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From romb...@apache.org
Subject [sling-org-apache-sling-discovery-api] 01/39: Move discovery to bundles section
Date Fri, 20 Oct 2017 14:36:26 GMT
This is an automated email from the ASF dual-hosted git repository.

rombert pushed a commit to branch master
in repository https://gitbox.apache.org/repos/asf/sling-org-apache-sling-discovery-api.git

commit a052911e6eeb27f3054a1c8e25c7e7858ae333c8
Author: Carsten Ziegeler <cziegeler@apache.org>
AuthorDate: Sun May 5 10:35:09 2013 +0000

    Move discovery to bundles section
    
    git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1479281 13f79535-47bb-0310-9956-ffa450edef68
---
 pom.xml                                            |  86 +++++
 .../org/apache/sling/discovery/ClusterView.java    |  57 +++
 .../apache/sling/discovery/DiscoveryService.java   |  50 +++
 .../sling/discovery/InstanceDescription.java       | 106 ++++++
 .../org/apache/sling/discovery/InstanceFilter.java |  37 ++
 .../apache/sling/discovery/PropertyProvider.java   |  55 +++
 .../org/apache/sling/discovery/TopologyEvent.java  | 158 +++++++++
 .../sling/discovery/TopologyEventListener.java     |  59 ++++
 .../org/apache/sling/discovery/TopologyView.java   |  71 ++++
 .../discovery/impl/NoClusterDiscoveryService.java  | 392 +++++++++++++++++++++
 .../discovery/impl/StandardPropertyProvider.java   | 219 ++++++++++++
 .../org/apache/sling/discovery/package-info.java   |  30 ++
 12 files changed, 1320 insertions(+)

diff --git a/pom.xml b/pom.xml
new file mode 100644
index 0000000..f30d328
--- /dev/null
+++ b/pom.xml
@@ -0,0 +1,86 @@
+<?xml version="1.0" encoding="ISO-8859-1"?>
+<!--
+  Licensed to the Apache Software Foundation (ASF) under one
+  or more contributor license agreements.  See the NOTICE file
+  distributed with this work for additional information
+  regarding copyright ownership.  The ASF licenses this file
+  to you under the Apache License, Version 2.0 (the
+  "License"); you may not use this file except in compliance
+  with the License.  You may obtain a copy of the License at
+
+   http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing,
+  software distributed under the License is distributed on an
+  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+  KIND, either express or implied.  See the License for the
+  specific language governing permissions and limitations
+  under the License.
+-->
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
+
+    <modelVersion>4.0.0</modelVersion>
+    <parent>
+        <groupId>org.apache.sling</groupId>
+        <artifactId>sling</artifactId>
+        <version>15</version>
+        <relativePath>../../../parent/pom.xml</relativePath>
+    </parent>
+
+    <artifactId>org.apache.sling.discovery.api</artifactId>
+    <packaging>bundle</packaging>
+    <version>0.1.0-SNAPSHOT</version>
+
+    <name>Apache Sling Discovery API</name>
+    <description>
+        Support for topology discovery of instances.
+    </description>
+
+    <scm>
+        <connection>scm:svn:http://svn.apache.org/repos/asf/sling/trunk/contrib/extensions/discovery/api</connection>
+        <developerConnection>scm:svn:https://svn.apache.org/repos/asf/sling/trunk/contrib/extensions/discovery/api</developerConnection>
+        <url>http://svn.apache.org/viewvc/sling/trunk/contrib/extensions/discovery/api</url>
+    </scm>
+
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.felix</groupId>
+                <artifactId>maven-scr-plugin</artifactId>
+            </plugin>
+            <plugin>
+                <groupId>org.apache.felix</groupId>
+                <artifactId>maven-bundle-plugin</artifactId>
+                <extensions>true</extensions>
+            </plugin>
+        </plugins>
+    </build>
+    <dependencies>
+        <dependency>
+            <groupId>org.apache.felix</groupId>
+            <artifactId>org.apache.felix.scr.annotations</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>biz.aQute</groupId>
+            <artifactId>bndlib</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>org.slf4j</groupId>
+            <artifactId>slf4j-api</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>org.osgi</groupId>
+            <artifactId>org.osgi.core</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>org.osgi</groupId>
+            <artifactId>org.osgi.compendium</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>org.apache.sling</groupId>
+            <artifactId>org.apache.sling.settings</artifactId>
+            <version>1.1.0</version>
+            <scope>provided</scope>
+        </dependency>
+    </dependencies>
+</project>
diff --git a/src/main/java/org/apache/sling/discovery/ClusterView.java b/src/main/java/org/apache/sling/discovery/ClusterView.java
new file mode 100644
index 0000000..64744a1
--- /dev/null
+++ b/src/main/java/org/apache/sling/discovery/ClusterView.java
@@ -0,0 +1,57 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.sling.discovery;
+
+import java.util.List;
+
+/**
+ * A ClusterView represents the instances of a cluster that are
+ * up and running and that all can see each other at a certain point in time.
+ * <p>
+ * A ClusterView can also consist of just one single instance.
+ */
+public interface ClusterView {
+
+	/**
+	 * Returns an id of this cluster view
+	 * @return an id of this cluster view
+	 */
+    String getId();
+
+    /**
+     * Provides the list of InstanceDescriptions with a stable ordering.
