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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.hbase.Cell;
28  import org.apache.hadoop.hbase.KeyValue.KVComparator;
29  import org.apache.hadoop.hbase.classification.InterfaceAudience;
30  import org.apache.hadoop.hbase.regionserver.ScannerContext.NextState;
31  
32  /**
33   * Implements a heap merge across any number of KeyValueScanners.
34   * <p>
35   * Implements KeyValueScanner itself.
36   * <p>
37   * This class is used at the Region level to merge across Stores
38   * and at the Store level to merge across the memstore and StoreFiles.
39   * <p>
40   * In the Region case, we also need InternalScanner.next(List), so this class
41   * also implements InternalScanner.  WARNING: As is, if you try to use this
42   * as an InternalScanner at the Store level, you will get runtime exceptions.
43   */
44  @InterfaceAudience.Private
45  public class KeyValueHeap extends NonReversedNonLazyKeyValueScanner
46      implements KeyValueScanner, InternalScanner {
47    protected PriorityQueue<KeyValueScanner> heap = null;
48  
49    /**
50     * The current sub-scanner, i.e. the one that contains the next key/value
51     * to return to the client. This scanner is NOT included in {@link #heap}
52     * (but we frequently add it back to the heap and pull the new winner out).
53     * We maintain an invariant that the current sub-scanner has already done
54     * a real seek, and that current.peek() is always a real key/value (or null)
55     * except for the fake last-key-on-row-column supplied by the multi-column
56     * Bloom filter optimization, which is OK to propagate to StoreScanner. In
57     * order to ensure that, always use {@link #pollRealKV()} to update current.
58     */
59    protected KeyValueScanner current = null;
60  
61    protected KVScannerComparator comparator;
62    
63    /**
64     * Constructor.  This KeyValueHeap will handle closing of passed in
65     * KeyValueScanners.
66     * @param scanners
67     * @param comparator
68     */
69    public KeyValueHeap(List<? extends KeyValueScanner> scanners,
70        KVComparator comparator) throws IOException {
71      this(scanners, new KVScannerComparator(comparator));
72    }
73  
74    /**
75     * Constructor.
76     * @param scanners
77     * @param comparator
78     * @throws IOException
79     */
80    KeyValueHeap(List<? extends KeyValueScanner> scanners,
81        KVScannerComparator comparator) throws IOException {
82      this.comparator = comparator;
83      if (!scanners.isEmpty()) {
84        this.heap = new PriorityQueue<KeyValueScanner>(scanners.size(),
85            this.comparator);
86        for (KeyValueScanner scanner : scanners) {
87          if (scanner.peek() != null) {
88            this.heap.add(scanner);
89          } else {
90            scanner.close();
91          }
92        }
93        this.current = pollRealKV();
94      }
95    }
96  
97    public Cell peek() {
98      if (this.current == null) {
99        return null;
100     }
101     return this.current.peek();
102   }
103 
104   public Cell next()  throws IOException {
105     if(this.current == null) {
106       return null;
107     }
108     Cell kvReturn = this.current.next();
109     Cell kvNext = this.current.peek();
110     if (kvNext == null) {
111       this.current.close();
112       this.current = pollRealKV();
113     } else {
114       KeyValueScanner topScanner = this.heap.peek();
115       // no need to add current back to the heap if it is the only scanner left
116       if (topScanner != null && this.comparator.compare(kvNext, topScanner.peek()) >= 0) {
117         this.heap.add(this.current);
118         this.current = pollRealKV();
119       }
120     }
121     return kvReturn;
122   }
123 
124   /**
125    * Gets the next row of keys from the top-most scanner.
126    * <p>
127    * This method takes care of updating the heap.
128    * <p>
129    * This can ONLY be called when you are using Scanners that implement InternalScanner as well as
130    * KeyValueScanner (a {@link StoreScanner}).
131    * @param result
132    * @return true if more rows exist after this one, false if scanner is done
133    */
134   @Override
135   public boolean next(List<Cell> result) throws IOException {
136     return next(result, NoLimitScannerContext.getInstance());
137   }
138 
139   @Override
140   public boolean next(List<Cell> result, ScannerContext scannerContext) throws IOException {
141     if (this.current == null) {
142       return scannerContext.setScannerState(NextState.NO_MORE_VALUES).hasMoreValues();
143     }
144     InternalScanner currentAsInternal = (InternalScanner)this.current;
145     boolean moreCells = currentAsInternal.next(result, scannerContext);
146     Cell pee = this.current.peek();
147 
148     /*
149      * By definition, any InternalScanner must return false only when it has no
150      * further rows to be fetched. So, we can close a scanner if it returns
151      * false. All existing implementations seem to be fine with this. It is much
152      * more efficient to close scanners which are not needed than keep them in
153      * the heap. This is also required for certain optimizations.
