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This README is just a fastquick start document. You can find more detailed documentation atredis.io.

Redis is often referred to as adata structures server. What this means is that Redis provides access to mutable data structures via a set of commands, which are sent using aserver-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 it is also non-volatile.
• The implementation of data structures emphasizes memory efficiency, so data structures inside Redis will likely use less memory compared to the same data structure modelled using a high-level programming language.
• Redis offers a number of features that are natural to find in a database, like replication, tunable levels of durability, clustering, and 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 that work with complex data types like Lists, Sets, ordered data structures, and so forth.

If you want to know more, this is a list of selected starting points:

• Introduction to Redis data types.https://redis.io/docs/latest/develop/data-types/

• The full list of Redis commands.https://redis.io/commands

• There is much more inside the official Redis documentation.https://redis.io/documentation

Redis OSS was renamed Redis Community Edition (CE) with the v7.4 release.

Redis Ltd. also offersRedis Software, a self-managed software with additional compliance, reliability, and resiliency for enterprise scaling,andRedis Cloud, a fully managed service integrated with Google Cloud, Azure, and AWS for production-ready apps.

Read more about the differences between Redis Community Edition and Redishere.

Redis can be compiled and used on Linux, OSX, OpenBSD, NetBSD, FreeBSD.We support big endian and little endian architectures, and both 32 bitand 64 bit systems.

It may compile on Solaris derived systems (for instance SmartOS) but oursupport for this platform isbest effort and Redis is not guaranteed towork as well as in Linux, OSX, and *BSD.

It is as simple as:

% make

To build with TLS support, you'll need OpenSSL development libraries (e.g.libssl-dev on Debian/Ubuntu) and run:

% make BUILD_TLS=yes

To build with systemd support, you'll need systemd development libraries (suchas libsystemd-dev on Debian/Ubuntu or systemd-devel on CentOS) and run:

% make USE_SYSTEMD=yes

To append a suffix to Redis program names, use:

% make PROG_SUFFIX="-alt"

You can build a 32 bit Redis binary using:

% make 32bit

After building Redis, it is a good idea to test it using:

% make test

If TLS is built, running the tests with TLS enabled (you will needtcl-tlsinstalled):

% ./utils/gen-test-certs.sh% ./runtest --tls

Redis has some dependencies which are included in thedeps directory.make does not automatically rebuild dependencies even if something inthe source code of dependencies changes.

When you update the source code withgit pull or when code inside thedependencies tree is modified in any other way, make sure to use the followingcommand in order to really clean everything and rebuild from scratch:

% make distclean

This will clean: jemalloc, lua, hiredis, linenoise and other dependencies.

Also if you force certain build options like 32bit target, no C compileroptimizations (for debugging purposes), and other similar build time options,those options are cached indefinitely until you issue amake distcleancommand.

If after building Redis with a 32 bit target you need to rebuild itwith a 64 bit target, or the other way around, you need to perform amake distclean in the root directory of the Redis distribution.

In case of build errors when trying to build a 32 bit binary of Redis, trythe following steps:

• Install the package libc6-dev-i386 (also try g++-multilib).
• Try using the following command line instead ofmake 32bit:make CFLAGS="-m32 -march=native" LDFLAGS="-m32"

Selecting a non-default memory allocator when building Redis is done by settingtheMALLOC environment variable. Redis is compiled and linked against libcmalloc by default, with the exception of jemalloc being the default on Linuxsystems. This default was picked because jemalloc has proven to have fewerfragmentation problems than libc malloc.

To force compiling against libc malloc, use:

% make MALLOC=libc

To compile against jemalloc on Mac OS X systems, use:

% make MALLOC=jemalloc

By default, Redis will build using the POSIX clock_gettime function as themonotonic clock source. On most modern systems, the internal processor clockcan be used to improve performance. Cautions can be found here:http://oliveryang.net/2015/09/pitfalls-of-TSC-usage/

To build with support for the processor's internal instruction clock, use:

% make CFLAGS="-DUSE_PROCESSOR_CLOCK"

Redis will build with a user-friendly colorized output by default.If you want to see a more verbose output, use the following:

% make V=1

To run Redis with the default configuration, just type:

% cd src% ./redis-server

If you want to provide your redis.conf, you have to run it using an additionalparameter (the path of the configuration file):

% cd src% ./redis-server /path/to/redis.conf

It is possible to alter the Redis configuration by passing parameters directlyas options using the command line. Examples:

% ./redis-server --port 9999 --replicaof 127.0.0.1 6379% ./redis-server /etc/redis/6379.conf --loglevel debug

All the options in redis.conf are also supported as options using the commandline, with exactly the same name.

Please consult theTLS.md file for more information onhow to use Redis with TLS.

You can use redis-cli to play with Redis. Start a redis-server instance,then in another terminal try the following:

% cd src% ./redis-cliredis> pingPONGredis> set foo barOKredis> get foo"bar"redis> incr mycounter(integer) 1redis> incr mycounter(integer) 2redis>

You can find the list of all the available commands athttps://redis.io/commands.