+     * <p>
+     * Stable ordering implies that unless an instance leaves the cluster
+     * (due to shutdown/crash/network problems) the instance keeps the
+     * relative position in the list.
+     * @return the list of InstanceDescriptions (with a stable ordering)
+     */
+    List<InstanceDescription> getInstances();
+
+	/**
+	 * Provides the InstanceDescription belonging to the leader instance.
+	 * <p>
+	 * Every ClusterView is guaranteed to have one and only one leader.
+	 * <p>
+	 * The leader is stable: once a leader is elected it stays leader
+	 * unless it leaves the cluster (due to shutdown/crash/network problems)
+	 * @return the InstanceDescription belonging to the leader instance
+	 */
+    InstanceDescription getLeader();
+}
diff --git a/src/main/java/org/apache/sling/discovery/DiscoveryService.java b/src/main/java/org/apache/sling/discovery/DiscoveryService.java
new file mode 100644
index 0000000..3477650
--- /dev/null
+++ b/src/main/java/org/apache/sling/discovery/DiscoveryService.java
@@ -0,0 +1,50 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.sling.discovery;
+
+/**
+ * The discovery service can be used to get the current topology view.
+ * <p>
+ * The discovery service is in charge of managing live instances that
+ * have announced themselves as being part of a topology view. The exact
+ * details of how this announcement occurs is implementation dependent.
+ */
+public interface DiscoveryService {
+
+	/**
+	 * Returns the topology that was last discovered by this service.
+	 * <p>
+	 * If for some reason the service is currently not able to do topology discovery
+	 * it will return the last valid topology marked with <code>false</code> in the call
+	 * to <codeTopologyView.isCurrent()</code>. This is also true if the service
+	 * has noticed a potential change in the topology and is in the process of
+	 * settling the change in the topology (eg with peers, ie voting).
+	 * <p>
+	 * Note that this call is synchronized with <code>TopologyEventListener.handleTopologyEvent()</code>
+	 * calls: ie if calls to <code>TopologyEventListener.handleTopologyEvent()</code> are currently
+	 * ongoing, then the call to this method will block until all <code>TopologyEventListener</code>s
+	 * have been called. Be careful not to cause deadlock situations.
+	 * <p>
+	 * @return the topology that was last discovered by this service. This will never
+	 * be null (ie even if a change in the topology is ongoing at the moment or the
+	 * cluster consists only of the local instance).
+	 */
+    TopologyView getTopology();
+
+}
diff --git a/src/main/java/org/apache/sling/discovery/InstanceDescription.java b/src/main/java/org/apache/sling/discovery/InstanceDescription.java
new file mode 100644
index 0000000..0c714a3
--- /dev/null
+++ b/src/main/java/org/apache/sling/discovery/InstanceDescription.java
@@ -0,0 +1,106 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.sling.discovery;
+
+import java.util.Map;
+
+/**
+ * An InstanceDescription represents and contains information about an
+ * instance that is part of a TopologyView.
+ * <p>
+ * Note that all methods are idempotent - they always return the same values
+ * on subsequent calls. Rather, on any change new InstanceDescriptions are created.
+ *
+ *
+ * @see TopologyView
+ */
+public interface InstanceDescription {
+
+    /**
+     * Property containing a name for the instance.
+     * The instance should provide this property.
+     */
+    String PROPERTY_NAME = "org.apache.sling.instance.name";
+
+    /**
+     * Property containing a description for the instance.
+     * The instance should provide this property.
+     */
+    String PROPERTY_DESCRIPTION = "org.apache.sling.instance.name";
+
+    /**
+     * Property containing endpoints to connect to the instance. The
+     * value is a comma separated list.
+     * The instance should provide this property.
+     */
+    String PROPERTY_ENDPOINTS = "org.apache.sling.instance.endpoints";
+
+    /**
+	 * Returns the ClusterView of which this instance is part of.
+	 * <p>
+	 * Every instance is part of a ClusterView even if it is standalone.
+	 * @return the ClusterView
+	 */
+    ClusterView getClusterView();
+
+	/**
+	 * If an instance is part of a cluster, it can potentially be a leader of that cluster -
+	 * this information is queried here.
+	 * <p>
+	 * If an instance is not part of a cluster, this method returns true.
+	 * <p>
+	 * Only one instance of a cluster is guaranteed to be the leader at any time.
+	 * This guarantee is provided by this service.
+	 * If the leader goes down, the service elects a new leader and announces it to
+	 * TopologyEventListener listeners.
+	 * @return true if this instance is the - only -  leader in this cluster,
+	 * false if it is one of the slaves, or true if it is not at all part of a cluster
+	 */
+	boolean isLeader();
+
+	/**
+	 * Determines whether this InstanceDescription is representing the local instance.
+	 * @return whether this InstanceDescription is representing the local instance.
+	 */
+	boolean isLocal();
+
+    /**
+     * The identifier of the running Sling instance.
+     */
+    String getSlingId();
+
+    /**
+     * Returns the value of a particular property.
+     * <p>
+     * Note that there are no hard guarantees or requirements as to how quickly
+     * a property is available once it is set on a distant instance.
+     * @param name The property name
+     * @return The value of the property or <code>null</code>
+     * @see DiscoveryService#setProperty(String, String)
+     */
+    String getProperty(final String name);
+
+    /**
+     * Returns a Map containing all properties of this instance.
