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 return next(result, limit, null); 125 } 126 127 /** 128 * Gets the next row of keys from the top-most scanner. 129 * <p> 130 * This method takes care of updating the heap. 131 * <p> 132 * This can ONLY be called when you are using Scanners that implement 133 * InternalScanner as well as KeyValueScanner (a {@link StoreScanner}). 134 * @param result output result list 135 * @param limit limit on row count to get 136 * @param metric the metric name 137 * @return true if there are more keys, false if all scanners are done 138 */ 139 public boolean next(List<KeyValue> result, int limit, String metric) throws IOException { 140 if (this.current == null) { 141 return false; 142 } 143 InternalScanner currentAsInternal = (InternalScanner)this.current; 144 boolean mayContainMoreRows = currentAsInternal.next(result, limit, metric); 145 KeyValue pee = this.current.peek(); 146 /* 147 * By definition, any InternalScanner must return false only when it has no 148 * further rows to be fetched. So, we can close a scanner if it returns 149 * false. All existing implementations seem to be fine with this. It is much 150 * more efficient to close scanners which are not needed than keep them in 151 * the heap. This is also required for certain optimizations. 152 */ 153 if (pee == null || !mayContainMoreRows) { 154 this.current.close(); 155 } else { 156 this.heap.add(this.current); 157 } 158 this.current = pollRealKV(); 159 return (this.current != null); 160 } 161 162 /** 163 * Gets the next row of keys from the top-most scanner. 164 * <p> 165 * This method takes care of updating the heap. 166 * <p> 167 * This can ONLY be called when you are using Scanners that implement 168 * InternalScanner as well as KeyValueScanner (a {@link StoreScanner}). 169 * @param result 170 * @return true if there are more keys, false if all scanners are done 171 */ 172 public boolean next(List<KeyValue> result) throws IOException { 173 return next(result, -1); 174 } 175 176 @Override 177 public boolean next(List<KeyValue> result, String metric) throws IOException { 178 return next(result, -1, metric); 179 } 180 181 private static class KVScannerComparator implements Comparator<KeyValueScanner> { 182 private KVComparator kvComparator; 183 /** 184 * Constructor 185 * @param kvComparator 186 */ 187 public KVScannerComparator(KVComparator kvComparator) { 188 this.kvComparator = kvComparator; 189 } 190 public int compare(KeyValueScanner left, KeyValueScanner right) { 191 int comparison = compare(left.peek(), right.peek()); 192 if (comparison != 0) { 193 return comparison; 194 } else { 195 // Since both the keys are exactly the same, we break the tie in favor 196 // of the key which came latest. 197 long leftSequenceID = left.getSequenceID(); 198 long rightSequenceID = right.getSequenceID(); 199 if (leftSequenceID > rightSequenceID) { 200 return -1; 201 } else if (leftSequenceID < rightSequenceID) { 202 return 1; 203 } else { 204 return 0; 205 } 206 } 207 } 208 /** 209 * Compares two KeyValue 210 * @param left 211 * @param right 212 * @return less than 0 if left is smaller, 0 if equal etc.. 213 */ 214 public int compare(KeyValue left, KeyValue right) { 215 return this.kvComparator.compare(left, right); 216 } 217 /** 218 * @return KVComparator 219 */ 220 public KVComparator getComparator() { 221 return this.kvComparator; 222 } 223 } 224 225 public void close() { 226 if (this.current != null) { 227 this.current.close(); 228 } 229 if (this.heap != null) { 230 KeyValueScanner scanner; 231 while ((scanner = this.heap.poll()) != null) { 232 scanner.close(); 233 } 234 } 235 } 236 237 /** 238 * Seeks all scanners at or below the specified seek key. If we earlied-out 239 * of a row, we may end up skipping values that were never reached yet. 240 * Rather than iterating down, we want to give the opportunity to re-seek. 241 * <p> 242 * As individual scanners may run past their ends, those scanners are 243 * automatically closed and removed from the heap. 244 * <p> 245 * This function (and {@link #reseek(KeyValue)}) does not do multi-column 246 * Bloom filter and lazy-seek optimizations. To enable those, call 247 * {@link #requestSeek(KeyValue, boolean, boolean)}. 248 * @param seekKey KeyValue to seek at or after 249 * @return true if KeyValues exist at or after specified key, false if not 250 * @throws IOException 251 */ 252 @Override 253 public boolean seek(KeyValue seekKey) throws IOException { 254 return generalizedSeek(false, // This is not a lazy seek 255 seekKey, 256 false, // forward (false: this is not a reseek) 257 false); // Not using Bloom filters 258 } 259 260 /** 261 * This function is identical to the {@link #seek(KeyValue)} function except 262 * that scanner.seek(seekKey) is changed to scanner.reseek(seekKey). 