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<main>
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<h1 id="解析数据帧"><a class="header" href="#解析数据帧">解析数据帧</a></h1>
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<p>现在，鉴于大家已经掌握了 Tokio 的基本 I/O 用法，我们可以开始实现 <code>mini-redis</code> 的帧 <code>frame</code>。通过帧可以将字节流转换成帧组成的流。每个帧就是一个数据单元，例如客户端发送的一次请求就是一个帧。</p>
|
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<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
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|
</span><span class="boring">fn main() {
|
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</span>use bytes::Bytes;
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|
enum Frame {
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|
Simple(String),
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|
Error(String),
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|
|
Integer(u64),
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|
|
Bulk(Bytes),
|
|
|
Null,
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|
Array(Vec<Frame>),
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|
}
|
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<span class="boring">}</span></code></pre>
|
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|
<p>可以看到帧除了数据之外，并不具备任何语义。命令解析和实现会在更高的层次进行(相比帧解析层）。我们再来通过 HTTP 的帧来帮大家加深下相关的理解：</p>
|
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<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>enum HttpFrame {
|
|
|
RequestHead {
|
|
|
method: Method,
|
|
|
uri: Uri,
|
|
|
version: Version,
|
|
|
headers: HeaderMap,
|
|
|
},
|
|
|
ResponseHead {
|
|
|
status: StatusCode,
|
|
|
version: Version,
|
|
|
headers: HeaderMap,
|
|
|
},
|
|
|
BodyChunk {
|
|
|
chunk: Bytes,
|
|
|
},
|
|
|
}
|
|
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<span class="boring">}</span></code></pre>
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|
|
<p>为了实现 <code>mini-redis</code> 的帧，我们需要一个 <code>Connection</code> 结构体，里面包含了一个 <code>TcpStream</code> 以及对帧进行读写的方法:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>use tokio::net::TcpStream;
|
|
|
use mini_redis::{Frame, Result};
|
|
|
|
|
|
struct Connection {
|
|
|
stream: TcpStream,
|
|
|
// ... 这里定义其它字段
|
|
|
}
|
|
|
|
|
|
impl Connection {
|
|
|
/// 从连接读取一个帧
|
|
|
///
|
|
|
/// 如果遇到EOF，则返回 None
|
|
|
pub async fn read_frame(&mut self)
|
|
|
-> Result<Option<Frame>>
|
|
|
{
|
|
|
// 具体实现
|
|
|
}
|
|
|
|
|
|
/// 将帧写入到连接中
|
|
|
pub async fn write_frame(&mut self, frame: &Frame)
|
|
|
-> Result<()>
|
|
|
{
|
|
|
// 具体实现