+     * This method always returns a map, it might be empty. The returned map
+     * is not modifiable.
+     * @return a Map containing all properties of this instance
+     */
+    Map<String,String> getProperties();
+}
diff --git a/src/main/java/org/apache/sling/discovery/InstanceFilter.java b/src/main/java/org/apache/sling/discovery/InstanceFilter.java
new file mode 100644
index 0000000..a6e5a51
--- /dev/null
+++ b/src/main/java/org/apache/sling/discovery/InstanceFilter.java
@@ -0,0 +1,37 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.sling.discovery;
+
+/**
+ * Used to filter InstanceDescriptions in a TopologyView.
+ * <p>
+ * @see DiscoveryService#findInstances(InstanceFilter)
+ */
+public interface InstanceFilter {
+
+	/**
+	 * Returns true if this InstanceFilter selects the given InstanceDescription.
+	 * <p>
+	 * @param instance the InstanceDescription for which to decide if this
+	 * InstanceFilter accepts it or not.
+	 * @return true if this InstanceFilter selects the given InstanceDescription
+	 */
+	boolean accept(InstanceDescription instance);
+
+}
diff --git a/src/main/java/org/apache/sling/discovery/PropertyProvider.java b/src/main/java/org/apache/sling/discovery/PropertyProvider.java
new file mode 100644
index 0000000..2fa21f8
--- /dev/null
+++ b/src/main/java/org/apache/sling/discovery/PropertyProvider.java
@@ -0,0 +1,55 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.sling.discovery;
+
+
+/**
+ * The <code>PropertyProvider</code> service interface may be implemented by
+ * components that wish to define properties on the local instance which then
+ * are broadcast to the <code>TopologyView</code> instances.
+ * <p>
+ * The provided properties are registered with the {@link #PROPERTY_PROPERTIES}
+ * service property.
+ * If the set of provided properties changes or one of the provided values
+ * change, the service registration of the provider should be updated.
+ * This avoids periodic polling for changes.
+ */
+public interface PropertyProvider {
+
+    /**
+     * The name of the service registration property containing the names
+     * of the properties provided by this provider.
+     * The value is either a string or an array of strings.
+     * A property name must only contain alphanumeric characters plus <code>.</code>,
+     * <code>_</code>, <code>-</code>.
+     */
+    String PROPERTY_PROPERTIES = "instance.properties";
+
+	/**
+	 * Retrieves a property that is subsequently set on the local instance
+	 * and broadcast to the <code>TopologyView</code> instances.
+	 * <p>
+	 * These properties are non-persistent and disappear after the local instance goes down.
+	 *
+	 * @return The value of the property or <code>null</code>. If the property
+	 *         value can't be provided or if the provider does not support this
+	 *         property, it must return <code>null</code>.
+	 */
+	String getProperty(final String name);
+}
diff --git a/src/main/java/org/apache/sling/discovery/TopologyEvent.java b/src/main/java/org/apache/sling/discovery/TopologyEvent.java
new file mode 100644
index 0000000..65dc13f
--- /dev/null
+++ b/src/main/java/org/apache/sling/discovery/TopologyEvent.java
@@ -0,0 +1,158 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.sling.discovery;
+
+/**
+ * A topology event is sent whenever a change in the topology occurs.
+ * 
+ * This event object might be extended in the future with new event types and
+ * methods.
+ * 
+ * @see TopologyEventListener
+ */
+public class TopologyEvent {
+
+    public static enum Type {
+        /**
+         * Informs the service about the initial topology state - this is only 
+         * sent once at bind-time and is the first one a TopologyEventListener
+         * receives (after the implementation bundle was activated).
+         */
+        TOPOLOGY_INIT,
+
+        /**
+         * Informs the service about the fact that a state change was detected in
+         * the topology/cluster and that the new state is in the process of
+         * being discovered. Once the discovery is finished, a TOPOLOGY_CHANGED
+         * is sent with the new topology view.
+         * <p>
+         * An implementation must always send a TOPOLOGY_CHANGING before a
+         * TOPOLOGY_CHANGED.
+         */
+        TOPOLOGY_CHANGING,
+
+        /**
+         * Informs the service about a state change in the topology.
+         * <p>
+         * A state change includes:
+         * <ul>
+         * <li>A joining or leaving instance</li>
+         * <li>A restart of an instance - or more precisely: when the
+         * corresponding implementation bundle is deactivated/activated</li>
+         * <li>A cluster structure - either its members or the cluster
+         * view id - changed. The cluster view id changes when an instance joins, 
+         * leaves or was restarted (its bundle deactivated/activated)</li>
+         * </ul>
+         * <p>
+         * Note that a TOPOLOGY_CHANGED can also include changes in the
+         * properties!
+         */
+        TOPOLOGY_CHANGED,
+
+        /**
+         * One or many properties have been changed on an instance which is part
+         * of the topology.
+         * <p>
+         * This event is sent when otherwise the topology remains identical.
