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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 >
< 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 >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use bytes::Bytes;
enum Frame {
Simple(String),
Error(String),
Integer(u64),
Bulk(Bytes),
Null,
Array(Vec< Frame> ),
}
< span class = "boring" > }< / span > < / code > < / pre >
< 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 >
< 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,
},
}
< span class = "boring" > }< / span > < / code > < / pre >
< 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，
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，
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， ，
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， ， ， < 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， < / 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 >
< 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 >
< ul >
< li > < code > write_u8< / code > 写入一个字节< / li >
< li > < code > write_all< / code > 写入所有数据< / li >
< li > < code > write_decimal< / code > 由 mini-redis 提供< / li >
< / ul >
< 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 >
< 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， ， ， < code > flush< / code > 放入到 < code > write_frame< / code > 中。< / p >
< / main >
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