KernelTransactionImplementation.java

/*
 * Copyright (c) 2002-2017 "Neo Technology,"
 * Network Engine for Objects in Lund AB [http://neotechnology.com]
 *
 * This file is part of Neo4j.
 *
 * Neo4j is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
package org.neo4j.kernel.impl.api;

import java.time.Clock;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.function.Supplier;
import java.util.stream.Stream;

import org.neo4j.collection.pool.Pool;
import org.neo4j.graphdb.TransactionTerminatedException;
import org.neo4j.io.pagecache.tracing.cursor.PageCursorTracer;
import org.neo4j.internal.kernel.api.ExplicitIndexRead;
import org.neo4j.internal.kernel.api.ExplicitIndexWrite;
import org.neo4j.internal.kernel.api.Read;
import org.neo4j.internal.kernel.api.SchemaRead;
import org.neo4j.internal.kernel.api.SchemaWrite;
import org.neo4j.internal.kernel.api.Write;
import org.neo4j.io.pagecache.tracing.cursor.PageCursorTracerSupplier;
import org.neo4j.kernel.api.KernelTransaction;
import org.neo4j.kernel.api.KeyReadTokenNameLookup;
import org.neo4j.kernel.api.exceptions.ConstraintViolationTransactionFailureException;
import org.neo4j.kernel.api.exceptions.InvalidTransactionTypeKernelException;
import org.neo4j.kernel.api.exceptions.Status;
import org.neo4j.kernel.api.exceptions.TransactionFailureException;
import org.neo4j.kernel.api.exceptions.schema.ConstraintValidationException;
import org.neo4j.kernel.api.exceptions.schema.CreateConstraintFailureException;
import org.neo4j.kernel.api.exceptions.schema.DropIndexFailureException;
import org.neo4j.kernel.api.schema.index.IndexDescriptor;
import org.neo4j.internal.kernel.api.security.SecurityContext;
import org.neo4j.kernel.api.txstate.ExplicitIndexTransactionState;
import org.neo4j.kernel.api.txstate.TransactionState;
import org.neo4j.kernel.api.txstate.TxStateHolder;
import org.neo4j.kernel.impl.api.state.ConstraintIndexCreator;
import org.neo4j.kernel.impl.api.state.TxState;
import org.neo4j.kernel.impl.factory.AccessCapability;
import org.neo4j.kernel.impl.locking.ActiveLock;
import org.neo4j.kernel.impl.locking.LockTracer;
import org.neo4j.kernel.impl.locking.Locks;
import org.neo4j.kernel.impl.locking.StatementLocks;
import org.neo4j.kernel.impl.newapi.Cursors;
import org.neo4j.kernel.impl.newapi.Operations;
import org.neo4j.kernel.impl.proc.Procedures;
import org.neo4j.kernel.impl.transaction.TransactionHeaderInformationFactory;
import org.neo4j.kernel.impl.transaction.TransactionMonitor;
import org.neo4j.kernel.impl.transaction.log.PhysicalTransactionRepresentation;
import org.neo4j.kernel.impl.transaction.tracing.CommitEvent;
import org.neo4j.kernel.impl.transaction.tracing.TransactionEvent;
import org.neo4j.kernel.impl.transaction.tracing.TransactionTracer;
import org.neo4j.resources.CpuClock;
import org.neo4j.resources.HeapAllocation;
import org.neo4j.storageengine.api.StorageCommand;
import org.neo4j.storageengine.api.StorageEngine;
import org.neo4j.storageengine.api.StorageStatement;
import org.neo4j.storageengine.api.StoreReadLayer;
import org.neo4j.storageengine.api.txstate.TxStateVisitor;

import static java.util.concurrent.TimeUnit.NANOSECONDS;
import static org.neo4j.storageengine.api.TransactionApplicationMode.INTERNAL;

