001    /**
002     * Licensed to the Apache Software Foundation (ASF) under one or more
003     * contributor license agreements.  See the NOTICE file distributed with
004     * this work for additional information regarding copyright ownership.
005     * The ASF licenses this file to You under the Apache License, Version 2.0
006     * (the "License"); you may not use this file except in compliance with
007     * the License.  You may obtain a copy of the License at
008     *
009     *      http://www.apache.org/licenses/LICENSE-2.0
010     *
011     * Unless required by applicable law or agreed to in writing, software
012     * distributed under the License is distributed on an "AS IS" BASIS,
013     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014     * See the License for the specific language governing permissions and
015     * limitations under the License.
016     */
017    package org.apache.camel.processor.resequencer;
018    
019    import java.util.Timer;
020    
021    /**
022     * Resequences elements based on a given {@link SequenceElementComparator}.
023     * This resequencer is designed for resequencing element streams. Stream-based
024     * resequencing has the advantage that the number of elements to be resequenced
025     * need not be known in advance. Resequenced elements are delivered via a
026     * {@link SequenceSender}.
027     * <p>
028     * The resequencer's behaviour for a given comparator is controlled by the
029     * <code>timeout</code> property. This is the timeout (in milliseconds) for a
030     * given element managed by this resequencer. An out-of-sequence element can
031     * only be marked as <i>ready-for-delivery</i> if it either times out or if it
032     * has an immediate predecessor (in that case it is in-sequence). If an
033     * immediate predecessor of a waiting element arrives the timeout task for the
034     * waiting element will be cancelled (which marks it as <i>ready-for-delivery</i>).
035     * <p>
036     * If the maximum out-of-sequence time difference between elements within a
037     * stream is known, the <code>timeout</code> value should be set to this
038     * value. In this case it is guaranteed that all elements of a stream will be
039     * delivered in sequence via the {@link SequenceSender}. The lower the
040     * <code>timeout</code> value is compared to the out-of-sequence time
041     * difference between elements within a stream the higher the probability is for
042     * out-of-sequence elements delivered by this resequencer. Delivery of elements
043     * must be explicitly triggered by applications using the {@link #deliver()} or
044     * {@link #deliverNext()} methods. Only elements that are <i>ready-for-delivery</i>
045     * are delivered by these methods. The longer an application waits to trigger a
046     * delivery the more elements may become <i>ready-for-delivery</i>.
047     * <p>
048     * The resequencer remembers the last-delivered element. If an element arrives
049     * which is the immediate successor of the last-delivered element it is
050     * <i>ready-for-delivery</i> immediately. After delivery the last-delivered
051     * element is adjusted accordingly. If the last-delivered element is
052     * <code>null</code> i.e. the resequencer was newly created the first arriving
053     * element needs <code>timeout</code> milliseconds in any case for becoming
054     * <i>ready-for-delivery</i>.
055     * <p>
056     *
057     * @author Martin Krasser
058     *
059     * @version $Revision: 1532 $
060     */
061    public class ResequencerEngine<E> {
062    
063        /**
064         * The element that most recently hash been delivered or <code>null</code>
065         * if no element has been delivered yet.
066         */
067        private Element<E> lastDelivered;
068    
069        /**
070         * Minimum amount of time to wait for out-of-sequence elements.
071         */
072        private long timeout;
073    
074        /**
075         * A sequence of elements for sorting purposes.
076         */
077        private Sequence<Element<E>> sequence;
078    
079        /**
080         * A timer for scheduling timeout notifications.
081         */
082        private Timer timer;
083    
084        /**
085         * A strategy for sending sequence elements.
086         */
087        private SequenceSender<E> sequenceSender;
088    
089        /**
090         * Creates a new resequencer instance with a default timeout of 2000
091         * milliseconds.
092         *
093         * @param comparator a sequence element comparator.
094         */
095        public ResequencerEngine(SequenceElementComparator<E> comparator) {
096            this.sequence = createSequence(comparator);
097            this.timeout = 2000L;
098            this.lastDelivered = null;
099        }
100    
101        public void start() {
102            timer = new Timer("Stream Resequencer Timer");
103        }
104    
105        /**
106         * Stops this resequencer (i.e. this resequencer's {@link Timer} instance).
107         */
108        public void stop() {
109            timer.cancel();
110        }
111    
112        /**
113         * Returns the number of elements currently maintained by this resequencer.
114         *
115         * @return the number of elements currently maintained by this resequencer.
116         */
117        public synchronized int size() {
118            return sequence.size();
119        }
120    
121        /**
122         * Returns this resequencer's timeout value.
123         *
124         * @return the timeout in milliseconds.
