diff --git a/lib/private/share/share.php b/lib/private/share/share.php
index 646511fd64db..759ad7a828b1 100644
--- a/lib/private/share/share.php
+++ b/lib/private/share/share.php
@@ -1074,6 +1074,15 @@ protected static function unshareItem(array $item) {
 		\OC_Hook::emit('OCP\Share', 'post_unshare', $hookParams);
 	}
 
+	/**
+	 * Check if a backend class has been registered for the specified item type
+	 * @param string $itemType
+	 * @return boolean True if a backend has been registered, false otherwise.
+	 */
+	public static function hasBackend($itemType) {
+		return isset(self::$backendTypes[$itemType]);
+	}
+
 	/**
 	 * Get the backend class for the specified item type
 	 * @param string $itemType
@@ -1130,7 +1139,7 @@ public static function isResharingAllowed() {
 	 * @param string $itemType
 	 * @return array
 	 */
-	private static function getCollectionItemTypes($itemType) {
+	public static function getCollectionItemTypes($itemType) {
 		$collectionTypes = array($itemType);
 		foreach (self::$backendTypes as $type => $backend) {
 			if (in_array($backend['collectionOf'], $collectionTypes)) {
@@ -1138,7 +1147,7 @@ private static function getCollectionItemTypes($itemType) {
 			}
 		}
 		// TODO Add option for collections to be collection of themselves, only 'folder' does it now...
-		if (!self::getBackend($itemType) instanceof \OCP\Share_Backend_Collection || $itemType != 'folder') {
+		if (isset(self::$backendTypes[$itemType]) && (!self::getBackend($itemType) instanceof \OCP\Share_Backend_Collection || $itemType != 'folder')) {
 			unset($collectionTypes[0]);
 		}
 		// Return array if collections were found or the item type is a
diff --git a/lib/private/tags.php b/lib/private/tags.php
index 0b62caf2dd89..22fa4c70138b 100644
--- a/lib/private/tags.php
+++ b/lib/private/tags.php
@@ -5,6 +5,7 @@
 * @author Thomas Tanghus
 * @copyright 2012-2013 Thomas Tanghus <thomas@tanghus.net>
 * @copyright 2012 Bart Visscher bartv@thisnet.nl
+* @copyright 2014 Bernhard Reiter <ockham@raz.or.at>
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU AFFERO GENERAL PUBLIC LICENSE
@@ -34,14 +35,14 @@
 
 namespace OC;
 
+use OC\Tags\Backend\Database;
+
 class Tags implements \OCP\ITags {
 
 	/**
-	 * Tags
-	 *
-	 * @var array
+	 * FIXME var Backend\AbstractBackend
 	 */
-	private $tags = array();
+	private $backends = array();
 
 	/**
 	 * Used for storing objectid/categoryname pairs while rescanning.
@@ -64,56 +65,26 @@ class Tags implements \OCP\ITags {
 	 */
 	private $user = null;
 
-	const TAG_TABLE = '*PREFIX*vcategory';
-	const RELATION_TABLE = '*PREFIX*vcategory_to_object';
+	public static $backendClasses = array(
+		'local' => 'OC\Tags\Backend\Database',
+		'shared' => 'OC\Tags\Backend\Shared'
+	);
 
 	const TAG_FAVORITE = '_$!<Favorite>!$_';
 
 	/**
 	* Constructor.
 	*
-	* @param string $user The user whos data the object will operate on.
+	* @param string $user The user whose data the object will operate on.
 	* @param string $type
 	*/
 	public function __construct($user, $type, $defaultTags = array()) {
 		$this->user = $user;
 		$this->type = $type;
-		$this->loadTags($defaultTags);
+		foreach (self::$backendClasses as $backendName => $class)
+			$this->backends[$backendName] = new $class($user, $type);
 	}
 
-	/**
-	* Load tags from db.
-	*
-	*/
-	protected function loadTags($defaultTags=array()) {
-		$this->tags = array();
-		$result = null;
-		$sql = 'SELECT `id`, `category` FROM `' . self::TAG_TABLE . '` '
-			. 'WHERE `uid` = ? AND `type` = ? ORDER BY `category`';
-		try {
-			$stmt = \OCP\DB::prepare($sql);
-			$result = $stmt->execute(array($this->user, $this->type));
-			if (\OCP\DB::isError($result)) {
-				\OCP\Util::writeLog('core', __METHOD__. ', DB error: ' . \OCP\DB::getErrorMessage($result), \OCP\Util::ERROR);
-			}
-		} catch(\Exception $e) {
-			\OCP\Util::writeLog('core', __METHOD__.', exception: '.$e->getMessage(),
-				\OCP\Util::ERROR);
-		}
-
-		if(!is_null($result)) {
-			while( $row = $result->fetchRow()) {
-				$this->tags[$row['id']] = $row['category'];
-			}
-		}
-
-		if(count($defaultTags) > 0 && count($this->tags) === 0) {
-			$this->addMultiple($defaultTags, true);
-		}
-		\OCP\Util::writeLog('core', __METHOD__.', tags: ' . print_r($this->tags, true),
-			\OCP\Util::DEBUG);
-
-	}
 
 	/**
 	* Check if any tags are saved for this type and user.
@@ -121,7 +92,7 @@ protected function loadTags($defaultTags=array()) {
 	* @return boolean.
 	*/
 	public function isEmpty() {
-		return count($this->tags) === 0;
+		return $this->backend->isEmpty();
 	}
 
 	/**
@@ -136,28 +107,30 @@ public function isEmpty() {
 	* @return array
 	*/
 	public function getTags() {
-		if(!count($this->tags)) {
+		$tags = array();
+		foreach ($this->backends as $name => $backend)
+			$tags += $backend->getUnsortedTags();
+
+		if(!count($tags))
 			return array();
-		}
 
-		$tags = array_values($this->tags);
 		uasort($tags, 'strnatcasecmp');
 		$tagMap = array();
 
-		foreach($tags as $tag) {
-			if($tag !== self::TAG_FAVORITE) {
+		foreach($tags as $tagId => $tagName) {
+			if($tagName !== self::TAG_FAVORITE) {
 				$tagMap[] = array(
-					'id' => $this->array_searchi($tag, $this->tags),
-					'name' => $tag
+					'id' => $tagId,
+					'name' => $tagName
 					);
 			}
 		}
-		return $tagMap;
 