/**
 * This class should replace the {@link org.neo4j.kernel.api.KernelTransaction} interface, and take its name, as soon
 * as
 * {@code TransitionalTxManagementKernelTransaction} is gone from {@code server}.
 */
public class KernelTransactionImplementation implements KernelTransaction, TxStateHolder
{
    /*
     * IMPORTANT:
     * This class is pooled and re-used. If you add *any* state to it, you *must* make sure that:
     *   - the #initialize() method resets that state for re-use
     *   - the #release() method releases resources acquired in #initialize() or during the transaction's life time
     */

    // default values for not committed tx id and tx commit time
    private static final long NOT_COMMITTED_TRANSACTION_ID = -1;
    private static final long NOT_COMMITTED_TRANSACTION_COMMIT_TIME = -1;

    // Logic
    private final SchemaWriteGuard schemaWriteGuard;
    private final TransactionHooks hooks;
    private final ConstraintIndexCreator constraintIndexCreator;
    private final StatementOperationParts statementOperations;
    private final StorageEngine storageEngine;
    private final TransactionTracer transactionTracer;
    private final Pool<KernelTransactionImplementation> pool;
    private final Supplier<ExplicitIndexTransactionState> explicitIndexTxStateSupplier;

    // For committing
    private final TransactionHeaderInformationFactory headerInformationFactory;
    private final TransactionCommitProcess commitProcess;
    private final TransactionMonitor transactionMonitor;
    private final PageCursorTracerSupplier cursorTracerSupplier;
    private final StoreReadLayer storeLayer;
    private final Clock clock;

    // State that needs to be reset between uses. Most of these should be cleared or released in #release(),
    // whereas others, such as timestamp or txId when transaction starts, even locks, needs to be set in #initialize().
    private TransactionState txState;
    private ExplicitIndexTransactionState explicitIndexTransactionState;
    private TransactionWriteState writeState;
    private TransactionHooks.TransactionHooksState hooksState;
    private final KernelStatement currentStatement;
    private final StorageStatement storageStatement;
    private final List<CloseListener> closeListeners = new ArrayList<>( 2 );
    private SecurityContext securityContext;
    private volatile StatementLocks statementLocks;
    private volatile long userTransactionId;
    private boolean beforeHookInvoked;
    private volatile boolean closing;
    private volatile boolean closed;
    private boolean failure;
    private boolean success;
    private volatile Status terminationReason;
    private long startTimeMillis;
    private long timeoutMillis;
    private long lastTransactionIdWhenStarted;
    private volatile long lastTransactionTimestampWhenStarted;
    private final Statistics statistics;
    private TransactionEvent transactionEvent;
    private Type type;
    private long transactionId;
    private long commitTime;
    private volatile int reuseCount;
    private volatile Map<String,Object> userMetaData;
    private final Operations operations;

    /**
     * Lock prevents transaction {@link #markForTermination(Status)}  transaction termination} from interfering with
     * {@link #close() transaction commit} and specifically with {@link #release()}.
     * Termination can run concurrently with commit and we need to make sure that it terminates the right lock client
     * and the right transaction (with the right {@link #reuseCount}) because {@link KernelTransactionImplementation}
     * instances are pooled.
     */
    private final Lock terminationReleaseLock = new ReentrantLock();

