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/*
* Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
* or more contributor license agreements. Licensed under the Elastic License
* 2.0 and the Server Side Public License, v 1; you may not use this file except
* in compliance with, at your election, the Elastic License 2.0 or the Server
* Side Public License, v 1.
*/
package org.elasticsearch.index.seqno;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.atomic.AtomicLong;
/**
* This class generates sequences numbers and keeps track of the so-called "local checkpoint" which is the highest number for which all
* previous sequence numbers have been processed (inclusive).
*/
public class LocalCheckpointTracker {
/**
* We keep a bit for each sequence number that is still pending. To optimize allocation, we do so in multiple sets allocating them on
* demand and cleaning up while completed. This constant controls the size of the sets.
*/
static final short BIT_SET_SIZE = 1024;
/**
* A collection of bit sets representing processed sequence numbers. Each sequence number is mapped to a bit set by dividing by the
* bit set size.
*/
final Map processedSeqNo = new HashMap<>();
/**
* A collection of bit sets representing durably persisted sequence numbers. Each sequence number is mapped to a bit set by dividing by
* the bit set size.
*/
final Map persistedSeqNo = new HashMap<>();
/**
* The current local checkpoint, i.e., all sequence numbers no more than this number have been processed.
*/
final AtomicLong processedCheckpoint = new AtomicLong();
/**
* The current persisted local checkpoint, i.e., all sequence numbers no more than this number have been durably persisted.
*/
final AtomicLong persistedCheckpoint = new AtomicLong();
/**
* The next available sequence number.
*/
final AtomicLong nextSeqNo = new AtomicLong();
/**
* Initialize the local checkpoint service. The {@code maxSeqNo} should be set to the last sequence number assigned, or
* {@link SequenceNumbers#NO_OPS_PERFORMED} and {@code localCheckpoint} should be set to the last known local checkpoint,
* or {@link SequenceNumbers#NO_OPS_PERFORMED}.
*
* @param maxSeqNo the last sequence number assigned, or {@link SequenceNumbers#NO_OPS_PERFORMED}
* @param localCheckpoint the last known local checkpoint, or {@link SequenceNumbers#NO_OPS_PERFORMED}
*/
public LocalCheckpointTracker(final long maxSeqNo, final long localCheckpoint) {
if (localCheckpoint < 0 && localCheckpoint != SequenceNumbers.NO_OPS_PERFORMED) {
throw new IllegalArgumentException(
"local checkpoint must be non-negative or [" + SequenceNumbers.NO_OPS_PERFORMED + "] " + "but was [" + localCheckpoint + "]"
);
}
if (maxSeqNo < 0 && maxSeqNo != SequenceNumbers.NO_OPS_PERFORMED) {
throw new IllegalArgumentException(
"max seq. no. must be non-negative or [" + SequenceNumbers.NO_OPS_PERFORMED + "] but was [" + maxSeqNo + "]"
);
}
nextSeqNo.set(maxSeqNo + 1);
processedCheckpoint.set(localCheckpoint);
persistedCheckpoint.set(localCheckpoint);
}
/**
* Issue the next sequence number.
*
* @return the next assigned sequence number
*/
public long generateSeqNo() {
return nextSeqNo.getAndIncrement();
}
/**
* Marks the provided sequence number as seen and updates the max_seq_no if needed.
*/
public void advanceMaxSeqNo(final long seqNo) {
nextSeqNo.accumulateAndGet(seqNo + 1, Math::max);
}
/**
* Marks the provided sequence number as processed and updates the processed checkpoint if possible.
*
* @param seqNo the sequence number to mark as processed
*/
public synchronized void markSeqNoAsProcessed(final long seqNo) {
markSeqNo(seqNo, processedCheckpoint, processedSeqNo);
}
/**
* Marks the provided sequence number as persisted and updates the checkpoint if possible.
*
* @param seqNo the sequence number to mark as persisted
*/
public synchronized void markSeqNoAsPersisted(final long seqNo) {
markSeqNo(seqNo, persistedCheckpoint, persistedSeqNo);
}
private void markSeqNo(final long seqNo, final AtomicLong checkPoint, final Map bitSetMap) {
assert Thread.holdsLock(this);
// make sure we track highest seen sequence number
advanceMaxSeqNo(seqNo);
if (seqNo <= checkPoint.get()) {
// this is possible during recovery where we might replay an operation that was also replicated
return;
}
final CountedBitSet bitSet = getBitSetForSeqNo(bitSetMap, seqNo);
final int offset = seqNoToBitSetOffset(seqNo);
bitSet.set(offset);
if (seqNo == checkPoint.get() + 1) {
updateCheckpoint(checkPoint, bitSetMap);
}
}
/**
* The current checkpoint which can be advanced by {@link #markSeqNoAsProcessed(long)}.