+		return $tagMap;
 	}
 
 	/**
-	* Get the a list if items tagged with $tag.
+	* Get the a list of items tagged with $tag.
 	*
 	* Throws an exception if the tag could not be found.
 	*
@@ -165,47 +138,13 @@ public function getTags() {
 	* @return array An array of object ids or false on error.
 	*/
 	public function getIdsForTag($tag) {
-		$result = null;
-		if(is_numeric($tag)) {
-			$tagId = $tag;
-		} elseif(is_string($tag)) {
-			$tag = trim($tag);
-			if($tag === '') {
-				\OCP\Util::writeLog('core', __METHOD__.', Cannot use empty tag names', \OCP\Util::DEBUG);
-				return false;
-			}
-			$tagId = $this->array_searchi($tag, $this->tags);
-		}
-
-		if($tagId === false) {
-			$l10n = \OC_L10N::get('core');
-			throw new \Exception(
-				$l10n->t('Could not find category "%s"', $tag)
-			);
-		}
+		foreach ($this->backends as $name => $backend)
+			if ($id = $backend->hasTag($tag))
+				break;
 
 		$ids = array();
-		$sql = 'SELECT `objid` FROM `' . self::RELATION_TABLE
-			. '` WHERE `categoryid` = ?';
-
-		try {
-			$stmt = \OCP\DB::prepare($sql);
-			$result = $stmt->execute(array($tagId));
-			if (\OCP\DB::isError($result)) {
-				\OCP\Util::writeLog('core', __METHOD__. 'DB error: ' . \OCP\DB::getErrorMessage($result), \OCP\Util::ERROR);
-				return false;
-			}
-		} catch(\Exception $e) {
-			\OCP\Util::writeLog('core', __METHOD__.', exception: '.$e->getMessage(),
-				\OCP\Util::ERROR);
-			return false;
-		}
-
-		if(!is_null($result)) {
-			while( $row = $result->fetchRow()) {
-				$ids[] = (int)$row['objid'];
-			}
-		}
+		foreach ($this->backends as $name => $backend)
+			$ids += $backend->getIdsForTag($id);
 
 		return $ids;
 	}
@@ -214,14 +153,19 @@ public function getIdsForTag($tag) {
 	* Checks whether a tag is already saved.
 	*
 	* @param string $name The name to check for.
-	* @return bool
+	* @return The tag's id, or false if no tag of that name has been saved yet.
 	*/
 	public function hasTag($name) {
-		return $this->in_arrayi($name, $this->tags);
+		$id = false;
+		foreach ($this->backends as $name => $backend)
+			if ($id = $backend->hasTag($name))
+				break;
+
+		return $id;
 	}
 
 	/**
-	* Add a new tag.
+	* Add a new tag, owned by the current user.
 	*
 	* @param string $name A string with a name of the tag
 	* @return false|string the id of the added tag or false on error.
@@ -237,31 +181,8 @@ public function add($name) {
 			\OCP\Util::writeLog('core', __METHOD__.', name: ' . $name. ' exists already', \OCP\Util::DEBUG);
 			return false;
 		}
-		try {
-			$result = \OCP\DB::insertIfNotExist(
-				self::TAG_TABLE,
-				array(
-					'uid' => $this->user,
-					'type' => $this->type,
-					'category' => $name,
-				)
-			);
-			if (\OCP\DB::isError($result)) {
-				\OCP\Util::writeLog('core', __METHOD__. 'DB error: ' . \OCP\DB::getErrorMessage($result), \OCP\Util::ERROR);
-				return false;
-			} elseif((int)$result === 0) {
-				\OCP\Util::writeLog('core', __METHOD__.', Tag already exists: ' . $name, \OCP\Util::DEBUG);
-				return false;
-			}
-		} catch(\Exception $e) {
-			\OCP\Util::writeLog('core', __METHOD__.', exception: '.$e->getMessage(),
-				\OCP\Util::ERROR);
-			return false;
-		}
-		$id = \OCP\DB::insertid(self::TAG_TABLE);
-		\OCP\Util::writeLog('core', __METHOD__.', id: ' . $id, \OCP\Util::DEBUG);
-		$this->tags[$id] = $name;
-		return $id;
+
+		return $this->backends['local']->add($name);
 	}
 
 	/**
@@ -280,28 +201,13 @@ public function rename($from, $to) {
 			return false;
 		}
 
-		$id = $this->array_searchi($from, $this->tags);
-		if($id === false) {
-			\OCP\Util::writeLog('core', __METHOD__.', tag: ' . $from. ' does not exist', \OCP\Util::DEBUG);
-			return false;
-		}
+		$id = false;
+		foreach ($this->backends as $name => $backend)
+			if ($id = $backend->hasTag($from))
+				return $backend->rename($id, $to);
 
-		$sql = 'UPDATE `' . self::TAG_TABLE . '` SET `category` = ? '
-			. 'WHERE `uid` = ? AND `type` = ? AND `id` = ?';
-		try {
-			$stmt = \OCP\DB::prepare($sql);
-			$result = $stmt->execute(array($to, $this->user, $this->type, $id));
-			if (\OCP\DB::isError($result)) {
-				\OCP\Util::writeLog('core', __METHOD__. 'DB error: ' . \OCP\DB::getErrorMessage($result), \OCP\Util::ERROR);
-				return false;
-			}
-		} catch(\Exception $e) {
-			\OCP\Util::writeLog('core', __METHOD__.', exception: '.$e->getMessage(),
-				\OCP\Util::ERROR);
-			return false;
-		}
-		$this->tags[$id] = $to;
-		return true;
+		\OCP\Util::writeLog('core', __METHOD__.', tag: ' . $from. ' does not exist', \OCP\Util::DEBUG);
+		return false;
 	}
 
 	/**
@@ -322,8 +228,7 @@ public function addMultiple($names, $sync=false, $id = null) {
 