    public KernelTransactionImplementation( StatementOperationParts statementOperations, SchemaWriteGuard schemaWriteGuard,
            TransactionHooks hooks, ConstraintIndexCreator constraintIndexCreator, Procedures procedures,
            TransactionHeaderInformationFactory headerInformationFactory, TransactionCommitProcess commitProcess,
            TransactionMonitor transactionMonitor, Supplier<ExplicitIndexTransactionState> explicitIndexTxStateSupplier,
            Pool<KernelTransactionImplementation> pool, Clock clock, CpuClock cpuClock, HeapAllocation heapAllocation,
            TransactionTracer transactionTracer, LockTracer lockTracer, PageCursorTracerSupplier cursorTracerSupplier,
            StorageEngine storageEngine, AccessCapability accessCapability, Cursors cursors )
    {
        this.statementOperations = statementOperations;
        this.schemaWriteGuard = schemaWriteGuard;
        this.hooks = hooks;
        this.constraintIndexCreator = constraintIndexCreator;
        this.headerInformationFactory = headerInformationFactory;
        this.commitProcess = commitProcess;
        this.transactionMonitor = transactionMonitor;
        this.storeLayer = storageEngine.storeReadLayer();
        this.storageEngine = storageEngine;
        this.explicitIndexTxStateSupplier = explicitIndexTxStateSupplier;
        this.pool = pool;
        this.clock = clock;
        this.transactionTracer = transactionTracer;
        this.cursorTracerSupplier = cursorTracerSupplier;
        this.storageStatement = storeLayer.newStatement();
        this.currentStatement = new KernelStatement( this, this, storageStatement,
                procedures, accessCapability, lockTracer, statementOperations );
        this.statistics = new Statistics( this, cpuClock, heapAllocation );
        this.userMetaData = new HashMap<>();
        this.operations = new Operations( storageEngine, storageStatement, this, explicitIndexTxStateSupplier, cursors );
    }

    /**
     * Reset this transaction to a vanilla state, turning it into a logically new transaction.
     */
    public KernelTransactionImplementation initialize( long lastCommittedTx, long lastTimeStamp, StatementLocks statementLocks, Type type,
            SecurityContext frozenSecurityContext, long transactionTimeout, long userTransactionId )
    {
        this.type = type;
        this.statementLocks = statementLocks;
        this.userTransactionId = userTransactionId;
        this.terminationReason = null;
        this.closing = false;
        this. closed = false;
        this.beforeHookInvoked = false;
        this.failure = false;
        this.success = false;
        this.beforeHookInvoked = false;
        this.writeState = TransactionWriteState.NONE;
        this.startTimeMillis = clock.millis();
        this.timeoutMillis = transactionTimeout;
        this.lastTransactionIdWhenStarted = lastCommittedTx;
        this.lastTransactionTimestampWhenStarted = lastTimeStamp;
        this.transactionEvent = transactionTracer.beginTransaction();
        assert transactionEvent != null : "transactionEvent was null!";
        this.securityContext = frozenSecurityContext;
        this.transactionId = NOT_COMMITTED_TRANSACTION_ID;
        this.commitTime = NOT_COMMITTED_TRANSACTION_COMMIT_TIME;
        PageCursorTracer pageCursorTracer = cursorTracerSupplier.get();
        this.statistics.init( Thread.currentThread().getId(), pageCursorTracer );
        this.currentStatement.initialize( statementLocks, pageCursorTracer );
        return this;
    }

    int getReuseCount()
    {
        return reuseCount;
    }

    @Override
    public long startTime()
    {
        return startTimeMillis;
    }

    @Override
    public long timeout()
    {
        return timeoutMillis;
    }

    @Override
    public long lastTransactionIdWhenStarted()
    {
        return lastTransactionIdWhenStarted;
    }

    @Override
    public void success()
    {
        this.success = true;
    }

    boolean isSuccess()
    {
        return success;
    }

    @Override
    public void failure()
    {
        failure = true;
    }

    @Override
    public Optional<Status> getReasonIfTerminated()
    {
        return Optional.ofNullable( terminationReason );
    }

    boolean markForTermination( long expectedReuseCount, Status reason )
    {
        terminationReleaseLock.lock();
        try
        {
            return expectedReuseCount == reuseCount && markForTerminationIfPossible( reason );
        }
        finally
        {
            terminationReleaseLock.unlock();
        }
    }

