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Server functionality for HBase
/*
* 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.Iterator;
import java.util.SortedSet;
import org.apache.commons.lang3.NotImplementedException;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.hbase.Cell;
import org.apache.hadoop.hbase.PrivateCellUtil;
import org.apache.hadoop.hbase.client.Scan;
import org.apache.yetus.audience.InterfaceAudience;
/**
* A scanner of a single memstore segment.
*/
@InterfaceAudience.Private
public class SegmentScanner implements KeyValueScanner {
// the observed structure
protected final Segment segment;
// the highest relevant MVCC
private long readPoint;
// the current iterator that can be reinitialized by
// seek(), backwardSeek(), or reseek()
protected Iterator iter;
// the pre-calculated cell to be returned by peek()
protected Cell current = null;
// or next()
// A flag represents whether could stop skipping KeyValues for MVCC
// if have encountered the next row. Only used for reversed scan
private boolean stopSkippingKVsIfNextRow = false;
// Stop skipping KeyValues for MVCC if finish this row. Only used for reversed scan
private Cell stopSkippingKVsRow;
// last iterated KVs by seek (to restore the iterator state after reseek)
private Cell last = null;
// flag to indicate if this scanner is closed
protected boolean closed = false;
/**
* Scanners are ordered from 0 (oldest) to newest in increasing order.
*/
protected SegmentScanner(Segment segment, long readPoint) {
this.segment = segment;
this.readPoint = readPoint;
// increase the reference count so the underlying structure will not be de-allocated
this.segment.incScannerCount();
iter = segment.iterator();
// the initialization of the current is required for working with heap of SegmentScanners
updateCurrent();
if (current == null) {
// nothing to fetch from this scanner
close();
}
}
/**
* Look at the next Cell in this scanner, but do not iterate the scanner
* @return the currently observed Cell
*/
@Override
public Cell peek() { // sanity check, the current should be always valid
if (closed) {
return null;
}
if (current != null && current.getSequenceId() > readPoint) {
throw new RuntimeException("current is invalid: read point is " + readPoint + ", "
+ "while current sequence id is " + current.getSequenceId());
}
return current;
}
/**
* Return the next Cell in this scanner, iterating the scanner
* @return the next Cell or null if end of scanner
*/
@Override
public Cell next() throws IOException {
if (closed) {
return null;
}
Cell oldCurrent = current;
updateCurrent(); // update the currently observed Cell
return oldCurrent;
}
/**
* Seek the scanner at or after the specified Cell.
* @param cell seek value
* @return true if scanner has values left, false if end of scanner
*/
@Override
public boolean seek(Cell cell) throws IOException {
if (closed) {
return false;
}
if (cell == null) {
close();
return false;
}
// restart the iterator from new key
iter = getIterator(cell);
// last is going to be reinitialized in the next getNext() call
last = null;
updateCurrent();
return (current != null);
}
protected Iterator getIterator(Cell cell) {
return segment.tailSet(cell).iterator();
}
/**
* Reseek the scanner at or after the specified KeyValue. This method is guaranteed to seek at or
* after the required key only if the key comes after the current position of the scanner. Should
* not be used to seek to a key which may come before the current position.
* @param cell seek value (should be non-null)
* @return true if scanner has values left, false if end of scanner
*/
@Override
public boolean reseek(Cell cell) throws IOException {
if (closed) {
return false;
}
/*
* See HBASE-4195 & HBASE-3855 & HBASE-6591 for the background on this implementation. This code
* is executed concurrently with flush and puts, without locks. The ideal implementation for
* performance would use the sub skip list implicitly pointed by the iterator. Unfortunately the
* Java API does not offer a method to get it. So we remember the last keys we iterated to and
* restore the reseeked set to at least that point.