+         */
+        PROPERTIES_CHANGED
+
+    }
+
+    private final Type type;
+    private final TopologyView oldView;
+    private final TopologyView newView;
+
+    public TopologyEvent(final Type type, final TopologyView oldView,
+            final TopologyView newView) {
+        if (type == null) {
+            throw new IllegalArgumentException("type must not be null");
+        }
+
+        if (type == Type.TOPOLOGY_INIT) {
+            // then oldView is null
+            if (oldView != null) {
+                throw new IllegalArgumentException("oldView must be null");
+            }
+            // and newView must be not null
+            if (newView == null) {
+                throw new IllegalArgumentException("newView must not be null");
+            }
+        } else if (type == Type.TOPOLOGY_CHANGING) {
+            // then newView is null
+            if (newView != null) {
+                throw new IllegalArgumentException("newView must be null");
+            }
+            // and oldView must not be null
+            if (oldView == null) {
+                throw new IllegalArgumentException("oldView must not be null");
+            }
+        } else {
+            // in all other cases both oldView and newView must not be null
+            if (oldView == null) {
+                throw new IllegalArgumentException("oldView must not be null");
+            }
+            if (newView == null) {
+                throw new IllegalArgumentException("newView must not be null");
+            }
+        }
+        this.type = type;
+        this.oldView = oldView;
+        this.newView = newView;
+    }
+
+    /**
+     * Returns the type of this event
+     * 
+     * @return the type of this event
+     */
+    public Type getType() {
+        return type;
+    }
+
+    /**
+     * Returns the view which was valid up until now.
+     * <p>
+     * This is null in case of <code>TOPOLOGY_INIT</code>
+     * 
+     * @return the view which was valid up until now, or null in case of a fresh
+     *         instance start
+     */
+    public TopologyView getOldView() {
+        return oldView;
+    }
+
+    /**
+     * Returns the view which is currently (i.e. newly) valid.
+     * <p>
+     * This is null in case of <code>TOPOLOGY_CHANGING</code>
+     * 
+     * @return the view which is currently valid, or null in case of
+     *         <code>TOPOLOGY_CHANGING</code>
+     */
+    public TopologyView getNewView() {
+        return newView;
+    }
+
+    @Override
+    public String toString() {
+        return "TopologyEvent [type=" + type + ", oldView=" + oldView
+                + ", newView=" + newView + "]";
+    }
+}
diff --git a/src/main/java/org/apache/sling/discovery/TopologyEventListener.java b/src/main/java/org/apache/sling/discovery/TopologyEventListener.java
new file mode 100644
index 0000000..aed95c6
--- /dev/null
+++ b/src/main/java/org/apache/sling/discovery/TopologyEventListener.java
@@ -0,0 +1,59 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.sling.discovery;
+
+/**
+ * The <code>TopologyEventListener</code> service interface may be implemented by
+ * components interested in being made aware of changes in the topology.
+ * <p>
+ * Upon registration and whenever changes in the topology occur, this
+ * service is informed.
+ */
+public interface TopologyEventListener {
+
+	/**
+	 * Inform the service about an event in the topology - or in the discovery
+	 * of the topology.
+	 * <p>
+	 * The <code>TopologyEvent</code> contains details about what changed.
+	 * The supported event types are:
+	 * <ul>
+	 *  <li><code>TOPOLOGY_INIT</code> sent when the <code>TopologyEventListener</code>
+	 *  was first bound to the discovery service - represents the initial state
+	 *  of the topology at that time.</li>
+	 *  <li><code>TOPOLOGY_CHANGING</code> sent when the discovery service
+	 *  discovered a change in the topology and has started to settle the change.
+	 *  This event is sent before <code>TOPOLOGY_CHANGED</code> but is optional</li>
+	 *  <li><code>TOPOLOGY_CHANGED</code> sent when the discovery service
+	 *  discovered a change in the topology and has settled it.</li>
+	 *  <li><code>PROPERTIES_CHANGED</code> sent when the one or many properties
+	 *  have changed in an instance in the current topology</li>
+	 * </ul>
+	 * A note on instance restarts: it is currently not a requirement on the
+	 * discovery service to send a TopologyEvent should an instance restart
+	 * occur rapidly (ie within the change detection timeout). A TopologyEvent
+	 * is only sent if the number of instances or any property changes.
+	 * Should there be a requirement to detect a restart in a guaranteed fashion,
+	 * it is always possible to set a particular property (using the PropertyProvider)
+	 * to the instance start time and have others detect a change in that property.
+	 * @param event The topology event
+	 */
+	void handleTopologyEvent(TopologyEvent event);
+
+}
diff --git a/src/main/java/org/apache/sling/discovery/TopologyView.java b/src/main/java/org/apache/sling/discovery/TopologyView.java
new file mode 100644
index 0000000..cc143ce
--- /dev/null
+++ b/src/main/java/org/apache/sling/discovery/TopologyView.java
@@ -0,0 +1,71 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.sling.discovery;
+
+import java.util.Set;
+
+/**
+ * A topology view is a cross-cluster list of instances and clusters
+ * that have announced themselves with the DiscoveryService.
+ *
+ */
+public interface TopologyView {
+
+	/**
+	 * Checks if this TopologyView is currently valid - or if the
+	 * service knows of a topology change just going on (or another
+	 * uncertainty about the topology such as IOException etc)
+	 * @return true if this TopologyView is currently valid, false
+	 * if the service knows of a topology change just going on (or
+	 * another issue with discovery like IOException etc)
+	 */
+	boolean isCurrent();
+
+	/**
+	 * Provides the InstanceDescription belonging to <b>this</b> instance.
