diff --git a/docs/changelog.rst b/docs/changelog.rst
index e3b4826f..48c7d822 100644
--- a/docs/changelog.rst
+++ b/docs/changelog.rst
@@ -20,6 +20,12 @@ v0.18.0 (UNRELEASED)
 - Add :class:`mopidy.models.Ref` class for use as a lightweight reference to
   other model types, containing just an URI, a name, and an object type.
 
+**Extension registry**
+
+- Switched to using a registry model for classes provided by extension. This
+  allows extensions to be extended as needed for plugable local libraries.
+  (Fixes :issue:`601`)
+
 **Pluggable local libraries**
 
 Fixes issues :issue:`44`, partially resolves :issue:`397`, and causes
@@ -38,6 +44,15 @@ a temporary regression of :issue:`527`.
 - Added support for deprecated config values in order to allow for
   graceful removal of :confval:`local/tag_cache_file`.
 
+- Added :confval:`local/library` to select which library to use.
+
+- Added :confval:`local/data_dir` to have a common setting for where to store
+  local library data. This is intended to avoid every single local library
+  provider having to have it's own setting for this.
+
+- Added :confval:`local/scan_flush_threshold` to control how often to tell
+  local libraries to store changes.
+
 **Streaming backend**
 
 - Live lookup of URI metadata has been added. (Fixes :issue:`540`)
diff --git a/docs/commands/mopidy.rst b/docs/commands/mopidy.rst
index 44e961e6..49c7b5b9 100644
--- a/docs/commands/mopidy.rst
+++ b/docs/commands/mopidy.rst
@@ -83,6 +83,10 @@ Additionally, extensions can provide extra commands. Run `mopidy --help`
 for a list of what is available on your system and command-specific help.
 Commands for disabled extensions will be listed, but can not be run.
 
+.. cmdoption:: local clear
+
+    Clear local media files from the local library.
+
 .. cmdoption:: local scan
 
     Scan local media files present in your library.
diff --git a/docs/ext/local.rst b/docs/ext/local.rst
index 0c16142c..aaaeb8e4 100644
--- a/docs/ext/local.rst
+++ b/docs/ext/local.rst
@@ -29,10 +29,20 @@ Configuration values
 
     If the local extension should be enabled or not.
 
+.. confval:: local/library
+
+    Local library provider to use, change this if you want to use a third party
+    library for local files.
+
 .. confval:: local/media_dir
 
     Path to directory with local media files.
 
+.. confval:: local/data_dir
+
+    Path to directory to store local metadata such as libraries and playlists
+    in.
+
 .. confval:: local/playlists_dir
 
     Path to playlists directory with m3u files for local media.
@@ -42,6 +52,11 @@ Configuration values
     Number of milliseconds before giving up scanning a file and moving on to
     the next file.
 
+.. confval:: local/scan_flush_threshold
+
+    Number of tracks to wait before telling library it should try and store
+    its progress so far. Some libraries might not respect this setting.
+
 .. confval:: local/excluded_file_extensions
 
     File extensions to exclude when scanning the media directory. Values
@@ -84,34 +99,13 @@ Pluggable library support
 -------------------------
 
 Local libraries are fully pluggable. What this means is that users may opt to
-disable the current default library ``local-json``, replacing it with a third
+disable the current default library ``json``, replacing it with a third
 party one. When running :command:`mopidy local scan` mopidy will populate
 whatever the current active library is with data. Only one library may be
 active at a time.
 
-
-*****************
-Mopidy-Local-JSON
-*****************
-
-Extension for storing local music library in a JSON file, default built in
-library for local files.
-
-
-Default configuration
-=====================
-
-.. literalinclude:: ../../mopidy/backends/local/json/ext.conf
-    :language: ini
-
-
-Configuration values
-====================
-
-.. confval:: local-json/enabled
-
-    If the local-json extension should be enabled or not.
-
-.. confval:: local-json/json_file
-
-    Path to a file to store the gzipped JSON data in.
+To create a new library provider you must create class that implements the
+:class:`~mopidy.backends.local.Libary` interface and install it in the
+extension registry under ``local:library``. Any data that the library needs
+to store on disc should be stored in :confval:`local/data_dir` using the
+library name as part of the filename or directory to avoid any conflicts.
diff --git a/mopidy/__main__.py b/mopidy/__main__.py
index 2e6a6cc5..1ddd76a4 100644
--- a/mopidy/__main__.py
+++ b/mopidy/__main__.py
@@ -40,11 +40,13 @@ def main():
     signal.signal(signal.SIGUSR1, pykka.debug.log_thread_tracebacks)
 
     try:
+        registry = ext.Registry()
+
         root_cmd = commands.RootCommand()
         config_cmd = commands.ConfigCommand()
         deps_cmd = commands.DepsCommand()
 
-        root_cmd.set(extension=None)
+        root_cmd.set(extension=None, registry=registry)
         root_cmd.add_child('config', config_cmd)
         root_cmd.add_child('deps', deps_cmd)
 
@@ -84,7 +86,6 @@ def main():
                 enabled_extensions.append(extension)
 
         log_extension_info(installed_extensions, enabled_extensions)
-        ext.register_gstreamer_elements(enabled_extensions)
 
         # Config and deps commands are simply special cased for now.
         if args.command == config_cmd:
@@ -108,10 +109,13 @@ def main():
                 args.extension.ext_name)
             return 1
 