    /**
     * {@inheritDoc}
     * <p>
     * This method is guarded by {@link #terminationReleaseLock} to coordinate concurrent
     * {@link #close()} and {@link #release()} calls.
     */
    @Override
    public void markForTermination( Status reason )
    {
        terminationReleaseLock.lock();
        try
        {
            markForTerminationIfPossible( reason );
        }
        finally
        {
            terminationReleaseLock.unlock();
        }
    }

    private boolean markForTerminationIfPossible( Status reason )
    {
        if ( canBeTerminated() )
        {
            failure = true;
            terminationReason = reason;
            if ( statementLocks != null )
            {
                statementLocks.stop();
            }
            transactionMonitor.transactionTerminated( hasTxStateWithChanges() );
            return true;
        }
        return false;
    }

    @Override
    public boolean isOpen()
    {
        return !closed && !closing;
    }

    @Override
    public SecurityContext securityContext()
    {
        return securityContext;
    }

    public void setMetaData( Map<String, Object> data )
    {
        this.userMetaData = data;
    }

    public Map<String, Object> getMetaData()
    {
        return userMetaData;
    }

    @Override
    public KernelStatement acquireStatement()
    {
        assertTransactionOpen();
        currentStatement.acquire();
        return currentStatement;
    }

    ExecutingQueryList executingQueries()
    {
        return currentStatement.executingQueryList();
    }

    void upgradeToDataWrites() throws InvalidTransactionTypeKernelException
    {
        writeState = writeState.upgradeToDataWrites();
    }

    void upgradeToSchemaWrites() throws InvalidTransactionTypeKernelException
    {
        schemaWriteGuard.assertSchemaWritesAllowed();
        writeState = writeState.upgradeToSchemaWrites();
    }

    private void dropCreatedConstraintIndexes() throws TransactionFailureException
    {
        if ( hasTxStateWithChanges() )
        {
            for ( IndexDescriptor createdConstraintIndex : txState().constraintIndexesCreatedInTx() )
            {
                try
                {
                    // TODO logically, which statement should this operation be performed on?
                    constraintIndexCreator.dropUniquenessConstraintIndex( createdConstraintIndex );
                }
                catch ( DropIndexFailureException e )
                {
                    throw new IllegalStateException( "Constraint index that was created in a transaction should be " +
                            "possible to drop during rollback of that transaction.", e );
                }
            }
        }
    }

    @Override
    public TransactionState txState()
    {
        if ( txState == null )
        {
            transactionMonitor.upgradeToWriteTransaction();
            txState = new TxState();
        }
        return txState;
    }

    @Override
    public ExplicitIndexTransactionState explicitIndexTxState()
    {
        return explicitIndexTransactionState != null ? explicitIndexTransactionState :
               (explicitIndexTransactionState = explicitIndexTxStateSupplier.get());
    }

    @Override
    public boolean hasTxStateWithChanges()
    {
        return txState != null && txState.hasChanges();
    }

    private void markAsClosed( long txId )
    {
        assertTransactionOpen();
        closed = true;
        notifyListeners( txId );
        closeCurrentStatementIfAny();
    }

    private void notifyListeners( long txId )
    {
        for ( CloseListener closeListener : closeListeners )
        {
            closeListener.notify( txId );
        }
    }

    private void closeCurrentStatementIfAny()
    {
        currentStatement.forceClose();
    }

    private void assertTransactionNotClosing()
    {
        if ( closing )
        {
            throw new IllegalStateException( "This transaction is already being closed." );
        }
    }

    private void assertTransactionOpen()
    {
        if ( closed )
        {
            throw new IllegalStateException( "This transaction has already been completed." );
        }
    }

    private boolean hasChanges()
    {
        return hasTxStateWithChanges() || hasExplicitIndexChanges();
    }

    private boolean hasExplicitIndexChanges()
    {
        return explicitIndexTransactionState != null && explicitIndexTransactionState.hasChanges();
    }

    private boolean hasDataChanges()
    {
        return hasTxStateWithChanges() && txState.hasDataChanges();
    }

