cassandra-trunk/src/java/org/apache/cassandra/db/ReadCommand.java
/*
 * 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.cassandra.db;

import java.io.IOException;
import java.util.*;
import java.util.concurrent.TimeUnit;
import java.util.function.BiFunction;
import java.util.function.LongPredicate;
import java.util.function.Function;

import javax.annotation.Nullable;

import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Preconditions;
import com.google.common.collect.Iterables;
import com.google.common.collect.Sets;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import io.netty.util.concurrent.FastThreadLocal;
import org.apache.cassandra.config.*;
import org.apache.cassandra.db.filter.*;
import org.apache.cassandra.exceptions.QueryCancelledException;
import org.apache.cassandra.net.MessageFlag;
import org.apache.cassandra.net.MessagingService;
import org.apache.cassandra.net.ParamType;
import org.apache.cassandra.net.Verb;
import org.apache.cassandra.db.partitions.*;
import org.apache.cassandra.db.rows.*;
import org.apache.cassandra.db.transform.RTBoundCloser;
import org.apache.cassandra.db.transform.RTBoundValidator;
import org.apache.cassandra.db.transform.RTBoundValidator.Stage;
import org.apache.cassandra.db.transform.StoppingTransformation;
import org.apache.cassandra.db.transform.Transformation;
import org.apache.cassandra.exceptions.UnknownIndexException;
import org.apache.cassandra.index.Index;
import org.apache.cassandra.index.IndexNotAvailableException;
import org.apache.cassandra.index.IndexRegistry;
import org.apache.cassandra.io.IVersionedSerializer;
import org.apache.cassandra.io.sstable.format.SSTableReader;
import org.apache.cassandra.io.util.DataInputPlus;
import org.apache.cassandra.io.util.DataOutputPlus;
import org.apache.cassandra.locator.Replica;
import org.apache.cassandra.metrics.TableMetrics;
import org.apache.cassandra.net.Message;
import org.apache.cassandra.schema.IndexMetadata;
import org.apache.cassandra.schema.Schema;
import org.apache.cassandra.schema.SchemaConstants;
import org.apache.cassandra.schema.TableId;
import org.apache.cassandra.schema.TableMetadata;
import org.apache.cassandra.schema.SchemaProvider;
import org.apache.cassandra.service.ActiveRepairService;
import org.apache.cassandra.service.ClientWarn;
import org.apache.cassandra.tracing.Tracing;
import org.apache.cassandra.utils.CassandraUInt;
import org.apache.cassandra.utils.FBUtilities;
import org.apache.cassandra.utils.ObjectSizes;
import org.apache.cassandra.utils.TimeUUID;

import static com.google.common.collect.Iterables.any;
import static com.google.common.collect.Iterables.filter;
import static org.apache.cassandra.utils.Clock.Global.nanoTime;
import static org.apache.cassandra.utils.MonotonicClock.Global.approxTime;
import static org.apache.cassandra.db.partitions.UnfilteredPartitionIterators.MergeListener.NOOP;

/**
 * General interface for storage-engine read commands (common to both range and
 * single partition commands).
 * <p>
 * This contains all the informations needed to do a local read.
 */
public abstract class ReadCommand extends AbstractReadQuery
{
    private static final int TEST_ITERATION_DELAY_MILLIS = CassandraRelevantProperties.TEST_READ_ITERATION_DELAY_MS.getInt();

    protected static final Logger logger = LoggerFactory.getLogger(ReadCommand.class);
    public static final IVersionedSerializer<ReadCommand> serializer = new Serializer();

    // Expose the active command running so transitive calls can lookup this command.
    // This is useful for a few reasons, but mainly because the CQL query is here.
    private static final FastThreadLocal<ReadCommand> COMMAND = new FastThreadLocal<>();

    private final Kind kind;

    private final boolean isDigestQuery;
    private final boolean acceptsTransient;
    // if a digest query, the version for which the digest is expected. Ignored if not a digest.
    private int digestVersion;

    private boolean trackWarnings;

    @Nullable
    private final IndexMetadata index;

    protected static abstract class SelectionDeserializer
    {
        public abstract ReadCommand deserialize(DataInputPlus in,
                                                int version,
                                                boolean isDigest,
                                                int digestVersion,
                                                boolean acceptsTransient,
                                                TableMetadata metadata,
                                                long nowInSec,
                                                ColumnFilter columnFilter,
                                                RowFilter rowFilter,
                                                DataLimits limits,
                                                IndexMetadata index) throws IOException;
    }

    protected enum Kind
    {
        SINGLE_PARTITION (SinglePartitionReadCommand.selectionDeserializer),
        PARTITION_RANGE  (PartitionRangeReadCommand.selectionDeserializer);

        private final SelectionDeserializer selectionDeserializer;