+        for extension in enabled_extensions:
+            extension.setup(registry)
+
         # Anything that wants to exit after this point must use
         # mopidy.utils.process.exit_process as actors can have been started.
         try:
-            return args.command.run(args, proxied_config, enabled_extensions)
+            return args.command.run(args, proxied_config)
         except NotImplementedError:
             print(root_cmd.format_help())
             return 1
diff --git a/mopidy/audio/scan.py b/mopidy/audio/scan.py
index f797a84d..0c8e3478 100644
--- a/mopidy/audio/scan.py
+++ b/mopidy/audio/scan.py
@@ -181,6 +181,12 @@ def audio_data_to_track(data):
     track_kwargs['uri'] = data['uri']
     track_kwargs['album'] = Album(**album_kwargs)
 
+    # TODO: this feels like a half assed workaround. we need to be sure that we
+    # don't suddenly have lists in our models where we expect strings etc
+    if ('genre' in track_kwargs and
+            not isinstance(track_kwargs['genre'], basestring)):
+        track_kwargs['genre'] = ', '.join(track_kwargs['genre'])
+
     if ('name' in artist_kwargs
             and not isinstance(artist_kwargs['name'], basestring)):
         track_kwargs['artists'] = [Artist(name=artist)
diff --git a/mopidy/backends/local/__init__.py b/mopidy/backends/local/__init__.py
index e9204338..7cb2f0d5 100644
--- a/mopidy/backends/local/__init__.py
+++ b/mopidy/backends/local/__init__.py
@@ -5,7 +5,6 @@ import os
 
 import mopidy
 from mopidy import config, ext
-from mopidy.utils import encoding, path
 
 logger = logging.getLogger(__name__)
 
@@ -22,25 +21,136 @@ class Extension(ext.Extension):
 
     def get_config_schema(self):
         schema = super(Extension, self).get_config_schema()
+        schema['library'] = config.String()
         schema['media_dir'] = config.Path()
+        schema['data_dir'] = config.Path()
         schema['playlists_dir'] = config.Path()
         schema['tag_cache_file'] = config.Deprecated()
         schema['scan_timeout'] = config.Integer(
             minimum=1000, maximum=1000*60*60)
+        schema['scan_flush_threshold'] = config.Integer(minimum=0)
         schema['excluded_file_extensions'] = config.List(optional=True)
         return schema
 
-    def validate_environment(self):
-        try:
-            path.get_or_create_dir(b'$XDG_DATA_DIR/mopidy/local')
-        except EnvironmentError as error:
-            error = encoding.locale_decode(error)
-            logger.warning('Could not create local data dir: %s', error)
-
-    def get_backend_classes(self):
+    def setup(self, registry):
         from .actor import LocalBackend
-        return [LocalBackend]
+        from .json import JsonLibrary
+
+        LocalBackend.libraries = registry['local:library']
+
+        registry.add('backend', LocalBackend)
+        registry.add('local:library', JsonLibrary)
 
     def get_command(self):
         from .commands import LocalCommand
         return LocalCommand()
+
+
+class Library(object):
+    """
+    Local library interface.
+
+    Extensions that wish to provide an alternate local library storage backend
+    need to sub-class this class and install and configure it with an
+    extension. Both scanning and library calls will use the active local
+    library.
+
+    :param config: Config dictionary
+    """
+
+    #: Name of the local library implementation, must be overriden.
+    name = None
+
+    def __init__(self, config):
+        self._config = config
+
+    def load(self):
+        """
+        (Re)load any tracks stored in memory, if any, otherwise just return
+        number of available tracks currently available. Will be called at
+        startup for both library and update use cases, so if you plan to store
+        tracks in memory this is when the should be (re)loaded.
+
+        :rtype: :class:`int` representing number of tracks in library.
+        """
+        return 0
+
+    def lookup(self, uri):
+        """
+        Lookup the given URI.
+
+        Unlike the core APIs, local tracks uris can only be resolved to a
+        single track.
+
+        :param string uri: track URI
+        :rtype: :class:`~mopidy.models.Track`
+        """
+        raise NotImplementedError
+
+    # TODO: remove uris, replacing it with support in query language.
+    # TODO: remove exact, replacing it with support in query language.
+    def search(self, query=None, limit=100, offset=0, exact=False, uris=None):
+        """
+        Search the library for tracks where ``field`` contains ``values``.
+
+        :param dict query: one or more queries to search for
+        :param int limit: maximum number of results to return
+        :param int offset: offset into result set to use.
+        :param bool exact: whether to look for exact matches
+        :param uris: zero or more URI roots to limit the search to
+        :type uris: list of strings or :class:`None`
+        :rtype: :class:`~mopidy.models.SearchResult`
+        """
+        raise NotImplementedError
+
+    # TODO: add file browsing support.
+
+    # Remaining methods are use for the update process.
+    def begin(self):
+        """
+        Prepare library for accepting updates. Exactly what this means is
+        highly implementation depended. This must however return an iterator
+        that generates all tracks in the library for efficient scanning.
+
+        :rtype: :class:`~mopidy.models.Track` iterator
+        """
+        raise NotImplementedError
+
+    def add(self, track):
+        """
+        Add the given track to library.
+
+        :param :class:`~mopidy.models.Track` track: Track to add to the library
+        """
+        raise NotImplementedError
+
+    def remove(self, uri):
+        """
+        Remove the given track from the library.
+
+        :param str uri: URI to remove from the library/
+        """
+        raise NotImplementedError
+
+    def flush(self):
+        """
+        Called for every n-th track indicating that work should be committed.
+        Sub-classes are free to ignore these hints.
+
+        :rtype: Boolean indicating if state was flushed.
+        """
+        return False
+
+    def close(self):
+        """
+        Close any resources used for updating, commit outstanding work etc.
+        """
+        pass
+
+    def clear(self):
+        """
+        Clear out whatever data storage is used by this backend.
+
+        :rtype: Boolean indicating if state was cleared.
+        """
+        return False
diff --git a/mopidy/backends/local/actor.py b/mopidy/backends/local/actor.py
index a9902c8b..c29a5dbe 100644
--- a/mopidy/backends/local/actor.py
+++ b/mopidy/backends/local/actor.py
@@ -8,13 +8,17 @@ import pykka
 from mopidy.backends import base
 from mopidy.utils import encoding, path
 
