Java tutorial
/** * * Licensed to the Apache Software Foundation (ASF) under one * or more contributor license agreements. See the NOTICE file * distributed with this work for additional information * regarding copyright ownership. The ASF licenses this file * to you under the Apache License, Version 2.0 (the * "License"); you may not use this file except in compliance * with the License. You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ package org.apache.hadoop.hbase.regionserver; import java.io.IOException; import java.util.ArrayList; import java.util.Comparator; import java.util.List; import java.util.PriorityQueue; import org.apache.commons.logging.Log; import org.apache.commons.logging.LogFactory; import org.apache.hadoop.hbase.Cell; import org.apache.hadoop.hbase.CellComparator; import org.apache.hadoop.hbase.classification.InterfaceAudience; import org.apache.hadoop.hbase.regionserver.ScannerContext.NextState; /** * Implements a heap merge across any number of KeyValueScanners. * <p> * Implements KeyValueScanner itself. * <p> * This class is used at the Region level to merge across Stores * and at the Store level to merge across the memstore and StoreFiles. * <p> * In the Region case, we also need InternalScanner.next(List), so this class * also implements InternalScanner. WARNING: As is, if you try to use this * as an InternalScanner at the Store level, you will get runtime exceptions. */ @InterfaceAudience.Private public class KeyValueHeap extends NonReversedNonLazyKeyValueScanner implements KeyValueScanner, InternalScanner { private static final Log LOG = LogFactory.getLog(KeyValueHeap.class); protected PriorityQueue<KeyValueScanner> heap = null; // Holds the scanners when a ever a eager close() happens. All such eagerly closed // scans are collected and when the final scanner.close() happens will perform the // actual close. protected List<KeyValueScanner> scannersForDelayedClose = null; /** * The current sub-scanner, i.e. the one that contains the next key/value * to return to the client. This scanner is NOT included in {@link #heap} * (but we frequently add it back to the heap and pull the new winner out). * We maintain an invariant that the current sub-scanner has already done * a real seek, and that current.peek() is always a real key/value (or null) * except for the fake last-key-on-row-column supplied by the multi-column * Bloom filter optimization, which is OK to propagate to StoreScanner. In * order to ensure that, always use {@link #pollRealKV()} to update current. */ protected KeyValueScanner current = null; protected KVScannerComparator comparator; /** * Constructor. This KeyValueHeap will handle closing of passed in * KeyValueScanners. * @param scanners * @param comparator */ public KeyValueHeap(List<? extends KeyValueScanner> scanners, CellComparator comparator) throws IOException { this(scanners, new KVScannerComparator(comparator)); } /** * Constructor. * @param scanners * @param comparator * @throws IOException */ KeyValueHeap(List<? extends KeyValueScanner> scanners, KVScannerComparator comparator) throws IOException { this.comparator = comparator; this.scannersForDelayedClose = new ArrayList<KeyValueScanner>(scanners.size()); if (!scanners.isEmpty()) { this.heap = new PriorityQueue<KeyValueScanner>(scanners.size(), this.comparator); for (KeyValueScanner scanner : scanners) { if (scanner.peek() != null) { this.heap.add(scanner); } else { this.scannersForDelayedClose.add(scanner); } } this.current = pollRealKV(); } } public Cell peek() { if (this.current == null) { return null; } return this.current.peek(); } public Cell next() throws IOException { if (this.current == null) { return null; } Cell kvReturn = this.current.next(); Cell kvNext = this.current.peek(); if (kvNext == null) { this.scannersForDelayedClose.add(this.current); this.current = null; this.current = pollRealKV(); } else { KeyValueScanner topScanner = this.heap.peek(); // no need to add current back to the heap if it is the only scanner left if (topScanner != null && this.comparator.compare(kvNext, topScanner.peek()) >= 0) { this.heap.add(this.current); this.current = null; this.current = pollRealKV(); } } return kvReturn; } /** * Gets the next row of keys from the top-most scanner. * <p> * This method takes care of updating the heap. * <p> * This can ONLY be called when you are using Scanners that implement InternalScanner as well as * KeyValueScanner (a {@link StoreScanner}). * @param result * @return