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1   /**
2    *
3    * Licensed to the Apache Software Foundation (ASF) under one
4    * or more contributor license agreements.  See the NOTICE file
5    * distributed with this work for additional information
6    * regarding copyright ownership.  The ASF licenses this file
7    * to you under the Apache License, Version 2.0 (the
8    * "License"); you may not use this file except in compliance
9    * with the License.  You may obtain a copy of the License at
10   *
11   *     http://www.apache.org/licenses/LICENSE-2.0
12   *
13   * Unless required by applicable law or agreed to in writing, software
14   * distributed under the License is distributed on an "AS IS" BASIS,
15   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
16   * See the License for the specific language governing permissions and
17   * limitations under the License.
18   */
19  
20  package org.apache.hadoop.hbase.regionserver;
21  
22  import java.io.IOException;
23  import java.util.Comparator;
24  import java.util.List;
25  import java.util.PriorityQueue;
26  
27  import org.apache.hadoop.classification.InterfaceAudience;
28  import org.apache.hadoop.hbase.KeyValue;
29  import org.apache.hadoop.hbase.KeyValue.KVComparator;
30  
31  /**
32   * Implements a heap merge across any number of KeyValueScanners.
33   * <p>
34   * Implements KeyValueScanner itself.
35   * <p>
36   * This class is used at the Region level to merge across Stores
37   * and at the Store level to merge across the memstore and StoreFiles.
38   * <p>
39   * In the Region case, we also need InternalScanner.next(List), so this class
40   * also implements InternalScanner.  WARNING: As is, if you try to use this
41   * as an InternalScanner at the Store level, you will get runtime exceptions.
42   */
43  @InterfaceAudience.Private
44  public class KeyValueHeap extends NonLazyKeyValueScanner
45      implements KeyValueScanner, InternalScanner {
46    private PriorityQueue<KeyValueScanner> heap = null;
47  
48    /**
49     * The current sub-scanner, i.e. the one that contains the next key/value
50     * to return to the client. This scanner is NOT included in {@link #heap}
51     * (but we frequently add it back to the heap and pull the new winner out).
52     * We maintain an invariant that the current sub-scanner has already done
53     * a real seek, and that current.peek() is always a real key/value (or null)
54     * except for the fake last-key-on-row-column supplied by the multi-column
55     * Bloom filter optimization, which is OK to propagate to StoreScanner. In
56     * order to ensure that, always use {@link #pollRealKV()} to update current.
57     */
58    private KeyValueScanner current = null;
59  
60    private KVScannerComparator comparator;
61  
62    /**
63     * Constructor.  This KeyValueHeap will handle closing of passed in
64     * KeyValueScanners.
65     * @param scanners
66     * @param comparator
67     */
68    public KeyValueHeap(List<? extends KeyValueScanner> scanners,
69        KVComparator comparator) throws IOException {
70      this.comparator = new KVScannerComparator(comparator);
71      if (!scanners.isEmpty()) {
72        this.heap = new PriorityQueue<KeyValueScanner>(scanners.size(),
73            this.comparator);
74        for (KeyValueScanner scanner : scanners) {
75          if (scanner.peek() != null) {
76            this.heap.add(scanner);
77          } else {
78            scanner.close();
79          }
80        }
81        this.current = pollRealKV();
82      }
83    }
84  
85    public KeyValue peek() {
86      if (this.current == null) {
87        return null;
88      }
89      return this.current.peek();
90    }
91  
92    public KeyValue next()  throws IOException {
93      if(this.current == null) {
94        return null;
95      }
96      KeyValue kvReturn = this.current.next();
97      KeyValue kvNext = this.current.peek();
98      if (kvNext == null) {
99        this.current.close();
100       this.current = pollRealKV();
101     } else {
102       KeyValueScanner topScanner = this.heap.peek();
103       if (topScanner == null ||
104           this.comparator.compare(kvNext, topScanner.peek()) >= 0) {
105         this.heap.add(this.current);
106         this.current = pollRealKV();
107       }
108     }
109     return kvReturn;
110   }
111 
112   /**
113    * Gets the next row of keys from the top-most scanner.
114    * <p>
115    * This method takes care of updating the heap.
116    * <p>
117    * This can ONLY be called when you are using Scanners that implement
118    * InternalScanner as well as KeyValueScanner (a {@link StoreScanner}).