+	 * @return the InstanceDescription belonging to <b>this</b> instance
+	 */
+	InstanceDescription getLocalInstance();
+
+    /**
+     * Provides the set of InstanceDescriptions in the entire topology,
+     * without any particular order
+     * @return the set of InstanceDescriptions in the entire topology,
+     * without any particular order
+     */
+	Set<InstanceDescription> getInstances();
+
+	/**
+	 * Searches through this topology and picks those accepted by the provided
+	 * <code>InstanceFilter</code> - and returns them without any particular order
+	 * @param filter the filter to use
+	 * @return the set of InstanceDescriptions which were accepted by the InstanceFilter,
+	 * without any particular order
+	 */
+	Set<InstanceDescription> findInstances(InstanceFilter filter);
+
+    /**
+     * Provides the collection of ClusterViews.
+     * <p>
+     * Note that all InstanceDescriptions belong to exactly one ClusterView -
+     * including InstanceDescriptions that form "a cluster of 1"
+     * @return the set of ClusterViews, without any particular order
+     */
+	Set<ClusterView> getClusterViews();
+}
diff --git a/src/main/java/org/apache/sling/discovery/impl/NoClusterDiscoveryService.java b/src/main/java/org/apache/sling/discovery/impl/NoClusterDiscoveryService.java
new file mode 100644
index 0000000..cfb3c6b
--- /dev/null
+++ b/src/main/java/org/apache/sling/discovery/impl/NoClusterDiscoveryService.java
@@ -0,0 +1,392 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.sling.discovery.impl;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+
+import org.apache.felix.scr.annotations.Activate;
+import org.apache.felix.scr.annotations.Component;
+import org.apache.felix.scr.annotations.ConfigurationPolicy;
+import org.apache.felix.scr.annotations.Deactivate;
+import org.apache.felix.scr.annotations.Reference;
+import org.apache.felix.scr.annotations.ReferenceCardinality;
+import org.apache.felix.scr.annotations.ReferencePolicy;
+import org.apache.felix.scr.annotations.Service;
+import org.apache.sling.discovery.ClusterView;
+import org.apache.sling.discovery.DiscoveryService;
+import org.apache.sling.discovery.InstanceDescription;
+import org.apache.sling.discovery.InstanceFilter;
+import org.apache.sling.discovery.PropertyProvider;
+import org.apache.sling.discovery.TopologyEvent;
+import org.apache.sling.discovery.TopologyEvent.Type;
+import org.apache.sling.discovery.TopologyEventListener;
+import org.apache.sling.discovery.TopologyView;
+import org.apache.sling.settings.SlingSettingsService;
+import org.osgi.framework.Constants;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * This is a simple implementation of the discovery service
+ * which can be used for a cluster less installation (= single instance).
+ * It is disabled by default and can be enabled through a OSGi configuration.
+ */
+@Component(policy = ConfigurationPolicy.REQUIRE, immediate=true)
+@Service(value = {DiscoveryService.class})
+public class NoClusterDiscoveryService implements DiscoveryService {
+
+    /** The logger. */
+    private final Logger logger = LoggerFactory.getLogger(this.getClass());
+
+    /**
+     * Sling settings service to get the Sling ID and run modes.
+     */
+    @Reference
+    private SlingSettingsService settingsService;
+
+    /**
+     * All topology event listeners.
+     */
+    @Reference(cardinality = ReferenceCardinality.OPTIONAL_MULTIPLE, policy = ReferencePolicy.DYNAMIC)
+    private TopologyEventListener[] listeners = new TopologyEventListener[0];
+
+    /**
+     * All property providers.
+     */
+    @Reference(cardinality = ReferenceCardinality.OPTIONAL_MULTIPLE, policy = ReferencePolicy.DYNAMIC,
+               referenceInterface=PropertyProvider.class, updated="updatedPropertyProvider")
+    private List<ProviderInfo> providerInfos = new ArrayList<ProviderInfo>();
+
+    /**
+     * Special lock object to sync data structure access
+     */
+    private final Object lock = new Object();
+
+    /**
+     * The current topology view.
+     */
+    private TopologyView topologyView;
+
+    private Map<String, String> cachedProperties = new HashMap<String, String>();
+
+    /**
+     * Activate this service
+     * Create a new description.