-from .playlists import LocalPlaylistsProvider
+from .library import LocalLibraryProvider
 from .playback import LocalPlaybackProvider
+from .playlists import LocalPlaylistsProvider
 
 logger = logging.getLogger(__name__)
 
 
 class LocalBackend(pykka.ThreadingActor, base.Backend):
+    uri_schemes = ['local']
+    libraries = []
+
     def __init__(self, config, audio):
         super(LocalBackend, self).__init__()
 
@@ -22,16 +26,33 @@ class LocalBackend(pykka.ThreadingActor, base.Backend):
 
         self.check_dirs_and_files()
 
+        libraries = dict((l.name, l) for l in self.libraries)
+        library_name = config['local']['library']
+
+        if library_name in libraries:
+            library = libraries[library_name](config)
+            logger.debug('Using %s as the local library', library_name)
+        else:
+            library = None
+            logger.warning('Local library %s not found', library_name)
+
         self.playback = LocalPlaybackProvider(audio=audio, backend=self)
         self.playlists = LocalPlaylistsProvider(backend=self)
-
-        self.uri_schemes = ['local']
+        self.library = LocalLibraryProvider(backend=self, library=library)
 
     def check_dirs_and_files(self):
         if not os.path.isdir(self.config['local']['media_dir']):
             logger.warning('Local media dir %s does not exist.' %
                            self.config['local']['media_dir'])
 
+        try:
+            path.get_or_create_dir(self.config['local']['data_dir'])
+        except EnvironmentError as error:
+            logger.warning(
+                'Could not create local data dir: %s',
+                encoding.locale_decode(error))
+
+        # TODO: replace with data dir?
         try:
             path.get_or_create_dir(self.config['local']['playlists_dir'])
         except EnvironmentError as error:
diff --git a/mopidy/backends/local/commands.py b/mopidy/backends/local/commands.py
index e9eb807c..5e4bfe62 100644
--- a/mopidy/backends/local/commands.py
+++ b/mopidy/backends/local/commands.py
@@ -13,45 +13,72 @@ from . import translator
 logger = logging.getLogger(__name__)
 
 
+def _get_library(args, config):
+    libraries = dict((l.name, l) for l in args.registry['local:library'])
+    library_name = config['local']['library']
+
+    if library_name not in libraries:
+        logger.warning('Local library %s not found', library_name)
+        return 1
+
+    logger.debug('Using %s as the local library', library_name)
+    return libraries[library_name](config)
+
+
 class LocalCommand(commands.Command):
     def __init__(self):
         super(LocalCommand, self).__init__()
         self.add_child('scan', ScanCommand())
+        self.add_child('clear', ClearCommand())
+
+
+class ClearCommand(commands.Command):
+    help = 'Clear local media files from the local library.'
+
+    def run(self, args, config):
+        library = _get_library(args, config)
+        prompt = 'Are you sure you want to clear the library? [y/N] '
+
+        if raw_input(prompt).lower() != 'y':
+            logging.info('Clearing library aborted.')
+            return 0
+
+        if library.clear():
+            logging.info('Library succesfully cleared.')
+            return 0
+
+        logging.warning('Unable to clear library.')
+        return 1
 
 
 class ScanCommand(commands.Command):
-    help = "Scan local media files and populate the local library."
+    help = 'Scan local media files and populate the local library.'
 
-    def run(self, args, config, extensions):
+    def __init__(self):
+        super(ScanCommand, self).__init__()
+        self.add_argument('--limit',
+                          action='store', type=int, dest='limit', default=None,
+                          help='Maxmimum number of tracks to scan')
+
+    def run(self, args, config):
         media_dir = config['local']['media_dir']
         scan_timeout = config['local']['scan_timeout']
+        flush_threshold = config['local']['scan_flush_threshold']
+        excluded_file_extensions = config['local']['excluded_file_extensions']
         excluded_file_extensions = set(
-            ext.lower() for ext in config['local']['excluded_file_extensions'])
+            file_ext.lower() for file_ext in excluded_file_extensions)
 
-        updaters = {}
-        for e in extensions:
-            for updater_class in e.get_library_updaters():
-                if updater_class and 'local' in updater_class.uri_schemes:
-                    updaters[e.ext_name] = updater_class
-
-        if not updaters:
-            logger.error('No usable library updaters found.')
-            return 1
-        elif len(updaters) > 1:
-            logger.error('More than one library updater found. '
-                         'Provided by: %s', ', '.join(updaters.keys()))
-            return 1
-
-        local_updater = updaters.values()[0](config)
+        library = _get_library(args, config)
 
         uri_path_mapping = {}
         uris_in_library = set()
         uris_to_update = set()
         uris_to_remove = set()
 
-        tracks = local_updater.load()
-        logger.info('Checking %d tracks from library.', len(tracks))
-        for track in tracks:
+        num_tracks = library.load()
+        logger.info('Checking %d tracks from library.', num_tracks)
+
+        for track in library.begin():
             uri_path_mapping[track.uri] = translator.local_track_uri_to_path(
                 track.uri, media_dir)
             try:
@@ -65,16 +92,17 @@ class ScanCommand(commands.Command):
 
         logger.info('Removing %d missing tracks.', len(uris_to_remove))
         for uri in uris_to_remove:
-            local_updater.remove(uri)
+            library.remove(uri)
 
         logger.info('Checking %s for unknown tracks.', media_dir)
         for relpath in path.find_files(media_dir):
+            uri = translator.path_to_local_track_uri(relpath)
             file_extension = os.path.splitext(relpath)[1]
+
             if file_extension.lower() in excluded_file_extensions:
                 logger.debug('Skipped %s: File extension excluded.', uri)
                 continue
 
-            uri = translator.path_to_local_track_uri(relpath)
             if uri not in uris_in_library:
                 uris_to_update.add(uri)
                 uri_path_mapping[uri] = os.path.join(media_dir, relpath)
@@ -82,36 +110,44 @@ class ScanCommand(commands.Command):
         logger.info('Found %d unknown tracks.', len(uris_to_update))
         logger.info('Scanning...')
 
