这篇文档是关于Redis内部实现的概述，它提供了对Redis源代码结构、重要函数和结构的高层次理解。文档的目的是在不深入细节的情况下，给读者一个Redis服务器内部如何工作的大致概念。以下是对文档内容的解读：
Redis源代码布局
- 根目录：包含README文件、Makefile（用于构建Redis）、示例配置文件以及一些用于执行单元测试的shell脚本。
- 重要目录
src：Redis用C语言实现的核心代码。tests：用Tcl实现的单元测试。deps：Redis使用的库，包括jemalloc内存分配器。
主要文件和结构
- server.h
- 定义了全局结构体
server，包含服务器配置和状态。 - 定义了
client结构体，表示连接的客户端，包含文件描述符、命令参数、数据库指针等。 robj结构体定义了Redis对象，可以表示不同类型的数据，如字符串、列表等。
- server.c
- Redis服务器的入口点，包含
main()函数。 - 描述了启动Redis服务器的步骤，如设置默认配置、分配数据结构、启动事件循环。
- 包含处理客户端命令、过期键删除、内存管理等功能的代码。
- 定义了所有Redis命令的全局变量
redisCommandTable。
- networking.c
- 定义了与客户端、主服务器和从服务器的I/O函数。
- 包含创建客户端、发送回复、读取查询、处理输入缓冲区等功能。
- aof.c 和 rdb.c
- 实现了Redis的RDB快照和AOF持久化。
- 使用
fork()系统调用来创建具有相同内存内容的线程，用于将内存内容转储到磁盘。
- db.c
- 实现了操作键的通用命令，如
DEL和EXPIRE。 - 提供了操作Redis数据集的API，如查找键、添加键、删除键等。
- object.c
- 包含操作Redis对象的函数，如增加或减少引用计数、创建新对象等。
- replication.c
- 实现了Redis的主从复制功能。
- 包含将主服务器上的写命令发送给从服务器的函数，以保持数据同步。
- 其他C文件
t_hash.c,t_list.c,t_set.c,t_string.c,t_zset.c：实现了Redis的数据类型和相关命令。ae.c：实现了Redis事件循环。sds.c：Redis字符串库。anet.c：简化POSIX网络编程的库。dict.c：实现了一个非阻塞哈希表。scripting.c：实现了Lua脚本功能。cluster.c：实现了Redis集群功能。
Redis命令的解剖
- Redis命令通常以
void返回类型定义，并且接受一个client *c参数。 - 命令在
server.c中的命令表里进行引用，指定了命令名称、实现函数、需要的参数数量和其他属性。
解读：
Redis internals
===
If you are reading this README you are likely in front of a Github page
or you just untarred the Redis distribution tar ball. In both the cases
you are basically one step away from the source code, so here we explain
the Redis source code layout, what is in each file as a general idea, the
most important functions and structures inside the Redis server and so forth.
We keep all the discussion at a high level without digging into the details
since this document would be huge otherwise and our code base changes
continuously, but a general idea should be a good starting point to
understand more. Moreover most of the code is heavily commented and easy
to follow.
Source code layout
---
The Redis root directory just contains this README, the Makefile which
calls the real Makefile inside the `src` directory and an example
configuration for Redis and Sentinel. You can find a few shell
scripts that are used in order to execute the Redis, Redis Cluster and
Redis Sentinel unit tests, which are implemented inside the `tests`
directory.
Inside the root are the following important directories:
* `src`: contains the Redis implementation, written in C.
* `tests`: contains the unit tests, implemented in Tcl.
* `deps`: contains libraries Redis uses. Everything needed to compile Redis is inside this directory; your system just needs to provide `libc`, a POSIX compatible interface and a C compiler. Notably `deps` contains a copy of `jemalloc`, which is the default allocator of Redis under Linux. Note that under `deps` there are also things which started with the Redis project, but for which the main repository is not `anitrez/redis`. An exception to this rule is `deps/geohash-int` which is the low level geocoding library used by Redis: it originated from a different project, but at this point it diverged so much that it is developed as a separated entity directly inside the Redis repository.
There are a few more directories but they are not very important for our goals
here. We'll focus mostly on `src`, where the Redis implementation is contained,
exploring what there is inside each file. The order in which files are
exposed is the logical one to follow in order to disclose different layers
of complexity incrementally.
