REDIS - REmote DIctionary Server

=============
WHAT'S REDIS?
=============

Redis is a database. To be more specific redis is a very simple database
implementing a dictionary where keys are associated with values. For example
I can set the key "surname_1992" to the string "Smith".

Redis takes the whole dataset in memory, but the dataset is persistent
since from time to time Redis writes a dump of the dataset on disk. The dump
is loaded every time the server is restarted.

This means that it can happen that after a system crash the last modifications
of the dataset are lost, but it's the price to pay for a lot of speed.
Redis is the right database for all the applications where it is acceptable
after a crash that some modification gets lost, but where speed is very
important.

However you can configure Redis to save the DB after a given number of
modifications and/or after a given amount of time since the last change
in the dataset. Saving happens in background so the DB will continue to
server queries while it is saving the DB dump on disk.

=================================
HOW REDIS DIFFERS FROM MEMCACHED?
=================================

Maily in two ways:

- Memcached is not persistent, it just holds everything in memory
  without saving since its main goal is to be used as a cache.
  Redis instead can be used as the main DB for the application.
- Like memcached Redis uses a key-value model, but while keys can just
  be strings, values in Redis can be lists and sets, and complex
  operations like intersections, set/get n-th element of lists, pop/push of
  elements, can be performed against sets and lists. It is possible to
  use lists as message queues.

===========================
DOES REDIS SUPPORT LOCKING?
===========================

No, the idea is to provide atomic primitives in order to make the programmer
able to use redis with locking free algorithms. For example imagine you have
10 computers and 1 redis server. You want to count words in a very large text.
This large text is splitted among the 10 computers, every computer will process
its part and use Redis's INCR command to atomically increment a counter
for every occurrence of the word found.

INCR/DECR are not the only atomic primitives, there are others like PUSH/POP
on lists, POP RANDOM KEY operations, UPDATE and so on. For example you can
use Redis like a Tuple Space (http://en.wikipedia.org/wiki/Tuple_space) in
order to implement distributed algorithms.

MULTIPLE DATABASES SUPPORT
==========================

Another synchronization primitive is the support for multiple DBs. By default DB 0 is selected for every new connection, but using the SELECT command it is possible to select a different database. The MOVE operation can move an item from one DB to another atomically. This can be used as a base for locking free algorithms together with the 'RANDOMKEY' or 'POPRANDOMKEY' commands.

================
REDIS DATA TYPES
================

Redis support the following three data types as values:

- Strings: just any sequence of bytes. Redis strings are binary safe so they
  can not just hold text, but images, compressed data and everything else.
- Lists: lists of strings, with support for operations like append a new string on head, on tail, list length, obtain a range of elements, truncate the list to a given length, sort the list, and so on.
- Sets: an unsorted set of strings. It is possible to add or delete elements from a set, to perform set intersection, union, subtraction, and so on.

Values can be Strings, Lists or Sets. Keys can be a subset of strings not containing newlines ("\n") and spaces (" ").

Note that sometimes strings may hold numeric vaules that must be parsed by
Redis. An example is the INCR command that atomically increments the number
stored at the specified key. In this case Redis is able to handle integers
that can be stored inside a 'long long' type, that is a 64-bit signed integer.

Implementation details
----------------------

Strings are implemented as dynamically allocated strings of characters.
Lists are implemented as doubly liked lists with cached length.
Sets are implemented using hash tables that use chaining to resolve collisions.

==============
REDIS TUTORIAL
==============

(note, you can skip this section if you are only interested in "formal" doc.)

Later in this document you can find detailed information about Redis commands,
the protocol specification, and so on. This kind of documentation is useful
but... if you are new to Redis it is also BORING! The Redis protocol is designed
so that is both pretty efficient to be parsed by computers, but simple enough
to be used by humans just pocking around with the 'telnet' command, so this
section will show to the reader how to play a bit with Redis to get an initial
feeling about it, and how it works.

To start just compile redis with 'make' and start it with './redis-server'.
The server will start and log stuff on the standard output, if you want
it to log more edit redis.conf, set the loglevel to debug, and restart it.

You can specify a configuration file as unique parameter:

    ./redis-server /etc/redis.conf

This is NOT required. The server will start even without a configuration file
using a default built-in configuration.

Now let's try to set a key to a given value:

$ telnet localhost 6379
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
SET foo 3  
bar
+OK

The first line we sent to the server is "set foo 3". This means "set the key
foo with the following three bytes I'll send you". The following line is
the "bar" string, that is, the three bytes. So the effect is to set the
key "foo" to the value "bar". Very simple!