-        scanner = scan.Scanner(scan_timeout)
-        progress = Progress(len(uris_to_update))
+        uris_to_update = sorted(uris_to_update)[:args.limit]
 
-        for uri in sorted(uris_to_update):
+        scanner = scan.Scanner(scan_timeout)
+        progress = _Progress(flush_threshold, len(uris_to_update))
+
+        for uri in uris_to_update:
             try:
                 data = scanner.scan(path.path_to_uri(uri_path_mapping[uri]))
                 track = scan.audio_data_to_track(data).copy(uri=uri)
-                local_updater.add(track)
+                library.add(track)
                 logger.debug('Added %s', track.uri)
             except exceptions.ScannerError as error:
                 logger.warning('Failed %s: %s', uri, error)
 
-            progress.increment()
+            if progress.increment():
+                progress.log()
+                if library.flush():
+                    logger.debug('Progress flushed.')
 
-        logger.info('Commiting changes.')
-        local_updater.commit()
+        progress.log()
+        library.close()
+        logger.info('Done scanning.')
         return 0
 
 
-# TODO: move to utils?
-class Progress(object):
-    def __init__(self, total):
+class _Progress(object):
+    def __init__(self, batch_size, total):
         self.count = 0
+        self.batch_size = batch_size
         self.total = total
         self.start = time.time()
 