 		$newones = array();
 		foreach($names as $name) {
-			if(($this->in_arrayi(
-				$name, $this->tags) == false) && $name !== '') {
+			if(($this->hasTag($name) == false) && $name !== '') {
 				$newones[] = $name;
 			}
 			if(!is_null($id) ) {
@@ -333,61 +238,12 @@ public function addMultiple($names, $sync=false, $id = null) {
 		}
 		$this->tags = array_merge($this->tags, $newones);
 		if($sync === true) {
-			$this->save();
+			$this->backend->save();
 		}
 
 		return true;
 	}
 
-	/**
-	 * Save the list of tags and their object relations
-	 */
-	protected function save() {
-		if(is_array($this->tags)) {
-			foreach($this->tags as $tag) {
-				try {
-					\OCP\DB::insertIfNotExist(self::TAG_TABLE,
-						array(
-							'uid' => $this->user,
-							'type' => $this->type,
-							'category' => $tag,
-						));
-				} catch(\Exception $e) {
-					\OCP\Util::writeLog('core', __METHOD__.', exception: '.$e->getMessage(),
-						\OCP\Util::ERROR);
-				}
-			}
-			// reload tags to get the proper ids.
-			$this->loadTags();
-			// Loop through temporarily cached objectid/tagname pairs
-			// and save relations.
-			$tags = $this->tags;
-			// For some reason this is needed or array_search(i) will return 0..?
-			ksort($tags);
-			foreach(self::$relations as $relation) {
-				$tagId = $this->array_searchi($relation['tag'], $tags);
-				\OCP\Util::writeLog('core', __METHOD__ . 'catid, ' . $relation['tag'] . ' ' . $tagId, \OCP\Util::DEBUG);
-				if($tagId) {
-					try {
-						\OCP\DB::insertIfNotExist(self::RELATION_TABLE,
-							array(
-								'objid' => $relation['objid'],
-								'categoryid' => $tagId,
-								'type' => $this->type,
-								));
-					} catch(\Exception $e) {
-						\OCP\Util::writeLog('core', __METHOD__.', exception: '.$e->getMessage(),
-							\OCP\Util::ERROR);
-					}
-				}
-			}
-			self::$relations = array(); // reset
-		} else {
-			\OCP\Util::writeLog('core', __METHOD__.', $this->tags is not an array! '
-				. print_r($this->tags, true), \OCP\Util::ERROR);
-		}
-	}
-
 	/**
 	* Delete tags and tag/object relations for a user.
 	*
@@ -396,48 +252,7 @@ protected function save() {
 	* @param array $arguments
 	*/
 	public static function post_deleteUser($arguments) {
-		// Find all objectid/tagId pairs.
-		$result = null;
-		try {
-			$stmt = \OCP\DB::prepare('SELECT `id` FROM `' . self::TAG_TABLE . '` '
-				. 'WHERE `uid` = ?');
-			$result = $stmt->execute(array($arguments['uid']));
-			if (\OCP\DB::isError($result)) {
-				\OCP\Util::writeLog('core', __METHOD__. 'DB error: ' . \OCP\DB::getErrorMessage($result), \OCP\Util::ERROR);
-			}
-		} catch(\Exception $e) {
-			\OCP\Util::writeLog('core', __METHOD__.', exception: '.$e->getMessage(),
-				\OCP\Util::ERROR);
-		}
-
-		if(!is_null($result)) {
-			try {
-				$stmt = \OCP\DB::prepare('DELETE FROM `' . self::RELATION_TABLE . '` '
-					. 'WHERE `categoryid` = ?');
-				while( $row = $result->fetchRow()) {
-					try {
-						$stmt->execute(array($row['id']));
-					} catch(\Exception $e) {
-						\OCP\Util::writeLog('core', __METHOD__.', exception: '.$e->getMessage(),
-							\OCP\Util::ERROR);
-					}
-				}
-			} catch(\Exception $e) {
-				\OCP\Util::writeLog('core', __METHOD__.', exception: '.$e->getMessage(),
-					\OCP\Util::ERROR);
-			}
-		}
-		try {
-			$stmt = \OCP\DB::prepare('DELETE FROM `' . self::TAG_TABLE . '` '
-				. 'WHERE `uid` = ?');
-			$result = $stmt->execute(array($arguments['uid']));
-			if (\OCP\DB::isError($result)) {
-				\OCP\Util::writeLog('core', __METHOD__. ', DB error: ' . \OCP\DB::getErrorMessage($result), \OCP\Util::ERROR);
-			}
-		} catch(\Exception $e) {
-			\OCP\Util::writeLog('core', __METHOD__ . ', exception: '
-				. $e->getMessage(), \OCP\Util::ERROR);
-		}
+		$this->backend->post_deleteUser($arguments);
 	}
 
 	/**
@@ -447,28 +262,7 @@ public static function post_deleteUser($arguments) {
 	* @return boolean Returns false on error.
 	*/
 	public function purgeObjects(array $ids) {
-		if(count($ids) === 0) {
-			// job done ;)
-			return true;
-		}
-		$updates = $ids;
-		try {
-			$query = 'DELETE FROM `' . self::RELATION_TABLE . '` ';
-			$query .= 'WHERE `objid` IN (' . str_repeat('?,', count($ids)-1) . '?) ';
-			$query .= 'AND `type`= ?';
-			$updates[] = $this->type;
-			$stmt = \OCP\DB::prepare($query);
-			$result = $stmt->execute($updates);
-			if (\OCP\DB::isError($result)) {
-				\OCP\Util::writeLog('core', __METHOD__. 'DB error: ' . \OCP\DB::getErrorMessage($result), \OCP\Util::ERROR);
-				return false;
-			}
-		} catch(\Exception $e) {
-			\OCP\Util::writeLog('core', __METHOD__.', exception: ' . $e->getMessage(),
-				\OCP\Util::ERROR);
-			return false;
-		}
-		return true;
+		return $this->backend->purgeObjects($ids);
 	}
 