125         */
126        public long getTimeout() {
127            return timeout;
128        }
129    
130        /**
131         * Sets this sequencer's timeout value.
132         *
133         * @param timeout the timeout in milliseconds.
134         */
135        public void setTimeout(long timeout) {
136            this.timeout = timeout;
137        }
138    
139        /**
140         * Returns the sequence sender.
141         *
142         * @return the sequence sender.
143         */
144        public SequenceSender<E> getSequenceSender() {
145            return sequenceSender;
146        }
147    
148        /**
149         * Sets the sequence sender.
150         *
151         * @param sequenceSender a sequence element sender.
152         */
153        public void setSequenceSender(SequenceSender<E> sequenceSender) {
154            this.sequenceSender = sequenceSender;
155        }
156    
157        /**
158         * Returns the last delivered element.
159         *
160         * @return the last delivered element or <code>null</code> if no delivery
161         *         has been made yet.
162         */
163        E getLastDelivered() {
164            if (lastDelivered == null) {
165                return null;
166            }
167            return lastDelivered.getObject();
168        }
169    
170        /**
171         * Sets the last delivered element. This is for testing purposes only.
172         *
173         * @param o an element.
174         */
175        void setLastDelivered(E o) {
176            lastDelivered = new Element<E>(o);
177        }
178    
179        /**
180         * Inserts the given element into this resequencer. If the element is not
181         * ready for immediate delivery and has no immediate presecessor then it is
182         * scheduled for timing out. After being timed out it is ready for delivery.
183         *
184         * @param o an element.
185         */
186        public synchronized void insert(E o) {
187            // wrap object into internal element
188            Element<E> element = new Element<E>(o);
189            // add element to sequence in proper order
190            sequence.add(element);
191    
192            Element<E> successor = sequence.successor(element);
193    
194            // check if there is an immediate successor and cancel
195            // timer task (no need to wait any more for timeout)
196            if (successor != null) {
197                successor.cancel();
198            }
199    
200            // start delivery if current element is successor of last delivered element
201            if (successorOfLastDelivered(element)) {
202                // nothing to schedule
203            } else if (sequence.predecessor(element) != null) {
204                // nothing to schedule
205            } else {
206                element.schedule(defineTimeout());
207            }
208        }
209    
210        /**
211         * Delivers all elements which are currently ready to deliver.
212         *
213         * @throws Exception thrown by {@link SequenceSender#sendElement(Object)}.
214         *
215         * @see ResequencerEngine#deliverNext() 
216         */
217        public synchronized void deliver() throws Exception {
218            while (deliverNext()) {
219                // do nothing here
220            }
221        }
222    
223        /**
224         * Attempts to deliver a single element from the head of the resequencer
225         * queue (sequence). Only elements which have not been scheduled for timing
226         * out or which already timed out can be delivered. Elements are deliveref via
227         * {@link SequenceSender#sendElement(Object)}.
228         *
229         * @return <code>true</code> if the element has been delivered
230         *         <code>false</code> otherwise.
231         *
232         * @throws Exception thrown by {@link SequenceSender#sendElement(Object)}.
233         *
234         */
235        public boolean deliverNext() throws Exception {
236            if (sequence.size() == 0) {
237                return false;
238            }
239            // inspect element with lowest sequence value
240            Element<E> element = sequence.first();
241    
242            // if element is scheduled do not deliver and return
243            if (element.scheduled()) {
244                return false;
245            }
246    
247            // remove deliverable element from sequence
248            sequence.remove(element);
249    
250            // set the delivered element to last delivered element
251            lastDelivered = element;
252    
253            // deliver the sequence element
254            sequenceSender.sendElement(element.getObject());
255    
256            // element has been delivered
257            return true;
258        }
259    
260        /**
261         * Returns <code>true</code> if the given element is the immediate
262         * successor of the last delivered element.
263         *
264         * @param element an element.
265         * @return <code>true</code> if the given element is the immediate
266         *         successor of the last delivered element.
267         */
268        private boolean successorOfLastDelivered(Element<E> element) {
269            if (lastDelivered == null) {
270                return false;
271            }
272            if (sequence.comparator().successor(element, lastDelivered)) {
273                return true;
274            }
275            return false;
276        }
277    
278        /**
279         * Creates a timeout task based on the timeout setting of this resequencer.
280         *
281         * @return a new timeout task.
282         */
283        private Timeout defineTimeout() {
284            return new Timeout(timer, timeout);
285        }
286    
287        private static <E> Sequence<Element<E>> createSequence(SequenceElementComparator<E> comparator) {
288            return new Sequence<Element<E>>(new ElementComparator<E>(comparator));
289        }
290    
291    }