    @Override
    public long closeTransaction() throws TransactionFailureException
    {
        assertTransactionOpen();
        assertTransactionNotClosing();
        closing = true;
        try
        {
            if ( failure || !success || isTerminated() )
            {
                rollback();
                failOnNonExplicitRollbackIfNeeded();
                return ROLLBACK;
            }
            else
            {
                return commit();
            }
        }
        finally
        {
            try
            {
                closed = true;
                closing = false;
                transactionEvent.setSuccess( success );
                transactionEvent.setFailure( failure );
                transactionEvent.setTransactionWriteState( writeState.name() );
                transactionEvent.setReadOnly( txState == null || !txState.hasChanges() );
                transactionEvent.close();
            }
            finally
            {
                release();
            }
        }
    }

    public boolean isClosing()
    {
        return closing;
    }

    /**
     * Throws exception if this transaction was marked as successful but failure flag has also been set to true.
     * <p>
     * This could happen when:
     * <ul>
     * <li>caller explicitly calls both {@link #success()} and {@link #failure()}</li>
     * <li>caller explicitly calls {@link #success()} but transaction execution fails</li>
     * <li>caller explicitly calls {@link #success()} but transaction is terminated</li>
     * </ul>
     * <p>
     *
     * @throws TransactionFailureException when execution failed
     * @throws TransactionTerminatedException when transaction was terminated
     */
    private void failOnNonExplicitRollbackIfNeeded() throws TransactionFailureException
    {
        if ( success && isTerminated() )
        {
            throw new TransactionTerminatedException( terminationReason );
        }
        if ( success )
        {
            // Success was called, but also failure which means that the client code using this
            // transaction passed through a happy path, but the transaction was still marked as
            // failed for one or more reasons. Tell the user that although it looked happy it
            // wasn't committed, but was instead rolled back.
            throw new TransactionFailureException( Status.Transaction.TransactionMarkedAsFailed,
                    "Transaction rolled back even if marked as successful" );
        }
    }

    private long commit() throws TransactionFailureException
    {
        boolean success = false;
        long txId = READ_ONLY;

        try ( CommitEvent commitEvent = transactionEvent.beginCommitEvent() )
        {
            // Trigger transaction "before" hooks.
            if ( hasDataChanges() )
            {
                try
                {
                    hooksState = hooks.beforeCommit( txState, this, storageEngine.storeReadLayer(), storageStatement );
                    if ( hooksState != null && hooksState.failed() )
                    {
                        Throwable cause = hooksState.failure();
                        throw new TransactionFailureException( Status.Transaction.TransactionHookFailed, cause, "" );
                    }
                }
                finally
                {
                    beforeHookInvoked = true;
                }
            }

            // Convert changes into commands and commit
            if ( hasChanges() )
            {
                // grab all optimistic locks now, locks can't be deferred any further
                statementLocks.prepareForCommit( currentStatement.lockTracer() );
                // use pessimistic locks for the rest of the commit process, locks can't be deferred any further
                Locks.Client commitLocks = statementLocks.pessimistic();

                // Gather up commands from the various sources
                Collection<StorageCommand> extractedCommands = new ArrayList<>();
                storageEngine.createCommands(
                        extractedCommands,
                        txState,
                        storageStatement,
                        commitLocks,
                        lastTransactionIdWhenStarted );
                if ( hasExplicitIndexChanges() )
                {
                    explicitIndexTransactionState.extractCommands( extractedCommands );
                }

                /* Here's the deal: we track a quick-to-access hasChanges in transaction state which is true
                 * if there are any changes imposed by this transaction. Some changes made inside a transaction undo
                 * previously made changes in that same transaction, and so at some point a transaction may have
                 * changes and at another point, after more changes seemingly,
                 * the transaction may not have any changes.
                 * However, to track that "undoing" of the changes is a bit tedious, intrusive and hard to maintain
                 * and get right.... So to really make sure the transaction has changes we re-check by looking if we
                 * have produced any commands to add to the logical log.
                 */
                if ( !extractedCommands.isEmpty() )
                {
                    // Finish up the whole transaction representation
                    PhysicalTransactionRepresentation transactionRepresentation =
                            new PhysicalTransactionRepresentation( extractedCommands );
                    TransactionHeaderInformation headerInformation = headerInformationFactory.create();
                    long timeCommitted = clock.millis();
                    transactionRepresentation.setHeader( headerInformation.getAdditionalHeader(),
                            headerInformation.getMasterId(),
                            headerInformation.getAuthorId(),
                            startTimeMillis, lastTransactionIdWhenStarted, timeCommitted,
                            commitLocks.getLockSessionId() );