 	/**
@@ -517,32 +311,39 @@ public function removeFromFavorites($objid) {
 	* @return boolean Returns false on error.
 	*/
 	public function tagAs($objid, $tag) {
+		$backendToUse = $this->backends['local']; // Default backend
 		if(is_string($tag) && !is_numeric($tag)) {
 			$tag = trim($tag);
 			if($tag === '') {
-				\OCP\Util::writeLog('core', __METHOD__.', Cannot add an empty tag', \OCP\Util::DEBUG);
+				\OCP\Util::writeLog('core', __METHOD__.', Cannot add an empty tag',
+									\OCP\Util::DEBUG);
 				return false;
 			}
-			if(!$this->hasTag($tag)) {
-				$this->add($tag);
+
+			$tagId = false;
+			foreach ($this->backends as $name => $backend) {
+				if ($tagId = $backend->hasTag($tag)) {
+					$backendToUse = $backend;
+					break;
+				}
+			}
+
+			if(!$tagId) {
+				// Add new tag to this user's tags (i.e. using the local backend).
+				$tagId = $backendToUse->add($tag);
 			}
-			$tagId =  $this->array_searchi($tag, $this->tags);
+
 		} else {
-			$tagId = $tag;
-		}
-		try {
-			\OCP\DB::insertIfNotExist(self::RELATION_TABLE,
-				array(
-					'objid' => $objid,
-					'categoryid' => $tagId,
-					'type' => $this->type,
-				));
-		} catch(\Exception $e) {
-			\OCP\Util::writeLog('core', __METHOD__.', exception: '.$e->getMessage(),
-				\OCP\Util::ERROR);
-			return false;
+			foreach ($this->backends as $name => $backend) {
+				if ($backend->hasTagId($tag)) {
+					$backendToUse = $backend;
+					$tagId = $tag;
+					break;
+				}
+			}
 		}
-		return true;
+
+		return $backendToUse->tagAs($objid, $tagId);
 	}
 
 	/**
@@ -559,31 +360,32 @@ public function unTag($objid, $tag) {
 				\OCP\Util::writeLog('core', __METHOD__.', Tag name is empty', \OCP\Util::DEBUG);
 				return false;
 			}
-			$tagId =  $this->array_searchi($tag, $this->tags);
-		} else {
-			$tagId = $tag;
-		}
 
-		try {
-			$sql = 'DELETE FROM `' . self::RELATION_TABLE . '` '
-					. 'WHERE `objid` = ? AND `categoryid` = ? AND `type` = ?';
-			$stmt = \OCP\DB::prepare($sql);
-			$stmt->execute(array($objid, $tagId, $this->type));
-		} catch(\Exception $e) {
-			\OCP\Util::writeLog('core', __METHOD__.', exception: '.$e->getMessage(),
-				\OCP\Util::ERROR);
+			foreach ($this->backends as $name => $backend)
+				if ($tagId = $backend->hasTag($tag))
+					return $backend->unTag($objid, $tagId);
+
+			// Tag with given name not found.
 			return false;
+
+		} else {
+			foreach ($this->backends as $name => $backend)
+				if ($backend->hasTagId($tag))
+					return $backend->unTag($objid, $tag);
 		}
-		return true;
+
+	// Tag with given ID not found.
+	return false;
 	}
 
 	/**
-	* Delete tags from the
+	* Delete tags from the db.
 	*
 	* @param string[] $names An array of tags to delete
 	* @return bool Returns false on error
 	*/
 	public function delete($names) {
+		$success = true;
 		if(!is_array($names)) {
 			$names = array($names);
 		}
@@ -591,47 +393,17 @@ public function delete($names) {
 		$names = array_map('trim', $names);
 		array_filter($names);
 
-		\OCP\Util::writeLog('core', __METHOD__ . ', before: '
-			. print_r($this->tags, true), \OCP\Util::DEBUG);
-		foreach($names as $name) {
-			$id = null;
-
-			if($this->hasTag($name)) {
-				$id = $this->array_searchi($name, $this->tags);
-				unset($this->tags[$id]);
-			}
-			try {
-				$stmt = \OCP\DB::prepare('DELETE FROM `' . self::TAG_TABLE . '` WHERE '
-					. '`uid` = ? AND `type` = ? AND `category` = ?');
-				$result = $stmt->execute(array($this->user, $this->type, $name));
-				if (\OCP\DB::isError($result)) {
-					\OCP\Util::writeLog('core', __METHOD__. 'DB error: ' . \OCP\DB::getErrorMessage($result), \OCP\Util::ERROR);
-				}
-			} catch(\Exception $e) {
-				\OCP\Util::writeLog('core', __METHOD__ . ', exception: '
-					. $e->getMessage(), \OCP\Util::ERROR);
-				return false;
-			}
-			if(!is_null($id) && $id !== false) {
-				try {
-					$sql = 'DELETE FROM `' . self::RELATION_TABLE . '` '
-							. 'WHERE `categoryid` = ?';
-					$stmt = \OCP\DB::prepare($sql);
-					$result = $stmt->execute(array($id));
-					if (\OCP\DB::isError($result)) {
-						\OCP\Util::writeLog('core',
-							__METHOD__. 'DB error: ' . \OCP\DB::getErrorMessage($result),
-							\OCP\Util::ERROR);
-						return false;
-					}
-				} catch(\Exception $e) {
-					\OCP\Util::writeLog('core', __METHOD__.', exception: '.$e->getMessage(),
-						\OCP\Util::ERROR);
-					return false;
+		foreach ($this->backends as $backendName => $backend) {
+			$tagsInBackend = array();
+			foreach ($names as $name) {
+				if ($backend->hasTag($name)) {
+					$tagsInBackend[] = $name;
+					unset($name);
 				}
 			}
+			$success &= $backend->delete($tagsInBackend);
 		}
-		return true;
+		return $success;
 	}
 
