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Java B+Tree Key-Value Store Library
/**
* Copyright © 2011-2012 Akiban Technologies, Inc. All rights reserved.
*
* This program and the accompanying materials are made available
* under the terms of the Eclipse Public License v1.0 which
* accompanies this distribution, and is available at
* http://www.eclipse.org/legal/epl-v10.html
*
* This program may also be available under different license terms.
* For more information, see www.akiban.com or contact [email protected].
*
* Contributors:
* Akiban Technologies, Inc.
*/
package com.persistit;
import static com.persistit.TransactionStatus.ABORTED;
import static com.persistit.TransactionStatus.UNCOMMITTED;
import java.util.concurrent.locks.ReentrantLock;
import com.persistit.Accumulator.Delta;
import com.persistit.exception.RetryException;
/**
*
* Representation of one {@link TransactionIndex} hash table bucket. This class
* contains much of the important logic for managing active and recently
* committed or aborted transactions. It contains four singly-linked lists of
* {@link TransactionStatus} objects:
*
* - current
* - List of transactions having start timestamps greater than or equal to the
* floor (defined below).
TransactionStatus objects leave
* this list when the floor is raised. This is done by the {@link #reduce()}
* method.
* - aborted
* - List of aborted transactions having start times less than the floor.
*
TransactionStatus objects leave this list once their MvvCount
* becomes zero, meaning that no MVV instances in the database containing
* updates from that transaction still exist.
* - longRunning
* - List of active transactions having start times less than the floor which
* have neither committed nor aborted. The {@link #reduce()} method moves long
* running transactions to this list so that it can increase the floor.
* - free
* - List of currently unused
TransactionStatus instances that
* can be allocated and reused for new transactions.
*
*
*
* The floor value is a timestamp that divides transactions on the
* current list from those moved to the longRunning, aborted or free list. The
* status of any transaction started after the current value of floor is denoted
* by a TransactionStatus object on the current list. For any
* transaction started before the current floor, the transaction is known to
* have committed unless a TransactionStatus object is found on
* either the aborted or longRunning lists. These are intended as exceptional
* cases, and their lists are expected to remain relatively short. The
* TransactionStatus for a committed transaction (the usual case)
* is simply moved to the free list once the floor is raised.
*
*
* @author peter
*
*/
class TransactionIndexBucket {
/**
* The owner of the hash table that contains this bucket
*/
final TransactionIndex _transactionIndex;
/**
* Index of this bucket in the TransactionIndex hash table
*/
final int _hashIndex;
/**
* Floor timestamp. A transaction that started before this time is usually
* committed, in which case there no longer exists a TransactionStatus
* object for it. There are two other cases: the transaction is still
* running, in which its TransactionStatus has been moved to the
* long_running list, or it aborted, in which case its TransactionStatus has
* moved to the aborted list.
*/
volatile long _floor = Long.MAX_VALUE;
/**
* Singly-linked list of TransactionStatus instances for transactions
* currently running or recently either committed or aborted. A
* TransactionStatus enters this list when the corresponding transaction is
* registered. It leaves this list after the _floor value has become larger
* than its start timestamp, going to (a) the _free list (because the
* transaction committed), the _aborted list (because it aborted) or the
* _longRunning list because it is still running.
*/
volatile TransactionStatus _current;
/**
* Current member count
*/
int _currentCount;
/**
* Singly-linked list of TransactionStatus instances for aborted
* transactions. A TransactionStatus enters this list after the transaction
* has aborted; it leaves this list when all of the MVV versions created by
* the transaction have been pruned.
*/
volatile TransactionStatus _aborted;
/**
* Aborted member count
*/
int _abortedCount;
/**
* Singly-linked list of TransactionStatus instances for transactions that
* started before the floor value. A TransactionStatus enters this list when
* its transaction has been running for a "long" time, meaning that the
* current list has a large number of TransactionStatus instances that could
* be removed by increasing the floor value.
*/
volatile TransactionStatus _longRunning;
/**
* Long running member count
*/
int _longRunningCount;
/**
* Singly-linked list of TransactionStatus instances that no longer
* represent concurrent or recent transactions. These are recycled when a
* new transaction is registered.
*/
volatile TransactionStatus _free;
/**
* Free member count
*/
int _freeCount;
/**
* Number of allocated TransactionStatus instances that were released for
* garbage collection due to full free list.
*/
int _droppedCount;
/**
* Last floor value received for all active transactions. Obtained from the
* current ActiveTransactionCache.
*/
long _activeTransactionFloor;
/**
* Lock used to prevent multi-threaded access to the lists in this
* structure. Fair to prevent barging.