                    // Commit the transaction
                    success = true;
                    TransactionToApply batch = new TransactionToApply( transactionRepresentation );
                    txId = transactionId = commitProcess.commit( batch, commitEvent, INTERNAL );
                    commitTime = timeCommitted;
                }
            }
            success = true;
            return txId;
        }
        catch ( ConstraintValidationException | CreateConstraintFailureException e )
        {
            throw new ConstraintViolationTransactionFailureException(
                    e.getUserMessage( new KeyReadTokenNameLookup( statementOperations.keyReadOperations() ) ), e );
        }
        finally
        {
            if ( !success )
            {
                rollback();
            }
            else
            {
                afterCommit( txId );
            }
        }
    }

    private void rollback() throws TransactionFailureException
    {
        try
        {
            try
            {
                dropCreatedConstraintIndexes();
            }
            catch ( IllegalStateException | SecurityException e )
            {
                throw new TransactionFailureException( Status.Transaction.TransactionRollbackFailed, e,
                        "Could not drop created constraint indexes" );
            }

            // Free any acquired id's
            if ( txState != null )
            {
                try
                {
                    txState.accept( new TxStateVisitor.Adapter()
                    {
                        @Override
                        public void visitCreatedNode( long id )
                        {
                            storeLayer.releaseNode( id );
                        }

                        @Override
                        public void visitCreatedRelationship( long id, int type, long startNode, long endNode )
                                throws ConstraintValidationException
                        {
                            storeLayer.releaseRelationship( id );
                        }
                    } );
                }
                catch ( ConstraintValidationException | CreateConstraintFailureException e )
                {
                    throw new IllegalStateException(
                            "Releasing locks during rollback should perform no constraints checking.", e );
                }
            }
        }
        finally
        {
            afterRollback();
        }
    }

    @Override
    public Read dataRead()
    {
        return operations;
    }

    @Override
    public Write dataWrite()
    {
        return operations;
    }

    @Override
    public ExplicitIndexRead indexRead()
    {
        return operations;
    }

    @Override
    public ExplicitIndexWrite indexWrite()
    {
        throw new UnsupportedOperationException( "not implemented" );
    }

    @Override
    public SchemaRead schemaRead()
    {
        return operations;
    }

    @Override
    public SchemaWrite schemaWrite()
    {
        throw new UnsupportedOperationException( "not implemented" );
    }

    @Override
    public org.neo4j.internal.kernel.api.Locks locks()
    {
        throw new UnsupportedOperationException( "not implemented" );
    }

    private void afterCommit( long txId )
    {
        try
        {
            markAsClosed( txId );
            if ( beforeHookInvoked )
            {
                hooks.afterCommit( txState, this, hooksState );
            }
        }
        finally
        {
            transactionMonitor.transactionFinished( true, hasTxStateWithChanges() );
        }
    }

    private void afterRollback()
    {
        try
        {
            markAsClosed( ROLLBACK );
            if ( beforeHookInvoked )
            {
                hooks.afterRollback( txState, this, hooksState );
            }
        }
        finally
        {
            transactionMonitor.transactionFinished( false, hasTxStateWithChanges() );
        }
    }