true if more rows exist after this one, false if scanner is done */ @Override public boolean next(List<Cell> result) throws IOException { return next(result, NoLimitScannerContext.getInstance()); } @Override public boolean next(List<Cell> result, ScannerContext scannerContext) throws IOException { if (this.current == null) { return scannerContext.setScannerState(NextState.NO_MORE_VALUES).hasMoreValues(); } InternalScanner currentAsInternal = (InternalScanner) this.current; boolean moreCells = currentAsInternal.next(result, scannerContext); Cell pee = this.current.peek(); /* * By definition, any InternalScanner must return false only when it has no * further rows to be fetched. So, we can close a scanner if it returns * false. All existing implementations seem to be fine with this. It is much * more efficient to close scanners which are not needed than keep them in * the heap. This is also required for certain optimizations. */ if (pee == null || !moreCells) { // add the scanner that is to be closed this.scannersForDelayedClose.add(this.current); } else { this.heap.add(this.current); } this.current = null; this.current = pollRealKV(); if (this.current == null) { moreCells = scannerContext.setScannerState(NextState.NO_MORE_VALUES).hasMoreValues(); } return moreCells; } protected static class KVScannerComparator implements Comparator<KeyValueScanner> { protected CellComparator kvComparator; /** * Constructor * @param kvComparator */ public KVScannerComparator(CellComparator kvComparator) { this.kvComparator = kvComparator; } public int compare(KeyValueScanner left, KeyValueScanner right) { int comparison = compare(left.peek(), right.peek()); if (comparison != 0) { return comparison; } else { // Since both the keys are exactly the same, we break the tie in favor of higher ordered // scanner since it'll have newer data. Since higher value should come first, we reverse // sort here. return Long.compare(right.getScannerOrder(), left.getScannerOrder()); } } /** * Compares two KeyValue * @param left * @param right * @return less than 0 if left is smaller, 0 if equal etc.. */ public int compare(Cell left, Cell right) { return this.kvComparator.compare(left, right); } /** * @return KVComparator */ public CellComparator getComparator() { return this.kvComparator; } } public void close() { for (KeyValueScanner scanner : this.scannersForDelayedClose) { scanner.close(); } this.scannersForDelayedClose.clear(); if (this.current != null) { this.current.close(); } if (this.heap != null) { KeyValueScanner scanner; while ((scanner = this.heap.poll()) != null) { scanner.close(); } } } /** * Seeks all scanners at or below the specified seek key. If we earlied-out * of a row, we may end up skipping values that were never reached yet. * Rather than iterating down, we want to give the opportunity to re-seek. * <p> * As individual scanners may run past their ends, those scanners are * automatically closed and removed from the heap. * <p> * This function (and {@link #reseek(Cell)}) does not do multi-column * Bloom filter and lazy-seek optimizations. To enable those, call * {@link #requestSeek(Cell, boolean, boolean)}. * @param seekKey KeyValue to seek at or after * @return true if KeyValues exist at or after specified key, false if not * @throws IOException */ @Override public boolean seek(Cell seekKey) throws IOException { return generalizedSeek(false, // This is not a lazy seek seekKey, false, // forward (false: this is not a reseek) false); // Not using Bloom filters } /** * This function is identical to the {@link #seek(Cell)} function except * that scanner.seek(seekKey) is changed to scanner.reseek(seekKey). */ @Override public boolean reseek(Cell seekKey) throws IOException { return generalizedSeek(false, // This is not a lazy seek seekKey, true, // forward (true because this is reseek) false); // Not using Bloom filters } /** * {@inheritDoc} */ @Override public boolean requestSeek(Cell key, boolean forward, boolean useBloom) throws IOException { return generalizedSeek(true, key, forward, useBloom); } /** * @param isLazy whether we are trying to seek to exactly the given row/col. * Enables Bloom filter and most-recent-file-first optimizations for * multi-column get/scan queries. * @param seekKey key to seek to * @param forward whether to seek forward (also known as reseek) * @param useBloom whether to optimize seeks using Bloom filters */ private boolean generalizedSeek(boolean isLazy, Cell seekKey, boolean