*/
ReentrantLock _lock = new ReentrantLock(true);
/**
* Singly-linked list of Delta objects available for reuse
*/
Delta _freeDeltaList;
/**
* Count of Delta instances on the free Delta list
*/
int _freeDeltaCount;
TransactionIndexBucket(final TransactionIndex transactionIndex, final int hashIndex) {
_transactionIndex = transactionIndex;
_hashIndex = hashIndex;
}
void lock() {
_lock.lock();
}
void unlock() {
_lock.unlock();
}
TransactionStatus allocateTransactionStatus() throws InterruptedException {
assert _lock.isHeldByCurrentThread();
final TransactionStatus status = _free;
if (status != null) {
if (status.isLocked()) {
status.briefLock();
}
assert !status.isLocked();
_free = status.getNext();
_freeCount--;
status.setNext(null);
return status;
} else {
return new TransactionStatus(this);
}
}
void addCurrent(final TransactionStatus status) {
assert _lock.isHeldByCurrentThread();
status.setNext(_current);
if (status.getTs() < _floor) {
_floor = status.getTs();
}
_current = status;
_currentCount++;
}
void addAborted(final TransactionStatus status) {
assert _lock.isHeldByCurrentThread();
status.setNext(_aborted);
_aborted = status;
_abortedCount++;
}
int getIndex() {
return _hashIndex;
}
TransactionStatus getCurrent() {
return _current;
}
TransactionStatus getAborted() {
return _aborted;
}
TransactionStatus getLongRunning() {
return _longRunning;
}
long getFloor() {
return _floor;
}
int getCurrentCount() {
return _currentCount;
}
int getLongRunningCount() {
return _longRunningCount;
}
int getAbortedCount() {
return _abortedCount;
}
int getFreeCount() {
return _freeCount;
}
int getDroppedCount() {
return _droppedCount;
}
TimestampAllocator getTimestampAllocator() {
return _transactionIndex.getTimestampAllocator();
}
boolean hasFloorMoved() {
return _activeTransactionFloor != _transactionIndex.getActiveTransactionFloor();
}
void notifyCompleted(final TransactionStatus status, final long timestamp) {
assert _lock.isHeldByCurrentThread();
final long ts = status.getTs();
if (ts >= getFloor()) {
for (TransactionStatus s = getCurrent(); s != null; s = s.getNext()) {
if (s == status) {
s.complete(timestamp);
status.wwUnlock();
if (s.getTs() == getFloor() || hasFloorMoved()) {
reduce();
}
return;
}
}
} else {
TransactionStatus previous = null;
for (TransactionStatus s = getLongRunning(); s != null; s = s.getNext()) {
if (s == status) {
final TransactionStatus next = s.getNext();
assert s.getTc() != UNCOMMITTED;
s.complete(timestamp);
status.wwUnlock();
boolean moved = false;
if (s.getTc() == ABORTED) {
aggregate(s, false);
s.setNext(_aborted);
_aborted = s;
_abortedCount++;
moved = true;
} else {
/*
* The activeTransactionFloor can only increase after
* the relevant transactions have been notified.
* Therefore any transaction now being notified must
* either be ABORTED or have a tc greater than or equal
* to the floor.
*/
assert s.getTc() >= _activeTransactionFloor;
}
if (moved) {
if (previous == null) {
_longRunning = next;
} else {
previous.setNext(next);
}
_longRunningCount--;
}
return;
}
previous = s;
}
}
throw new IllegalStateException("No such transaction " + status);
}
/**
*
* Reduce the size of the {@link #_current} list by moving the oldest
* TransactionStatus to the {@link #_aborted} list or the
* {@link #_longRunning} list, and then increasing the {@link #_floor} to
* reflect the start timestamp of the next oldest remaining transaction. If
* this method determines that the next oldest transaction is also aborted,
* it repeats the process and disposes of that transaction as well.
*
*
* The {@link #clean()} method moves TransactionStatus
* instances from the _aborted and _longRunning
* lists back to the {@link #_free} list.
*
*/
void reduce() {
assert _lock.isHeldByCurrentThread();
final boolean hasMoved = hasFloorMoved();
_activeTransactionFloor = _transactionIndex.getActiveTransactionFloor();
boolean more = true;
while (more) {
more = false;
TransactionStatus previous = null;
long newFloor = Long.MAX_VALUE;
for (TransactionStatus status = _current; status != null;) {
final TransactionStatus next = status.getNext();
assert status.getTs() >= _floor;
/*
* Is this TransactionStatus aborted and notified?
*/
final boolean aborted = isAborted(status);
final boolean committed = isCommitted(status);
if (status.getTs() == _floor) {
/*
* Essential invariant: the TransactionStatus is added to
* its new list before being removed from _current.
