Apache CouchDB

Couch is an acronym for cluster of unreliable commodity hardware.[4] The CouchDB project was created in April 2005 by Damien Katz, a former Lotus Notes developer at IBM. He self-funded the project for almost two years and released it as an open-source project under the GNU General Public License.

In February 2008, it became an Apache Incubator project and was offered under the Apache License instead.[5] A few months after, it graduated to a top-level project.[6] This led to the first stable version being released in July 2010.[7]

In early 2012, Katz left the project to focus on Couchbase Server.[8]

Since Katz's departure, the Apache CouchDB project has continued, releasing 1.2 in April 2012 and 1.3 in April 2013. In July 2013, the CouchDB community merged the codebase for BigCouch, Cloudant's clustered version of CouchDB, into the Apache project.[9] The BigCouch clustering framework is included in the current release of Apache CouchDB.[10]

Native clustering is supported at version 2.0.0. And the new Mango Query Server provides a simple JSON-based way to perform CouchDB queries without JavaScript or MapReduce.

ACID Semantics

CouchDB provides ACID semantics.[11] It does this by implementing a form of Multi-Version Concurrency Control, meaning that CouchDB can handle a high volume of concurrent readers and writers without conflict.

Built for Offline

CouchDB can replicate to devices (like smartphones) that can go offline and handle data sync for you when the device is back online.

Distributed Architecture with Replication

CouchDB was designed with bi-directional replication (or synchronization) and off-line operation in mind. That means multiple replicas can have their own copies of the same data, modify it, and then sync those changes at a later time.

Document Storage

CouchDB stores data as "documents", as one or more field/value pairs expressed as JSON. Field values can be simple things like strings, numbers, or dates; but ordered lists and associative arrays can also be used. Every document in a CouchDB database has a unique id and there is no required document schema.

Eventual Consistency

CouchDB guarantees eventual consistency to be able to provide both availability and partition tolerance.

Map/Reduce Views and Indexes

The stored data is structured using views. In CouchDB, each view is constructed by a JavaScript function that acts as the Map half of a map/reduce operation. The function takes a document and transforms it into a single value that it returns. CouchDB can index views and keep those indexes updated as documents are added, removed, or updated.

HTTP API

All items have a unique URI that gets exposed via HTTP. It uses the HTTP methods POST, GET, PUT and DELETE for the four basic CRUD (Create, Read, Update, Delete) operations on all resources.

CouchDB also offers a built-in administration interface accessible via Web called Fauxton.[12]

Replication and synchronization capabilities of CouchDB make it ideal for using it in mobile devices, where network connection is not guaranteed, and the application must keep on working offline.

CouchDB is well suited for applications with accumulating, occasionally changing data, on which pre-defined queries are to be run and where versioning is important (CRM, CMS systems, by example). Master-master replication is an especially interesting feature, allowing easy multi-site deployments.[13]

CouchDB manages a collection of JSON documents. The documents are organised via views. Views are defined with aggregate functions and filters are computed in parallel, much like MapReduce.

Views are generally stored in the database and their indexes are updated continuously. CouchDB supports a view system using external socket servers and a JSON-based protocol.[26] As a consequence, view servers have been developed in a variety of languages (JavaScript is the default, but there are also PHP, Ruby, Python and Erlang).

curl http://127.0.0.1:5984/

{
  "couchdb": "Welcome",
  "version":"1.1.0"
}

curl -X PUT http://127.0.0.1:5984/wiki

{"ok": true}

curl -X PUT http://127.0.0.1:5984/wiki

{
  "error":"file_exists",
  "reason":"The database could not be created, the file already exists."
}

curl http://127.0.0.1:5984/wiki

{
  "db_name": "wiki",
  "doc_count": 0,
  "doc_del_count": 0,
  "update_seq": 0,
  "purge_seq": 0,
  "compact_running": false,
  "disk_size": 79,
  "instance_start_time": "1272453873691070",
  "disk_format_version": 5
}

curl -X DELETE http://127.0.0.1:5984/wiki

{"ok": true}

curl -X POST -H "Content-Type: application/json" --data \
'{ "text" : "Wikipedia on CouchDB", "rating": 5 }' \
http://127.0.0.1:5984/wiki

{
  "ok": true,
  "id": "123BAC",
  "rev": "946B7D1C"
}

curl http://127.0.0.1:5984/_all_dbs

["_replicator","_users","wiki"]

CouchDB includes a number of other open source projects as part of its default package.

• Document-oriented database
• XML database

• Anderson, J. Chris; Slater, Noah; Lehnardt, Jan (November 15, 2009), CouchDB: The Definitive Guide (1st ed.), O'Reilly Media, p. 300, ISBN 978-0-596-15816-3
• Lennon, Joe (December 15, 2009), Beginning CouchDB (1st ed.), Apress, p. 300, ISBN 978-1-4302-7237-3, archived from the original on December 5, 2010, retrieved November 1, 2009
• Holt, Bradley (March 7, 2011), Writing and Querying MapReduce Views in CouchDB (1st ed.), O'Reilly Media, p. 76, ISBN 978-1-4493-0312-9
• Holt, Bradley (April 11, 2011), Scaling CouchDB (1st ed.), O'Reilly Media, p. 72, ISBN 978-1-4493-0343-3
• Brown, MC (October 31, 2011), Getting Started with CouchDB (1st ed.), O'Reilly Media, p. 50, ISBN 978-1-4493-0755-4
• Thompson, Mick (August 2, 2011), Getting Started with GEO, CouchDB, and Node.js (1st ed.), O'Reilly Media, p. 64, ISBN 978-1-4493-0752-3

• Official website
• CouchDB: The Definitive Guide
• Complete HTTP API Reference
• Simple PHP5 library to communicate with CouchDB
• Asynchronous CouchDB client for Java
• Asynchronous CouchDB client for Scala
• Lehnardt, Jan (2008). "Couch DB at 10,000 feet". Erlang eXchange 2008. Archived from the original on 9 November 2012. Retrieved 15 April 2012.
• Lenhardt, Jan (2009). "CouchDB for Erlang Developers". Erlang Factory London 2009. Archived from the original on 19 June 2011. Retrieved 15 April 2012.
• Katz, Damien (January 2009). "CouchDB and Me". RubyFringe. InfoQ. Archived from the original on 27 April 2011. Retrieved 15 April 2012.