forward, boolean useBloom) throws IOException { if (!isLazy && useBloom) { throw new IllegalArgumentException("Multi-column Bloom filter " + "optimization requires a lazy seek"); } if (current == null) { return false; } KeyValueScanner scanner = current; try { while (scanner != null) { Cell topKey = scanner.peek(); if (comparator.getComparator().compare(seekKey, topKey) <= 0) { // Top KeyValue is at-or-after Seek KeyValue. We only know that all // scanners are at or after seekKey (because fake keys of // scanners where a lazy-seek operation has been done are not greater // than their real next keys) but we still need to enforce our // invariant that the top scanner has done a real seek. This way // StoreScanner and RegionScanner do not have to worry about fake // keys. heap.add(scanner); scanner = null; current = pollRealKV(); return current != null; } boolean seekResult; if (isLazy && heap.size() > 0) { // If there is only one scanner left, we don't do lazy seek. seekResult = scanner.requestSeek(seekKey, forward, useBloom); } else { seekResult = NonLazyKeyValueScanner.doRealSeek(scanner, seekKey, forward); } if (!seekResult) { this.scannersForDelayedClose.add(scanner); } else { heap.add(scanner); } scanner = heap.poll(); if (scanner == null) { current = null; } } } catch (Exception e) { if (scanner != null) { try { scanner.close(); } catch (Exception ce) { LOG.warn("close KeyValueScanner error", ce); } } throw e; } // Heap is returning empty, scanner is done return false; } /** * Fetches the top sub-scanner from the priority queue, ensuring that a real * seek has been done on it. Works by fetching the top sub-scanner, and if it * has not done a real seek, making it do so (which will modify its top KV), * putting it back, and repeating this until success. Relies on the fact that * on a lazy seek we set the current key of a StoreFileScanner to a KV that * is not greater than the real next KV to be read from that file, so the * scanner that bubbles up to the top of the heap will have global next KV in * this scanner heap if (1) it has done a real seek and (2) its KV is the top * among all top KVs (some of which are fake) in the scanner heap. */ protected KeyValueScanner pollRealKV() throws IOException { KeyValueScanner kvScanner = heap.poll(); if (kvScanner == null) { return null; } while (kvScanner != null && !kvScanner.realSeekDone()) { if (kvScanner.peek() != null) { try { kvScanner.enforceSeek(); } catch (IOException ioe) { // Add the item to delayed close set in case it is leak from close this.scannersForDelayedClose.add(kvScanner); throw ioe; } Cell curKV = kvScanner.peek(); if (curKV != null) { KeyValueScanner nextEarliestScanner = heap.peek(); if (nextEarliestScanner == null) { // The heap is empty. Return the only possible scanner. return kvScanner; } // Compare the current scanner to the next scanner. We try to avoid // putting the current one back into the heap if possible. Cell nextKV = nextEarliestScanner.peek(); if (nextKV == null || comparator.compare(curKV, nextKV) < 0) { // We already have the scanner with the earliest KV, so return it. return kvScanner; } // Otherwise, put the scanner back into the heap and let it compete // against all other scanners (both those that have done a "real // seek" and a "lazy seek"). heap.add(kvScanner); } else { // Close the scanner because we did a real seek and found out there // are no more KVs. this.scannersForDelayedClose.add(kvScanner); } } else { // Close the scanner because it has already run out of KVs even before // we had to do a real seek on it. this.scannersForDelayedClose.add(kvScanner); } kvScanner = heap.poll(); } return kvScanner; } /** * @return the current Heap */ public PriorityQueue<KeyValueScanner> getHeap() { return this.heap; } /** * @see KeyValueScanner#getScannerOrder() */ @Override public long getScannerOrder() { return 0; } KeyValueScanner getCurrentForTesting() { return current; } @Override public Cell getNextIndexedKey() { // here we return the next index key from the top scanner return current == null ? null : current.getNextIndexedKey(); } @Override public void shipped() throws IOException { for (KeyValueScanner scanner : this.scannersForDelayedClose) { scanner.close(); // There wont be further fetch of Cells from these scanners. Just close. } this.scannersForDelayedClose.clear(); if (this.current != null) { this.current.shipped(); } if (this.heap != null) { for (KeyValueScanner scanner : this.heap) { scanner.shipped(); } } } }