 	// case-insensitive in_array
diff --git a/lib/private/tags/backend/abstractbackend.php b/lib/private/tags/backend/abstractbackend.php
new file mode 100644
index 000000000000..3e3f6073ceb4
--- /dev/null
+++ b/lib/private/tags/backend/abstractbackend.php
@@ -0,0 +1,191 @@
+<?php
+/**
+* ownCloud - Abstract backend for tags
+*
+* @author Thomas Tanghus
+* @copyright 2012-2013 Thomas Tanghus <thomas@tanghus.net>
+* @copyright 2012 Bart Visscher bartv@thisnet.nl
+* @copyright 2014 Bernhard Reiter <ockham@raz.or.at>
+*
+* This library is free software; you can redistribute it and/or
+* modify it under the terms of the GNU AFFERO GENERAL PUBLIC LICENSE
+* License as published by the Free Software Foundation; either
+* version 3 of the License, or any later version.
+*
+* This library is distributed in the hope that it will be useful,
+* but WITHOUT ANY WARRANTY; without even the implied warranty of
+* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+* GNU AFFERO GENERAL PUBLIC LICENSE for more details.
+*
+* You should have received a copy of the GNU Affero General Public
+* License along with this library.  If not, see <http://www.gnu.org/licenses/>.
+*
+*/
+
+namespace OC\Tags\Backend;
+
+/**
+ * Subclass this class for tags backends.
+ */
+abstract class AbstractBackend {
+
+	/**
+	 * The name of the backend.
+	 *
+	 * @var string
+	 */
+	public $name;
+
+	/**
+	 * The tags this backend holds.
+	 *
+	 * @var array
+	 */
+	protected $tags = array();
+
+	/**
+	 * The type of the items for which this backends holds tags
+	 *
+	 * @var string
+	 */
+	protected $type;
+
+	/**
+	 * The user whose tags this backend holds
+	 *
+	 * @var string
+	 */
+	protected $user;
+
+	/**
+	* Constructor.
+	*
+	* @param string $user The user whose data the object will operate on.
+	* @param string $type
+	*/
+	public function __construct(
+		$user,
+		$type,
+		$defaultTags = array()
+	) {
+		$this->user = $user;
+		$this->type = $type;
+		$this->loadTags($this->user, $this->type, $defaultTags);
+	}
+
+	/**
+	* Load tags from db.
+	*
+	*/
+	public abstract function loadTags($user, $type, $defaultTags=array());
+
+	/**
+	* Check if any tags are saved for this type and user.
+	*
+	* @return boolean.
+	*/
+	public function isEmpty() {
+		return count($this->tags) === 0;
+	}
+
+	/**
+	 * Get a list of all tags stored in the backend.
+	 */
+	public function getUnsortedTags() {
+		return $this->tags;
+	}
+
+	/**
+	* Get the list of items tagged with $tag.
+	*
+	* Throws an exception if the tag could not be found.
+	*
+	* @param string $tag Tag id or name.
+	* @return array An array of object ids or false on error.
+	*/
+	public abstract function getIdsForTag($tag);
+
+	/**
+	* Checks whether a tag is already saved.
+	*
+	* @param string $name The name to check for.
+	* @return string The tag's id or false if it hasn't been saved yet.
+	*/
+	public function hasTag($name) {
+		return $this->array_searchi($name, $this->tags);
+	}
+
+	/**
+	* Checks whether a tag given by id is already saved.
+	*
+	* @param int $id The id to check for.
+	* @return bool False if the tag hasn't been saved yet.
+	*/
+	public function hasTagId($id) {
+		return array_key_exists($id, $this->tags);
+	}
+
+	/**
+	* Add a new tag.
+	*
+	* @param string $name A string with a name of the tag
+	* @return false|string the id of the added tag or false on error.
+	*/
+	public abstract function add($name);
+
+	/**
+	* Rename tag.
+	*
+	* @param string $id The id of the existing tag
+	* @param string $to The new name of the tag.
+	* @return bool
+	*/
+	public abstract function rename($id, $to);
+
+	/**
+	 * Save the list of tags and their object relations
+	 */
+	public abstract function save();
+
+	/**
+	* Delete tag/object relations from the db
+	*
+	* @param array $ids The ids of the objects
+	* @return boolean Returns false on error.
+	*/
+	public abstract function purgeObjects(array $ids);
+
+	/**
+	* Creates a tag/object relation.
+	*
+	* @param int $objid The id of the object
+	* @param string $tag The id or name of the tag
+	* @return boolean Returns false on error.
+	*/
+	public abstract function tagAs($objid, $tag);
+
+	/**
+	* Delete single tag/object relation from the db
+	*
+	* @param int $objid The id of the object
+	* @param string $tag The id or name of the tag
+	* @return boolean
+	*/
+	public abstract function unTag($objid, $tag);
+
+	/**
+	* Delete tags from the
+	*
+	* @param string[] $names An array of tags to delete
+	* @return bool Returns false on error
+	*/
+	public abstract function delete($names);
+
+	// case-insensitive array_search
+	protected function array_searchi($needle, $haystack) {
+		if(!is_array($haystack)) {
+			return false;
+		}
+		return array_search(strtolower($needle), array_map('strtolower', $haystack));
+	}
+}
diff --git a/lib/private/tags/backend/database.php b/lib/private/tags/backend/database.php
new file mode 100644
index 000000000000..235afe5b3655
--- /dev/null
+++ b/lib/private/tags/backend/database.php
@@ -0,0 +1,431 @@
+<?php
+/**
+* ownCloud - Database backend for tags
+*
+* @author Thomas Tanghus
+* @copyright 2012-2013 Thomas Tanghus <thomas@tanghus.net>
+* @copyright 2012 Bart Visscher bartv@thisnet.nl
+* @copyright 2014 Bernhard Reiter <ockham@raz.or.at>
+*
+* This library is free software; you can redistribute it and/or
+* modify it under the terms of the GNU AFFERO GENERAL PUBLIC LICENSE
+* License as published by the Free Software Foundation; either
+* version 3 of the License, or any later version.
+*