154      */
155 
156     if (pee == null || !moreCells) {
157       this.current.close();
158     } else {
159       this.heap.add(this.current);
160     }
161     this.current = pollRealKV();
162     if (this.current == null) {
163       moreCells = scannerContext.setScannerState(NextState.NO_MORE_VALUES).hasMoreValues();
164     }
165     return moreCells;
166   }
167 
168   protected static class KVScannerComparator implements Comparator<KeyValueScanner> {
169     protected KVComparator kvComparator;
170     /**
171      * Constructor
172      * @param kvComparator
173      */
174     public KVScannerComparator(KVComparator kvComparator) {
175       this.kvComparator = kvComparator;
176     }
177     public int compare(KeyValueScanner left, KeyValueScanner right) {
178       int comparison = compare(left.peek(), right.peek());
179       if (comparison != 0) {
180         return comparison;
181       } else {
182         // Since both the keys are exactly the same, we break the tie in favor
183         // of the key which came latest.
184         long leftSequenceID = left.getSequenceID();
185         long rightSequenceID = right.getSequenceID();
186         if (leftSequenceID > rightSequenceID) {
187           return -1;
188         } else if (leftSequenceID < rightSequenceID) {
189           return 1;
190         } else {
191           return 0;
192         }
193       }
194     }
195     /**
196      * Compares two KeyValue
197      * @param left
198      * @param right
199      * @return less than 0 if left is smaller, 0 if equal etc..
200      */
201     public int compare(Cell left, Cell right) {
202       return this.kvComparator.compare(left, right);
203     }
204     /**
205      * @return KVComparator
206      */
207     public KVComparator getComparator() {
208       return this.kvComparator;
209     }
210   }
211 
212   public void close() {
213     if (this.current != null) {
214       this.current.close();
215     }
216     if (this.heap != null) {
217       KeyValueScanner scanner;
218       while ((scanner = this.heap.poll()) != null) {
219         scanner.close();
220       }
221     }
222   }
223 
224   /**
225    * Seeks all scanners at or below the specified seek key.  If we earlied-out
226    * of a row, we may end up skipping values that were never reached yet.
227    * Rather than iterating down, we want to give the opportunity to re-seek.
228    * <p>
229    * As individual scanners may run past their ends, those scanners are
230    * automatically closed and removed from the heap.
231    * <p>
232    * This function (and {@link #reseek(Cell)}) does not do multi-column
233    * Bloom filter and lazy-seek optimizations. To enable those, call
234    * {@link #requestSeek(Cell, boolean, boolean)}.
235    * @param seekKey KeyValue to seek at or after
236    * @return true if KeyValues exist at or after specified key, false if not
237    * @throws IOException
238    */
239   @Override
240   public boolean seek(Cell seekKey) throws IOException {
241     return generalizedSeek(false,    // This is not a lazy seek
242                            seekKey,
243                            false,    // forward (false: this is not a reseek)
244                            false);   // Not using Bloom filters
245   }
246 
247   /**
248    * This function is identical to the {@link #seek(Cell)} function except
249    * that scanner.seek(seekKey) is changed to scanner.reseek(seekKey).
250    */
251   @Override
252   public boolean reseek(Cell seekKey) throws IOException {
253     return generalizedSeek(false,    // This is not a lazy seek
254                            seekKey,
255                            true,     // forward (true because this is reseek)
256                            false);   // Not using Bloom filters
257   }
258 
259   /**
260    * {@inheritDoc}
261    */
262   @Override
263   public boolean requestSeek(Cell key, boolean forward,
264       boolean useBloom) throws IOException {
265     return generalizedSeek(true, key, forward, useBloom);
266   }
267 
268   /**
269    * @param isLazy whether we are trying to seek to exactly the given row/col.
270    *          Enables Bloom filter and most-recent-file-first optimizations for
271    *          multi-column get/scan queries.