     def increment(self):
         self.count += 1
-        if self.count % 1000 == 0 or self.count == self.total:
-            duration = time.time() - self.start
-            remainder = duration / self.count * (self.total - self.count)
-            logger.info('Scanned %d of %d files in %ds, ~%ds left.',
-                        self.count, self.total, duration, remainder)
+        return self.count % self.batch_size == 0
+
+    def log(self):
+        duration = time.time() - self.start
+        remainder = duration / self.count * (self.total - self.count)
+        logger.info('Scanned %d of %d files in %ds, ~%ds left.',
+                    self.count, self.total, duration, remainder)
diff --git a/mopidy/backends/local/ext.conf b/mopidy/backends/local/ext.conf
index f906a04f..8f1e860c 100644
--- a/mopidy/backends/local/ext.conf
+++ b/mopidy/backends/local/ext.conf
@@ -1,8 +1,11 @@
 [local]
 enabled = true
+library = json
 media_dir = $XDG_MUSIC_DIR
+data_dir = $XDG_DATA_DIR/mopidy/local
 playlists_dir = $XDG_DATA_DIR/mopidy/local/playlists
 scan_timeout = 1000
+scan_flush_threshold = 1000
 excluded_file_extensions =
   .html
   .jpeg
diff --git a/mopidy/backends/local/json.py b/mopidy/backends/local/json.py
new file mode 100644
index 00000000..7bccf101
--- /dev/null
+++ b/mopidy/backends/local/json.py
@@ -0,0 +1,91 @@
+from __future__ import absolute_import, unicode_literals
+
+import gzip
+import json
+import logging
+import os
+import tempfile
+
+import mopidy
+from mopidy import models
+from mopidy.backends import local
+from mopidy.backends.local import search
+
+logger = logging.getLogger(__name__)
+
+
+# TODO: move to load and dump in models?
+def load_library(json_file):
+    try:
+        with gzip.open(json_file, 'rb') as fp:
+            return json.load(fp, object_hook=models.model_json_decoder)
+    except (IOError, ValueError) as e:
+        logger.warning('Loading JSON local library failed: %s', e)
+        return {}
+
+
+def write_library(json_file, data):
+    data['version'] = mopidy.__version__
+    directory, basename = os.path.split(json_file)
+
+    # TODO: cleanup directory/basename.* files.
+    tmp = tempfile.NamedTemporaryFile(
+        prefix=basename + '.', dir=directory, delete=False)
+
+    try:
+        with gzip.GzipFile(fileobj=tmp, mode='wb') as fp:
+            json.dump(data, fp, cls=models.ModelJSONEncoder,
+                      indent=2, separators=(',', ': '))
+        os.rename(tmp.name, json_file)
+    finally:
+        if os.path.exists(tmp.name):
+            os.remove(tmp.name)
+
+
+class JsonLibrary(local.Library):
+    name = b'json'
+
+    def __init__(self, config):
+        self._tracks = {}
+        self._media_dir = config['local']['media_dir']
+        self._json_file = os.path.join(
+            config['local']['data_dir'], b'library.json.gz')
+
+    def load(self):
+        logger.debug('Loading json library from %s', self._json_file)
+        library = load_library(self._json_file)
+        self._tracks = dict((t.uri, t) for t in library.get('tracks', []))
+        return len(self._tracks)
+
+    def lookup(self, uri):
+        try:
+            return self._tracks[uri]
+        except KeyError:
+            return None
+
+    def search(self, query=None, limit=100, offset=0, uris=None, exact=False):
+        tracks = self._tracks.values()
+        # TODO: pass limit and offset into search helpers
+        if exact:
+            return search.find_exact(tracks, query=query, uris=uris)
+        else:
+            return search.search(tracks, query=query, uris=uris)
+
+    def begin(self):
+        return self._tracks.itervalues()
+
+    def add(self, track):
+        self._tracks[track.uri] = track
+
+    def remove(self, uri):
+        self._tracks.pop(uri, None)
+
+    def close(self):
+        write_library(self._json_file, {'tracks': self._tracks.values()})
+
+    def clear(self):
+        try:
+            os.remove(self._json_file)
+            return True
+        except OSError:
+            return False
diff --git a/mopidy/backends/local/json/__init__.py b/mopidy/backends/local/json/__init__.py
deleted file mode 100644
index 031dae51..00000000
--- a/mopidy/backends/local/json/__init__.py
+++ /dev/null
@@ -1,30 +0,0 @@
-from __future__ import unicode_literals
-
-import os
-
-import mopidy
-from mopidy import config, ext
-
-
-class Extension(ext.Extension):
-
-    dist_name = 'Mopidy-Local-JSON'
-    ext_name = 'local-json'
-    version = mopidy.__version__
-
-    def get_default_config(self):
-        conf_file = os.path.join(os.path.dirname(__file__), 'ext.conf')
-        return config.read(conf_file)
-
-    def get_config_schema(self):
-        schema = super(Extension, self).get_config_schema()
-        schema['json_file'] = config.Path()
-        return schema
-
-    def get_backend_classes(self):
-        from .actor import LocalJsonBackend
-        return [LocalJsonBackend]
-
-    def get_library_updaters(self):
-        from .library import LocalJsonLibraryUpdateProvider
-        return [LocalJsonLibraryUpdateProvider]
diff --git a/mopidy/backends/local/json/actor.py b/mopidy/backends/local/json/actor.py
deleted file mode 100644
index 4fc46417..00000000
--- a/mopidy/backends/local/json/actor.py
+++ /dev/null
@@ -1,30 +0,0 @@
-from __future__ import unicode_literals
-
-import logging
-import os
-
-import pykka
-
-from mopidy.backends import base
-from mopidy.utils import encoding
-
-from . import library
-
-logger = logging.getLogger(__name__)
-
-
-class LocalJsonBackend(pykka.ThreadingActor, base.Backend):
-    def __init__(self, config, audio):
-        super(LocalJsonBackend, self).__init__()
-
-        self.config = config
-        self.library = library.LocalJsonLibraryProvider(backend=self)
-        self.uri_schemes = ['local']
-
-        if not os.path.exists(config['local-json']['json_file']):
-            try:
-                library.write_library(config['local-json']['json_file'], {})
-                logger.info('Created empty local JSON library.')
-            except EnvironmentError as error:
-                error = encoding.locale_decode(error)
-                logger.warning('Could not create local library: %s', error)
diff --git a/mopidy/backends/local/json/ext.conf b/mopidy/backends/local/json/ext.conf
deleted file mode 100644
index db0b784a..00000000
--- a/mopidy/backends/local/json/ext.conf
+++ /dev/null
@@ -1,3 +0,0 @@
-[local-json]
-enabled = true
-json_file = $XDG_DATA_DIR/mopidy/local/library.json.gz
diff --git a/mopidy/backends/local/json/library.py b/mopidy/backends/local/json/library.py
deleted file mode 100644
index 99640543..00000000
--- a/mopidy/backends/local/json/library.py
+++ /dev/null
@@ -1,108 +0,0 @@
-from __future__ import unicode_literals
-
-import gzip
-import json
-import logging
-import os
-import tempfile
-
-import mopidy
-from mopidy import models
-from mopidy.backends import base
-from mopidy.backends.local import search
-
-logger = logging.getLogger(__name__)
-
-
-def load_library(json_file):
-    try:
-        with gzip.open(json_file, 'rb') as fp:
-            return json.load(fp, object_hook=models.model_json_decoder)
-    except (IOError, ValueError) as e:
-        logger.warning('Loading JSON local library failed: %s', e)
-        return {}
-
-
-def write_library(json_file, data):
-    data['version'] = mopidy.__version__
-    directory, basename = os.path.split(json_file)
-
-    # TODO: cleanup directory/basename.* files.
-    tmp = tempfile.NamedTemporaryFile(
-        prefix=basename + '.', dir=directory, delete=False)
-
-    try:
-        with gzip.GzipFile(fileobj=tmp, mode='wb') as fp:
-            json.dump(data, fp, cls=models.ModelJSONEncoder,
-                      indent=2, separators=(',', ': '))
-        os.rename(tmp.name, json_file)
-    finally:
-        if os.path.exists(tmp.name):
-            os.remove(tmp.name)
-
-
-class LocalJsonLibraryProvider(base.BaseLibraryProvider):
-    def __init__(self, *args, **kwargs):
-        super(LocalJsonLibraryProvider, self).__init__(*args, **kwargs)
-        self._uri_mapping = {}
-        self._media_dir = self.backend.config['local']['media_dir']
-        self._json_file = self.backend.config['local-json']['json_file']
-        self.refresh()
-
-    def refresh(self, uri=None):
-        logger.debug(
-            'Loading local tracks from %s using %s',
-            self._media_dir, self._json_file)
-
-        tracks = load_library(self._json_file).get('tracks', [])
-        uris_to_remove = set(self._uri_mapping)
-
-        for track in tracks:
-            self._uri_mapping[track.uri] = track
-            uris_to_remove.discard(track.uri)
-
-        for uri in uris_to_remove:
-            del self._uri_mapping[uri]
-
-        logger.info(
-            'Loaded %d local tracks from %s using %s',
-            len(tracks), self._media_dir, self._json_file)
-
-    def lookup(self, uri):
-        try:
-            return [self._uri_mapping[uri]]
-        except KeyError:
-            logger.debug('Failed to lookup %r', uri)
-            return []
-
-    def find_exact(self, query=None, uris=None):
-        tracks = self._uri_mapping.values()
-        return search.find_exact(tracks, query=query, uris=uris)
-
-    def search(self, query=None, uris=None):
-        tracks = self._uri_mapping.values()
-        return search.search(tracks, query=query, uris=uris)
-
-
-class LocalJsonLibraryUpdateProvider(base.BaseLibraryProvider):
-    uri_schemes = ['local']
-
-    def __init__(self, config):
-        self._tracks = {}
-        self._media_dir = config['local']['media_dir']
-        self._json_file = config['local-json']['json_file']
-
-    def load(self):
-        for track in load_library(self._json_file).get('tracks', []):
-            self._tracks[track.uri] = track
-        return self._tracks.values()
-
-    def add(self, track):
-        self._tracks[track.uri] = track
-
-    def remove(self, uri):
-        if uri in self._tracks:
-            del self._tracks[uri]
-
-    def commit(self):
-        write_library(self._json_file, {'tracks': self._tracks.values()})
diff --git a/mopidy/backends/local/library.py b/mopidy/backends/local/library.py
new file mode 100644
index 00000000..c83bec20
--- /dev/null
+++ b/mopidy/backends/local/library.py
@@ -0,0 +1,42 @@
+from __future__ import unicode_literals
+
+import logging
+
+from mopidy.backends import base
+
+logger = logging.getLogger(__name__)
+
+
+class LocalLibraryProvider(base.BaseLibraryProvider):
+    """Proxy library that delegates work to our active local library."""
+
+    def __init__(self, backend, library):
+        super(LocalLibraryProvider, self).__init__(backend)
+        self._library = library
+        self.refresh()
+
+    def refresh(self, uri=None):
+        if not self._library:
+            return 0
+        num_tracks = self._library.load()
+        logger.info('Loaded %d local tracks using %s',
+                    num_tracks, self._library.name)
+
+    def lookup(self, uri):
+        if not self._library:
+            return []
+        track = self._library.lookup(uri)
+        if track is None:
+            logger.debug('Failed to lookup %r', uri)
+            return []
+        return [track]
+
+    def find_exact(self, query=None, uris=None):
+        if not self._library:
+            return None
+        return self._library.search(query=query, uris=uris, exact=True)
+
+    def search(self, query=None, uris=None):
+        if not self._library:
+            return None
+        return self._library.search(query=query, uris=uris, exact=False)
diff --git a/mopidy/commands.py b/mopidy/commands.py
index 46989c9c..e73f9373 100644
--- a/mopidy/commands.py
+++ b/mopidy/commands.py
@@ -257,22 +257,26 @@ class RootCommand(Command):
             type=config_override_type, metavar='OPTIONS',
             help='`section/key=value` values to override config options')
 
