Worker` struct. Each `Worker` will store a single
`JoinHandle<()>` instance. Then we’ll implement a method on `Worker` that will
take a closure of code to run and send it to the already running thread for
execution. We’ll also give each `Worker` an `id` so we can distinguish between
the different instances of `Worker` in the pool when logging or debugging.

Here is the new process that will happen when we create a `ThreadPool`. We’ll
implement the code that sends the closure to the thread after we have `Worker`
set up in this way:

1. Define a `Worker` struct that holds an `id` and a `JoinHandle<()>`.
1. Change `ThreadPool` to hold a vector of `Worker` instances.
1. Define a `Worker::new` function that takes an `id` number and returns a
`Worker` instance that holds the `id` and a thread spawned with an empty
closure.
1. In `ThreadPool::new`, use the `for` loop counter to generate an `id`, create
a new `Worker` with that `id`, and store the `Worker` in the vector.

If you’re up for a challenge, try implementing these changes on your own before
looking at the code in Listing 20-15.

Ready? Here is Listing 20-15 with one way to make the preceding modifications.

Filename: src/lib.rs

```
use std::thread;

pub struct ThreadPool {
  1 workers: Vec<Worker>,
}

impl ThreadPool {
    --snip--
    pub fn new(size: usize) -> ThreadPool {
        assert!(size > 0);

        let mut workers = Vec::with_capacity(size);

      2 for id in 0..size {
          3 workers.push(Worker::new(id));
        }

        ThreadPool { workers }
    }
    --snip--
}

4 struct Worker {
    id: usize,
    thread: thread::JoinHandle<()>,
}

impl Worker {
  5 fn new(id: usize) -> Worker {
      6 let thread = thread::spawn(|| {});

        Worker { 7 id, 8 thread }
    }
}
```

Listing 20-15: Modifying `ThreadPool` to hold `Worker` instances instead of
holding threads directly

We’ve changed the name of the field on `ThreadPool` from `threads` to `workers`
because it’s now holding `Worker` instances instead of `JoinHandle<()>`
instances [1]. We use the counter in the `for` loop [2] as an argument to
`Worker::new`, and we store each new `Worker` in the vector named `workers` [3].

External code (like our server in *src/main.rs*) doesn’t need to know the
implementation details regarding using a `Worker` struct within `ThreadPool`,
so we make the `Worker` struct [4] and its `new` function [5] private. The
`Worker::new` function uses the `id` we give it [7] and stores a
`JoinHandle<()>` instance [8] that is created by spawning a new thread using an
empty closure [6].

> Note: If the operating system can’t create a thread because there aren’t
enough system resources, `thread::spawn` will panic. That will cause our whole
server to panic, even though the creation of some threads might succeed. For
simplicity’s sake, this behavior is fine, but in a production thread pool
implementation, you’d likely want to use `std::thread::Builder` and its `spawn`
method that returns `Result` instead.

This code will compile and will store the number of `Worker` instances we
specified as an argument to `ThreadPool::new`. But we’re *still* not processing
the closure that we get in `execute`. Let’s look at how to do that next.

#### Sending Requests to Threads via Channels

The next problem we’ll tackle is that the closures given to `thread::spawn` do
absolutely nothing. Currently, we get the closure we want to execute in the
`execute` method. But we need to give `thread::spawn` a closure to run when we
create each `Worker` during the creation of the `ThreadPool`.

We want the `Worker` structs that we just created to fetch the code to run from
a queue held in the `ThreadPool` and send that code to its thread to run.

The channels we learned about in Chapter 16—a simple way to communicate between
two threads—would be perfect for this use case. We’ll use a channel to function
as the queue of jobs, and `execute` will send a job from the `ThreadPool` to
the `Worker` instances, which will send the job to its thread. Here is the plan:

1. The `ThreadPool` will create a channel and hold on to the sender.
1. Each `Worker` will hold on to the receiver.
1. We’ll create a new `Job` struct that will hold the closures we want to send
down the channel.
1. The `execute` method will send the job it wants to execute through the
sender.
1. In its thread, the `Worker` will loop over its receiver and execute the
closures of any jobs it receives.

Let’s start by creating a channel in `ThreadPool::new` and holding the sender
in the `ThreadPool` instance, as shown in Listing 20-16. The `Job` struct
doesn’t hold anything for now but will be the type of item we’re sending down
the channel.