Note: lately Redis was refactored quite a bit. Function names and file
names have been changed, so you may find that this documentation reflects the
`unstable` branch more closely. For instance in Redis 3.0 the `server.c`
and `server.h` files were named to `redis.c` and `redis.h`. However the overall
structure is the same. Keep in mind that all the new developments and pull
requests should be performed against the `unstable` branch.
server.h
---
The simplest way to understand how a program works is to understand the
data structures it uses. So we'll start from the main header file of
Redis, which is `server.h`.
All the server configuration and in general all the shared state is
defined in a global structure called `server`, of type `struct redisServer`.
A few important fields in this structure are:
* `server.db` is an array of Redis databases, where data is stored.
* `server.commands` is the command table.
* `server.clients` is a linked list of clients connected to the server.
* `server.master` is a special client, the master, if the instance is a slave.
There are tons of other fields. Most fields are commented directly inside
the structure definition.
Another important Redis data structure is the one defining a client.
In the past it was called `redisClient`, now just `client`. The structure
has many fields, here we'll just show the main ones:
struct client {
int fd;
sds querybuf;
int argc;
robj **argv;
redisDb *db;
int flags;
list *reply;
char buf[PROTO_REPLY_CHUNK_BYTES];
... many other fields ...
}
The client structure defines a *connected client*:
* The `fd` field is the client socket file descriptor.
* `argc` and `argv` are populated with the command the client is executing, so that functions implementing a given Redis command can read the arguments.
* `querybuf` accumulates the requests from the client, which are parsed by the Redis server according to the Redis protocol and executed by calling the implementations of the commands the client is executing.
* `reply` and `buf` are dynamic and static buffers that accumulate the replies the server sends to the client. These buffers are incrementally written to the socket as soon as the file descriptor is writable.
As you can see in the client structure above, arguments in a command
are described as `robj` structures. The following is the full `robj`
structure, which defines a *Redis object*:
typedef struct redisObject {
unsigned type:4;
unsigned encoding:4;
unsigned lru:LRU_BITS; /* lru time (relative to server.lruclock) */
int refcount;
void *ptr;
} robj;
Basically this structure can represent all the basic Redis data types like
strings, lists, sets, sorted sets and so forth. The interesting thing is that
it has a `type` field, so that it is possible to know what type a given
object has, and a `refcount`, so that the same object can be referenced
in multiple places without allocating it multiple times. Finally the `ptr`
field points to the actual representation of the object, which might vary
even for the same type, depending on the `encoding` used.
Redis objects are used extensively in the Redis internals, however in order
to avoid the overhead of indirect accesses, recently in many places
we just use plain dynamic strings not wrapped inside a Redis object.
server.c
---
This is the entry point of the Redis server, where the `main()` function
is defined. The following are the most important steps in order to startup
the Redis server.
* `initServerConfig()` setups the default values of the `server` structure.
* `initServer()` allocates the data structures needed to operate, setup the listening socket, and so forth.
* `aeMain()` starts the event loop which listens for new connections.
There are two special functions called periodically by the event loop:
1. `serverCron()` is called periodically (according to `server.hz` frequency), and performs tasks that must be performed from time to time, like checking for timedout clients.
2. `beforeSleep()` is called every time the event loop fired, Redis served a few requests, and is returning back into the event loop.
Inside server.c you can find code that handles other vital things of the Redis server:
* `call()` is used in order to call a given command in the context of a given client.
* `activeExpireCycle()` handles eviciton of keys with a time to live set via the `EXPIRE` command.
* `freeMemoryIfNeeded()` is called when a new write command should be performed but Redis is out of memory according to the `maxmemory` directive.
* The global variable `redisCommandTable` defines all the Redis commands, specifying the name of the command, the function implementing the command, the number of arguments required, and other properties of each command.
networking.c
---
This file defines all the I/O functions with clients, masters and slaves
(which in Redis are just special clients):
* `createClient()` allocates and initializes a new client.
* the `addReply*()` family of functions are used by commands implementations in order to append data to the client structure, that will be transmitted to the client as a reply for a given command executed.
* `writeToClient()` transmits the data pending in the output buffers to the client and is called by the *writable event handler* `sendReplyToClient()`.
* `readQueryFromClient()` is the *readable event handler* and accumulates data from read from the client into the query buffer.
* `processInputBuffer()` is the entry point in order to parse the client query buffer according to the Redis protocol. Once commands are ready to be processed, it calls `processCommand()` which is defined inside `server.c` in order to actually execute the command.