-    def run(self, args, config, extensions):
+    def run(self, args, config):
         loop = gobject.MainLoop()
+
+        backend_classes = args.registry['backend']
+        frontend_classes = args.registry['frontend']
+
         try:
             audio = self.start_audio(config)
-            backends = self.start_backends(config, extensions, audio)
+            backends = self.start_backends(config, backend_classes, audio)
             core = self.start_core(audio, backends)
-            self.start_frontends(config, extensions, core)
+            self.start_frontends(config, frontend_classes, core)
             loop.run()
         except KeyboardInterrupt:
             logger.info('Interrupted. Exiting...')
             return
         finally:
             loop.quit()
-            self.stop_frontends(extensions)
+            self.stop_frontends(frontend_classes)
             self.stop_core()
-            self.stop_backends(extensions)
+            self.stop_backends(backend_classes)
             self.stop_audio()
             process.stop_remaining_actors()
 
@@ -280,11 +284,7 @@ class RootCommand(Command):
         logger.info('Starting Mopidy audio')
         return Audio.start(config=config).proxy()
 
-    def start_backends(self, config, extensions, audio):
-        backend_classes = []
-        for extension in extensions:
-            backend_classes.extend(extension.get_backend_classes())
-
+    def start_backends(self, config, backend_classes, audio):
         logger.info(
             'Starting Mopidy backends: %s',
             ', '.join(b.__name__ for b in backend_classes) or 'none')
@@ -300,11 +300,7 @@ class RootCommand(Command):
         logger.info('Starting Mopidy core')
         return Core.start(audio=audio, backends=backends).proxy()
 
-    def start_frontends(self, config, extensions, core):
-        frontend_classes = []
-        for extension in extensions:
-            frontend_classes.extend(extension.get_frontend_classes())
-
+    def start_frontends(self, config, frontend_classes, core):
         logger.info(
             'Starting Mopidy frontends: %s',
             ', '.join(f.__name__ for f in frontend_classes) or 'none')
@@ -312,21 +308,19 @@ class RootCommand(Command):
         for frontend_class in frontend_classes:
             frontend_class.start(config=config, core=core)
 
-    def stop_frontends(self, extensions):
+    def stop_frontends(self, frontend_classes):
         logger.info('Stopping Mopidy frontends')
-        for extension in extensions:
-            for frontend_class in extension.get_frontend_classes():
-                process.stop_actors_by_class(frontend_class)
+        for frontend_class in frontend_classes:
+            process.stop_actors_by_class(frontend_class)
 
     def stop_core(self):
         logger.info('Stopping Mopidy core')
         process.stop_actors_by_class(Core)
 
-    def stop_backends(self, extensions):
+    def stop_backends(self, backend_classes):
         logger.info('Stopping Mopidy backends')
-        for extension in extensions:
-            for backend_class in extension.get_backend_classes():
-                process.stop_actors_by_class(backend_class)
+        for backend_class in backend_classes:
+            process.stop_actors_by_class(backend_class)
 
     def stop_audio(self):
         logger.info('Stopping Mopidy audio')
diff --git a/mopidy/core/actor.py b/mopidy/core/actor.py
index 4924cca2..dba8d76d 100644
--- a/mopidy/core/actor.py
+++ b/mopidy/core/actor.py
@@ -91,25 +91,24 @@ class Backends(list):
         self.with_playback = collections.OrderedDict()
         self.with_playlists = collections.OrderedDict()
 