* Concurrently executing threads can read the _current,
* _aborted and _longRunning members outside of locks and
* therefore _aborted or _longRunning needs to be updated
* before _current becomes null.
*/
boolean moved = false;
/*
* Move the TransactionStatus somewhere if any of these
* conditions hold
*/
if (committed && isObsolete(status)) {
aggregate(status, true);
/*
* committed
*/
free(status);
moved = true;
} else if (aborted) {
aggregate(status, false);
/*
* aborted
*/
status.setNext(_aborted);
_aborted = status;
_abortedCount++;
moved = true;
} else if (_currentCount > _transactionIndex.getLongRunningThreshold()) {
/**
* long-running
*/
status.setNext(_longRunning);
_longRunning = status;
_longRunningCount++;
moved = true;
}
if (moved) {
/*
* Unlink the TransactionStatus from the current list.
*/
if (previous != null) {
previous.setNext(next);
} else {
_current = next;
}
_currentCount--;
status = next;
more = true;
} else {
newFloor = _floor;
previous = status;
}
} else {
if (status.getTs() < newFloor) {
newFloor = status.getTs();
}
previous = status;
}
status = next;
}
_floor = newFloor;
}
if (hasMoved) {
cleanup(_activeTransactionFloor);
}
}
/**
* Move TransactionStatus instances from the
* _aborted and _longRunning lists back to the
* {@link #_free} list when there are no longer any active transactions that
* need them. This method is called periodically whenever the
* {@link #reduce()} method detects that the ActiveTransactionCache has been
* updated. It is also called by
* TransactionIndex{@link #cleanup(long)} for unit tests.
*
* An aborted TransactionStatus can be freed once its MVV count
* becomes zero indicating that no versions written by that transaction
* remain in the database and it is no longer concurrent with any
* active transactions. This second condition is important because a
* concurrent transaction may be in the process of reading an MVV while this
* method is executing; the TransactionStatus must be retained
* until that transaction (and any other transactions that might also read
* the same MVV) has seen that the transaction aborted.
*
*
* A long-running TransactionStatus can be removed once it has
* committed or aborted and there exists no other possibly concurrent active
* transaction.
*
*
* @param activeTransactionFloor
* Lower bound of start timestamps of all currently executing
* transactions.
*/
void cleanup(final long activeTransactionFloor) {
assert _lock.isHeldByCurrentThread();
TransactionStatus previous;
/*
* Remove every aborted transaction whose mvvCount became zero before
* the start of any active transaction.
*/
previous = null;
for (TransactionStatus status = _aborted; status != null;) {
final TransactionStatus next = status.getNext();
assert status.getTc() == ABORTED;
if (status.getMvvCount() == 0 && status.getTa() < activeTransactionFloor && status.isNotified()) {
aggregate(status, false);
if (previous == null) {
_aborted = next;
} else {
previous.setNext(next);
}
_abortedCount--;
free(status);
} else {
previous = status;
}
status = next;
}
/*
* Remove every long-running transaction whose commit timestamp is no
* longer concurrent with any active transaction.
*/
previous = null;
for (TransactionStatus status = _longRunning; status != null;) {
final TransactionStatus next = status.getNext();
if (status.isNotified() && isCommitted(status) && isObsolete(status)) {
aggregate(status, true);
if (previous == null) {
_longRunning = next;
} else {
previous.setNext(next);
}
_longRunningCount--;
free(status);
} else {
previous = status;
}
status = next;
}
}
int resetMVVCounts(final long timestamp) {
int count = 0;
for (TransactionStatus status = _current; status != null;) {
if (status.getTc() == ABORTED && status.getTs() < timestamp && status.getMvvCount() > 0) {
status.setMvvCount(0);
count++;
}
status = status.getNext();
}
for (TransactionStatus status = _aborted; status != null;) {
assert status.getTc() == ABORTED;
if (status.getTs() < timestamp && status.getMvvCount() > 0) {
status.setMvvCount(0);
count++;
}
status = status.getNext();
}
return count;
}
boolean isEmpty() {
return _abortedCount + _currentCount + _longRunningCount == 0;
}
private boolean isAborted(final TransactionStatus status) {
return status.getTc() == ABORTED && status.isNotified();
}
private boolean isCommitted(final TransactionStatus status) {
return status.getTc() > 0 && status.getTc() != UNCOMMITTED && status.isNotified();
}
private boolean isObsolete(final TransactionStatus status) {
return status.getTc() < _activeTransactionFloor;
}
private void aggregate(final TransactionStatus status, final boolean committed) {
assert _lock.isHeldByCurrentThread();
for (Delta delta = status.takeDelta(); delta != null; delta = freeDelta(delta)) {
if (committed) {
delta.getAccumulator().aggregate(_hashIndex, delta);
}
delta.setAccumulator(null);
}
}
/**
* Compute and return the snapshot value of an Accumulator
*
* @throws RetryException
* if a TransactionStatus started but had not finished
* committing.