In order to install Redis binaries into /usr/local/bin, just use:

% make install

You can usemake PREFIX=/some/other/directory install if you wish to use adifferent destination.

make install will just install binaries in your system, but will not configureinit scripts and configuration files in the appropriate place. This is notneeded if you just want to play a bit with Redis, but if you are installingit the proper way for a production system, we have a script that does thisfor Ubuntu and Debian systems:

% cd utils% ./install_server.sh

Note:install_server.sh will not work on Mac OSX; it is built for Linux only.

The script will ask you a few questions and will setup everything you needto run Redis properly as a background daemon that will start again onsystem reboots.

You'll be able to stop and start Redis using the script named/etc/init.d/redis_<portnumber>, for instance/etc/init.d/redis_6379.

By contributing code to the Redis project in any form, including sending a pull request via GitHub,a code fragment or patch via private email or public discussion groups, you agree to release yourcode under the terms of theRedis Software Grant and Contributor License Agreement. Redis softwarecontains contributions to the original Redis core project, which are owned by their contributors andlicensed under the 3BSD license. Any copy of that license in this repository applies only to thosecontributions. Redis releases all Redis Community Edition versions from 7.4.x and thereafter under theRSALv2/SSPL dual-license as described in theLICENSE.txt file included in the Redis Community Edition source distribution.

Please see theCONTRIBUTING.md file in this source distribution for more information. Forsecurity bugs and vulnerabilities, please seeSECURITY.md.

The purpose of a trademark is to identify the goods and services of a person or company withoutcausing confusion. As the registered owner of its name and logo, Redis accepts certain limited usesof its trademarks but it has requirements that must be followed as described in its TrademarkGuidelines available at:https://redis.com/legal/trademark-guidelines/.

If you are reading this README you are likely in front of a GitHub pageor you just untarred the Redis distribution tar ball. In both the casesyou are basically one step away from the source code, so here we explainthe Redis source code layout, what is in each file as a general idea, themost important functions and structures inside the Redis server and so forth.We keep all the discussion at a high level without digging into the detailssince this document would be huge otherwise and our code base changescontinuously, but a general idea should be a good starting point tounderstand more. Moreover most of the code is heavily commented and easyto follow.

The Redis root directory just contains this README, the Makefile whichcalls the real Makefile inside thesrc directory and an exampleconfiguration for Redis and Redis Sentinel. You can find a few shellscripts that are used in order to execute the Redis, Redis Cluster andRedis Sentinel unit tests, which are implemented inside thetestsdirectory.

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 providelibc, a POSIX compatible interface and a C compiler. Notablydeps contains a copy ofjemalloc, which is the default allocator of Redis under Linux. Note that underdeps there are also things which started with the Redis project, but for which the main repository is notredis/redis.

There are a few more directories but they are not very important for our goalshere. We'll focus mostly onsrc, where the Redis implementation is contained,exploring what there is inside each file. The order in which files areexposed is the logical one to follow in order to disclose different layersof complexity incrementally.

Note: lately Redis was refactored quite a bit. Function names and filenames have been changed, so you may find that this documentation reflects theunstable branch more closely. For instance, in Redis 3.0 theserver.candserver.h files were namedredis.c andredis.h. However the overallstructure is the same. Keep in mind that all the new developments and pullrequests should be performed against theunstable branch.

The simplest way to understand how a program works is to understand thedata structures it uses. So we'll start from the main header file ofRedis, which isserver.h.

All the server configuration and in general all the shared state isdefined in a global structure calledserver, of typestruct 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 replica.

There are tons of other fields. Most fields are commented directly insidethe structure definition.

Another important Redis data structure is the one defining a client.In the past it was calledredisClient, now justclient. The structurehas many fields, here we'll just show the main ones:

structclient {intfd;sdsquerybuf;intargc;robj**argv;redisDb*db;intflags;list*reply;// ... many other fields ...charbuf[PROTO_REPLY_CHUNK_BYTES];}

The client structure defines aconnected client:

• Thefd field is the client socket file descriptor.
• argc andargv 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 andbuf 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 commandare described asrobj structures. The following is the fullrobjstructure, which defines aRedis object:

structredisObject {unsignedtype:4;unsignedencoding:4;unsignedlru:LRU_BITS;/* LRU time (relative to global lru_clock) or                            * LFU data (least significant 8 bits frequency                            * and most significant 16 bits access time). */intrefcount;void*ptr;};

Basically this structure can represent all the basic Redis data types likestrings, lists, sets, sorted sets and so forth. The interesting thing is thatit has atype field, so that it is possible to know what type a givenobject has, and arefcount, so that the same object can be referencedin multiple places without allocating it multiple times. Finally theptrfield points to the actual representation of the object, which might varyeven for the same type, depending on theencoding used.

Redis objects are used extensively in the Redis internals, however in orderto avoid the overhead of indirect accesses, recently in many placeswe just use plain dynamic strings not wrapped inside a Redis object.