263 */ 264 @Override 265 public boolean reseek(KeyValue seekKey) throws IOException { 266 return generalizedSeek(false, // This is not a lazy seek 267 seekKey, 268 true, // forward (true because this is reseek) 269 false); // Not using Bloom filters 270 } 271 272 /** 273 * {@inheritDoc} 274 */ 275 @Override 276 public boolean requestSeek(KeyValue key, boolean forward, 277 boolean useBloom) throws IOException { 278 return generalizedSeek(true, key, forward, useBloom); 279 } 280 281 /** 282 * @param isLazy whether we are trying to seek to exactly the given row/col. 283 * Enables Bloom filter and most-recent-file-first optimizations for 284 * multi-column get/scan queries. 285 * @param seekKey key to seek to 286 * @param forward whether to seek forward (also known as reseek) 287 * @param useBloom whether to optimize seeks using Bloom filters 288 */ 289 private boolean generalizedSeek(boolean isLazy, KeyValue seekKey, 290 boolean forward, boolean useBloom) throws IOException { 291 if (!isLazy && useBloom) { 292 throw new IllegalArgumentException("Multi-column Bloom filter " + 293 "optimization requires a lazy seek"); 294 } 295 296 if (current == null) { 297 return false; 298 } 299 heap.add(current); 300 current = null; 301 302 KeyValueScanner scanner; 303 while ((scanner = heap.poll()) != null) { 304 KeyValue topKey = scanner.peek(); 305 if (comparator.getComparator().compare(seekKey, topKey) <= 0) { 306 // Top KeyValue is at-or-after Seek KeyValue. We only know that all 307 // scanners are at or after seekKey (because fake keys of 308 // scanners where a lazy-seek operation has been done are not greater 309 // than their real next keys) but we still need to enforce our 310 // invariant that the top scanner has done a real seek. This way 311 // StoreScanner and RegionScanner do not have to worry about fake keys. 312 heap.add(scanner); 313 current = pollRealKV(); 314 return current != null; 315 } 316 317 boolean seekResult; 318 if (isLazy) { 319 seekResult = scanner.requestSeek(seekKey, forward, useBloom); 320 } else { 321 seekResult = NonLazyKeyValueScanner.doRealSeek( 322 scanner, seekKey, forward); 323 } 324 325 if (!seekResult) { 326 scanner.close(); 327 } else { 328 heap.add(scanner); 329 } 330 } 331 332 // Heap is returning empty, scanner is done 333 return false; 334 } 335 336 /** 337 * Fetches the top sub-scanner from the priority queue, ensuring that a real 338 * seek has been done on it. Works by fetching the top sub-scanner, and if it 339 * has not done a real seek, making it do so (which will modify its top KV), 340 * putting it back, and repeating this until success. Relies on the fact that 341 * on a lazy seek we set the current key of a StoreFileScanner to a KV that 342 * is not greater than the real next KV to be read from that file, so the 343 * scanner that bubbles up to the top of the heap will have global next KV in 344 * this scanner heap if (1) it has done a real seek and (2) its KV is the top 345 * among all top KVs (some of which are fake) in the scanner heap. 346 */ 347 private KeyValueScanner pollRealKV() throws IOException { 348 KeyValueScanner kvScanner = heap.poll(); 349 if (kvScanner == null) { 350 return null; 351 } 352 353 while (kvScanner != null && !kvScanner.realSeekDone()) { 354 if (kvScanner.peek() != null) { 355 kvScanner.enforceSeek(); 356 KeyValue curKV = kvScanner.peek(); 357 if (curKV != null) { 358 KeyValueScanner nextEarliestScanner = heap.peek(); 359 if (nextEarliestScanner == null) { 360 // The heap is empty. Return the only possible scanner. 361 return kvScanner; 362 } 363 364 // Compare the current scanner to the next scanner. We try to avoid 365 // putting the current one back into the heap if possible. 366 KeyValue nextKV = nextEarliestScanner.peek(); 367 if (nextKV == null || comparator.compare(curKV, nextKV) < 0) { 368 // We already have the scanner with the earliest KV, so return it. 369 return kvScanner; 370 } 371 372 // Otherwise, put the scanner back into the heap and let it compete 373 // against all other scanners (both those that have done a "real 374 // seek" and a "lazy seek"). 375 heap.add(kvScanner); 376 } else { 377 // Close the scanner because we did a real seek and found out there 378 // are no more KVs. 379 kvScanner.close(); 380 } 381 } else { 382 // Close the scanner because it has already run out of KVs even before 383 // we had to do a real seek on it. 384 kvScanner.close(); 385 } 386 kvScanner = heap.poll(); 387 } 388 389 return kvScanner; 390 } 391 392 /** 393 * @return the current Heap 394 */ 395 public PriorityQueue<KeyValueScanner> getHeap() { 396 return this.heap; 397 } 398 399 @Override 400 public long getSequenceID() { 401 return 0; 402 } 403 404 KeyValueScanner getCurrentForTesting() { 405 return current; 406 } 407 }