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From Apache Wiki <>
Subject [Couchdb Wiki] Update of "Introduction" by RobertKowalski
Date Sun, 25 May 2014 13:02:04 GMT
Dear Wiki user,

You have subscribed to a wiki page or wiki category on "Couchdb Wiki" for change notification.

The "Introduction" page has been changed by RobertKowalski:

move to cwiki

+ The content of this page has moved to
- <<Include(EditTheWiki)>>
- <<TableOfContents()>>
- = Introduction =
- == Meet CouchDB ==
- CouchDB is often categorized as a “NoSQL” database, a term that became increasingly
popular in late 2009, and early 2010. While this term is a rather generic characterization
of a database, or data store, it does clearly define a break from traditional SQL-based databases.
A CouchDB database lacks a schema, or rigid pre-defined data structures such as tables. Data
stored in CouchDB is a JSON document(s). The structure of the data, or document(s), can change
dynamically to accommodate evolving needs.
- == What it is Not ==
- To better understand what CouchDB is, it may be helpful to understand a few things that
CouchDB isn't:
-  * A relational database. These differences are articulated above in the Meet CouchDB section,
and other portions of this Wiki.
-  * A replacement for all databases. When developing and designing a good information system
you should select the __best tool for the job__.  While CouchDB can be used in a wide variety
of applications you may find that another data store is a better fit for your problem.  If
you are new to CouchDB, and aren't sure if it's a good fit for your data management problem,
please ask others on the mailing list and the #couchdb IRC channel for advice.
-  * An object-oriented database. While CouchDB stores JSON objects, it isn't meant to function
as a seamless persistence layer for an object-oriented programming language.
- == Key Characteristics ==
- Let's review some of the basic elements of CouchDB.  
- === Documents ===
- A CouchDB document is a [[|JSON]] object that consists of named fields.
Field values may be strings, numbers, dates, or even ordered lists and associative maps. An
example of a document would be a blog post:
- {{{#!highlight javascript
- {
-     "Subject": "I like Plankton",
-     "Author": "Rusty",
-     "PostedDate": "5/23/2006",
-     "Tags": ["plankton", "baseball", "decisions"],
-     "Body": "I decided today that I don't like baseball. I like plankton."
- }
- }}}
- In the above example document, `Subject` is a field that contains a single string value
"I like plankton". `Tags` is a field containing the list of values "plankton",  "baseball",
and "decisions".
- A CouchDB database is a flat collection of these documents. Each document is identified
by a unique ID.
- === Views ===
- To address this problem of adding structure back to semi-structured data, CouchDB integrates
a view model using JavaScript for description. Views are the method of aggregating and reporting
on the documents in a database, and are built on-demand to aggregate, join and report on database
documents. Views are built dynamically and don’t affect the underlying document; you can
have as many different view representations of the same data as you like. Incremental updates
to documents do not require full re-indexing of views.
- === Schema-Free ===
- Unlike SQL databases, which are designed to store and report on highly structured, interrelated
data, CouchDB is designed to store and report on large amounts of semi-structured, document
oriented data. CouchDB greatly simplifies the development of document oriented applications,
such as collaborative web applications.
- In an SQL database, the schema and storage of the existing data must be updated as needs
evolve. With CouchDB, no schema is required, so new document types with new meaning can be
safely added alongside the old. However, for applications requiring robust validation of new
documents custom validation functions are possible. The view engine is designed to easily
handle new document types and disparate but similar documents.
- === Distributed ===
- CouchDB is a peer based distributed database system. Any number of CouchDB hosts (servers
and offline-clients) can have independent "replica copies" of the same database, where applications
have full database interactivity (query, add, edit, delete). When back online or on a schedule,
database changes can be replicated bi-directionally.
- CouchDB has built-in conflict detection and management and the replication process is incremental
and fast, copying only documents changed since the previous replication. Most applications
require no special planning to take advantage of distributed updates and replication.
- Unlike cumbersome attempts to bolt distributed features on top of the same legacy models
and databases, replication in CouchDB is the result of careful ground-up design, engineering
and integration. This replication framework provides a comprehensive set of features:
-  * Master → Slave replication
-  * Master ↔ Master replication
-  * Filtered Replication
-  * Incremental and bi-directional replication
-  * Conflict management
- These replication features can be used in combination to create powerful solutions to many
problems in the IT industry. In addition to the fantastic replication features, CouchDB's
reliability and scalability is further enhanced by being implemented in the Erlang programming
language. Erlang has built-in support for concurrency, distribution, fault tolerance, and
has been used for years to build reliable systems in the telecommunications industry. By design,
the Erlang language and runtime are able to take advantage of newer hardware with multiple
CPU cores.
- = Next Steps =
- For installation instructions consult the [[Installation|installation instructions]] or
dive right into the [[Basics|basics]] if CouchDB is already installed. Detailed information
for the more curious can be found in the [[Technical Overview|technical overview]].

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