        Kind(SelectionDeserializer selectionDeserializer)
        {
            this.selectionDeserializer = selectionDeserializer;
        }
    }

    protected ReadCommand(Kind kind,
                          boolean isDigestQuery,
                          int digestVersion,
                          boolean acceptsTransient,
                          TableMetadata metadata,
                          long nowInSec,
                          ColumnFilter columnFilter,
                          RowFilter rowFilter,
                          DataLimits limits,
                          IndexMetadata index,
                          boolean trackWarnings)
    {
        super(metadata, nowInSec, columnFilter, rowFilter, limits);
        if (acceptsTransient && isDigestQuery)
            throw new IllegalArgumentException("Attempted to issue a digest response to transient replica");

        this.kind = kind;
        this.isDigestQuery = isDigestQuery;
        this.digestVersion = digestVersion;
        this.acceptsTransient = acceptsTransient;
        this.index = index;
        this.trackWarnings = trackWarnings;
    }

    public static ReadCommand getCommand()
    {
        return COMMAND.get();
    }

    protected abstract void serializeSelection(DataOutputPlus out, int version) throws IOException;
    protected abstract long selectionSerializedSize(int version);

    public abstract boolean isLimitedToOnePartition();

    public abstract boolean isRangeRequest();

    /**
     * Creates a new <code>ReadCommand</code> instance with new limits.
     *
     * @param newLimits the new limits
     * @return a new <code>ReadCommand</code> with the updated limits
     */
    public abstract ReadCommand withUpdatedLimit(DataLimits newLimits);

    /**
     * The configured timeout for this command.
     *
     * @return the configured timeout for this command.
     */
    public abstract long getTimeout(TimeUnit unit);

    /**
     * Whether this query is a digest one or not.
     *
     * @return Whether this query is a digest query.
     */
    public boolean isDigestQuery()
    {
        return isDigestQuery;
    }

    /**
     * If the query is a digest one, the requested digest version.
     *
     * @return the requested digest version if the query is a digest. Otherwise, this can return
     * anything.
     */
    public int digestVersion()
    {
        return digestVersion;
    }

    /**
     * Sets the digest version, for when digest for that command is requested.
     * <p>
     * Note that we allow setting this independently of setting the command as a digest query as
     * this allows us to use the command as a carrier of the digest version even if we only call
     * setIsDigestQuery on some copy of it.
     *
     * @param digestVersion the version for the digest is this command is used for digest query..
     * @return this read command.
     */
    public ReadCommand setDigestVersion(int digestVersion)
    {
        this.digestVersion = digestVersion;
        return this;
    }

    /**
     * @return Whether this query expects only a transient data response, or a full response
     */
    public boolean acceptsTransient()
    {
        return acceptsTransient;
    }

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

    public boolean isTrackingWarnings()
    {
        return trackWarnings;
    }

    /**
     * Index (metadata) chosen for this query. Can be null.
     *
     * @return index (metadata) chosen for this query
     */
    @Nullable
    public IndexMetadata indexMetadata()
    {
        return index;
    }

    /**
     * The clustering index filter this command to use for the provided key.
     * <p>
     * Note that that method should only be called on a key actually queried by this command
     * and in practice, this will almost always return the same filter, but for the sake of
     * paging, the filter on the first key of a range command might be slightly different.
     *
     * @param key a partition key queried by this command.
     *
     * @return the {@code ClusteringIndexFilter} to use for the partition of key {@code key}.
     */
    public abstract ClusteringIndexFilter clusteringIndexFilter(DecoratedKey key);

    /**
     * Returns a copy of this command.
     *
     * @return a copy of this command.
     */
    public abstract ReadCommand copy();

    /**
     * Returns a copy of this command with acceptsTransient set to true.
     */
    public ReadCommand copyAsTransientQuery(Replica replica)
    {
        Preconditions.checkArgument(replica.isTransient(),
                                    "Can't make a transient request on a full replica: " + replica);
        return copyAsTransientQuery();
    }

    /**
     * Returns a copy of this command with acceptsTransient set to true.
     */
    public ReadCommand copyAsTransientQuery(Iterable<Replica> replicas)
    {
        if (any(replicas, Replica::isFull))
            throw new IllegalArgumentException("Can't make a transient request on full replicas: " + Iterables.toString(filter(replicas, Replica::isFull)));
        return copyAsTransientQuery();
    }

    protected abstract ReadCommand copyAsTransientQuery();