+     */
+    @Activate
+    protected void activate() {
+        logger.debug("NoClusterDiscoveryService started.");
+        final InstanceDescription myDescription = new InstanceDescription() {
+
+            public boolean isLocal() {
+                return true;
+            }
+
+            public boolean isLeader() {
+                return true;
+            }
+
+            public String getSlingId() {
+                return settingsService.getSlingId();
+            }
+
+            public String getProperty(final String name) {
+            	synchronized(lock) {
+            		return cachedProperties.get(name);
+            	}
+            }
+
+			public Map<String, String> getProperties() {
+				synchronized(lock) {
+					return Collections.unmodifiableMap(cachedProperties);
+				}
+			}
+
+			public ClusterView getClusterView() {
+				final Collection<ClusterView> clusters = topologyView.getClusterViews();
+				if (clusters==null || clusters.size()==0) {
+					return null;
+				}
+				return clusters.iterator().next();
+			}
+        };
+        final Set<InstanceDescription> instances = new HashSet<InstanceDescription>();
+        instances.add(myDescription);
+
+        final TopologyEventListener[] registeredServices;
+		synchronized ( lock ) {
+            registeredServices = this.listeners;
+            final ClusterView clusterView = new ClusterView() {
+
+                public InstanceDescription getLeader() {
+                    return myDescription;
+                }
+
+                public List<InstanceDescription> getInstances() {
+                    return new LinkedList<InstanceDescription>(instances);
+                }
+
+				public String getId() {
+					return "0";
+				}
+            };
+            this.topologyView = new TopologyView() {
+
+    			public InstanceDescription getLocalInstance() {
+    				return myDescription;
+    			}
+
+    			public boolean isCurrent() {
+    				return true;
+    			}
+
+    			public Set<InstanceDescription> getInstances() {
+    				return instances;
+    			}
+
+    			public Set<InstanceDescription> findInstances(InstanceFilter picker) {
+    				Set<InstanceDescription> result = new HashSet<InstanceDescription>();
+    				for (Iterator<InstanceDescription> it = getTopology().getInstances().iterator(); it.hasNext();) {
+    					InstanceDescription instance = it.next();
+    					if (picker.accept(instance)) {
+    						result.add(instance);
+    					}
+    				}
+    				return result;
+    			}
+
+    			public Set<ClusterView> getClusterViews() {
+    				Set<ClusterView> clusters = new HashSet<ClusterView>();
+    				clusters.add(clusterView);
+    				return clusters;
+    			}
+
+    		};
+        }
+        for(final TopologyEventListener da: registeredServices) {
+        	da.handleTopologyEvent(new TopologyEvent(Type.TOPOLOGY_INIT, null, topologyView));
+        }
+    }
+
+    /**
+     * Deactivate this service.
+     */
+    @Deactivate
+    protected void deactivate() {
+        logger.debug("NoClusterDiscoveryService stopped.");
+        this.topologyView = null;
+    }
+
+    /**
+     * Bind a new property provider.
+     */
+    @SuppressWarnings("unused")
+	private void bindPropertyProvider(final PropertyProvider propertyProvider, final Map<String, Object> props) {
+    	logger.debug("bindPropertyProvider: Binding PropertyProvider {}", propertyProvider);
+
+        final TopologyEventListener[] awares;
+        synchronized (lock) {
+            final ProviderInfo info = new ProviderInfo(propertyProvider, props);
+            this.providerInfos.add(info);
+            Collections.sort(this.providerInfos);
+            this.updatePropertiesCache();
+            if ( this.topologyView == null ) {
+                awares = null;
+            } else {
+                awares = this.listeners;
+            }
+        }
+        if ( awares != null ) {
+            for(final TopologyEventListener da : awares) {
+                da.handleTopologyEvent(new TopologyEvent(Type.PROPERTIES_CHANGED, this.topologyView, this.topologyView));
+            }
+        }
+    }
+
+    /**
+     * Update a property provider.
+     */
+    @SuppressWarnings("unused")
+    private void updatedPropertyProvider(final PropertyProvider propertyProvider, final Map<String, Object> props) {
+        logger.debug("bindPropertyProvider: Updating PropertyProvider {}", propertyProvider);
+
+        this.unbindPropertyProvider(propertyProvider, props, false);
+        this.bindPropertyProvider(propertyProvider, props);
+    }
+
+    /**
+     * Unbind a property provider
+     */
+    @SuppressWarnings("unused")
+	private void unbindPropertyProvider(final PropertyProvider propertyProvider, final Map<String, Object> props) {
+        this.unbindPropertyProvider(propertyProvider, props, true);
+    }
+
+    /**
+     * Unbind a property provider
+     */
+    @SuppressWarnings("unused")
+    private void unbindPropertyProvider(final PropertyProvider propertyProvider,
+            final Map<String, Object> props,
+            final boolean inform) {
+    	logger.debug("unbindPropertyProvider: Releasing PropertyProvider {}", propertyProvider);
+
+    	final TopologyEventListener[] awares;
+        synchronized (lock) {
+            final ProviderInfo info = new ProviderInfo(propertyProvider, props);
+            this.providerInfos.remove(info);
+            this.updatePropertiesCache();
+            if ( this.topologyView == null ) {
+                awares = null;
+            } else {
+                awares = this.listeners;
+            }
+        }
+        if ( inform && awares != null ) {
+            for(final TopologyEventListener da : awares) {
+                da.handleTopologyEvent(new TopologyEvent(Type.PROPERTIES_CHANGED, this.topologyView, this.topologyView));
+            }
+        }
+    }
+
+    private void updatePropertiesCache() {
+        final Map<String, String> newProps = new HashMap<String, String>();
+        for(final ProviderInfo info : this.providerInfos) {
+            newProps.putAll(info.properties);
+        }
+        this.cachedProperties = newProps;
+        if ( this.logger.isDebugEnabled() ) {
+            this.logger.debug("New properties: {}", this.cachedProperties);
+        }
+    }
+
+    @SuppressWarnings("unused")
+    private void bindTopologyEventListener(final TopologyEventListener clusterAware) {
+
+        logger.debug("bindTopologyEventListener: Binding TopologyEventListener {}", clusterAware);
+
+        boolean inform = true;
+        synchronized (lock) {
+            List<TopologyEventListener> currentList = new ArrayList<TopologyEventListener>(
+                Arrays.asList(listeners));
+            currentList.add(clusterAware);
+            this.listeners = currentList.toArray(new TopologyEventListener[currentList.size()]);
+            if ( this.topologyView == null ) {
+                inform = false;
+            }
+        }
+
+        if ( inform ) {
+        	clusterAware.handleTopologyEvent(new TopologyEvent(Type.TOPOLOGY_INIT, null, topologyView));
+        }
+    }
+
+    @SuppressWarnings("unused")
+    private void unbindTopologyEventListener(final TopologyEventListener clusterAware) {
+
+        logger.debug("unbindTopologyEventListener: Releasing TopologyEventListener {}", clusterAware);
+
+        synchronized (lock) {
+            List<TopologyEventListener> currentList = new ArrayList<TopologyEventListener>(
+                Arrays.asList(listeners));
+            currentList.remove(clusterAware);
+            this.listeners = currentList.toArray(new TopologyEventListener[currentList.size()]);
+        }
+    }
+
+    /**
+     * @see DiscoveryService#getTopology()
+     */
+    public TopologyView getTopology() {
+    	return topologyView;
+    }
+
+    /**
+     * Internal class caching some provider infos like service id and ranking.