Filename: src/lib.rs

```
use std::{sync::mpsc, thread};

pub struct ThreadPool {
    workers: Vec<Worker>,
    sender: mpsc::Sender<Job>,
}

struct Job;

impl ThreadPool {
    --snip--
    pub fn new(size: usize) -> ThreadPool {
        assert!(size > 0);

      1 let (sender, receiver) = mpsc::channel();

        let mut workers = Vec::with_capacity(size);

        for id in 0..size {
            workers.push(Worker::new(id));
        }

        ThreadPool { workers, 2 sender }
    }
    --snip--
}
```

Listing 20-16: Modifying `ThreadPool` to store the sender of a channel that
transmits `Job` instances

In `ThreadPool::new`, we create our new channel [1] and have the pool hold the
sender [2]. This will successfully compile.

Let’s try passing a receiver of the channel into each `Worker` as the thread
pool creates the channel. We know we want to use the receiver in the thread
that the `Worker` instances spawn, so we’ll reference the `receiver` parameter
in the closure. The code in Listing 20-17 won’t quite compile yet.

Filename: src/lib.rs

```
impl ThreadPool {
    --snip--
    pub fn new(size: usize) -> ThreadPool {
        assert!(size > 0);

        let (sender, receiver) = mpsc::channel();

        let mut workers = Vec::with_capacity(size);

        for id in 0..size {
          1 workers.push(Worker::new(id, receiver));
        }

        ThreadPool { workers, sender }
    }
    --snip--
}

--snip--

impl Worker {
    fn new(id: usize, receiver: mpsc::Receiver<Job>) -> Worker {
        let thread = thread::spawn(|| {
          2 receiver;
        });

        Worker { id, thread }
    }
}
```

Listing 20-17: Passing the receiver to each `Worker`

We’ve made some small and straightforward changes: we pass the receiver into
`Worker::new` [1], and then we use it inside the closure [2].

When we try to check this code, we get this error:

```
$ cargo check
    Checking hello v0.1.0 (file:///projects/hello)
error[E0382]: use of moved value: `receiver`
  --> src/lib.rs:26:42
   |
21 |         let (sender, receiver) = mpsc::channel();
   |                      -------- move occurs because `receiver` has type
`std::sync::mpsc::Receiver<Job>`, which does not implement the `Copy` trait
...
26 |             workers.push(Worker::new(id, receiver));
   |                                          ^^^^^^^^ value moved here, in
previous iteration of loop
```

The code is trying to pass `receiver` to multiple `Worker` instances. This
won’t work, as you’ll recall from Chapter 16: the channel implementation that
Rust provides is multiple *producer*, single *consumer*. This means we can’t
just clone the consuming end of the channel to fix this code. We also don’t
want to send a message multiple times to multiple consumers; we want one list
of messages with multiple `Worker` instances such that each message gets
processed once.

Additionally, taking a job off the channel queue involves mutating the
`receiver`, so the threads need a safe way to share and modify `receiver`;
otherwise, we might get race conditions (as covered in Chapter 16).

Recall the thread-safe smart pointers discussed in Chapter 16: to share
ownership across multiple threads and allow the threads to mutate the value, we
need to use `Arc<Mutex<T>>`. The `Arc` type will let multiple `Worker`
instances own the receiver, and `Mutex` will ensure that only one `Worker` gets
a job from the receiver at a time. Listing 20-18 shows the changes we need to
make.

Filename: src/lib.rs

```
use std::{
    sync::{mpsc, Arc, Mutex},
    thread,
};
--snip--

impl ThreadPool {
    --snip--
    pub fn new(size: usize) -> ThreadPool {
        assert!(size > 0);

        let (sender, receiver) = mpsc::channel();

      1 let receiver = Arc::new(Mutex::new(receiver));

        let mut workers = Vec::with_capacity(size);

        for id in 0..size {
            workers.push(
                Worker::new(id, Arc::clone(& 2 receiver))
            );
        }

        ThreadPool { workers, sender }
    }

    --snip--
}

--snip--

impl Worker {
    fn new(
        id: usize,
        receiver: Arc<Mutex<mpsc::Receiver<Job>>>,
    ) -> Worker {
        --snip--
    }
}
```

Listing 20-18: Sharing the receiver among the `Worker` instances using `Arc`
and `Mutex`

In `ThreadPool::new`, we put the receiver in an `Arc` and a `Mutex` [1]. For
each new `Worker`, we clone the `Arc` to bump the reference count so the
`Worker` instances can share ownership of the receiver [2].

With these changes, the code compiles! We’re getting there!