    /**
     * Returns a copy of this command with isDigestQuery set to true.
     */
    public ReadCommand copyAsDigestQuery(Replica replica)
    {
        Preconditions.checkArgument(replica.isFull(),
                                    "Can't make a digest request on a transient replica " + replica);
        return copyAsDigestQuery();
    }

    /**
     * Returns a copy of this command with isDigestQuery set to true.
     */
    public ReadCommand copyAsDigestQuery(Iterable<Replica> replicas)
    {
        if (any(replicas, Replica::isTransient))
            throw new IllegalArgumentException("Can't make a digest request on a transient replica " + Iterables.toString(filter(replicas, Replica::isTransient)));

        return copyAsDigestQuery();
    }

    protected abstract ReadCommand copyAsDigestQuery();

    protected abstract UnfilteredPartitionIterator queryStorage(ColumnFamilyStore cfs, ReadExecutionController executionController);

    /**
     * Whether the underlying {@code ClusteringIndexFilter} is reversed or not.
     *
     * @return whether the underlying {@code ClusteringIndexFilter} is reversed or not.
     */
    public abstract boolean isReversed();

    @SuppressWarnings("resource")
    public ReadResponse createResponse(UnfilteredPartitionIterator iterator, RepairedDataInfo rdi)
    {
        // validate that the sequence of RT markers is correct: open is followed by close, deletion times for both
        // ends equal, and there are no dangling RT bound in any partition.
        iterator = RTBoundValidator.validate(iterator, Stage.PROCESSED, true);

        return isDigestQuery()
               ? ReadResponse.createDigestResponse(iterator, this)
               : ReadResponse.createDataResponse(iterator, this, rdi);
    }

    @SuppressWarnings("resource") // We don't need to close an empty iterator.
    public ReadResponse createEmptyResponse()
    {
        UnfilteredPartitionIterator iterator = EmptyIterators.unfilteredPartition(metadata());
        
        return isDigestQuery()
               ? ReadResponse.createDigestResponse(iterator, this)
               : ReadResponse.createDataResponse(iterator, this, RepairedDataInfo.NO_OP_REPAIRED_DATA_INFO);
    }

    long indexSerializedSize(int version)
    {
        return null != index
             ? IndexMetadata.serializer.serializedSize(index, version)
             : 0;
    }

    public Index getIndex(ColumnFamilyStore cfs)
    {
        return null != index
             ? cfs.indexManager.getIndex(index)
             : null;
    }

    static IndexMetadata findIndex(TableMetadata table, RowFilter rowFilter)
    {
        if (table.indexes.isEmpty() || rowFilter.isEmpty())
            return null;

        ColumnFamilyStore cfs = Keyspace.openAndGetStore(table);

        Index index = cfs.indexManager.getBestIndexFor(rowFilter);

        return null != index
             ? index.getIndexMetadata()
             : null;
    }

    /**
     * If the index manager for the CFS determines that there's an applicable
     * 2i that can be used to execute this command, call its (optional)
     * validation method to check that nothing in this command's parameters
     * violates the implementation specific validation rules.
     */
    public void maybeValidateIndex()
    {
        if (null != index)
            IndexRegistry.obtain(metadata()).getIndex(index).validate(this);
    }

    /**
     * Executes this command on the local host.
     *
     * @param executionController the execution controller spanning this command
     *
     * @return an iterator over the result of executing this command locally.
     */
    @SuppressWarnings("resource") // The result iterator is closed upon exceptions (we know it's fine to potentially not close the intermediary
                                  // iterators created inside the try as long as we do close the original resultIterator), or by closing the result.
    public UnfilteredPartitionIterator executeLocally(ReadExecutionController executionController)
    {
        long startTimeNanos = nanoTime();

        COMMAND.set(this);
        try
        {
            ColumnFamilyStore cfs = Keyspace.openAndGetStore(metadata());
            Index index = getIndex(cfs);

            Index.Searcher searcher = null;
            if (index != null)
            {
                if (!cfs.indexManager.isIndexQueryable(index))
                    throw new IndexNotAvailableException(index);

                searcher = index.searcherFor(this);
                Tracing.trace("Executing read on {}.{} using index {}", cfs.metadata.keyspace, cfs.metadata.name, index.getIndexMetadata().name);
            }