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>关于 Redis 协议的说明，可以看看<a href="https://redis.io/topics/protocol">官方文档</a>，<code>Connection</code> 代码的完整实现见<a href="https://github.com/tokio-rs/mini-redis/blob/tutorial/src/connection.rs">这里</a>.</p>
|
|
|
<h2 id="缓冲读取buffered-reads"><a class="header" href="#缓冲读取buffered-reads">缓冲读取(Buffered Reads)</a></h2>
|
|
|
<p><code>read_frame</code> 方法会等到一个完整的帧都读取完毕后才返回，与之相比，它底层调用的<code>TcpStream::read</code> 只会返回任意多的数据(填满传入的缓冲区 buffer )，它可能返回帧的一部分、一个帧、多个帧，总之这种读取行为是不确定的。</p>
|
|
|
<p>当 <code>read_frame</code> 的底层调用 <code>TcpStream::read</code> 读取到部分帧时，会将数据先缓冲起来，接着继续等待并读取数据。如果读到多个帧，那第一个帧会被返回，然后剩下的数据依然被缓冲起来，等待下一次 <code>read_frame</code> 被调用。</p>
|
|
|
<p>为了实现这种功能，我们需要为 <code>Connection</code> 增加一个读取缓冲区。数据首先从 <code>socket</code> 中读取到缓冲区中，接着这些数据会被解析为帧，当一个帧被解析后，该帧对应的数据会从缓冲区被移除。</p>
|
|
|
<p>这里使用 <a href="https://docs.rs/bytes/1/bytes/struct.BytesMut.html"><code>BytesMut</code></a> 作为缓冲区类型，它是 <a href="https://docs.rs/bytes/1/bytes/struct.Bytes.html"><code>Bytes</code></a> 的可变版本。</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>use bytes::BytesMut;
|
|
|
use tokio::net::TcpStream;
|
|
|
|
|
|
pub struct Connection {
|
|
|
stream: TcpStream,
|
|
|
buffer: BytesMut,
|
|
|
}
|
|
|
|
|
|
impl Connection {
|
|
|
pub fn new(stream: TcpStream) -> Connection {
|
|
|
Connection {
|
|
|
stream,
|
|
|
// 分配一个缓冲区，具有4kb的缓冲长度
|
|
|
buffer: BytesMut::with_capacity(4096),
|
|
|
}
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>接下来，实现 <code>read_frame</code> 方法:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>use tokio::io::AsyncReadExt;
|
|
|
use bytes::Buf;
|
|
|
use mini_redis::Result;
|
|
|
|
|
|
pub async fn read_frame(&mut self)
|
|
|
-> Result<Option<Frame>>
|
|
|
{
|
|
|
loop {
|
|
|
// 尝试从缓冲区的数据中解析出一个数据帧，
|
|
|
// 只有当数据足够被解析时，才返回对应的帧
|
|
|
if let Some(frame) = self.parse_frame()? {
|
|
|
return Ok(Some(frame));
|
|
|
}
|
|
|
|
|
|
// 如果缓冲区中的数据还不足以被解析为一个数据帧，
|
|
|
// 那么我们需要从 socket 中读取更多的数据
|
|
|
//
|
|
|
// 读取成功时，会返回读取到的字节数，0 代表着读到了数据流的末尾
|
|
|
if 0 == self.stream.read_buf(&mut self.buffer).await? {
|
|
|
// 代码能执行到这里，说明了对端关闭了连接，
|
|
|
// 需要看看缓冲区是否还有数据，若没有数据，说明所有数据成功被处理，
|
|
|
// 若还有数据，说明对端在发送帧的过程中断开了连接，导致只发送了部分数据
|
|
|
if self.buffer.is_empty() {
|
|
|
return Ok(None);
|
|
|
} else {
|
|
|
return Err("connection reset by peer".into());
|
|
|
}
|
|
|
}
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p><code>read_frame</code> 内部使用循环的方式读取数据，直到一个完整的帧被读取到时，才会返回。当然，当远程的对端关闭了连接后，也会返回。</p>
|
|
|
<h4 id="buf-特征"><a class="header" href="#buf-特征"><code>Buf</code> 特征</a></h4>
|
|
|
<p>在上面的 <code>read_frame</code> 方法中，我们使用了 <code>read_buf</code> 来读取 socket 中的数据，该方法的参数是来自 <a href="https://docs.rs/bytes/"><code>bytes</code></a> 包的 <code>BufMut</code>。</p>