* @throws InterruptedException
*/
long getAccumulatorSnapshot(final Accumulator accumulator, final long timestamp, final int step)
throws RetryException, InterruptedException {
assert _lock.isHeldByCurrentThread();
long value = accumulator.getBucketValue(_hashIndex);
value = accumulatorSnapshotHelper(_current, accumulator, timestamp, step, value);
value = accumulatorSnapshotHelper(_longRunning, accumulator, timestamp, step, value);
return value;
}
private long accumulatorSnapshotHelper(final TransactionStatus start, final Accumulator accumulator,
final long timestamp, final int step, final long initialValue) throws RetryException, InterruptedException {
long value = initialValue;
for (TransactionStatus status = start; status != null; status = status.getNext()) {
final long tc = status.getTc();
if (status.getTs() == timestamp) {
for (Delta delta = status.getDelta(); delta != null; delta = delta.getNext()) {
if (delta.getAccumulator() == accumulator && (delta.getStep() <= step)) {
value = accumulator.applyValue(value, delta.getValue());
}
}
} else if (tc > 0 && tc != UNCOMMITTED && tc < timestamp) {
for (Delta delta = status.getDelta(); delta != null; delta = delta.getNext()) {
if (delta.getAccumulator() == accumulator) {
value = accumulator.applyValue(value, delta.getValue());
}
}
} else if (tc < 0 && tc != ABORTED && -tc < timestamp) {
status.briefLock();
_transactionIndex.incrementAccumulatorSnapshotRetryCounter();
throw RetryException.SINGLE;
}
}
return value;
}
void checkpointAccumulatorSnapshots(final long timestamp) throws RetryException, InterruptedException {
assert _lock.isHeldByCurrentThread();
accumulatorCheckpointHelper(_current, timestamp);
accumulatorCheckpointHelper(_longRunning, timestamp);
}
private void accumulatorCheckpointHelper(final TransactionStatus start, final long timestamp)
throws RetryException, InterruptedException {
for (TransactionStatus status = start; status != null; status = status.getNext()) {
final long tc = status.getTc();
if (tc > 0 && tc != UNCOMMITTED && tc < timestamp) {
for (Delta delta = status.getDelta(); delta != null; delta = delta.getNext()) {
final Accumulator accumulator = delta.getAccumulator();
final long newValue = accumulator.applyValue(accumulator.getCheckpointTemp(), delta.getValue());
accumulator.setCheckpointTemp(newValue);
}
} else if (tc < 0 && tc != ABORTED && -tc < timestamp) {
boolean locked = false;
try {
locked = status.wwLock(TransactionIndex.SHORT_TIMEOUT);
} finally {
if (locked) {
status.wwUnlock();
}
}
_transactionIndex.incrementAccumulatorCheckpointRetryCounter();
throw RetryException.SINGLE;
}
}
}
private void free(final TransactionStatus status) {
assert _lock.isHeldByCurrentThread();
/*
* May be held by another thread briefly while status being checked
*/
assert !status.isHeldByCurrentThread();
if (_freeCount < _transactionIndex.getMaxFreeListSize()) {
status.setNext(_free);
_free = status;
_freeCount++;
} else {
_droppedCount++;
}
}
private Delta freeDelta(final Delta delta) {
final Delta next = delta.getNext();
/*
* If the free Delta list is already full then simply drop this one and
* let it be garbage collected
*/
if (_freeDeltaCount < _transactionIndex.getMaxFreeDeltaListSize()) {
delta.setNext(_freeDeltaList);
_freeDeltaList = delta;
_freeDeltaCount++;
}
return next;
}
Delta allocateDelta() {
final Delta delta = _freeDeltaList;
if (delta != null) {
_freeDeltaList = delta.getNext();
_freeDeltaCount--;
return delta;
} else {
return new Delta();
}
}
@Override
public String toString() {
return String.format("",
TransactionIndex.minMaxString(_floor), listString(_current), listString(_aborted),
listString(_longRunning), listString(_free));
}
String listString(final TransactionStatus list) {
final StringBuilder sb = new StringBuilder();
for (TransactionStatus status = list; status != null; status = status.getNext()) {
if (sb.length() != 0) {
sb.append(",");
}
sb.append(status);
}
return sb.toString();
}
}