* `freeClient()` deallocates, disconnects and removes a client.
aof.c and rdb.c
---
As you can guess from the names these files implement the RDB and AOF
persistence for Redis. Redis uses a persistence model based on the `fork()`
system call in order to create a thread with the same (shared) memory
content of the main Redis thread. This secondary thread dumps the content
of the memory on disk. This is used by `rdb.c` to create the snapshots
on disk and by `aof.c` in order to perform the AOF rewrite when the
append only file gets too big.
The implementation inside `aof.c` has additional functions in order to
implement an API that allows commands to append new commands into the AOF
file as clients execute them.
The `call()` function defined inside `server.c` is responsible to call
the functions that in turn will write the commands into the AOF.
db.c
---
Certain Redis commands operate on specific data types, others are general.
Examples of generic commands are `DEL` and `EXPIRE`. They operate on keys
and not on their values specifically. All those generic commands are
defined inside `db.c`.
Moreover `db.c` implements an API in order to perform certain operations
on the Redis dataset without directly accessing the internal data structures.
The most important functions inside `db.c` which are used in many commands
implementations are the following:
* `lookupKeyRead()` and `lookupKeyWrite()` are used in order to get a pointer to the value associated to a given key, or `NULL` if the key does not exist.
* `dbAdd()` and its higher level counterpart `setKey()` create a new key in a Redis database.
* `dbDelete()` removes a key and its associated value.
* `emptyDb()` removes an entire single database or all the databases defined.
The rest of the file implements the generic commands exposed to the client.
object.c
---
The `robj` structure defining Redis objects was already described. Inside
`object.c` there are all the functions that operate with Redis objects at
a basic level, like functions to allocate new objects, handle the reference
counting and so forth. Notable functions inside this file:
* `incrRefcount()` and `decrRefCount()` are used in order to increment or decrement an object reference count. When it drops to 0 the object is finally freed.
* `createObject()` allocates a new object. There are also specialized functions to allocate string objects having a specific content, like `createStringObjectFromLongLong()` and similar functions.
This file also implements the `OBJECT` command.
replication.c
---
This is one of the most complex files inside Redis, it is recommended to
approach it only after getting a bit familiar with the rest of the code base.
In this file there is the implementation of both the master and slave role
of Redis.
One of the most important functions inside this file is `replicationFeedSlaves()` that writes commands to the clients representing slave instances connected
to our master, so that the slaves can get the writes performed by the clients:
this way their data set will remain synchronized with the one in the master.
This file also implements both the `SYNC` and `PSYNC` commands that are
used in order to perform the first synchronization between masters and
slaves, or to continue the replication after a disconnection.
Other C files
---
* `t_hash.c`, `t_list.c`, `t_set.c`, `t_string.c` and `t_zset.c` contains the implementation of the Redis data types. They implement both an API to access a given data type, and the client commands implementations for these data types.
* `ae.c` implements the Redis event loop, it's a self contained library which is simple to read and understand.
* `sds.c` is the Redis string library, check http://github.com/antirez/sds for more information.
* `anet.c` is a library to use POSIX networking in a simpler way compared to the raw interface exposed by the kernel.
* `dict.c` is an implementation of a non-blocking hash table which rehashes incrementally.
* `scripting.c` implements Lua scripting. It is completely self contained from the rest of the Redis implementation and is simple enough to understand if you are familar with the Lua API.
* `cluster.c` implements the Redis Cluster. Probably a good read only after being very familiar with the rest of the Redis code base. If you want to read `cluster.c` make sure to read the [Redis Cluster specification][3].
[3]: http://redis.io/topics/cluster-spec
Anatomy of a Redis command
---
All the Redis commands are defined in the following way:
void foobarCommand(client *c) {
printf("%s",c->argv[1]->ptr); /* Do something with the argument. */
addReply(c,shared.ok); /* Reply something to the client. */
}
The command is then referenced inside `server.c` in the command table:
{"foobar",foobarCommand,2,"rtF",0,NULL,0,0,0,0,0},
In the above example `2` is the number of arguments the command takes,
while `"rtF"` are the command flags, as documented in the command table
top comment inside `server.c`.
After the command operates in some way, it returns a reply to the client,
usually using `addReply()` or a similar function defined inside `networking.c`.
There are tons of commands implementations inside th Redis source code
that can serve as examples of actual commands implementations. To write
a few toy commands can be a good exercise to familiarize with the code base.
There are also many other files not described here, but it is useless to
cover everything. We want to just help you with the first steps.
Eventually you'll find your way inside the Redis code base :-)
Enjoy!