+     */
+    private final static class ProviderInfo implements Comparable<ProviderInfo> {
+
+        public final PropertyProvider provider;
+        public final int ranking;
+        public final long serviceId;
+        public final Map<String, String> properties = new HashMap<String, String>();
+
+        public ProviderInfo(final PropertyProvider provider, final Map<String, Object> serviceProps) {
+            this.provider = provider;
+            final Object sr = serviceProps.get(Constants.SERVICE_RANKING);
+            if ( sr == null || !(sr instanceof Integer)) {
+                this.ranking = 0;
+            } else {
+                this.ranking = (Integer)sr;
+            }
+            this.serviceId = (Long)serviceProps.get(Constants.SERVICE_ID);
+            final Object namesObj = serviceProps.get(PropertyProvider.PROPERTY_PROPERTIES);
+            if ( namesObj instanceof String ) {
+                final String val = provider.getProperty((String)namesObj);
+                if ( val != null ) {
+                    this.properties.put((String)namesObj, val);
+                }
+            } else if ( namesObj instanceof String[] ) {
+                for(final String name : (String[])namesObj ) {
+                    final String val = provider.getProperty(name);
+                    if ( val != null ) {
+                        this.properties.put(name, val);
+                    }
+                }
+            }
+        }
+
+        /**
+         * @see java.lang.Comparable#compareTo(java.lang.Object)
+         */
+        public int compareTo(final ProviderInfo o) {
+            // Sort by rank in ascending order.
+            if ( this.ranking < o.ranking ) {
+                return -1; // lower rank
+            } else if (this.ranking > o.ranking ) {
+                return 1; // higher rank
+            }
+            // If ranks are equal, then sort by service id in descending order.
+            return (this.serviceId < o.serviceId) ? 1 : -1;
+        }
+
+        @Override
+        public boolean equals(final Object obj) {
+            if ( obj instanceof ProviderInfo ) {
+                return ((ProviderInfo)obj).serviceId == this.serviceId;
+            }
+            return false;
+        }
+
+        @Override
+        public int hashCode() {
+            return provider.hashCode();
+        }
+    }
+}
diff --git a/src/main/java/org/apache/sling/discovery/impl/StandardPropertyProvider.java b/src/main/java/org/apache/sling/discovery/impl/StandardPropertyProvider.java
new file mode 100644
index 0000000..798ff0a
--- /dev/null
+++ b/src/main/java/org/apache/sling/discovery/impl/StandardPropertyProvider.java
@@ -0,0 +1,219 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.sling.discovery.impl;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Dictionary;
+import java.util.HashMap;
+import java.util.Hashtable;
+import java.util.List;
+import java.util.Map;
+
+import org.apache.felix.scr.annotations.Activate;
+import org.apache.felix.scr.annotations.Component;
+import org.apache.felix.scr.annotations.Deactivate;
+import org.apache.felix.scr.annotations.Modified;
+import org.apache.felix.scr.annotations.Reference;
+import org.apache.felix.scr.annotations.ReferenceCardinality;
+import org.apache.felix.scr.annotations.ReferencePolicy;
+import org.apache.sling.discovery.InstanceDescription;
+import org.apache.sling.discovery.PropertyProvider;
+import org.osgi.framework.Constants;
+import org.osgi.framework.ServiceReference;
+import org.osgi.framework.ServiceRegistration;
+import org.osgi.service.component.ComponentContext;
+import org.osgi.service.http.HttpService;
+
+/**
+ * This service provides the standard instance properties (if available)
+ */
+@Component(immediate=true)
+@Reference(referenceInterface=HttpService.class,
+           cardinality=ReferenceCardinality.OPTIONAL_MULTIPLE,
+           policy=ReferencePolicy.DYNAMIC)
+public class StandardPropertyProvider {
+
+    /** Endpoint service registration property from RFC 189 */
+    private static final String REG_PROPERTY_ENDPOINTS = "osgi.http.service.endpoints";
+
+    private volatile long changeCount;
+
+    private String instanceName;
+
+    private String instanceDescription;
+
+    private ServiceRegistration propagationService;
+
+    private final Map<Long, String[]> endpoints = new HashMap<Long, String[]>();
+
+    private String endpointString;
+
+    private Dictionary<String, Object> getRegistrationProperties() {
+        final List<String> names = new ArrayList<String>();
+        if ( this.instanceName != null ) {
+            names.add(InstanceDescription.PROPERTY_NAME);
+        }
+        if ( this.instanceDescription != null ) {
+            names.add(InstanceDescription.PROPERTY_DESCRIPTION);
+        }
+        names.add(InstanceDescription.PROPERTY_ENDPOINTS);
+
+        final StringBuilder sb = new StringBuilder();
+        boolean first = true;
+        synchronized ( this.endpoints ) {
+            for(final String[] points : endpoints.values()) {
+                for(final String point : points) {
+                    if ( first ) {
+                        first = false;
+                    } else {
+                        sb.append(",");
+                    }
+                    sb.append(point);
+                }
+            }
+        }
+        this.endpointString = sb.toString();