*
* @return the current checkpoint
*/
public long getProcessedCheckpoint() {
return processedCheckpoint.get();
}
/**
* The current persisted checkpoint which can be advanced by {@link #markSeqNoAsPersisted(long)}.
*
* @return the current persisted checkpoint
*/
public long getPersistedCheckpoint() {
return persistedCheckpoint.get();
}
/**
* The maximum sequence number issued so far.
*
* @return the maximum sequence number
*/
public long getMaxSeqNo() {
return nextSeqNo.get() - 1;
}
/**
* constructs a {@link SeqNoStats} object, using local state and the supplied global checkpoint
*
* This is needed to make sure the persisted local checkpoint and max seq no are consistent
*/
public synchronized SeqNoStats getStats(final long globalCheckpoint) {
return new SeqNoStats(getMaxSeqNo(), getPersistedCheckpoint(), globalCheckpoint);
}
/**
* Checks if the given sequence number was marked as processed in this tracker.
*/
public boolean hasProcessed(final long seqNo) {
assert seqNo >= 0 : "invalid seq_no=" + seqNo;
if (seqNo >= nextSeqNo.get()) {
return false;
}
if (seqNo <= processedCheckpoint.get()) {
return true;
}
final long bitSetKey = getBitSetKey(seqNo);
final int bitSetOffset = seqNoToBitSetOffset(seqNo);
synchronized (this) {
// check again under lock
if (seqNo <= processedCheckpoint.get()) {
return true;
}
final CountedBitSet bitSet = processedSeqNo.get(bitSetKey);
return bitSet != null && bitSet.get(bitSetOffset);
}
}
/**
* Moves the checkpoint to the last consecutively processed sequence number. This method assumes that the sequence number
* following the current checkpoint is processed.
*/
private void updateCheckpoint(AtomicLong checkPoint, Map bitSetMap) {
assert Thread.holdsLock(this);
assert getBitSetForSeqNo(bitSetMap, checkPoint.get() + 1).get(seqNoToBitSetOffset(checkPoint.get() + 1))
: "updateCheckpoint is called but the bit following the checkpoint is not set";
// keep it simple for now, get the checkpoint one by one; in the future we can optimize and read words
long bitSetKey = getBitSetKey(checkPoint.get());
CountedBitSet current = bitSetMap.get(bitSetKey);
if (current == null) {
// the bit set corresponding to the checkpoint has already been removed, set ourselves up for the next bit set
assert checkPoint.get() % BIT_SET_SIZE == BIT_SET_SIZE - 1;
current = bitSetMap.get(++bitSetKey);
}
do {
checkPoint.incrementAndGet();
/*
* The checkpoint always falls in the current bit set or we have already cleaned it; if it falls on the last bit of the
* current bit set, we can clean it.
*/
if (checkPoint.get() == lastSeqNoInBitSet(bitSetKey)) {
assert current != null;
final CountedBitSet removed = bitSetMap.remove(bitSetKey);
assert removed == current;
current = bitSetMap.get(++bitSetKey);
}
} while (current != null && current.get(seqNoToBitSetOffset(checkPoint.get() + 1)));
}
private static long lastSeqNoInBitSet(final long bitSetKey) {
return (1 + bitSetKey) * BIT_SET_SIZE - 1;
}
/**
* Return the bit set for the provided sequence number, possibly allocating a new set if needed.
*
* @param seqNo the sequence number to obtain the bit set for
* @return the bit set corresponding to the provided sequence number
*/
private static long getBitSetKey(final long seqNo) {
return seqNo / BIT_SET_SIZE;
}
private CountedBitSet getBitSetForSeqNo(final Map bitSetMap, final long seqNo) {
assert Thread.holdsLock(this);
final long bitSetKey = getBitSetKey(seqNo);
return bitSetMap.computeIfAbsent(bitSetKey, k -> new CountedBitSet(BIT_SET_SIZE));
}
/**
* Obtain the position in the bit set corresponding to the provided sequence number. The bit set corresponding to the sequence number
* can be obtained via {@link #getBitSetForSeqNo(Map, long)}.
*
* @param seqNo the sequence number to obtain the position for
* @return the position in the bit set corresponding to the provided sequence number
*/
private static int seqNoToBitSetOffset(final long seqNo) {
return Math.toIntExact(seqNo % BIT_SET_SIZE);
}
}