+        backends_by_scheme = {}
+        name = lambda backend: backend.actor_ref.actor_class.__name__
+
         for backend in backends:
             has_library = backend.has_library().get()
             has_playback = backend.has_playback().get()
             has_playlists = backend.has_playlists().get()
 
             for scheme in backend.uri_schemes.get():
-                self.add(self.with_library, has_library, scheme, backend)
-                self.add(self.with_playback, has_playback, scheme, backend)
-                self.add(self.with_playlists, has_playlists, scheme, backend)
+                assert scheme not in backends_by_scheme, (
+                    'Cannot add URI scheme %s for %s, '
+                    'it is already handled by %s'
+                ) % (scheme, name(backend), name(backends_by_scheme[scheme]))
+                backends_by_scheme[scheme] = backend
 
-    def add(self, registry, supported, uri_scheme, backend):
-        if not supported:
-            return
-
-        if uri_scheme not in registry:
-            registry[uri_scheme] = backend
-            return
-
-        get_name = lambda actor: actor.actor_ref.actor_class.__name__
-        raise AssertionError(
-            'Cannot add URI scheme %s for %s, it is already handled by %s' %
-            (uri_scheme, get_name(backend), get_name(registry[uri_scheme])))
+                if has_library:
+                    self.with_library[scheme] = backend
+                if has_playback:
+                    self.with_playback[scheme] = backend
+                if has_playlists:
+                    self.with_playlists[scheme] = backend
diff --git a/mopidy/ext.py b/mopidy/ext.py
index 33b9497d..b29523a7 100644
--- a/mopidy/ext.py
+++ b/mopidy/ext.py
@@ -1,5 +1,6 @@
 from __future__ import unicode_literals
 
+import collections
 import logging
 import pkg_resources
 
@@ -61,6 +62,15 @@ class Extension(object):
         """
         pass
 
+    def setup(self, registry):
+        for backend_class in self.get_backend_classes():
+            registry.add('backend', backend_class)
+
+        for frontend_class in self.get_frontend_classes():
+            registry.add('frontend', frontend_class)
+
+        self.register_gstreamer_elements()
+
     def get_frontend_classes(self):
         """List of frontend actor classes
 
@@ -79,6 +89,7 @@ class Extension(object):
         """
         return []
 
+    # TODO: remove
     def get_library_updaters(self):
         """List of library updater classes
 
@@ -112,6 +123,24 @@ class Extension(object):
         pass
 
 
+# TODO: document
+class Registry(collections.Mapping):
+    def __init__(self):
+        self._registry = {}
+
+    def add(self, name, cls):
+        self._registry.setdefault(name, []).append(cls)
+
+    def __getitem__(self, name):
+        return self._registry.setdefault(name, [])
+
+    def __iter__(self):
+        return iter(self._registry)
+
+    def __len__(self):
+        return len(self._registry)
+
+
 def load_extensions():
     """Find all installed extensions.
 
@@ -166,15 +195,3 @@ def validate_extension(extension):
         return False
 
     return True
-
-
-def register_gstreamer_elements(enabled_extensions):
-    """Registers custom GStreamer elements from extensions.
-
-    :param enabled_extensions: list of enabled extensions
-    """
-
-    for extension in enabled_extensions:
-        logger.debug(
-            'Registering GStreamer elements for: %s', extension.ext_name)
-        extension.register_gstreamer_elements()
diff --git a/setup.py b/setup.py
index b6857d4e..607496b7 100644
--- a/setup.py
+++ b/setup.py
@@ -43,7 +43,6 @@ setup(
         'mopidy.ext': [
             'http = mopidy.http:Extension [http]',
             'local = mopidy.backends.local:Extension',
-            'local-json = mopidy.backends.local.json:Extension',
             'mpd = mopidy.mpd:Extension',
             'stream = mopidy.backends.stream:Extension',
         ],
diff --git a/tests/backends/local/events_test.py b/tests/backends/local/events_test.py
index 1e26a68c..967d4cdb 100644
--- a/tests/backends/local/events_test.py
+++ b/tests/backends/local/events_test.py
@@ -17,7 +17,9 @@ class LocalBackendEventsTest(unittest.TestCase):
     config = {
         'local': {
             'media_dir': path_to_data_dir(''),
+            'data_dir': path_to_data_dir(''),
             'playlists_dir': b'',
+            'library': 'json',
         }
     }
 
diff --git a/tests/backends/local/library_test.py b/tests/backends/local/library_test.py
index e4c00570..4ca5abf0 100644
--- a/tests/backends/local/library_test.py
+++ b/tests/backends/local/library_test.py
@@ -1,13 +1,14 @@
 from __future__ import unicode_literals
 
-import copy
+import os
+import shutil
 import tempfile
 import unittest
 
 import pykka
 
 from mopidy import core
-from mopidy.backends.local.json import actor
+from mopidy.backends.local import actor, json
 from mopidy.models import Track, Album, Artist
 
 from tests import path_to_data_dir
@@ -61,21 +62,22 @@ class LocalLibraryProviderTest(unittest.TestCase):
     config = {
         'local': {
             'media_dir': path_to_data_dir(''),
+            'data_dir': path_to_data_dir(''),
             'playlists_dir': b'',
-        },
-        'local-json': {
-            'json_file': path_to_data_dir('library.json.gz'),
+            'library': 'json',
         },
     }
 
     def setUp(self):
-        self.backend = actor.LocalJsonBackend.start(
+        actor.LocalBackend.libraries = [json.JsonLibrary]
+        self.backend = actor.LocalBackend.start(
             config=self.config, audio=None).proxy()
         self.core = core.Core(backends=[self.backend])
         self.library = self.core.library
 
     def tearDown(self):
         pykka.ActorRegistry.stop_all()
+        actor.LocalBackend.libraries = []
 
     def test_refresh(self):
         self.library.refresh()
@@ -88,28 +90,30 @@ class LocalLibraryProviderTest(unittest.TestCase):
         # Verifies that https://github.com/mopidy/mopidy/issues/500
         # has been fixed.
 