This is the entry point of the Redis server, where themain() functionis defined. The following are the most important steps in order to startupthe Redis server.

• initServerConfig() sets up the default values of theserver 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 toserver.hz frequency), and performs tasks that must be performed from time to time, like checking for timed out 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 eviction of keys with a time to live set via theEXPIRE command.
• performEvictions() is called when a new write command should be performed but Redis is out of memory according to themaxmemory directive.
• The global variableredisCommandTable 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.

This file is auto generated by utils/generate-command-code.py, the content is based on the JSON files in the src/commands folder.These are meant to be the single source of truth about the Redis commands, and all the metadata about them.These JSON files are not meant to be used by anyone directly, instead that metadata can be obtained via theCOMMAND command.

This file defines all the I/O functions with clients, masters and replicas(which in Redis are just special clients):

• createClient() allocates and initializes a new client.
• TheaddReply*() family of functions are used by command 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 thewritable event handlersendReplyToClient().
• readQueryFromClient() is thereadable event handler and accumulates data 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 callsprocessCommand() which is defined insideserver.c in order to actually execute the command.
• freeClient() deallocates, disconnects and removes a client.

As you can guess from the names, these files implement the RDB and AOFpersistence for Redis. Redis uses a persistence model based on thefork()system call in order to create a process with the same (shared) memorycontent of the main Redis process. This secondary process dumps the contentof the memory on disk. This is used byrdb.c to create the snapshotson disk and byaof.c in order to perform the AOF rewrite when theappend only file gets too big.

The implementation insideaof.c has additional functions in order toimplement an API that allows commands to append new commands into the AOFfile as clients execute them.

Thecall() function defined insideserver.c is responsible for callingthe functions that in turn will write the commands into the AOF.

Certain Redis commands operate on specific data types; others are general.Examples of generic commands areDEL andEXPIRE. They operate on keysand not on their values specifically. All those generic commands aredefined insidedb.c.

Moreoverdb.c implements an API in order to perform certain operationson the Redis dataset without directly accessing the internal data structures.

The most important functions insidedb.c which are used in many commandimplementations are the following:

• lookupKeyRead() andlookupKeyWrite() are used in order to get a pointer to the value associated to a given key, orNULL if the key does not exist.
• dbAdd() and its higher level counterpartsetKey() create a new key in a Redis database.
• dbDelete() removes a key and its associated value.
• emptyData() removes an entire single database or all the databases defined.

The rest of the file implements the generic commands exposed to the client.

Therobj structure defining Redis objects was already described. Insideobject.c there are all the functions that operate with Redis objects ata basic level, like functions to allocate new objects, handle the referencecounting and so forth. Notable functions inside this file:

• incrRefCount() anddecrRefCount() 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, likecreateStringObjectFromLongLong() and similar functions.

This file also implements theOBJECT command.

This is one of the most complex files inside Redis, it is recommended toapproach 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 replica roleof Redis.

One of the most important functions inside this file isreplicationFeedSlaves() that writes commands to the clients representing replica instances connectedto our master, so that the replicas 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 theSYNC andPSYNC commands that areused in order to perform the first synchronization between masters andreplicas, or to continue the replication after a disconnection.

The script unit is composed of 3 units:

• script.c - integration of scripts with Redis (commands execution, set replication/resp, ...)
• script_lua.c - responsible to execute Lua code, usesscript.c to interact with Redis from within the Lua code.
• function_lua.c - contains the Lua engine implementation, usesscript_lua.c to execute the Lua code.
• functions.c - contains Redis Functions implementation (FUNCTION command), usesfunctions_lua.c if the function it wants to invoke needs the Lua engine.
• eval.c - contains theeval implementation usingscript_lua.c to invoke the Lua code.

• t_hash.c,t_list.c,t_set.c,t_string.c,t_zset.c andt_stream.c contains the implementation of the Redis data types. They implement both an API to access a given data type, and the client command 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, checkhttps://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.
• 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 readcluster.c make sure to read theRedis Cluster specification.

All the Redis commands are defined in the following way:

voidfoobarCommand(client*c) {printf("%s",c->argv[1]->ptr);/* Do something with the argument. */addReply(c,shared.ok);/* Reply something to the client. */}

The command function is referenced by a JSON file, together with its metadata, seecommands.c described above for details.The command flags are documented in the comment above thestruct redisCommand inserver.h.For other details, please refer to theCOMMAND command.https://redis.io/commands/command/

After the command operates in some way, it returns a reply to the client,usually usingaddReply() or a similar function defined insidenetworking.c.

There are tons of command implementations inside the Redis source codethat can serve as examples of actual commands implementations (e.g. pingCommand). Writinga few toy commands can be a good exercise to get familiar with the code base.

There are also many other files not described here, but it is useless tocover everything. We just want to help you with the first steps.Eventually you'll find your way inside the Redis code base :-)

Enjoy!