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