-        with tempfile.NamedTemporaryFile() as library:
-            with open(self.config['local-json']['json_file']) as fh:
-                library.write(fh.read())
-            library.flush()
+        tmpdir = tempfile.mkdtemp()
+        try:
+            tmplib = os.path.join(tmpdir, 'library.json.gz')
+            shutil.copy(path_to_data_dir('library.json.gz'), tmplib)
 
-            config = copy.deepcopy(self.config)
-            config['local-json']['json_file'] = library.name
-            backend = actor.LocalJsonBackend(config=config, audio=None)
+            config = {'local': self.config['local'].copy()}
+            config['local']['data_dir'] = tmpdir
+            backend = actor.LocalBackend(config=config, audio=None)
 
             # Sanity check that value is in the library
             result = backend.library.lookup(self.tracks[0].uri)
             self.assertEqual(result, self.tracks[0:1])
 
-            # Clear library and refresh
-            library.seek(0)
-            library.truncate()
+            # Clear and refresh.
+            open(tmplib, 'w').close()
             backend.library.refresh()
 
             # Now it should be gone.
             result = backend.library.lookup(self.tracks[0].uri)
             self.assertEqual(result, [])
 
+        finally:
+            shutil.rmtree(tmpdir)
+
     def test_lookup(self):
         tracks = self.library.lookup(self.tracks[0].uri)
         self.assertEqual(tracks, self.tracks[0:1])
diff --git a/tests/backends/local/playback_test.py b/tests/backends/local/playback_test.py
index 4c3dd70d..7d48cfea 100644
--- a/tests/backends/local/playback_test.py
+++ b/tests/backends/local/playback_test.py
@@ -22,7 +22,9 @@ class LocalPlaybackProviderTest(unittest.TestCase):
     config = {
         'local': {
             'media_dir': path_to_data_dir(''),
+            'data_dir': path_to_data_dir(''),
             'playlists_dir': b'',
+            'library': 'json',
         }
     }
 
diff --git a/tests/backends/local/playlists_test.py b/tests/backends/local/playlists_test.py
index c02e1d23..6c602282 100644
--- a/tests/backends/local/playlists_test.py
+++ b/tests/backends/local/playlists_test.py
@@ -20,6 +20,8 @@ class LocalPlaylistsProviderTest(unittest.TestCase):
     config = {
         'local': {
             'media_dir': path_to_data_dir(''),
+            'data_dir': path_to_data_dir(''),
+            'library': 'json',
         }
     }
 
diff --git a/tests/backends/local/tracklist_test.py b/tests/backends/local/tracklist_test.py
index c7cfe51f..28def50c 100644
--- a/tests/backends/local/tracklist_test.py
+++ b/tests/backends/local/tracklist_test.py
@@ -18,7 +18,9 @@ class LocalTracklistProviderTest(unittest.TestCase):
     config = {
         'local': {
             'media_dir': path_to_data_dir(''),
+            'data_dir': path_to_data_dir(''),
             'playlists_dir': b'',
+            'library': 'json',
         }
     }
     tracks = [
diff --git a/tests/core/actor_test.py b/tests/core/actor_test.py
index e9e5f396..4a808cad 100644
--- a/tests/core/actor_test.py
+++ b/tests/core/actor_test.py
@@ -13,9 +13,11 @@ class CoreActorTest(unittest.TestCase):
     def setUp(self):
         self.backend1 = mock.Mock()
         self.backend1.uri_schemes.get.return_value = ['dummy1']
+        self.backend1.actor_ref.actor_class.__name__ = b'B1'
 
         self.backend2 = mock.Mock()
         self.backend2.uri_schemes.get.return_value = ['dummy2']
+        self.backend2.actor_ref.actor_class.__name__ = b'B2'
 
         self.core = Core(audio=None, backends=[self.backend1, self.backend2])
 
@@ -29,32 +31,12 @@ class CoreActorTest(unittest.TestCase):
         self.assertIn('dummy2', result)
 
     def test_backends_with_colliding_uri_schemes_fails(self):
-        self.backend1.actor_ref.actor_class.__name__ = b'B1'
-        self.backend2.actor_ref.actor_class.__name__ = b'B2'
         self.backend2.uri_schemes.get.return_value = ['dummy1', 'dummy2']
+
         self.assertRaisesRegexp(
             AssertionError,
             'Cannot add URI scheme dummy1 for B2, it is already handled by B1',
             Core, audio=None, backends=[self.backend1, self.backend2])
 
-    def test_backends_with_colliding_uri_schemes_passes(self):
-        """
-        Checks that backends with overlapping schemes, but distinct sub parts
-        provided can co-exist.
-        """
-
-        self.backend1.has_library().get.return_value = False
-        self.backend1.has_playlists().get.return_value = False
-
-        self.backend2.uri_schemes.get.return_value = ['dummy1']
-        self.backend2.has_playback().get.return_value = False
-        self.backend2.has_playlists().get.return_value = False
-
-        core = Core(audio=None, backends=[self.backend1, self.backend2])
-        self.assertEqual(core.backends.with_playback,
-                         {'dummy1': self.backend1})
-        self.assertEqual(core.backends.with_library,
-                         {'dummy1': self.backend2})
-
     def test_version(self):
         self.assertEqual(self.core.version, versioning.get_version())