+
+        final Dictionary<String, Object> serviceProps = new Hashtable<String, Object>();
+        serviceProps.put(PropertyProvider.PROPERTY_PROPERTIES, names.toArray(new String[names.size()]));
+        // we add a changing property to the service registration
+        // to make sure a modification event is really sent
+        synchronized ( this ) {
+            serviceProps.put("changeCount", this.changeCount++);
+        }
+        return serviceProps;
+    }
+
+    private String getPropertyValue(final ComponentContext bc, final String key) {
+        Object value = bc.getProperties().get(key);
+        if ( value == null ) {
+            value = bc.getBundleContext().getProperty(key);
+        }
+        if ( value != null ) {
+            return value.toString();
+        }
+        return null;
+    }
+
+    @Activate
+    protected void activate(final ComponentContext cc) {
+        this.modified(cc);
+    }
+
+    @Modified
+    protected void modified(final ComponentContext cc) {
+        this.instanceName = this.getPropertyValue(cc, "sling.name");
+        this.instanceDescription = this.getPropertyValue(cc, "sling.description");
+
+        this.propagationService = cc.getBundleContext().registerService(PropertyProvider.class.getName(),
+                new PropertyProvider() {
+
+                    public String getProperty(final String name) {
+                        if ( InstanceDescription.PROPERTY_DESCRIPTION.equals(name) ) {
+                            return instanceDescription;
+                        }
+                        if ( InstanceDescription.PROPERTY_NAME.equals(name) ) {
+                            return instanceName;
+                        }
+                        if ( InstanceDescription.PROPERTY_ENDPOINTS.equals(name) ) {
+                            return endpointString;
+                        }
+                        return null;
+                    }
+                }, this.getRegistrationProperties());
+    }
+
+    @Deactivate
+    protected void deactivate() {
+        if ( this.propagationService != null ) {
+            this.propagationService.unregister();
+            this.propagationService = null;
+        }
+    }
+
+    /**
+     * Bind a http service
+     */
+    protected void bindHttpService(final ServiceReference reference) {
+        final String[] endpointUrls = toStringArray(reference.getProperty(REG_PROPERTY_ENDPOINTS));
+        if ( endpointUrls != null ) {
+            synchronized ( this.endpoints ) {
+                this.endpoints.put((Long)reference.getProperty(Constants.SERVICE_ID), endpointUrls);
+            }
+            if ( this.propagationService != null ) {
+                this.propagationService.setProperties(this.getRegistrationProperties());
+            }
+        }
+    }
+
+    /**
+     * Unbind a http service
+     */
+    protected void unbindHttpService(final ServiceReference reference) {
+        boolean changed = false;
+        synchronized ( this.endpoints ) {
+            if ( this.endpoints.remove(reference.getProperty(Constants.SERVICE_ID)) != null ) {
+                changed = true;
+            }
+        }
+        if ( changed && this.propagationService != null ) {
+            this.propagationService.setProperties(this.getRegistrationProperties());
+        }
+    }
+
+    private String[] toStringArray(final Object propValue) {
+        if (propValue == null) {
+            // no value at all
+            return null;
+
+        } else if (propValue instanceof String) {
+            // single string
+            return new String[] { (String) propValue };
+
+        } else if (propValue instanceof String[]) {
+            // String[]
+            return (String[]) propValue;
+
+        } else if (propValue.getClass().isArray()) {
+            // other array
+            Object[] valueArray = (Object[]) propValue;
+            List<String> values = new ArrayList<String>(valueArray.length);
+            for (Object value : valueArray) {
+                if (value != null) {
+                    values.add(value.toString());
+                }
+            }
+            return values.toArray(new String[values.size()]);
+
+        } else if (propValue instanceof Collection<?>) {
+            // collection
+            Collection<?> valueCollection = (Collection<?>) propValue;
+            List<String> valueList = new ArrayList<String>(valueCollection.size());
+            for (Object value : valueCollection) {
+                if (value != null) {
+                    valueList.add(value.toString());
+                }
+            }
+            return valueList.toArray(new String[valueList.size()]);
+        }
+
+        return null;
+    }
+}
diff --git a/src/main/java/org/apache/sling/discovery/package-info.java b/src/main/java/org/apache/sling/discovery/package-info.java
new file mode 100644
index 0000000..387bb0f
--- /dev/null
+++ b/src/main/java/org/apache/sling/discovery/package-info.java
@@ -0,0 +1,30 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+/**
+ * Provides a service to interface which may be implemented by applications
+ * to get notified on cluster topology changes.
+ *
+ * @version 1.0
+ */
+@Version("1.0")
+package org.apache.sling.discovery;
+
+import aQute.bnd.annotation.Version;
+

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