+* This library is distributed in the hope that it will be useful,
+* but WITHOUT ANY WARRANTY; without even the implied warranty of
+* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+* GNU AFFERO GENERAL PUBLIC LICENSE for more details.
+*
+* You should have received a copy of the GNU Affero General Public
+* License along with this library.  If not, see <http://www.gnu.org/licenses/>.
+*
+*/
+
+namespace OC\Tags\Backend;
+
+class Database extends AbstractBackend {
+
+	/**
+	 * The name of the backend.
+	 *
+	 * @var string
+	 */
+	public $name = 'local';
+
+	/**
+	 * Tags
+	 *
+	 * @var array
+	 */
+	protected $tags = array();
+
+	/**
+	 * Used for storing objectid/categoryname pairs while rescanning.
+	 *
+	 * @var array
+	 */
+	private static $relations = array();
+
+	const TAG_TABLE = '*PREFIX*vcategory';
+	const RELATION_TABLE = '*PREFIX*vcategory_to_object';
+
+	/**
+	* Load tags from db.
+	*
+	*/
+	public function loadTags($user, $type, $defaultTags=array()) {
+		$this->tags = array();
+		$result = null;
+		$sql = 'SELECT `id`, `category` FROM `' . self::TAG_TABLE . '` '
+			. 'WHERE `uid` = ? AND `type` = ? ORDER BY `category`';
+		try {
+			$stmt = \OCP\DB::prepare($sql);
+			$result = $stmt->execute(array($user, $type));
+			if (\OCP\DB::isError($result)) {
+				\OCP\Util::writeLog('core', __METHOD__. ', DB error: ' . \OCP\DB::getErrorMessage($result), \OCP\Util::ERROR);
+			}
+		} catch(\Exception $e) {
+			\OCP\Util::writeLog('core', __METHOD__.', exception: '.$e->getMessage(),
+				\OCP\Util::ERROR);
+		}
+
+		if(!is_null($result)) {
+			while( $row = $result->fetchRow()) {
+				$this->tags[$row['id']] = $row['category'];
+			}
+		}
+
+		if(count($defaultTags) > 0 && count($this->tags) === 0) {
+			$this->addMultiple($defaultTags, true);
+		}
+
+		return $this->tags;
+	}
+
+	/**
+	* Get the list of items tagged with $tag.
+	*
+	* Throws an exception if the tag could not be found.
+	*
+	* @param string $tag Tag id or name.
+	* @return array An array of object ids or false on error.
+	*/
+	public function getIdsForTag($tag) {
+		return $this->getIdsForTagAndOwner($tag, $this->user);
+	}
+
+	public function getIdsForTagAndOwner($tag, $user) {
+		$result = null;
+		$tagId = false;
+		if(is_numeric($tag)) {
+			$tagId = $tag;
+		} elseif(is_string($tag)) {
+			$tag = trim($tag);
+			if($tag === '') {
+				\OCP\Util::writeLog('core', __METHOD__.', Cannot use empty tag names', \OCP\Util::DEBUG);
+				return false;
+			}
+			$tagId = $this->array_searchi($tag, $this->tags);
+		}
+
+		if($tagId === false) {
+			$l10n = \OC_L10N::get('core');
+			throw new \Exception(
+				$l10n->t('Could not find category "%s"', $tag)
+			);
+		}
+
+		$ids = array();
+		$sql = 'SELECT `objid` FROM `' . self::RELATION_TABLE
+			. '` WHERE `categoryid` = ?';
+		// TODO: At this point, we'd need to find out the respective owners of the objects, as we're
+		// only looking for objects owned by $user. Unfortunately, there doesn't seem to be any
+		// straight-forward solution to this problem.
+
+		try {
+			$stmt = \OCP\DB::prepare($sql);
+			$result = $stmt->execute(array($tagId));
+			if (\OCP\DB::isError($result)) {
+				\OCP\Util::writeLog('core', __METHOD__. 'DB error: ' . \OCP\DB::getErrorMessage($result), \OCP\Util::ERROR);
+				return false;
+			}
+		} catch(\Exception $e) {
+			\OCP\Util::writeLog('core', __METHOD__.', exception: '.$e->getMessage(),
+				\OCP\Util::ERROR);
+			return false;
+		}
+
+		if(!is_null($result)) {
+			while( $row = $result->fetchRow()) {
+				$ids[] = (int)$row['objid'];
+			}
+		}
+
+		return $ids;
+	}
+
+	/**
+	* Add a new tag.
+	*
+	* @param string $name A string with a name of the tag
+	* @return false|string the id of the added tag or false on error.
+	*/
+	public function add($name) {
+		try {
+			$result = \OCP\DB::insertIfNotExist(
+				self::TAG_TABLE,
+				array(
+					'uid' => $this->user,
+					'type' => $this->type,
+					'category' => $name,
+				)
+			);
+			if (\OCP\DB::isError($result)) {
+				\OCP\Util::writeLog('core', __METHOD__. 'DB error: ' . \OCP\DB::getErrorMessage($result), \OCP\Util::ERROR);
+				return false;
+			} elseif((int)$result === 0) {
+				\OCP\Util::writeLog('core', __METHOD__.', Tag already exists: ' . $name, \OCP\Util::DEBUG);
+				return false;
+			}
+		} catch(\Exception $e) {
+			\OCP\Util::writeLog('core', __METHOD__.', exception: '.$e->getMessage(),
+				\OCP\Util::ERROR);
+			return false;
+		}
+		$id = \OCP\DB::insertid(self::TAG_TABLE);
+		\OCP\Util::writeLog('core', __METHOD__.', id: ' . $id, \OCP\Util::DEBUG);
+		$this->tags[$id] = $name;
+		return $id;
+	}
+
+	/**
+	* Rename tag.
+	*
+	* @param string $from The id of the existing tag
+	* @param string $to The new name of the tag.
+	* @return bool
+	*/
+	public function rename($id, $to) {
+		$sql = 'UPDATE `' . self::TAG_TABLE . '` SET `category` = ? '
+			. 'WHERE `type` = ? AND `id` = ?';
+		try {
+			$stmt = \OCP\DB::prepare($sql);
+			$result = $stmt->execute(array($to, $this->type, $id));
+			if (\OCP\DB::isError($result)) {
+				\OCP\Util::writeLog('core', __METHOD__. 'DB error: ' . \OCP\DB::getErrorMessage($result), \OCP\Util::ERROR);
+				return false;
+			}
+		} catch(\Exception $e) {
+			\OCP\Util::writeLog('core', __METHOD__.', exception: '.$e->getMessage(),
+				\OCP\Util::ERROR);
+			return false;
+		}
+		$this->tags[$id] = $to;
+		return true;
+	}
+
+	/**
+	 * Save the list of tags and their object relations
+	 */
+	public function save() {
+		if(is_array($this->tags)) {