#### Implementing the execute Method

Let’s finally implement the `execute` method on `ThreadPool`. We’ll also change
`Job` from a struct to a type alias for a trait object that holds the type of
closure that `execute` receives. As discussed in “Creating Type Synonyms with
Type Aliases” on page XX, type aliases allow us to make long types shorter for
ease of use. Look at Listing 20-19.

Filename: src/lib.rs

```
--snip--

type Job = Box<dyn FnOnce() + Send + 'static>;

impl ThreadPool {
    --snip--

    pub fn execute<F>(&self, f: F)
    where
        F: FnOnce() + Send + 'static,
    {
      1 let job = Box::new(f);

      2 self.sender.send(job).unwrap();
    }
}

--snip--
```

Listing 20-19: Creating a `Job` type alias for a `Box` that holds each closure
and then sending the job down the channel

After creating a new `Job` instance using the closure we get in `execute` [1],
we send that job down the sending end of the channel [2]. We’re calling
`unwrap` on `send` for the case that sending fails. This might happen if, for
example, we stop all our threads from executing, meaning the receiving end has
stopped receiving new messages. At the moment, we can’t stop our threads from
executing: our threads continue executing as long as the pool exists. The
reason we use `unwrap` is that we know the failure case won’t happen, but the
compiler doesn’t know that.

But we’re not quite done yet! In the `Worker`, our closure being passed to
`thread::spawn` still only *references* the receiving end of the channel.
Instead, we need the closure to loop forever, asking the receiving end of the
channel for a job and running the job when it gets one. Let’s make the change
shown in Listing 20-20 to `Worker::new`.

Filename: src/lib.rs

```
--snip--

impl Worker {
    fn new(
        id: usize,
        receiver: Arc<Mutex<mpsc::Receiver<Job>>>,
    ) -> Worker {
        let thread = thread::spawn(move || loop {
            let job = receiver
              1 .lock()
              2 .unwrap()
              3 .recv()
              4 .unwrap();

            println!("Worker {id} got a job; executing.");

            job();
        });

        Worker { id, thread }
    }
}
```

Listing 20-20: Receiving and executing the jobs in the `Worker` instance’s
thread

Here, we first call `lock` on the `receiver` to acquire the mutex [1], and then
we call `unwrap` to panic on any errors [2]. Acquiring a lock might fail if the
mutex is in a *poisoned* state, which can happen if some other thread panicked
while holding the lock rather than releasing the lock. In this situation,
calling `unwrap` to have this thread panic is the correct action to take. Feel
free to change this `unwrap` to an `expect` with an error message that is
meaningful to you.

If we get the lock on the mutex, we call `recv` to receive a `Job` from the
channel [3]. A final `unwrap` moves past any errors here as well [4], which
might occur if the thread holding the sender has shut down, similar to how the
`send` method returns `Err` if the receiver shuts down.

The call to `recv` blocks, so if there is no job yet, the current thread will
wait until a job becomes available. The `Mutex<T>` ensures that only one
`Worker` thread at a time is trying to request a job.

Our thread pool is now in a working state! Give it a `cargo run` and make some
requests:

```
$ cargo run
   Compiling hello v0.1.0 (file:///projects/hello)
warning: field is never read: `workers`
 --> src/lib.rs:7:5
  |
7 |     workers: Vec<Worker>,
  |     ^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: field is never read: `id`
  --> src/lib.rs:48:5
   |
48 |     id: usize,
   |     ^^^^^^^^^

warning: field is never read: `thread`
  --> src/lib.rs:49:5
   |
49 |     thread: thread::JoinHandle<()>,
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

warning: `hello` (lib) generated 3 warnings
    Finished dev [unoptimized + debuginfo] target(s) in 1.40s
     Running `target/debug/hello`
Worker 0 got a job; executing.
Worker 2 got a job; executing.
Worker 1 got a job; executing.
Worker 3 got a job; executing.
Worker 0 got a job; executing.
Worker 2 got a job; executing.
Worker 1 got a job; executing.
Worker 3 got a job; executing.
Worker 0 got a job; executing.
Worker 2 got a job; executing.
```

Success! We now have a thread pool that executes connections asynchronously.
There are never more than four threads created, so our system won’t get
overloaded if the server receives a lot of requests. If we make a request to
*/sleep*, the server will be able to serve other requests by having another
thread run them.

