zzimma

REDIS - REmote DIctionary Server

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WHAT'S REDIS?
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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 it writes a dump of the dataset on disk. The dump
is loaded every time the server is restarted.

This means that it can happens 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.

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HOW REDIS DIFFERS FROM MEMCACHED?
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Maily in two ways:

- Memcached is not persistent, it just holds everything in memory
  without saving since its main goal is to be uesd 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 and concatenations can be performed
  against sets and lists.

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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, copressed 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 (" ").

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.