+			foreach($this->tags as $tag) {
+				try {
+					\OCP\DB::insertIfNotExist(self::TAG_TABLE,
+						array(
+							'uid' => $this->user,
+							'type' => $this->type,
+							'category' => $tag,
+						));
+				} catch(\Exception $e) {
+					\OCP\Util::writeLog('core', __METHOD__.', exception: '.$e->getMessage(),
+						\OCP\Util::ERROR);
+				}
+			}
+			// reload tags to get the proper ids.
+			$this->loadTags();
+			// Loop through temporarily cached objectid/tagname pairs
+			// and save relations.
+			$tags = $this->tags;
+			// For some reason this is needed or array_search(i) will return 0..?
+			ksort($tags);
+			foreach(self::$relations as $relation) {
+				$tagId = $this->array_searchi($relation['tag'], $tags);
+				\OCP\Util::writeLog('core', __METHOD__ . 'catid, ' . $relation['tag'] . ' ' . $tagId, \OCP\Util::DEBUG);
+				if($tagId) {
+					try {
+						\OCP\DB::insertIfNotExist(self::RELATION_TABLE,
+							array(
+								'objid' => $relation['objid'],
+								'categoryid' => $tagId,
+								'type' => $this->type,
+								));
+					} catch(\Exception $e) {
+						\OCP\Util::writeLog('core', __METHOD__.', exception: '.$e->getMessage(),
+							\OCP\Util::ERROR);
+					}
+				}
+			}
+			self::$relations = array(); // reset
+		} else {
+			\OCP\Util::writeLog('core', __METHOD__.', $this->tags is not an array! '
+				. print_r($this->tags, true), \OCP\Util::ERROR);
+		}
+	}
+
+	/**
+	* Delete tags and tag/object relations for a user.
+	*
+	* For hooking up on post_deleteUser
+	*
+	* @param array $arguments
+	*/
+	public static function post_deleteUser($arguments) {
+		// Find all objectid/tagId pairs.
+		$result = null;
+		try {
+			$stmt = \OCP\DB::prepare('SELECT `id` FROM `' . self::TAG_TABLE . '` '
+				. 'WHERE `uid` = ?');
+			$result = $stmt->execute(array($arguments['uid']));
+			if (\OCP\DB::isError($result)) {
+				\OCP\Util::writeLog('core', __METHOD__. 'DB error: ' . \OCP\DB::getErrorMessage($result), \OCP\Util::ERROR);
+			}
+		} catch(\Exception $e) {
+			\OCP\Util::writeLog('core', __METHOD__.', exception: '.$e->getMessage(),
+				\OCP\Util::ERROR);
+		}
+
+		if(!is_null($result)) {
+			try {
+				$stmt = \OCP\DB::prepare('DELETE FROM `' . self::RELATION_TABLE . '` '
+					. 'WHERE `categoryid` = ?');
+				while( $row = $result->fetchRow()) {
+					try {
+						$stmt->execute(array($row['id']));
+					} catch(\Exception $e) {
+						\OCP\Util::writeLog('core', __METHOD__.', exception: '.$e->getMessage(),
+							\OCP\Util::ERROR);
+					}
+				}
+			} catch(\Exception $e) {
+				\OCP\Util::writeLog('core', __METHOD__.', exception: '.$e->getMessage(),
+					\OCP\Util::ERROR);
+			}
+		}
+		try {
+			$stmt = \OCP\DB::prepare('DELETE FROM `' . self::TAG_TABLE . '` '
+				. 'WHERE `uid` = ?');
+			$result = $stmt->execute(array($arguments['uid']));
+			if (\OCP\DB::isError($result)) {
+				\OCP\Util::writeLog('core', __METHOD__. ', DB error: ' . \OCP\DB::getErrorMessage($result), \OCP\Util::ERROR);
+			}
+		} catch(\Exception $e) {
+			\OCP\Util::writeLog('core', __METHOD__ . ', exception: '
+				. $e->getMessage(), \OCP\Util::ERROR);
+		}
+	}
+
+	/**
+	* Delete tag/object relations from the db
+	*
+	* @param array $ids The ids of the objects
+	* @return boolean Returns false on error.
+	*/
+	public function purgeObjects(array $ids) {
+		if(count($ids) === 0) {
+			// job done ;)
+			return true;
+		}
+		$updates = $ids;
+		try {
+			$query = 'DELETE FROM `' . self::RELATION_TABLE . '` ';
+			$query .= 'WHERE `objid` IN (' . str_repeat('?,', count($ids)-1) . '?) ';
+			$query .= 'AND `type`= ?';
+			$updates[] = $this->type;
+			$stmt = \OCP\DB::prepare($query);
+			$result = $stmt->execute($updates);
+			if (\OCP\DB::isError($result)) {
+				\OCP\Util::writeLog('core', __METHOD__. 'DB error: ' . \OCP\DB::getErrorMessage($result), \OCP\Util::ERROR);
+				return false;
+			}
+		} catch(\Exception $e) {
+			\OCP\Util::writeLog('core', __METHOD__.', exception: ' . $e->getMessage(),
+				\OCP\Util::ERROR);
+			return false;
+		}
+		return true;
+	}
+
+	/**
+	* Creates a tag/object relation.
+	*
+	* @param int $objid The id of the object
+	* @param int $tagId The id of the tag
+	* @return boolean Returns false on error.
+	*/
+	public function tagAs($objid, $tagId) {
+		try {
+			\OCP\DB::insertIfNotExist(self::RELATION_TABLE,
+				array(
+					'objid' => $objid,
+					'categoryid' => $tagId,
+					'type' => $this->type,
+				));
+		} catch(\Exception $e) {
+			\OCP\Util::writeLog('core', __METHOD__.', exception: '.$e->getMessage(),
+				\OCP\Util::ERROR);
+			return false;
+		}
+		return true;
+	}
+
+	/**
+	* Delete single tag/object relation from the db
+	*
+	* @param int $objid The id of the object
+	* @param string $tag The id or name of the tag
+	* @return boolean
+	*/
+	public function unTag($objid, $tagId) {
+		try {
+			$sql = 'DELETE FROM `' . self::RELATION_TABLE . '` '
+					. 'WHERE `objid` = ? AND `categoryid` = ? AND `type` = ?';
+			$stmt = \OCP\DB::prepare($sql);
+			$stmt->execute(array($objid, $tagId, $this->type));
+		} catch(\Exception $e) {
+			\OCP\Util::writeLog('core', __METHOD__.', exception: '.$e->getMessage(),
+				\OCP\Util::ERROR);
+			return false;
+		}
+		return true;
+	}
+
+	/**
+	* Delete tags from the db.
+	*
+	* @param string[] $names An array of tags to delete
+	* @return bool Returns false on error
+	*/
+	public function delete($names) {
+		foreach($names as $name) {
+			$id = null;
+
+			if($this->hasTag($name)) {
+				$id = $this->array_searchi($name, $this->tags);