> Note: If you open */sleep* in multiple browser windows simultaneously, they
might load one at a time in five-second intervals. Some web browsers execute
multiple instances of the same request sequentially for caching reasons. This
limitation is not caused by our web server.

After learning about the `while let` loop in Chapter 18, you might be wondering
why we didn’t write the `Worker` thread code as shown in Listing 20-21.

Filename: src/lib.rs

```
--snip--

impl Worker {
    fn new(
        id: usize,
        receiver: Arc<Mutex<mpsc::Receiver<Job>>>,
    ) -> Worker {
        let thread = thread::spawn(move || {
            while let Ok(job) = receiver.lock().unwrap().recv() {
                println!("Worker {id} got a job; executing.");

                job();
            }
        });

        Worker { id, thread }
    }
}
```

Listing 20-21: An alternative implementation of `Worker::new` using `while let`

This code compiles and runs but doesn’t result in the desired threading
behavior: a slow request will still cause other requests to wait to be
processed. The reason is somewhat subtle: the `Mutex` struct has no public
`unlock` method because the ownership of the lock is based on the lifetime of
the `MutexGuard<T>` within the `LockResult<MutexGuard<T>>` that the `lock`
method returns. At compile time, the borrow checker can then enforce the rule
that a resource guarded by a `Mutex` cannot be accessed unless we hold the
lock. However, this implementation can also result in the lock being held
longer than intended if we aren’t mindful of the lifetime of the
`MutexGuard<T>`.

The code in Listing 20-20 that uses `let job =
receiver.lock().unwrap().recv().unwrap();` works because with `let`, any
temporary values used in the expression on the right-hand side of the equal
sign are immediately dropped when the `let` statement ends. However, `while
let` (and `if let` and `match`) does not drop temporary values until the end of
the associated block. In Listing 20-21, the lock remains held for the duration
of the call to `job()`, meaning other `Worker` instances cannot receive jobs.

## Graceful Shutdown and Cleanup

The code in Listing 20-20 is responding to requests asynchronously through the
use of a thread pool, as we intended. We get some warnings about the `workers`,
`id`, and `thread` fields that we’re not using in a direct way that reminds us
we’re not cleaning up anything. When we use the less elegant ctrl-C method to
halt the main thread, all other threads are stopped immediately as well, even
if they’re in the middle of serving a request.

Next, then, we’ll implement the `Drop` trait to call `join` on each of the
threads in the pool so they can finish the requests they’re working on before
closing. Then we’ll implement a way to tell the threads they should stop
accepting new requests and shut down. To see this code in action, we’ll modify
our server to accept only two requests before gracefully shutting down its
thread pool.

### Implementing the Drop Trait on ThreadPool

Let’s start with implementing `Drop` on our thread pool. When the pool is
dropped, our threads should all join to make sure they finish their work.
Listing 20-22 shows a first attempt at a `Drop` implementation; this code won’t
quite work yet.

Filename: src/lib.rs

```
impl Drop for ThreadPool {
    fn drop(&mut self) {
      1 for worker in &mut self.workers {
          2 println!("Shutting down worker {}", worker.id);

          3 worker.thread.join().unwrap();
        }
    }
}
```

Listing 20-22: Joining each thread when the thread pool goes out of scope

First we loop through each of the thread pool `workers` [1]. We use `&mut` for
this because `self` is a mutable reference, and we also need to be able to
mutate `worker`. For each `worker`, we print a message saying that this
particular `Worker` instance is shutting down [2], and then we call `join` on
that `Worker` instance’s thread [3]. If the call to `join` fails, we use
`unwrap` to make Rust panic and go into an ungraceful shutdown.

Here is the error we get when we compile this code:

```
error[E0507]: cannot move out of `worker.thread` which is behind a mutable
reference
    --> src/lib.rs:52:13
     |
52   |             worker.thread.join().unwrap();
     |             ^^^^^^^^^^^^^ ------ `worker.thread` moved due to this
method call
     |             |
     |             move occurs because `worker.thread` has type
`JoinHandle<()>`, which does not implement the `Copy` trait
     |
note: this function takes ownership of the receiver `self`, which moves
`worker.thread`
```

The error tells us we can’t call `join` because we only have a mutable borrow
of each `worker` and `join` takes ownership of its argument. To solve this
issue, we need to move the thread out of the `Worker` instance that owns
`thread` so `join` can consume the thread. We did this in Listing 17-15: if
`Worker` holds an `Option<thread::JoinHandle<()>>` instead, we can call the
`take` method on the `Option` to move the value out of the `Some` variant and
leave a `None` variant in its place. In other words, a `Worker` that is running
will have a `Some` variant in `thread`, and when we want to clean up a
`Worker`, we’ll replace `Some` with `None` so the `Worker` doesn’t have a
thread to run.