(note that you can send commands in lowercase and it will work anyway,
commands are not case sensitive)

Note that after the first and the second line we sent to the server there
is a newline at the end. The server expects commands terminated by "\r\n"
and sequence of bytes terminated by "\r\n". This is a minimal overhead from
the point of view of both the server and client but allow us to play with
Redis with the telnet command easily.

The last line of the chat between server and client is "+OK". This means
our key was added without problems. Actually SET can never fail but
the "+OK" sent makes us sure that the server received everything and
the command was actually executed.

Let's try to get the key content now:

GET foo
3
bar

Ok that's very similar to 'set', just the other way around. We sent "get foo",
the server replied with a first line that is just a number of bytes the value
stored at key contained, followed by the actual bytes. Again "\r\n" are appended
both to the bytes count and the actual data.

What about requesting a non existing key?

GET blabla
nil

When the key does not exist instead of the length just the "nil" string is sent.
Another way to check if a given key exists or not is indeed the EXISTS command:

EXISTS nokey
0
EXISTS foo
1

As you can see the server replied '0' the first time since 'nokey' does not
exist, and '1' for 'foo', a key that actually exists.

Ok... now you know the basics, read the "REDIS COMMANDS REFERENCE" section to
learn all the commands supported by Redis and the "PROTOCOL SPECIFICATION"
section for more details about the protocol used if you plan to implement one
for a language missing a decent client implementation.

========================
REDIS COMMANDS REFERENCE
========================

Connection handling
-------------------

QUIT
    Ask the server to silently close the connection.

Commands operating on string values
-----------------------------------

SET <key> <value>
Time complexity: O(1)
    Set the string <value> as value of the key.
    The string can't be longer than 1073741824 bytes (1 GB).

GET <key>
Time complexity: O(1)
    Get the value of the specified key. If the key
    does not exist the special value 'nil' is returned.
    If the value stored at <key> is not a string an error
    is returned because GET can only handle string values.

SETNX <key> <value>
Time complexity: O(1)
    SETNX works exactly like SET with the only difference that
    if the key already exists no operation is performed.
    SETNX actually means "SET if Not eXists".

INCR <key>
INCRBY <key> <value>
Time complexity: O(1)
    Increment the number stored at <key> by one. If the key does
    not exist set the key to the value of "1" (like if the previous
    value was zero). If the value at <key> is not a string value
    an error is returned.

    INCRBY works just like INCR but instead to increment by 1 the
    increment is <value>.

DECR <key>
DECRBY <key> <value>
Time complexity: O(1)
    Decrement the number stored at <key> by one. If the key does
    not exist set the key to the value of "-1" (like if the previous
    value was zero). If the value at <key> is not a string value
    an error is returned.

    DECRBY works just like DECR but instead to decrement by 1 the
    decrement is <value>.

Commands operating on every value
---------------------------------

EXISTS <key>
Time complexity: O(1)
    Test of the specified key exists. The command returns
    "0" if the key exists, otherwise "1" is returned.
    Note that even keys set with an empty string as value will
    return "1".

DEL <key>
Time complexity: O(1)
    Remove the specified key. If the key does not exist
    no operation is performed. The command always returns success.

TYPE <key>
Time complexity: O(1)
    Return the type of the value stored at <key> in form of a
    string. The type can be one of "NONE","STRING","LIST","SET".
    NONE is returned if the key does not exist.

KEYS <pattern>
Time complexity: O(n) (with n being the number of keys in the DB)
    Returns all the keys matching the glob-style <pattern> as
    space separated strings. For example if you have in the
    database the keys "foo" and "foobar" the command "KEYS foo*"
    will return "foo foobar".

    Note that while the time complexity for this operation is O(n)
    the constant times are pretty low. For example Redis running
    on an entry level laptop can scan a 1 million keys database
    in 40 milliseconds. Still it's better to consider this one of
    the slow commands that may ruin the DB performance if not used
    with care.

RANDOMKEY
Time complexity: O(1)
    Returns a random key from the currently seleted DB.

RENAME <oldkey> <newkey>
    Atomically renames the key <oldkey> to <newkey>. If the source and
    destination name are the same an error is returned. If <newkey>
    already exists it is overwritten.

RENAMENX <oldkey> <newkey>
    Just like RENAME but fails if the destination key <newkey>
    already exists.

Commands operating on lists
---------------------------

RPUSH <key> <string>
Time complexity: O(1)
    Add the given string to the head of the list contained at key.
    If the key does not exist an empty list is created just before
    the append operation. If the key exists but is not a List an error
    is returned.