            UnfilteredPartitionIterator iterator = (null == searcher) ? queryStorage(cfs, executionController) : searcher.search(executionController);
            iterator = RTBoundValidator.validate(iterator, Stage.MERGED, false);

            try
            {
                iterator = withQuerySizeTracking(iterator);
                iterator = maybeSlowDownForTesting(iterator);
                iterator = withQueryCancellation(iterator);
                iterator = RTBoundValidator.validate(withoutPurgeableTombstones(iterator, cfs, executionController), Stage.PURGED, false);
                iterator = withMetricsRecording(iterator, cfs.metric, startTimeNanos);

                // If we've used a 2ndary index, we know the result already satisfy the primary expression used, so
                // no point in checking it again.
                RowFilter filter = (null == searcher) ? rowFilter() : index.getPostIndexQueryFilter(rowFilter());

                /*
                 * TODO: We'll currently do filtering by the rowFilter here because it's convenient. However,
                 * we'll probably want to optimize by pushing it down the layer (like for dropped columns) as it
                 * would be more efficient (the sooner we discard stuff we know we don't care, the less useless
                 * processing we do on it).
                 */
                iterator = filter.filter(iterator, nowInSec());

                // apply the limits/row counter; this transformation is stopping and would close the iterator as soon
                // as the count is observed; if that happens in the middle of an open RT, its end bound will not be included.
                // If tracking repaired data, the counter is needed for overreading repaired data, otherwise we can
                // optimise the case where this.limit = DataLimits.NONE which skips an unnecessary transform
                if (executionController.isTrackingRepairedStatus())
                {
                    DataLimits.Counter limit =
                    limits().newCounter(nowInSec(), false, selectsFullPartition(), metadata().enforceStrictLiveness());
                    iterator = limit.applyTo(iterator);
                    // ensure that a consistent amount of repaired data is read on each replica. This causes silent
                    // overreading from the repaired data set, up to limits(). The extra data is not visible to
                    // the caller, only iterated to produce the repaired data digest.
                    iterator = executionController.getRepairedDataInfo().extend(iterator, limit);
                }
                else
                {
                    iterator = limits().filter(iterator, nowInSec(), selectsFullPartition());
                }

                // because of the above, we need to append an aritifical end bound if the source iterator was stopped short by a counter.
                return RTBoundCloser.close(iterator);
            }
            catch (RuntimeException | Error e)
            {
                iterator.close();
                throw e;
            }
        }
        finally
        {
            COMMAND.set(null);
        }
    }

    protected abstract void recordLatency(TableMetrics metric, long latencyNanos);

    public ReadExecutionController executionController(boolean trackRepairedStatus)
    {
        return ReadExecutionController.forCommand(this, trackRepairedStatus);
    }

    public ReadExecutionController executionController()
    {
        return ReadExecutionController.forCommand(this, false);
    }

    /**
     * Wraps the provided iterator so that metrics on what is scanned by the command are recorded.
     * This also log warning/trow TombstoneOverwhelmingException if appropriate.
     */
    private UnfilteredPartitionIterator withMetricsRecording(UnfilteredPartitionIterator iter, final TableMetrics metric, final long startTimeNanos)
    {
        class MetricRecording extends Transformation<UnfilteredRowIterator>
        {
            private final int failureThreshold = DatabaseDescriptor.getTombstoneFailureThreshold();
            private final int warningThreshold = DatabaseDescriptor.getTombstoneWarnThreshold();

            private final boolean respectTombstoneThresholds = !SchemaConstants.isLocalSystemKeyspace(ReadCommand.this.metadata().keyspace);
            private final boolean enforceStrictLiveness = metadata().enforceStrictLiveness();

            private int liveRows = 0;
            private int lastReportedLiveRows = 0;
            private int tombstones = 0;
            private int lastReportedTombstones = 0;

            private DecoratedKey currentKey;

            @Override
            public UnfilteredRowIterator applyToPartition(UnfilteredRowIterator iter)
            {
                currentKey = iter.partitionKey();
                return Transformation.apply(iter, this);
            }

            @Override
            public Row applyToStatic(Row row)
            {
                return applyToRow(row);
            }

