README now has info about Redis codebase layout.

1 files changed, 257 insertions, 2 deletions

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+++ b/README.md
@@ -185,7 +185,262 @@ source distribution.
 Please see the [CONTRIBUTING][2] file in this source distribution for more
 information.
 
-Enjoy!
-
 [1]: https://github.com/antirez/redis/blob/unstable/COPYING
 [2]: https://github.com/antirez/redis/blob/unstable/CONTRIBUTING
+
+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 funcitons and structures inside the Redis server and so forth.
+We keep all the discussion at an 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
+actually calls the real Makefile inside the `src` directory, an example
+configuration for Redis and Sentinel. Finally 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 directory the 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 needs to provide just the `libc`, a POSIX compatible interface, and a C compiler. Notaly `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 discose differet layers
+of complexity incrementally.
+
+Note: lately Redis was refactored quite a bit. Function names and file
+names 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` where called `redis.c` and `redis.h`. However the overall
+structure is the same. Keep in mind that all the new developmetns and pull
+requests should be performed against the `unstable` branch.
+
+sever.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:
+
+* `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`
+strucutre, 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 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.
+
+db.c
+---
+
+Certain Redis operations operate on specific data types, others are general.
+Examples of general operations are `DEL` and `EXPIRE`. They operate on keys
+and not on their values specifically. All those generic operations 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.$a
+* `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 defined Redis objects was alraedy 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` contain 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 implementation and is simple enough to understand.
+* `cluster.c` implements the Redis Cluster. Probably a good read only after being very familiar with the rest of the Redis code base.
+
+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 operated 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 here, we want just to help you with the first steps,
+eventually you'll find your way inside the Redis code base :-)
+
+Enjoy!
+