|
|
|
<p>可以先来考虑下该如何使用 <code>read()</code> 和 <code>Vec<u8></code> 来实现同样的功能 :</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>use tokio::net::TcpStream;
|
|
|
|
|
|
pub struct Connection {
|
|
|
stream: TcpStream,
|
|
|
buffer: Vec<u8>,
|
|
|
cursor: usize,
|
|
|
}
|
|
|
|
|
|
impl Connection {
|
|
|
pub fn new(stream: TcpStream) -> Connection {
|
|
|
Connection {
|
|
|
stream,
|
|
|
// 4kb 大小的缓冲区
|
|
|
buffer: vec![0; 4096],
|
|
|
cursor: 0,
|
|
|
}
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>下面是相应的 <code>read_frame</code> 方法:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>use mini_redis::{Frame, Result};
|
|
|
|
|
|
pub async fn read_frame(&mut self)
|
|
|
-> Result<Option<Frame>>
|
|
|
{
|
|
|
loop {
|
|
|
if let Some(frame) = self.parse_frame()? {
|
|
|
return Ok(Some(frame));
|
|
|
}
|
|
|
|
|
|
// 确保缓冲区长度足够
|
|
|
if self.buffer.len() == self.cursor {
|
|
|
// 若不够，需要增加缓冲区长度
|
|
|
self.buffer.resize(self.cursor * 2, 0);
|
|
|
}
|
|
|
|
|
|
// 从游标位置开始将数据读入缓冲区
|
|
|
let n = self.stream.read(
|
|
|
&mut self.buffer[self.cursor..]).await?;
|
|
|
|
|
|
if 0 == n {
|
|
|
if self.cursor == 0 {
|
|
|
return Ok(None);
|
|
|
} else {
|
|
|
return Err("connection reset by peer".into());
|
|
|
}
|
|
|
} else {
|
|
|
// 更新游标位置
|
|
|
self.cursor += n;
|
|
|
}
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>在这段代码中，我们使用了非常重要的技术：通过游标( cursor )跟踪已经读取的数据，并将下次读取的数据写入到游标之后的缓冲区中，只有这样才不会让新读取的数据将之前读取的数据覆盖掉。</p>
|
|
|
<p>一旦缓冲区满了，还需要增加缓冲区的长度，这样才能继续写入数据。还有一点值得注意，在 <code>parse_frame</code> 方法的内部实现中，也需要通过游标来解析数据: <code>self.buffer[..self.cursor]</code>，通过这种方式，我们可以准确获取到目前已经读取的全部数据。</p>
|
|
|
<p>在网络编程中，通过字节数组和游标的方式读取数据是非常普遍的，因此 <code>bytes</code> 包提供了一个 <code>Buf</code> 特征，如果一个类型可以被读取数据，那么该类型需要实现 <code>Buf</code> 特征。与之对应，当一个类型可以被写入数据时，它需要实现 <code>BufMut</code> 。</p>
|
|
|
<p>当 <code>T: BufMut</code> ( 特征约束，说明类型 <code>T</code> 实现了 <code>BufMut</code> 特征 ) 被传给 <code>read_buf()</code> 方法时，缓冲区 <code>T</code> 的内部游标会自动进行更新。正因为如此，在使用了 <code>BufMut</code> 版本的 <code>read_frame</code> 中，我们并不需要管理自己的游标。</p>
|
|
|
<p>除了游标之外，<code>Vec<u8></code> 的使用也值得关注，该缓冲区在使用时必须要被初始化: <code>vec![0; 4096]</code>，该初始化会创建一个 4096 字节长度的数组，然后将数组的每个元素都填充上 0 。当缓冲区长度不足时，新创建的缓冲区数组依然会使用 0 被重新填充一遍。 事实上，这种初始化过程会存在一定的性能开销。</p>
|
|
|
<p>与 <code>Vec<u8></code> 相反， <code>BytesMut</code> 和 <code>BufMut</code> 就没有这个问题，它们无需被初始化，而且 <code>BytesMut</code> 还会阻止我们读取未初始化的内存。</p>
|
|
|
<h2 id="帧解析"><a class="header" href="#帧解析">帧解析</a></h2>
|
|
|
<p>在理解了该如何读取数据后， 再来看看该如何通过两个部分解析出一个帧：</p>
|
|
|
<ul>
|
|
|
<li>确保有一个完整的帧已经被写入了缓冲区，找到该帧的最后一个字节所在的位置</li>
|
|
|
<li>解析帧</li>
|
|
|
</ul>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>use mini_redis::{Frame, Result};