*/
iter = getIterator(getHighest(cell, last));
updateCurrent();
return (current != null);
}
/**
* Seek the scanner at or before the row of specified Cell, it firstly tries to seek the scanner
* at or after the specified Cell, return if peek KeyValue of scanner has the same row with
* specified Cell, otherwise seek the scanner at the first Cell of the row which is the previous
* row of specified KeyValue
* @param key seek Cell
* @return true if the scanner is at the valid KeyValue, false if such Cell does not exist
*/
@Override
public boolean backwardSeek(Cell key) throws IOException {
if (closed) {
return false;
}
seek(key); // seek forward then go backward
if (peek() == null || segment.compareRows(peek(), key) > 0) {
return seekToPreviousRow(key);
}
return true;
}
/**
* Seek the scanner at the first Cell of the row which is the previous row of specified key
* @param cell seek value
* @return true if the scanner at the first valid Cell of previous row, false if not existing such
* Cell
*/
@Override
public boolean seekToPreviousRow(Cell cell) throws IOException {
if (closed) {
return false;
}
boolean keepSeeking;
Cell key = cell;
do {
Cell firstKeyOnRow = PrivateCellUtil.createFirstOnRow(key);
SortedSet| cellHead = segment.headSet(firstKeyOnRow);
Cell lastCellBeforeRow = cellHead.isEmpty() ? null : cellHead.last();
if (lastCellBeforeRow == null) {
current = null;
return false;
}
Cell firstKeyOnPreviousRow = PrivateCellUtil.createFirstOnRow(lastCellBeforeRow);
this.stopSkippingKVsIfNextRow = true;
this.stopSkippingKVsRow = firstKeyOnPreviousRow;
seek(firstKeyOnPreviousRow);
this.stopSkippingKVsIfNextRow = false;
if (
peek() == null || segment.getComparator().compareRows(peek(), firstKeyOnPreviousRow) > 0
) {
keepSeeking = true;
key = firstKeyOnPreviousRow;
continue;
} else {
keepSeeking = false;
}
} while (keepSeeking);
return true;
}
/**
* Seek the scanner at the first KeyValue of last row
* @return true if scanner has values left, false if the underlying data is empty
*/
@Override
public boolean seekToLastRow() throws IOException {
if (closed) {
return false;
}
Cell higherCell = segment.isEmpty() ? null : segment.last();
if (higherCell == null) {
return false;
}
Cell firstCellOnLastRow = PrivateCellUtil.createFirstOnRow(higherCell);
if (seek(firstCellOnLastRow)) {
return true;
} else {
return seekToPreviousRow(higherCell);
}
}
/**
* Close the KeyValue scanner.
*/
@Override
public void close() {
if (closed) {
return;
}
getSegment().decScannerCount();
closed = true;
}
/**
* This functionality should be resolved in the higher level which is MemStoreScanner, currently
* returns true as default. Doesn't throw IllegalStateException in order not to change the
* signature of the overridden method
*/
@Override
public boolean shouldUseScanner(Scan scan, HStore store, long oldestUnexpiredTS) {
return getSegment().shouldSeek(scan.getColumnFamilyTimeRange().getOrDefault(
store.getColumnFamilyDescriptor().getName(), scan.getTimeRange()), oldestUnexpiredTS);
}
@Override
public boolean requestSeek(Cell c, boolean forward, boolean useBloom) throws IOException {
return NonLazyKeyValueScanner.doRealSeek(this, c, forward);
}
/**
* This scanner is working solely on the in-memory MemStore and doesn't work on store files,
* MutableCellSetSegmentScanner always does the seek, therefore always returning true.
*/
@Override
public boolean realSeekDone() {
return true;
}
/**
* This function should be never called on scanners that always do real seek operations (i.e. most
* of the scanners and also this one). The easiest way to achieve this is to call
* {@link #realSeekDone()} first.
*/
@Override
public void enforceSeek() throws IOException {
throw new NotImplementedException("enforceSeek cannot be called on a SegmentScanner");
}
/** Returns true if this is a file scanner. Otherwise a memory scanner is assumed. */
@Override
public boolean isFileScanner() {
return false;
}
@Override
public Path getFilePath() {
return null;
}
/**
* @return the next key in the index (the key to seek to the next block) if known, or null
* otherwise Not relevant for in-memory scanner
*/
@Override
public Cell getNextIndexedKey() {
return null;
}
/**
* Called after a batch of rows scanned (RPC) and set to be returned to client. Any in between
* cleanup can be done here. Nothing to be done for MutableCellSetSegmentScanner.
*/
@Override
public void shipped() throws IOException {
// do nothing
}
// debug method
@Override
public String toString() {
String res = "Store segment scanner of type " + this.getClass().getName() + "; ";
res += "Scanner order " + getScannerOrder() + "; ";
res += getSegment().toString();
return res;
}
/********************* Private Methods **********************/
private Segment getSegment() {
return segment;
}
/**
* Private internal method for iterating over the segment, skipping the cells with irrelevant MVCC
*/
protected void updateCurrent() {
Cell next = null;
try {
while (iter.hasNext()) {
next = iter.next();
if (next.getSequenceId() <= this.readPoint) {
current = next;
return;// skip irrelevant versions
}
// for backwardSeek() stay in the boundaries of a single row
if (stopSkippingKVsIfNextRow && segment.compareRows(next, stopSkippingKVsRow) > 0) {
current = null;
return;
}
} // end of while
current = null; // nothing found
} finally {
if (next != null) {
// in all cases, remember the last KV we iterated to, needed for reseek()
last = next;
}
}
}
/**
* Private internal method that returns the higher of the two key values, or null if they are both
* null
*/
private Cell getHighest(Cell first, Cell second) {
if (first == null && second == null) {
return null;
}
if (first != null && second != null) {
int compare = segment.compare(first, second);
return (compare > 0 ? first : second);
}
return (first != null ? first : second);
}
}
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