    /**
     * Release resources held up by this transaction & return it to the transaction pool.
     * This method is guarded by {@link #terminationReleaseLock} to coordinate concurrent
     * {@link #markForTermination(Status)} calls.
     */
    private void release()
    {
        terminationReleaseLock.lock();
        try
        {
            statementLocks.close();
            statementLocks = null;
            terminationReason = null;
            type = null;
            securityContext = null;
            transactionEvent = null;
            explicitIndexTransactionState = null;
            txState = null;
            hooksState = null;
            closeListeners.clear();
            reuseCount++;
            userMetaData = Collections.emptyMap();
            userTransactionId = 0;
            statistics.reset();
            pool.release( this );
        }
        finally
        {
            terminationReleaseLock.unlock();
        }
    }

    /**
     * Transaction can be terminated only when it is not closed and not already terminated.
     * Otherwise termination does not make sense.
     */
    private boolean canBeTerminated()
    {
        return !closed && !isTerminated();
    }

    @Override
    public boolean isTerminated()
    {
        return terminationReason != null;
    }

    @Override
    public long lastTransactionTimestampWhenStarted()
    {
        return lastTransactionTimestampWhenStarted;
    }

    @Override
    public void registerCloseListener( CloseListener listener )
    {
        assert listener != null;
        closeListeners.add( listener );
    }

    @Override
    public Type transactionType()
    {
        return type;
    }

    @Override
    public long getTransactionId()
    {
        if ( transactionId == NOT_COMMITTED_TRANSACTION_ID )
        {
            throw new IllegalStateException( "Transaction id is not assigned yet. " +
                                             "It will be assigned during transaction commit." );
        }
        return transactionId;
    }

    @Override
    public long getCommitTime()
    {
        if ( commitTime == NOT_COMMITTED_TRANSACTION_COMMIT_TIME )
        {
            throw new IllegalStateException( "Transaction commit time is not assigned yet. " +
                                             "It will be assigned during transaction commit." );
        }
        return commitTime;
    }

    @Override
    public Revertable overrideWith( SecurityContext context )
    {
        SecurityContext oldContext = this.securityContext;
        this.securityContext = context;
        return () -> this.securityContext = oldContext;
    }

    @Override
    public String toString()
    {
        String lockSessionId = statementLocks == null
                               ? "statementLocks == null"
                               : String.valueOf( statementLocks.pessimistic().getLockSessionId() );

        return "KernelTransaction[" + lockSessionId + "]";
    }

    public void dispose()
    {
        storageStatement.close();
    }

    /**
     * This method will be invoked by concurrent threads for inspecting the locks held by this transaction.
     * <p>
     * The fact that {@link #statementLocks} is a volatile fields, grants us enough of a read barrier to get a good
     * enough snapshot of the lock state (as long as the underlying methods give us such guarantees).
     *
     * @return the locks held by this transaction.
     */
    public Stream<? extends ActiveLock> activeLocks()
    {
        StatementLocks locks = this.statementLocks;
        return locks == null ? Stream.empty() : locks.activeLocks();
    }

    public long userTransactionId()
    {
        return userTransactionId;
    }

    public Statistics getStatistics()
    {
        return statistics;
    }

    public static class Statistics
    {
        private volatile long cpuTimeNanosWhenQueryStarted;
        private volatile long heapAllocatedBytesWhenQueryStarted;
        private volatile long waitingTimeNanos;
        private volatile long transactionThreadId;
        private final KernelTransactionImplementation transaction;
        private final CpuClock cpuClock;
        private final HeapAllocation heapAllocation;
        private volatile PageCursorTracer pageCursorTracer = PageCursorTracer.NULL;

        public Statistics( KernelTransactionImplementation transaction, CpuClock cpuClock, HeapAllocation heapAllocation )
        {
            this.transaction = transaction;
            this.cpuClock = cpuClock;
            this.heapAllocation = heapAllocation;
        }

        void init( long threadId, PageCursorTracer pageCursorTracer )
        {
            this.transactionThreadId = threadId;
            this.pageCursorTracer = pageCursorTracer;
            this.cpuTimeNanosWhenQueryStarted = cpuClock.cpuTimeNanos( transactionThreadId );
            this.heapAllocatedBytesWhenQueryStarted = heapAllocation.allocatedBytes( transactionThreadId );
        }