119    * @param result
120    * @param limit
121    * @return true if there are more keys, false if all scanners are done
122    */
123   public boolean next(List<KeyValue> result, int limit) throws IOException {
124     if (this.current == null) {
125       return false;
126     }
127     InternalScanner currentAsInternal = (InternalScanner)this.current;
128     boolean mayContainMoreRows = currentAsInternal.next(result, limit);
129     KeyValue pee = this.current.peek();
130     /*
131      * By definition, any InternalScanner must return false only when it has no
132      * further rows to be fetched. So, we can close a scanner if it returns
133      * false. All existing implementations seem to be fine with this. It is much
134      * more efficient to close scanners which are not needed than keep them in
135      * the heap. This is also required for certain optimizations.
136      */
137     if (pee == null || !mayContainMoreRows) {
138       this.current.close();
139     } else {
140       this.heap.add(this.current);
141     }
142     this.current = pollRealKV();
143     return (this.current != null);
144   }
145 
146   /**
147    * Gets the next row of keys from the top-most scanner.
148    * <p>
149    * This method takes care of updating the heap.
150    * <p>
151    * This can ONLY be called when you are using Scanners that implement
152    * InternalScanner as well as KeyValueScanner (a {@link StoreScanner}).
153    * @param result
154    * @return true if there are more keys, false if all scanners are done
155    */
156   public boolean next(List<KeyValue> result) throws IOException {
157     return next(result, -1);
158   }
159 
160   private static class KVScannerComparator implements Comparator<KeyValueScanner> {
161     private KVComparator kvComparator;
162     /**
163      * Constructor
164      * @param kvComparator
165      */
166     public KVScannerComparator(KVComparator kvComparator) {
167       this.kvComparator = kvComparator;
168     }
169     public int compare(KeyValueScanner left, KeyValueScanner right) {
170       int comparison = compare(left.peek(), right.peek());
171       if (comparison != 0) {
172         return comparison;
173       } else {
174         // Since both the keys are exactly the same, we break the tie in favor
175         // of the key which came latest.
176         long leftSequenceID = left.getSequenceID();
177         long rightSequenceID = right.getSequenceID();
178         if (leftSequenceID > rightSequenceID) {
179           return -1;
180         } else if (leftSequenceID < rightSequenceID) {
181           return 1;
182         } else {
183           return 0;
184         }
185       }
186     }
187     /**
188      * Compares two KeyValue
189      * @param left
190      * @param right
191      * @return less than 0 if left is smaller, 0 if equal etc..
192      */
193     public int compare(KeyValue left, KeyValue right) {
194       return this.kvComparator.compare(left, right);
195     }
196     /**
197      * @return KVComparator
198      */
199     public KVComparator getComparator() {
200       return this.kvComparator;
201     }
202   }
203 
204   public void close() {
205     if (this.current != null) {
206       this.current.close();
207     }
208     if (this.heap != null) {
209       KeyValueScanner scanner;
210       while ((scanner = this.heap.poll()) != null) {
211         scanner.close();
212       }
213     }
214   }
215 
216   /**
217    * Seeks all scanners at or below the specified seek key.  If we earlied-out
218    * of a row, we may end up skipping values that were never reached yet.
219    * Rather than iterating down, we want to give the opportunity to re-seek.
220    * <p>
221    * As individual scanners may run past their ends, those scanners are
222    * automatically closed and removed from the heap.
223    * <p>
224    * This function (and {@link #reseek(KeyValue)}) does not do multi-column
225    * Bloom filter and lazy-seek optimizations. To enable those, call
226    * {@link #requestSeek(KeyValue, boolean, boolean)}.
227    * @param seekKey KeyValue to seek at or after
228    * @return true if KeyValues exist at or after specified key, false if not
229    * @throws IOException
230    */
231   @Override
232   public boolean seek(KeyValue seekKey) throws IOException {
233     return generalizedSeek(false,    // This is not a lazy seek
234                            seekKey,
235                            false,    // forward (false: this is not a reseek)
236                            false);   // Not using Bloom filters
237   }
238 
239   /**
240    * This function is identical to the {@link #seek(KeyValue)} function except
241    * that scanner.seek(seekKey) is changed to scanner.reseek(seekKey).
242    */
243   @Override
244   public boolean reseek(KeyValue seekKey) throws IOException {
245     return generalizedSeek(false,    // This is not a lazy seek
246                            seekKey,
247                            true,     // forward (true because this is reseek)
248                            false);   // Not using Bloom filters
249   }
250 
251   /**
252    * {@inheritDoc}
253    */
254   @Override
255   public boolean requestSeek(KeyValue key, boolean forward,
256       boolean useBloom) throws IOException {
257     return generalizedSeek(true, key, forward, useBloom);
258   }
259 
260   /**
261    * @param isLazy whether we are trying to seek to exactly the given row/col.