|
|
|
use mini_redis::frame::Error::Incomplete;
|
|
|
use bytes::Buf;
|
|
|
use std::io::Cursor;
|
|
|
|
|
|
fn parse_frame(&mut self)
|
|
|
-> Result<Option<Frame>>
|
|
|
{
|
|
|
// 创建 `T: Buf` 类型
|
|
|
let mut buf = Cursor::new(&self.buffer[..]);
|
|
|
|
|
|
// 检查是否读取了足够解析出一个帧的数据
|
|
|
match Frame::check(&mut buf) {
|
|
|
Ok(_) => {
|
|
|
// 获取组成该帧的字节数
|
|
|
let len = buf.position() as usize;
|
|
|
|
|
|
// 在解析开始之前，重置内部的游标位置
|
|
|
buf.set_position(0);
|
|
|
|
|
|
// 解析帧
|
|
|
let frame = Frame::parse(&mut buf)?;
|
|
|
|
|
|
// 解析完成，将缓冲区该帧的数据移除
|
|
|
self.buffer.advance(len);
|
|
|
|
|
|
// 返回解析出的帧
|
|
|
Ok(Some(frame))
|
|
|
}
|
|
|
// 缓冲区的数据不足以解析出一个完整的帧
|
|
|
Err(Incomplete) => Ok(None),
|
|
|
// 遇到一个错误
|
|
|
Err(e) => Err(e.into()),
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>完整的 <code>Frame::check</code> 函数实现在<a href="https://github.com/tokio-rs/mini-redis/blob/tutorial/src/frame.rs#L63-L100">这里</a>，感兴趣的同学可以看看，在这里我们不会对它进行完整的介绍。</p>
|
|
|
<p>值得一提的是, <code>Frame::check</code> 使用了 <code>Buf</code> 的字节迭代风格的 API。例如，为了解析一个帧，首先需要检查它的第一个字节，该字节用于说明帧的类型。这种首字节检查是通过 <code>Buf::get_u8</code> 函数完成的，该函数会获取游标所在位置的字节，然后将游标位置向右移动一个字节。</p>
|
|
|
<h2 id="缓冲写入buffered-writes"><a class="header" href="#缓冲写入buffered-writes">缓冲写入(Buffered writes)</a></h2>
|
|
|
<p>关于帧操作的另一个 API 是 <code>write_frame(frame)</code> 函数，它会将一个完整的帧写入到 socket 中。 每一次写入，都会触发一次或数次系统调用，当程序中有大量的连接和写入时，系统调用的开销将变得非常高昂，具体可以看看 SyllaDB 团队写过的一篇<a href="https://www.scylladb.com/2022/01/12/async-rust-in-practice-performance-pitfalls-profiling/">性能调优文章</a>。</p>
|
|
|
<p>为了降低系统调用的次数，我们需要使用一个写入缓冲区，当写入一个帧时，首先会写入该缓冲区，然后等缓冲区数据足够多时，再集中将其中的数据写入到 socket 中，这样就将多次系统调用优化减少到一次。</p>
|
|
|
<p>还有，缓冲区也不总是能提升性能。 例如，考虑一个 <code>bulk</code> 帧(多个帧放在一起组成一个 bulk，通过批量发送提升效率)，该帧的特点就是：由于由多个帧组合而成，因此帧体数据可能会很大。所以我们不能将其帧体数据写入到缓冲区中，因为数据较大时，先写入缓冲区再写入 socket 会有较大的性能开销(实际上缓冲区就是为了批量写入，既然 bulk 已经是批量了，因此不使用缓冲区也很正常)。</p>
|
|
|
<p>为了实现缓冲写，我们将使用 <a href="https://docs.rs/tokio/1/tokio/io/struct.BufWriter.html"><code>BufWriter</code></a> 结构体。该结构体实现了 <code>AsyncWrite</code> 特征，当 <code>write</code> 方法被调用时，不会直接写入到 socket 中，而是先写入到缓冲区中。当缓冲区被填满时，其中的内容会自动刷到(写入到)内部的 socket 中，然后再将缓冲区清空。当然，其中还存在某些优化，通过这些优化可以绕过缓冲区直接访问 socket。</p>
|
|
|
<p>由于篇幅有限，我们不会实现完整的 <code>write_frame</code> 函数，想要看完整代码可以访问<a href="https://github.com/tokio-rs/mini-redis/blob/tutorial/src/connection.rs#L159-L184">这里</a>。</p>
|
|
|
<p>首先，更新下 <code>Connection</code> 的结构体:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>use tokio::io::BufWriter;
|
|
|
use tokio::net::TcpStream;
|
|
|
use bytes::BytesMut;
|
|
|
|
|
|
pub struct Connection {
|
|
|
stream: BufWriter<TcpStream>,
|
|
|