LPUSH <key> <string>
Time complexity: O(1)
    Add the given string to the tail of the list contained at key.
    If the key does not exist an empty list is created just before
    the append operation. If the key exists but is not a List an error
    is returned.

LLEN <key>
Time complexity: O(1)
    Return the length of the list stored at the specified key. If the
    key does not exist zero is returned (the same behaviour as for
    empty lists). If the value stored at key is not a list an error
    is returned.

LRANGE <key> <start> <end>
Time complexity: O(n) (with n being the length of the range)
    Return the specified elements of the list stored at the specified
    key. Start and end are zero-based indexes. 0 is the first element
    of the list (the list head), 1 the next element and so on.

    For example LRANGE foobar 0 2 will return the first three elements
    of the list.

    <start> and <end> can also be negative numbers indicating offsets
    from the end of the list. For example -1 is the last element of
    the list, -2 the penultimate element and so on.

    Indexes out of range will not produce an error: if start is over
    the end of the list, or start > end, an empty list is returned.
    If end over the end of the list Redis will threat it just like
    the last element of the list.

LTRIM <key> <start> <end>
Time complexity: O(n) (with n being len of list - len of range)
    Trim an existing list so that it will contain only the specified
    range of elements specified. Start and end are zero-based indexes.
    0 is the first element of the list (the list head), 1 the next element
    and so on.

    For example LTRIM foobar 0 2 will modify the list stored at foobar
    key so that only the first three elements of the list will remain.

    <start> and <end> can also be negative numbers indicating offsets
    from the end of the list. For example -1 is the last element of
    the list, -2 the penultimate element and so on.

    Indexes out of range will not produce an error: if start is over
    the end of the list, or start > end, an empty list is left as value.
    If end over the end of the list Redis will threat it just like
    the last element of the list.

    Hint: the obvious use of LTRIM is together with LPUSH/RPUSH. For example:

        LPUSH mylist <someelement>
        LTRIM mylist 0 99

    The above two commands will push elements in the list taking care that
    the list will not grow without limits. This is very useful when using
    Redis to store logs for example. It is important to note that when used
    in this way LTRIM is an O(1) operation because in the average case
    just one element is removed from the tail of the list.

LINDEX <key> <index>
Time complexity: O(n) (with n being the length of the list)
    Return the specified element of the list stored at the specified
    key. 0 is the first element, 1 the second and so on. Negative indexes
    are supported, for example -1 is the last element, -2 the penultimate
    and so on.

    If the value stored at key is not of list type an error is returned.
    If the index is out of range an empty string is returned.

    Note that even if the average time complexity is O(n) asking for
    the first or the last element of the list is O(1).

LPOP <key>
Time complexity: O(1)
    Atomically return and remove the first element of the list.
    For example if the list contains the elements "a","b","c" LPOP
    will return "a" and the list will become "b","c".

    If the <key> does not exist or the list is already empty the special
    value 'nil' is returned.

RPOP <key>
    This command works exactly like LPOP, but the last element instead
    of the first element of the list is returned/deleted.

Commands operating on sets
--------------------------

.... Sorry, this is a work in progress

Multiple DB commands
--------------------

SELECT <index>
    Select the DB with having the specified zero-based numeric index.
    For default every new client connection is automatically selected
    to DB 0.

MOVE <key> <index>
    Move the specified key from the currently selected DB to the specified
    destination DB. If a key with the same name exists in the destination
    DB an error is returned.

Persistence control commands
----------------------------

SAVE
    Save the DB on disk. The server hangs while the saving is not
    completed, no connection is served in the meanwhile. An OK code
    is returned when the DB was fully stored in disk.

BGSAVE
    Save the DB in background. The OK code is immediately returned.
    Redis forks, the parent continues to server the clients, the child
    saves the DB on disk then exit. A client my be able to check if the
    operation succeeded using the LASTSAVE command.

LASTSAVE
    Return the UNIX TIME of the last DB save executed with success.
    A client may check if a BGSAVE command succeeded reading the LASTSAVE
    value, then issuing a BGSAVE command and checking at regular intervals
    every N seconds if LASTSAVE changed.

SHUTDOWN
    Stop all the clients, save the DB, then quit the server. This commands
    makes sure that the DB is switched off without the lost of any data.
    This is not guaranteed if the client uses simply "SAVE" and then
    "QUIT" because other clients may alter the DB data between the two
    commands.

