Journaled Event implementation based on MongoDB

    * initial commit

org.apache.aries.events.mongo/pom.xml

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+<!--
+  ~ 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 SF 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/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <parent>
+        <groupId>org.apache.aries.events</groupId>
+        <artifactId>org.apache.aries.events</artifactId>
+        <version>0.1.0-SNAPSHOT</version>
+    </parent>
+    <groupId>org.apache.aries.events.mongo</groupId>
+    <artifactId>org.apache.aries.events.mongo</artifactId>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.apache.aries.events</groupId>
+            <artifactId>org.apache.aries.events.api</artifactId>
+            <version>${project.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.mongodb</groupId>
+            <artifactId>mongo-java-driver</artifactId>
+            <version>3.8.2</version>
+        </dependency>
+    </dependencies>
+</project>

org.apache.aries.events.mongo/src/main/java/org/apache/aries/events/mongo/CachingFactory.java

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+/*
+ * 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 SF 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.aries.events.mongo;
+
+import org.slf4j.Logger;
+
+import java.io.Closeable;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.function.Function;
+
+import static org.slf4j.LoggerFactory.getLogger;
+
+/**
+ * A factory that keeps previously created instances in a cache so that
+ * they will get reused if requested repeatedly.
+ * Currently there is no cache size limit implemented so this implementation
+ * is only good for the use case with limited parameter space.
+ * @param <K> key type. Serves as cache key as well as an input parameter for the
+ *           factory method. Must provide sensible implementations for
+ *          equals and hashCode methods
+ * @param <V> result type.
+ */
+public final class CachingFactory<K, V extends AutoCloseable> implements Closeable {
+
+
+    public static <K2, V2 extends AutoCloseable> CachingFactory<K2, V2> cachingFactory(Function<K2, V2> create) {
+        return new CachingFactory<K2, V2>(create);
+    }
+
+    /**
+     * Find or created a value for the specified key
+     * @param arg key instance
+     * @return either an existing (cached) value of newly created one.
+     */
+    public synchronized V get(K arg) {
+        return cache.computeIfAbsent(arg, create);
+    }
+
+    /**
+     * Clears all cached instances properly disposing them.
+     */
+    public synchronized void clear() {
+        cache.values().stream()
+            .forEach(CachingFactory::safeClose);
+        cache.clear();
+    }
+
+    /**
+     * Closing this factory properly disposing all cached instances
+     */
+    @Override
+    public void close() {
+        clear();
+    }
+
+    //*********************************************
+    // Private
+    //*********************************************
+
+    private static final Logger LOG = getLogger(CachingFactory.class);
+    private final Map<K, V> cache = new HashMap<K, V>();
+    private final Function<K, V> create;
+
+    private CachingFactory(Function<K, V> create) {
+        this.create = create;
+    }
+
+    private static void safeClose(AutoCloseable closable) {
+        try {
+            closable.close();
+        } catch (Exception e) {
+            LOG.warn(e.getMessage(), e);
+        }
+    }
+
+
+}

org.apache.aries.events.mongo/src/main/java/org/apache/aries/events/mongo/Common.java

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+/*
+ * 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 SF 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.aries.events.mongo;
+
+import com.mongodb.client.MongoCollection;
+import org.bson.Document;
+
+import static com.mongodb.client.model.Filters.lte;
+import static com.mongodb.client.model.Indexes.descending;
+import static org.apache.aries.events.mongo.Common.Fields.INDEX;
+
+/**
+ * Common string definitions
+ */
+@SuppressWarnings({"HardCodedStringLiteral", "InterfaceNeverImplemented"})
+interface Common {
+
+    String DEFAULT_DB_NAME = "aem-replication";
+
+    /** MongoDB field names */
+    interface Fields {
+        String INDEX = "i";
+        String TIME_STAMP = "t";
+        String PAYLOAD = "d";
+        String PROPS = "p";
+    }
+
+    /**
+     * Returns the next available index in the collection
+     * @param col collection to check. The collection must contain
+     *            log messages published by a Publisher instance
+     * @return the index that should be assigned to the next message when
+     * it gets published
+     */
+    static long upcomingIndex(MongoCollection<Document> col) {
+        Document doc = col.find(lte(INDEX, Long.MAX_VALUE))
+                          .sort(descending(INDEX))
+                          .first();
+        if (doc != null) {
+            long latestAvailable = doc.getLong(INDEX);
+            return latestAvailable + 1L;
+        } else {
+            return 0L;
+        }
+    }
+
+}

org.apache.aries.events.mongo/src/main/java/org/apache/aries/events/mongo/MessageReceiver.java

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+/*
+ * 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 SF 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.aries.events.mongo;
+
+import org.apache.aries.events.api.Message;
+
+public interface MessageReceiver extends AutoCloseable {
+
+    /** returns data entry for the specified offset.
+     * If necessary waits until data is available.
+     * If data entry at the specified offset has
+     * been evicted, throws NoSuchElement exception
+     * @param index an offset to the desired entry
+     * @return requested data entry together with the
+     *         offset for the next data entry
+     */
+    Message receive(long index) throws InterruptedException;
+
+    /** returns the index of the earliest available
+     * data entry. It also causes the receiver to
+     * pre-fetch and cache a batch of earliest available
+     * entries thus giving the user a chance to consume
+     * them and catch up before they get evicted
+     * @return an index of the first available data entry or
+     * 0 if the log is empty
+     */
+    long earliestIndex();
+
+    /** returns the index of the next available data
+     *  entry.
+     *  The returned index points to the entry yet to
+     *  be inserted into the log.
+     *  @return index of the data entry that will be
+     *  inserted next. 0 if the log is empty.
+     */
+    long latestIndex();
+
+    @Override
+    void close();
+
+}