X-Git-Url: https://git.saurik.com/redis.git/blobdiff_plain/ed9b544e10b84cd43348ddfab7068b610a5df1f7..7492bbe9f52a655bac2480f069083aaf220ac01b:/doc/README.html diff --git a/doc/README.html b/doc/README.html index 402038d7..4ad8c0ed 100644 --- a/doc/README.html +++ b/doc/README.html @@ -28,21 +28,23 @@

Introduction

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 asynchronously. The dump is loaded every time the server is restarted. This means that if a system crash occurs the last few queries can get lost (that is acceptable in many applications), so we supported master-slave replication from the early days.

Beyond key-value databases

In most key-value databases keys and values are simple strings. In Redis keys are just strings too, but the associated values can be Strings, Lists and Sets, and there are commands to perform complex atomic operations against this data types, so you can think at Redis as a data structures server.

For example you can append elements to a list stored at the key "mylist" using the LPUSH or RPUSH operation in O(1). Later you'll be able to get a range of elements with LRANGE or trim the list with LTRIM. Sets are very flexible too, it is possible to add and remove elements from Sets (unsorted collections of strings), and then ask for server-side intersection of Sets.

All this features, the support for sorting Lists and Sets, allow to use Redis as the sole DB for your scalable application without the need of any relational database. We wrote a simple Twitter clone in PHP + Redis to show a real world example, the link points to an article explaining the design and internals in very simple words.

What are the differences between Redis and Memcached?

In the following ways:

+I can set the key "surname_1992" to the string "Smith". The interesting thing about Redis is that values associated to keys are not limited to simple strings, they can also be lists and sets, with a number of server-side atomic operations associated to this data types.

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 asynchronously. The dump is loaded every time the server is restarted.

Redis can be configured to save the dataset after a given number of seconds elapzed and changes to the data set. For example you can tell Redis to save after 1000 changes and at least 60 seconds sinde the same save. You can specify a number of this combinatins.

Because data is written asynchronously, If a system crash occurs the last few queries can get lost (that is acceptable in many applications). Redis supports master-slave replication from the early days in order to make this a non issue if your application is of the kind where even few lost records are not acceptable.

Beyond key-value databases

In most key-value databases keys and values are simple strings. In Redis keys are just strings too, but the associated values can be Strings, Lists and Sets, and there are commands to perform complex atomic operations against this data types, so you can think at Redis as a data structures server.

For example you can append elements to a list stored at the key "mylist" using the LPUSH or RPUSH operation in O(1). Later you'll be able to get a range of elements with LRANGE or trim the list with LTRIM. Sets are very flexible too, it is possible to add and remove elements from Sets (unsorted collections of strings), and then ask for server-side intersection, union, difference of Sets.

All this features, the support for sorting Lists and Sets, allow to use Redis as the sole DB for your scalable application without the need of any relational database. We wrote a simple Twitter clone in PHP + Redis to show a real world example, the link points to an article explaining the design and internals in very simple words.

What are the differences between Redis and Memcached?

In the following ways:

-

What are the differences between Redis and Tokyo Cabinet / Tyrant?

Redis and Tokyo can be used for the same applications, but actually they are ery different beasts:

- - +

What are the differences between Redis and Tokyo Cabinet / Tyrant?

Redis and Tokyo Cabinet can be used for the same applications, but actually they are very different beasts. If you read twitter messages of people involved in scalable things both products are reported to work well, but surely there are times where one or the other can be the best choice. Some differences are the followings (I may be biased, make sure to check yourself both the products).

+ + + +

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. +10 computers and one Redis server. You want to count words in a very large text. This large text is split 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.

(News: locking with key-granularity is now planned)

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 supports the following three data types as values:

+order to implement distributed algorithms.

(News: locking with key-granularity is now planned)

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' commands.

Redis Data Types

Redis supports the following three data types as values:

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 @@ -79,22 +81,21 @@ our key was added without problems. Actually SET can never fail but the "+OK" sent lets us know that the server received everything and the command was actually executed.

Let's try to get the key content now:

 GET foo
-3
+$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?

+the server replied with a first line that is just the $ character follwed by
+the 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. In Redis slang this is called a bulk reply.

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:

+$-1
+
When the key does not exist instead of the length, just the "$-1" string is sent. Since a -1 length of a bulk reply has no meaning it is used in order to specifiy a 'nil' value and distinguish it from a zero length value. Another way to check if a given key exists or not is indeed the EXISTS command:

 EXISTS nokey
-0
+: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 COMMAND REFERENCE section to
+: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. Replies starting with the colon character are integer reply.

Ok... now you know the basics, read the REDIS COMMAND 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.

License

Redis is released under the BSD license. See the COPYING file for more information.

Credits

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

Enjoy,