buffer: BytesMut,
|
|
|
}
|
|
|
|
|
|
impl Connection {
|
|
|
pub fn new(stream: TcpStream) -> Connection {
|
|
|
Connection {
|
|
|
stream: BufWriter::new(stream),
|
|
|
buffer: BytesMut::with_capacity(4096),
|
|
|
}
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>接着来实现 <code>write_frame</code> 函数:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>use tokio::io::{self, AsyncWriteExt};
|
|
|
use mini_redis::Frame;
|
|
|
|
|
|
async fn write_frame(&mut self, frame: &Frame)
|
|
|
-> io::Result<()>
|
|
|
{
|
|
|
match frame {
|
|
|
Frame::Simple(val) => {
|
|
|
self.stream.write_u8(b'+').await?;
|
|
|
self.stream.write_all(val.as_bytes()).await?;
|
|
|
self.stream.write_all(b"\r\n").await?;
|
|
|
}
|
|
|
Frame::Error(val) => {
|
|
|
self.stream.write_u8(b'-').await?;
|
|
|
self.stream.write_all(val.as_bytes()).await?;
|
|
|
self.stream.write_all(b"\r\n").await?;
|
|
|
}
|
|
|
Frame::Integer(val) => {
|
|
|
self.stream.write_u8(b':').await?;
|
|
|
self.write_decimal(*val).await?;
|
|
|
}
|
|
|
Frame::Null => {
|
|
|
self.stream.write_all(b"$-1\r\n").await?;
|
|
|
}
|
|
|
Frame::Bulk(val) => {
|
|
|
let len = val.len();
|
|
|
|
|
|
self.stream.write_u8(b'$').await?;
|
|
|
self.write_decimal(len as u64).await?;
|
|
|
self.stream.write_all(val).await?;
|
|
|
self.stream.write_all(b"\r\n").await?;
|
|
|
}
|
|
|
Frame::Array(_val) => unimplemented!(),
|
|
|
}
|
|
|
|
|
|
self.stream.flush().await;
|
|
|
|
|
|
Ok(())
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
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<p>这里使用的方法由 <code>AsyncWriteExt</code> 提供，它们在 <code>TcpStream</code> 中也有对应的函数。但是在没有缓冲区的情况下最好避免使用这种逐字节的写入方式！不然，每写入几个字节就会触发一次系统调用，写完整个数据帧可能需要几十次系统调用，可以说是丧心病狂！</p>
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<ul>
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<li><code>write_u8</code> 写入一个字节</li>
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<li><code>write_all</code> 写入所有数据</li>
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<li><code>write_decimal</code>由 mini-redis 提供</li>
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</ul>
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<p>在函数结束前，我们还额外的调用了一次 <code>self.stream.flush().await</code>，原因是缓冲区可能还存在数据，因此需要手动刷一次数据：<code>flush</code> 的调用会将缓冲区中剩余的数据立刻写入到 socket 中。</p>
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<p>当然，当帧比较小的时候，每写一次帧就 flush 一次的模式性能开销会比较大，此时我们可以选择在 <code>Connection</code> 中实现 <code>flush</code> 函数，然后将等帧积累多个后，再一次性在 <code>Connection</code> 中进行 flush。当然，对于我们的例子来说，简洁性是非常重要的，因此选了将 <code>flush</code> 放入到 <code>write_frame</code> 中。</p>
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