        /**
         * Returns number of allocated bytes by current transaction.
         * @return number of allocated bytes by the thread.
         */
        public long heapAllocateBytes()
        {
            return heapAllocation.allocatedBytes( transactionThreadId ) - heapAllocatedBytesWhenQueryStarted;
        }

        /**
         * Return CPU time used by current transaction in milliseconds
         * @return the current CPU time used by the transaction, in milliseconds.
         */
        public long cpuTimeMillis()
        {
            long cpuTimeNanos = cpuClock.cpuTimeNanos( transactionThreadId ) - cpuTimeNanosWhenQueryStarted;
            return NANOSECONDS.toMillis( cpuTimeNanos );
        }

        /**
         * Return total number of page cache hits that current transaction performed
         * @return total page cache hits
         */
        public long totalTransactionPageCacheHits()
        {
            return pageCursorTracer.accumulatedHits();
        }

        /**
         * Return total number of page cache faults that current transaction performed
         * @return total page cache faults
         */
        public long totalTransactionPageCacheFaults()
        {
            return pageCursorTracer.accumulatedFaults();
        }

        /**
         * Report how long any particular query was waiting during it's execution
         * @param waitTimeNanos query waiting time in nanoseconds
         */
        @SuppressWarnings( "NonAtomicOperationOnVolatileField" )
        void addWaitingTime( long waitTimeNanos )
        {
            waitingTimeNanos += waitTimeNanos;
        }

        /**
         * Accumulated transaction waiting time that includes waiting time of all already executed queries
         * plus waiting time of currently executed query.
         * @return accumulated transaction waiting time
         * @param nowNanos current moment in nanoseconds
         */
        public long getWaitingTimeNanos( long nowNanos )
        {
            ExecutingQueryList queryList = transaction.executingQueries();
            long waitingTime = waitingTimeNanos;
            if ( queryList != null )
            {
                Long latestQueryWaitingNanos = queryList.top( executingQuery ->
                        executingQuery.totalWaitingTimeNanos( nowNanos ) );
                waitingTime = latestQueryWaitingNanos != null ? waitingTime + latestQueryWaitingNanos : waitingTime;
            }
            return waitingTime;
        }

        void reset()
        {
            pageCursorTracer = PageCursorTracer.NULL;
            cpuTimeNanosWhenQueryStarted = 0;
            heapAllocatedBytesWhenQueryStarted = 0;
            waitingTimeNanos = 0;
            transactionThreadId = -1;
        }
    }

    /**
     * It is not allowed for the same transaction to perform database writes as well as schema writes.
     * This enum tracks the current write transactionStatus of the transaction, allowing it to transition from
     * no writes (NONE) to data writes (DATA) or schema writes (SCHEMA), but it cannot transition between
     * DATA and SCHEMA without throwing an InvalidTransactionTypeKernelException. Note that this behavior
     * is orthogonal to the SecurityContext which manages what the transaction or statement is allowed to do
     * based on authorization.
     */
    private enum TransactionWriteState
    {
        NONE,
        DATA
                {
                    @Override
                    TransactionWriteState upgradeToSchemaWrites() throws InvalidTransactionTypeKernelException
                    {
                        throw new InvalidTransactionTypeKernelException(
                                "Cannot perform schema updates in a transaction that has performed data updates." );
                    }
                },
        SCHEMA
                {
                    @Override
                    TransactionWriteState upgradeToDataWrites() throws InvalidTransactionTypeKernelException
                    {
                        throw new InvalidTransactionTypeKernelException(
                                "Cannot perform data updates in a transaction that has performed schema updates." );
                    }
                };

        TransactionWriteState upgradeToDataWrites() throws InvalidTransactionTypeKernelException
        {
            return DATA;
        }

        TransactionWriteState upgradeToSchemaWrites() throws InvalidTransactionTypeKernelException
        {
            return SCHEMA;
        }
    }
}