Redis is often referred as a *data structures* server. What this means is that Redis provides access to mutable data structures via a set of commands, which are sent using a *server-client* model with TCP sockets and a simple protocol. So different processes can query and modify the same data structures in a shared way.
Data structures implemented into Redis have a few special properties:
* Redis cares to store them on disk, even if they are always served and modified into the server memory. This means that Redis is fast, but that is also non-volatile.
* Implementation of data structures stress on memory efficiency, so data structures inside Redis will likely use less memory compared to the same data structure modeled using an high level programming language.
* Redis offers a number of features that are natural to find in a database, like replication, tunable levels of durability, cluster, high availability.
Another good example is to think of Redis as a more complex version of memcached, where the operations are not just SETs and GETs, but operations to work with complex data types like Lists, Sets, ordered data structures, and so forth.
*`deps`: contains libraries Redis uses. Everything needed to compile Redis is inside this directory, your system needs to provide just the `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.
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:
*`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 show just 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 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`
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 is, 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, that may 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.
sever.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()` enters the event loop listening 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.
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.
`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.
*`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.
*`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].