+				unset($this->tags[$id]);
+			}
+			try {
+				$stmt = \OCP\DB::prepare('DELETE FROM `' . self::TAG_TABLE . '` WHERE '
+					. '`type` = ? AND `category` = ?');
+				$result = $stmt->execute(array($this->type, $name));
+				if (\OCP\DB::isError($result)) {
+					\OCP\Util::writeLog('core', __METHOD__. 'DB error: ' . \OCP\DB::getErrorMessage($result), \OCP\Util::ERROR);
+				}
+			} catch(\Exception $e) {
+				\OCP\Util::writeLog('core', __METHOD__ . ', exception: '
+					. $e->getMessage(), \OCP\Util::ERROR);
+				return false;
+			}
+			if(!is_null($id) && $id !== false) {
+				try {
+					$sql = 'DELETE FROM `' . self::RELATION_TABLE . '` '
+							. 'WHERE `categoryid` = ?';
+					$stmt = \OCP\DB::prepare($sql);
+					$result = $stmt->execute(array($id));
+					if (\OCP\DB::isError($result)) {
+						\OCP\Util::writeLog('core',
+							__METHOD__. 'DB error: ' . \OCP\DB::getErrorMessage($result),
+							\OCP\Util::ERROR);
+						return false;
+					}
+				} catch(\Exception $e) {
+					\OCP\Util::writeLog('core', __METHOD__.', exception: '.$e->getMessage(),
+						\OCP\Util::ERROR);
+					return false;
+				}
+			}
+		}
+		return true;
+	}
+}
diff --git a/lib/private/tags/backend/shared.php b/lib/private/tags/backend/shared.php
new file mode 100644
index 000000000000..b3f83ab17d9f
--- /dev/null
+++ b/lib/private/tags/backend/shared.php
@@ -0,0 +1,99 @@
+<?php
+/**
+* ownCloud - Backend for shared tags
+*
+* @author Thomas Tanghus
+* @copyright 2012-2013 Thomas Tanghus <thomas@tanghus.net>
+* @copyright 2012 Bart Visscher bartv@thisnet.nl
+* @copyright 2014 Bernhard Reiter <ockham@raz.or.at>
+*
+* This library is free software; you can redistribute it and/or
+* modify it under the terms of the GNU AFFERO GENERAL PUBLIC LICENSE
+* License as published by the Free Software Foundation; either
+* version 3 of the License, or any later version.
+*
+* This library is distributed in the hope that it will be useful,
+* but WITHOUT ANY WARRANTY; without even the implied warranty of
+* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+* GNU AFFERO GENERAL PUBLIC LICENSE for more details.
+*
+* You should have received a copy of the GNU Affero General Public
+* License along with this library.  If not, see <http://www.gnu.org/licenses/>.
+*
+*/
+
+namespace OC\Tags\Backend;
+
+use OC\Tags\Backend\Database;
+use OC\Share\Share;
+
+class Shared extends Database {
+
+	/**
+	 * The name of the backend.
+	 *
+	 * @var string
+	 */
+	public $name = 'shared';
+
+	/**
+	 * The shared tags' owners;
+	 *
+	 * @var string
+	 */
+	private $owners;
+
+	/**
+	* Load shared tags for a given user and type.
+	*
+	* @param string $user The user whose tags we're querying for.
+	* @param string $type The type for which we're querying the corresponding tags.
+	*/
+	public function loadTags($user, $type, $defaultTags=array()) {
+	// First, we find out if $type is part of a collection (and if that collection is part of
+	// another one and so on). Of these collection types, along with our original $type, we make a
+	// list of the ones for which a sharing backend has been registered.
+	//
+	// FIXME: Ideally, we wouldn't need to nest getItemsSharedWith in this loop but just call it
+	// with its $includeCollections parameter set to true. Unfortunately, this fails currently.
+	$this->owners = array();
+	foreach (Share::getCollectionItemTypes($type) as $collectionType) {
+		if (\OCP\Share::hasBackend($collectionType)) {
+			$backend = \OCP\Share::getBackend($collectionType);
+			$allMaybeSharedItems[$collectionType] = \OCP\Share::getItemsSharedWith(
+				$collectionType,
+				\OCP\Share::FORMAT_NONE
+			);
+		}
+	}
+
+	// We take a look at all shared items of the given $type (or of the collections it is part of)
+	// and find out their owners. Then, we gather the tags for the original $type from all owners,
+	// and return them as elements of a list that look like "Tag (owner)".
+	$tags = array();
+	foreach ($allMaybeSharedItems as $collectionType => $maybeSharedItems) {
+		foreach ($maybeSharedItems as $sharedItem) {
+			if (isset($sharedItem['id'])) { //workaround for https://github.com/owncloud/core/issues/2814
+				$owner = $sharedItem['uid_owner'];
+				$this->owners[] = $owner;
+				// $displayname_owner = $sharedItem['displayname_owner'];
+				foreach (parent::loadTags($owner, $type, $defaultTags) as $id => $tag) {
+					// We might want to use some regular tag sharing backend for setting the target name.
+					$this->tags[$id] = $tag; // . ' (' . $owner . ')';
+				}
+			}
+		}
+	}
+
+	return $this->tags;
+	}
+
+	public function getIdsForTag($tag) {
+	$ids = array();
+	foreach ($this->owners as $owner)
+		$ids += parent::getIdsForTagAndOwner($tag, $owner);
+
+	return $ids;
+	}
+
+}
diff --git a/lib/public/share.php b/lib/public/share.php
index e6519dd3e3a8..23e3ca35b3eb 100644
--- a/lib/public/share.php
+++ b/lib/public/share.php
@@ -305,6 +305,15 @@ public static function setExpirationDate($itemType, $itemSource, $date, $shareTi
 		return \OC\Share\Share::setExpirationDate($itemType, $itemSource, $date, $shareTime);
 	}
 
+	/**
+	 * Check if a backend class has been registered for the specified item type
+	 * @param string $itemType
+	 * @return boolean True if a backend has been registered, false otherwise.
+	 */
+	public static function hasBackend($itemType) {
+		return \OC\Share\Share::hasBackend($itemType);
+	}
+
 	/**
 	 * Get the backend class for the specified item type
 	 * @param string $itemType