======================
PROTOCOL SPECIFICATION
======================

The Redis protocol is a compromise between being easy to parse by a computer
and being easy to parse by an human. Before to read this section you are
strongly encouraged to read the "REDIS TUTORIAL" section of this README in order
to get a first feeling of the protocol playing with it by TELNET.

Networking layer
----------------

A client connects to a Redis server creating a TCP connection to the port 6973.
Every redis command or data transmitted by the client and the server is
terminated by "\r\n" (CRLF).

Simple INLINE commands
----------------------

The simplest commands are the inline commands. This is an example of a
server/client chat (the server chat starts with S:, the client chat with C:)

C: PING
S: +PONG

An inline command is a CRLF-terminated string sent to the client. The server
usually replies to inline commands with a single line that can be a number
or a return code.

When the server replies with a return code, if the first character of the
reply is a "+" then the command succeeded, if it is a "-" then the following
part of the string is an error.

The following is another example of an INLINE command returning an integer:

C: EXISTS somekey
S: 0

Since 'somekey' does not exist the server returned '0'.

Note that the EXISTS command takes one argument. Arguments are separated
simply by spaces.

Bulk commands
-------------

A bulk command is exactly like an inline command, but the last argument
of the command must be a stream of bytes in order to send data to the server.
the "SET" command is a bulk command, see the following example:

C: SET mykey 6
C: foobar
S: +OK

The last argument of the commnad is '6'. This specify the number of DATA
bytes that will follow (note that even this bytes are terminated by two
additional bytes of CRLF).

All the bulk commands are in this exact form: instead of the last argument
the number of bytes that will follow is specified, followed by the bytes,
and CRLF. In order to be more clear for the programmer this is the string
sent by the client in the above sample:

    "SET mykey 6\r\nfoobar\r\n"

Bulk replies
------------

The server may reply to an inline or bulk command with a bulk reply. See
the following example:

C: GET mykey
S: 6
S: foobar

A bulk reply is very similar to the last argument of a bulk command. The
server sends as the first line the number of bytes of the actual reply
followed by CRLF, then the bytes are sent followed by additional two bytes
for the final CRLF. The exact sequence sent by the server is:

    "6\r\nfoobar\r\n"

If the requested value does not exist the bulk reply will use the special
value 'nil' instead to send the line containing the number of bytes to read.
This is an example:

C: GET nonexistingkey
S: nil

The client library API should not return an empty string, but a nil object.
For example a Ruby library should return 'nil' while a C library should return
NULL.

Bulk reply error reporting
--------------------------

Bulk replies can signal errors, for example trying to use GET against a list
value is not permitted. Bulk replies use a negative bytes count in order to
signal an error. An error string of ABS(bytes_count) bytes will follow. See
the following example:

S: GET alistkey
S: -38
S: -ERR Requested element is not a string

-38 means: sorry your operation resulted in an error, but a 38 bytes string
that explains this error will follow. Client APIs should abort on this kind
of errors, for example a PHP client should call the die() function.

Multi-Bulk replies
------------------

In the specific case of the LRANGE command the server needs to return multiple
values (every element of the list is a value, and LRANGE needs to return
more than a single element). This is accomplished using multiple bulk writes,
prefixed by an initial line indicating how many bulk writes will follow.
Example:

C: LRANGE mylist 0 3
S: 4
S: 3
S: foo
S: 3
S: bar
S: 5
S: Hello
S: 5
S: World

The first line the server sent is "4\r\n" in order to specify that four bulk
write will follow. Then every bulk write is transmitted.

If the specified key does not exist instead of the number of elements in the
list, the special value 'nil' is sent. Example:

C: LRANGE nokey 0 1
S: nil

A client library API should return a nil object and not an empty list when this
happens.

Multi-Bulk replies errors
-------------------------

Like bulk reply errors Multi-bulk reply errors are reported using a negative
count. Example:

C: LRANGE stringkey 0 1
S: -38
S: -ERR Requested element is not a string

Check the Bulk replies errors section for more information.

Multiple commands and pipelining
--------------------------------

A client can use the same connection in order to issue multiple commands.
Pipelining is supported so multiple commands can be sent with a single
write operation by the client, it is not needed to read the server reply
in order to issue the next command. All the replies can be read at the end.

Usually Redis server and client will have a very fast link so this is not
very important to support this feature in a client implementation, still
if an application needs to issue a very large number of commands in short
time to use pipelining can be much faster.

=======
LICENSE
=======

Redis is released under the GPL license, version 2. See the COPYING file for
more information.

=======
CREDITS
=======

Redis is written and maintained by Salvatore Sanfilippo, Aka 'antirez'.

Enjoy,
antirez