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/**
*
* 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.Comparator;
import java.util.List;
import java.util.PriorityQueue;
import org.apache.hadoop.hbase.Cell;
import org.apache.hadoop.hbase.KeyValue.KVComparator;
import org.apache.hadoop.hbase.classification.InterfaceAudience;
import org.apache.hadoop.hbase.regionserver.ScannerContext.NextState;
/**
* Implements a heap merge across any number of KeyValueScanners.
*
* Implements KeyValueScanner itself.
*
* This class is used at the Region level to merge across Stores
* and at the Store level to merge across the memstore and StoreFiles.
*
* 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 {
protected PriorityQueue heap = 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 scanners,
KVComparator comparator) throws IOException {
this(scanners, new KVScannerComparator(comparator));
}
/**
* Constructor.
* @param scanners
* @param comparator
* @throws IOException
*/
KeyValueHeap(List scanners,
KVScannerComparator comparator) throws IOException {
this.comparator = comparator;
if (!scanners.isEmpty()) {
this.heap = new PriorityQueue(scanners.size(),
this.comparator);
for (KeyValueScanner scanner : scanners) {
if (scanner.peek() != null) {
this.heap.add(scanner);
} else {
scanner.close();
}
}
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.current.close();
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.
*
* This method takes care of updating the heap.
*
* 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 result) throws IOException {
return next(result, NoLimitScannerContext.getInstance());
}
@Override
public boolean next(List 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) {
this.current.close();
} 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 {
protected KVComparator kvComparator;
/**
* Constructor
* @param kvComparator
*/
public KVScannerComparator(KVComparator 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 the key which came latest.
long leftSequenceID = left.getSequenceID();
long rightSequenceID = right.getSequenceID();
if (leftSequenceID > rightSequenceID) {
return -1;
} else if (leftSequenceID < rightSequenceID) {
return 1;
} else {
return 0;
}
}
}
/**
* 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 KVComparator getComparator() {
return this.kvComparator;
}
}
public void close() {
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.
*
* As individual scanners may run past their ends, those scanners are
* automatically closed and removed from the heap.
*
* 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;
}
heap.add(current);
current = null;
KeyValueScanner scanner;
while ((scanner = heap.poll()) != 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);
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) {
scanner.close();
} else {
heap.add(scanner);
}
}
// 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) {
kvScanner.close();
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.
kvScanner.close();
}
} else {
// Close the scanner because it has already run out of KVs even before
// we had to do a real seek on it.
kvScanner.close();
}
kvScanner = heap.poll();
}
return kvScanner;
}
/**
* @return the current Heap
*/
public PriorityQueue getHeap() {
return this.heap;
}
@Override
public long getSequenceID() {
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();
}
}
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