So we know we want to update the definition of `Worker` like this:

Filename: src/lib.rs

```
struct Worker {
    id: usize,
    thread: Option<thread::JoinHandle<()>>,
}
```

Now let’s lean on the compiler to find the other places that need to change.
Checking this code, we get two errors:

```
error[E0599]: no method named `join` found for enum `Option` in the current
scope
  --> src/lib.rs:52:27
   |
52 |             worker.thread.join().unwrap();
   |                           ^^^^ method not found in
`Option<JoinHandle<()>>`

error[E0308]: mismatched types
  --> src/lib.rs:72:22
   |
72 |         Worker { id, thread }
   |                      ^^^^^^ expected enum `Option`, found struct
`JoinHandle`
   |
   = note: expected enum `Option<JoinHandle<()>>`
            found struct `JoinHandle<_>`
help: try wrapping the expression in `Some`
   |
72 |         Worker { id, thread: Some(thread) }
   |                      +++++++++++++      +
```

Let’s address the second error, which points to the code at the end of
`Worker::new`; we need to wrap the `thread` value in `Some` when we create a
new `Worker`. Make the following changes to fix this error:

Filename: src/lib.rs

```
impl Worker {
    fn new(
        id: usize,
        receiver: Arc<Mutex<mpsc::Receiver<Job>>>,
    ) -> Worker {
        --snip--

        Worker {
            id,
            thread: Some(thread),
        }
    }
}
```

The first error is in our `Drop` implementation. We mentioned earlier that we
intended to call `take` on the `Option` value to move `thread` out of `worker`.
The following changes will do so:

Filename: src/lib.rs

```
impl Drop for ThreadPool {
    fn drop(&mut self) {
        for worker in &mut self.workers {
            println!("Shutting down worker {}", worker.id);

          1 if let Some(thread) = worker.thread.take() {
              2 thread.join().unwrap();
            }
        }
    }
}
```

As discussed in Chapter 17, the `take` method on `Option` takes the `Some`
variant out and leaves `None` in its place. We’re using `if let` to destructure
the `Some` and get the thread [1]; then we call `join` on the thread [2]. If a
`Worker` instance’s thread is already `None`, we know that `Worker` has already
had its thread cleaned up, so nothing happens in that case.

### Signaling to the Threads to Stop Listening for Jobs

With all the changes we’ve made, our code compiles without any warnings.
However, the bad news is that this code doesn’t function the way we want it to
yet. The key is the logic in the closures run by the threads of the `Worker`
instances: at the moment, we call `join`, but that won’t shut down the threads,
because they `loop` forever looking for jobs. If we try to drop our
`ThreadPool` with our current implementation of `drop`, the main thread will
block forever, waiting for the first thread to finish.

To fix this problem, we’ll need a change in the `ThreadPool` `drop`
implementation and then a change in the `Worker` loop.

First we’ll change the `ThreadPool` `drop` implementation to explicitly drop
the `sender` before waiting for the threads to finish. Listing 20-23 shows the
changes to `ThreadPool` to explicitly drop `sender`. We use the same `Option`
and `take` technique as we did with the thread to be able to move `sender` out
of `ThreadPool`.

Filename: src/lib.rs

```
pub struct ThreadPool {
    workers: Vec<Worker>,
    sender: Option<mpsc::Sender<Job>>,
}
--snip--
impl ThreadPool {
    pub fn new(size: usize) -> ThreadPool {
        --snip--

        ThreadPool {
            workers,
            sender: Some(sender),
        }
    }

    pub fn execute<F>(&self, f: F)
    where
        F: FnOnce() + Send + 'static,
    {
        let job = Box::new(f);

        self.sender
            .as_ref()
            .unwrap()
            .send(job)
            .unwrap();
    }
}

impl Drop for ThreadPool {
    fn drop(&mut self) {
      1 drop(self.sender.take());

        for worker in &mut self.workers {
            println!("Shutting down worker {}", worker.id);

            if let Some(thread) = worker.thread.take() {
                thread.join().unwrap();
            }
        }
    }
}
```

Listing 20-23: Explicitly dropping `sender` before joining the `Worker` threads

Dropping `sender` [1] closes the channel, which indicates no more messages will
be sent. When that happens, all the calls to `recv` that the `Worker` instances
do in the infinite loop will return an error. In Listing 20-24, we change the
`Worker` loop to gracefully exit the loop in that case, which means the threads
will finish when the `ThreadPool` `drop` implementation calls `join` on them.