262    *          Enables Bloom filter and most-recent-file-first optimizations for
263    *          multi-column get/scan queries.
264    * @param seekKey key to seek to
265    * @param forward whether to seek forward (also known as reseek)
266    * @param useBloom whether to optimize seeks using Bloom filters
267    */
268   private boolean generalizedSeek(boolean isLazy, KeyValue seekKey,
269       boolean forward, boolean useBloom) throws IOException {
270     if (!isLazy && useBloom) {
271       throw new IllegalArgumentException("Multi-column Bloom filter " +
272           "optimization requires a lazy seek");
273     }
274 
275     if (current == null) {
276       return false;
277     }
278     heap.add(current);
279     current = null;
280 
281     KeyValueScanner scanner;
282     while ((scanner = heap.poll()) != null) {
283       KeyValue topKey = scanner.peek();
284       if (comparator.getComparator().compare(seekKey, topKey) <= 0) {
285         // Top KeyValue is at-or-after Seek KeyValue. We only know that all
286         // scanners are at or after seekKey (because fake keys of
287         // scanners where a lazy-seek operation has been done are not greater
288         // than their real next keys) but we still need to enforce our
289         // invariant that the top scanner has done a real seek. This way
290         // StoreScanner and RegionScanner do not have to worry about fake keys.
291         heap.add(scanner);
292         current = pollRealKV();
293         return current != null;
294       }
295 
296       boolean seekResult;
297       if (isLazy && heap.size() > 0) {
298         // If there is only one scanner left, we don't do lazy seek.
299         seekResult = scanner.requestSeek(seekKey, forward, useBloom);
300       } else {
301         seekResult = NonLazyKeyValueScanner.doRealSeek(
302             scanner, seekKey, forward);
303       }
304 
305       if (!seekResult) {
306         scanner.close();
307       } else {
308         heap.add(scanner);
309       }
310     }
311 
312     // Heap is returning empty, scanner is done
313     return false;
314   }
315 
316   /**
317    * Fetches the top sub-scanner from the priority queue, ensuring that a real
318    * seek has been done on it. Works by fetching the top sub-scanner, and if it
319    * has not done a real seek, making it do so (which will modify its top KV),
320    * putting it back, and repeating this until success. Relies on the fact that
321    * on a lazy seek we set the current key of a StoreFileScanner to a KV that
322    * is not greater than the real next KV to be read from that file, so the
323    * scanner that bubbles up to the top of the heap will have global next KV in
324    * this scanner heap if (1) it has done a real seek and (2) its KV is the top
325    * among all top KVs (some of which are fake) in the scanner heap.
326    */
327   private KeyValueScanner pollRealKV() throws IOException {
328     KeyValueScanner kvScanner = heap.poll();
329     if (kvScanner == null) {
330       return null;
331     }
332 
333     while (kvScanner != null && !kvScanner.realSeekDone()) {
334       if (kvScanner.peek() != null) {
335         kvScanner.enforceSeek();
336         KeyValue curKV = kvScanner.peek();
337         if (curKV != null) {
338           KeyValueScanner nextEarliestScanner = heap.peek();
339           if (nextEarliestScanner == null) {
340             // The heap is empty. Return the only possible scanner.
341             return kvScanner;
342           }
343 
344           // Compare the current scanner to the next scanner. We try to avoid
345           // putting the current one back into the heap if possible.
346           KeyValue nextKV = nextEarliestScanner.peek();
347           if (nextKV == null || comparator.compare(curKV, nextKV) < 0) {
348             // We already have the scanner with the earliest KV, so return it.
349             return kvScanner;
350           }
351 
352           // Otherwise, put the scanner back into the heap and let it compete
353           // against all other scanners (both those that have done a "real
354           // seek" and a "lazy seek").
355           heap.add(kvScanner);
356         } else {
357           // Close the scanner because we did a real seek and found out there
358           // are no more KVs.
359           kvScanner.close();
360         }
361       } else {
362         // Close the scanner because it has already run out of KVs even before
363         // we had to do a real seek on it.
364         kvScanner.close();
365       }
366       kvScanner = heap.poll();
367     }
368 
369     return kvScanner;
370   }
371 
372   /**
373    * @return the current Heap
374    */
375   public PriorityQueue<KeyValueScanner> getHeap() {
376     return this.heap;
377   }
378 
379   @Override
380   public long getSequenceID() {
381     return 0;
382   }
383 
384   KeyValueScanner getCurrentForTesting() {
385     return current;
386   }
387 }