272    * @param seekKey key to seek to
273    * @param forward whether to seek forward (also known as reseek)
274    * @param useBloom whether to optimize seeks using Bloom filters
275    */
276   private boolean generalizedSeek(boolean isLazy, Cell seekKey,
277       boolean forward, boolean useBloom) throws IOException {
278     if (!isLazy && useBloom) {
279       throw new IllegalArgumentException("Multi-column Bloom filter " +
280           "optimization requires a lazy seek");
281     }
282 
283     if (current == null) {
284       return false;
285     }
286     heap.add(current);
287     current = null;
288 
289     KeyValueScanner scanner;
290     while ((scanner = heap.poll()) != null) {
291       Cell topKey = scanner.peek();
292       if (comparator.getComparator().compare(seekKey, topKey) <= 0) {
293         // Top KeyValue is at-or-after Seek KeyValue. We only know that all
294         // scanners are at or after seekKey (because fake keys of
295         // scanners where a lazy-seek operation has been done are not greater
296         // than their real next keys) but we still need to enforce our
297         // invariant that the top scanner has done a real seek. This way
298         // StoreScanner and RegionScanner do not have to worry about fake keys.
299         heap.add(scanner);
300         current = pollRealKV();
301         return current != null;
302       }
303 
304       boolean seekResult;
305       if (isLazy && heap.size() > 0) {
306         // If there is only one scanner left, we don't do lazy seek.
307         seekResult = scanner.requestSeek(seekKey, forward, useBloom);
308       } else {
309         seekResult = NonLazyKeyValueScanner.doRealSeek(
310             scanner, seekKey, forward);
311       }
312 
313       if (!seekResult) {
314         scanner.close();
315       } else {
316         heap.add(scanner);
317       }
318     }
319 
320     // Heap is returning empty, scanner is done
321     return false;
322   }
323 
324   /**
325    * Fetches the top sub-scanner from the priority queue, ensuring that a real
326    * seek has been done on it. Works by fetching the top sub-scanner, and if it
327    * has not done a real seek, making it do so (which will modify its top KV),
328    * putting it back, and repeating this until success. Relies on the fact that
329    * on a lazy seek we set the current key of a StoreFileScanner to a KV that
330    * is not greater than the real next KV to be read from that file, so the
331    * scanner that bubbles up to the top of the heap will have global next KV in
332    * this scanner heap if (1) it has done a real seek and (2) its KV is the top
333    * among all top KVs (some of which are fake) in the scanner heap.
334    */
335   protected KeyValueScanner pollRealKV() throws IOException {
336     KeyValueScanner kvScanner = heap.poll();
337     if (kvScanner == null) {
338       return null;
339     }
340 
341     while (kvScanner != null && !kvScanner.realSeekDone()) {
342       if (kvScanner.peek() != null) {
343         kvScanner.enforceSeek();
344         Cell curKV = kvScanner.peek();
345         if (curKV != null) {
346           KeyValueScanner nextEarliestScanner = heap.peek();
347           if (nextEarliestScanner == null) {
348             // The heap is empty. Return the only possible scanner.
349             return kvScanner;
350           }
351 
352           // Compare the current scanner to the next scanner. We try to avoid
353           // putting the current one back into the heap if possible.
354           Cell nextKV = nextEarliestScanner.peek();
355           if (nextKV == null || comparator.compare(curKV, nextKV) < 0) {
356             // We already have the scanner with the earliest KV, so return it.
357             return kvScanner;
358           }
359 
360           // Otherwise, put the scanner back into the heap and let it compete
361           // against all other scanners (both those that have done a "real
362           // seek" and a "lazy seek").
363           heap.add(kvScanner);
364         } else {
365           // Close the scanner because we did a real seek and found out there
366           // are no more KVs.
367           kvScanner.close();
368         }
369       } else {
370         // Close the scanner because it has already run out of KVs even before
371         // we had to do a real seek on it.
372         kvScanner.close();
373       }
374       kvScanner = heap.poll();
375     }
376 
377     return kvScanner;
378   }
379 
380   /**
381    * @return the current Heap
382    */
383   public PriorityQueue<KeyValueScanner> getHeap() {
384     return this.heap;
385   }
386 
387   @Override
388   public long getSequenceID() {
389     return 0;
390   }
391 
392   KeyValueScanner getCurrentForTesting() {
393     return current;
394   }
395 
396   @Override
397   public Cell getNextIndexedKey() {
398     // here we return the next index key from the top scanner
399     return current == null ? null : current.getNextIndexedKey();
400   }
401 }