Filename: src/lib.rs

```
impl Worker {
    fn new(
        id: usize,
        receiver: Arc<Mutex<mpsc::Receiver<Job>>>,
    ) -> Worker {
        let thread = thread::spawn(move || loop {
            let message = receiver.lock().unwrap().recv();

            match message {
                Ok(job) => {
                    println!(
                        "Worker {id} got a job; executing."
                    );

                    job();
                }
                Err(_) => {
                    println!(
                        "Worker {id} shutting down."
                    );
                    break;
                }
            }
        });

        Worker {
            id,
            thread: Some(thread),
        }
    }
}
```

Listing 20-24: Explicitly breaking out of the loop when `recv` returns an error

To see this code in action, let’s modify `main` to accept only two requests
before gracefully shutting down the server, as shown in Listing 20-25.

Filename: src/main.rs

```
fn main() {
    let listener = TcpListener::bind("127.0.0.1:7878").unwrap();
    let pool = ThreadPool::new(4);

    for stream in listener.incoming().take(2) {
        let stream = stream.unwrap();

        pool.execute(|| {
            handle_connection(stream);
        });
    }

    println!("Shutting down.");
}
```

Listing 20-25: Shutting down the server after serving two requests by exiting
the loop

You wouldn’t want a real-world web server to shut down after serving only two
requests. This code just demonstrates that the graceful shutdown and cleanup is
in working order.

The `take` method is defined in the `Iterator` trait and limits the iteration
to the first two items at most. The `ThreadPool` will go out of scope at the
end of `main`, and the `drop` implementation will run.

Start the server with `cargo run`, and make three requests. The third request
should error, and in your terminal you should see output similar to this:

```
$ cargo run
   Compiling hello v0.1.0 (file:///projects/hello)
    Finished dev [unoptimized + debuginfo] target(s) in 1.0s
     Running `target/debug/hello`
Worker 0 got a job; executing.
Shutting down.
Shutting down worker 0
Worker 3 got a job; executing.
Worker 1 disconnected; shutting down.
Worker 2 disconnected; shutting down.
Worker 3 disconnected; shutting down.
Worker 0 disconnected; shutting down.
Shutting down worker 1
Shutting down worker 2
Shutting down worker 3
```

You might see a different ordering of `Worker` IDs and messages printed. We can
see how this code works from the messages: `Worker` instances 0 and 3 got the
first two requests. The server stopped accepting connections after the second
connection, and the `Drop` implementation on `ThreadPool` starts executing
before `Worker` 3 even starts its job. Dropping the `sender` disconnects all
the `Worker` instances and tells them to shut down. The `Worker` instances each
print a message when they disconnect, and then the thread pool calls `join` to
wait for each `Worker` thread to finish.

Notice one interesting aspect of this particular execution: the `ThreadPool`
dropped the `sender`, and before any `Worker` received an error, we tried to
join `Worker` 0. `Worker` 0 had not yet gotten an error from `recv`, so the
main thread blocked, waiting for `Worker` 0 to finish. In the meantime,
`Worker` 3 received a job and then all threads received an error. When `Worker`
0 finished, the main thread waited for the rest of the `Worker` instances to
finish. At that point, they had all exited their loops and stopped.

Congrats! We’ve now completed our project; we have a basic web server that uses
a thread pool to respond asynchronously. We’re able to perform a graceful
shutdown of the server, which cleans up all the threads in the pool. See
*https://www.nostarch.com/Rust2021* to download the full code for this chapter
for reference.

We could do more here! If you want to continue enhancing this project, here are
some ideas:

* Add more documentation to `ThreadPool` and its public methods.
* Add tests of the library’s functionality.
* Change calls to `unwrap` to more robust error handling.
* Use `ThreadPool` to perform some task other than serving web requests.
* Find a thread pool crate on *https://crates.io* and implement a similar web
server using the crate instead. Then compare its API and robustness to the
thread pool we implemented.

## Summary

Well done! You’ve made it to the end of the book! We want to thank you for
joining us on this tour of Rust. You’re now ready to implement your own Rust
projects and help with other people’s projects. Keep in mind that there is a
welcoming community of other Rustaceans who would love to help you with any
challenges you encounter on your Rust journey.