            @Override
            public Row applyToRow(Row row)
            {
                boolean hasTombstones = false;
                for (Cell<?> cell : row.cells())
                {
                    if (!cell.isLive(ReadCommand.this.nowInSec()))
                    {
                        countTombstone(row.clustering());
                        hasTombstones = true; // allows to avoid counting an extra tombstone if the whole row expired
                    }
                }

                if (row.hasLiveData(ReadCommand.this.nowInSec(), enforceStrictLiveness))
                    ++liveRows;
                else if (!row.primaryKeyLivenessInfo().isLive(ReadCommand.this.nowInSec())
                        && row.hasDeletion(ReadCommand.this.nowInSec())
                        && !hasTombstones)
                {
                    // We're counting primary key deletions only here.
                    countTombstone(row.clustering());
                }

                return row;
            }

            @Override
            public RangeTombstoneMarker applyToMarker(RangeTombstoneMarker marker)
            {
                countTombstone(marker.clustering());
                return marker;
            }

            private void countTombstone(ClusteringPrefix<?> clustering)
            {
                ++tombstones;
                if (tombstones > failureThreshold && respectTombstoneThresholds)
                {
                    String query = ReadCommand.this.toCQLString();
                    Tracing.trace("Scanned over {} tombstones for query {}; query aborted (see tombstone_failure_threshold)", failureThreshold, query);
                    metric.tombstoneFailures.inc();
                    if (trackWarnings)
                    {
                        MessageParams.remove(ParamType.TOMBSTONE_WARNING);
                        MessageParams.add(ParamType.TOMBSTONE_FAIL, tombstones);
                    }
                    throw new TombstoneOverwhelmingException(tombstones, query, ReadCommand.this.metadata(), currentKey, clustering);
                }
            }

            @Override
            protected void onPartitionClose()
            {
                int lr = liveRows - lastReportedLiveRows;
                int ts = tombstones - lastReportedTombstones;

                if (lr > 0)
                    metric.topReadPartitionRowCount.addSample(currentKey.getKey(), lr);

                if (ts > 0)
                    metric.topReadPartitionTombstoneCount.addSample(currentKey.getKey(), ts);

                lastReportedLiveRows = liveRows;
                lastReportedTombstones = tombstones;
            }

            @Override
            public void onClose()
            {
                recordLatency(metric, nanoTime() - startTimeNanos);

                metric.tombstoneScannedHistogram.update(tombstones);
                metric.liveScannedHistogram.update(liveRows);

                boolean warnTombstones = tombstones > warningThreshold && respectTombstoneThresholds;
                if (warnTombstones)
                {
                    String msg = String.format(
                            "Read %d live rows and %d tombstone cells for query %1.512s; token %s (see tombstone_warn_threshold)",
                            liveRows, tombstones, ReadCommand.this.toCQLString(), currentKey.getToken());
                    if (trackWarnings)
                        MessageParams.add(ParamType.TOMBSTONE_WARNING, tombstones);
                    else
                        ClientWarn.instance.warn(msg);
                    if (tombstones < failureThreshold)
                    {
                        metric.tombstoneWarnings.inc();
                    }

                    logger.warn(msg);
                }

                Tracing.trace("Read {} live rows and {} tombstone cells{}",
                        liveRows, tombstones,
                        (warnTombstones ? " (see tombstone_warn_threshold)" : ""));
            }
        }

        return Transformation.apply(iter, new MetricRecording());
    }

    private boolean shouldTrackSize(DataStorageSpec.LongBytesBound warnThresholdBytes, DataStorageSpec.LongBytesBound abortThresholdBytes)
    {
        return trackWarnings
               && !SchemaConstants.isSystemKeyspace(metadata().keyspace)
               && !(warnThresholdBytes == null && abortThresholdBytes == null);
    }

    private UnfilteredPartitionIterator withQuerySizeTracking(UnfilteredPartitionIterator iterator)
    {
        DataStorageSpec.LongBytesBound warnThreshold = DatabaseDescriptor.getLocalReadSizeWarnThreshold();
        DataStorageSpec.LongBytesBound failThreshold = DatabaseDescriptor.getLocalReadSizeFailThreshold();
        if (!shouldTrackSize(warnThreshold, failThreshold))
            return iterator;
        final long warnBytes = warnThreshold == null ? -1 : warnThreshold.toBytes();
        final long failBytes = failThreshold == null ? -1 : failThreshold.toBytes();
        class QuerySizeTracking extends Transformation<UnfilteredRowIterator>
        {
            private long sizeInBytes = 0;

            @Override
            public UnfilteredRowIterator applyToPartition(UnfilteredRowIterator iter)
            {
                sizeInBytes += ObjectSizes.sizeOnHeapOf(iter.partitionKey().getKey());
                return Transformation.apply(iter, this);
            }

            @Override
            protected Row applyToStatic(Row row)
            {
                return applyToRow(row);
            }

            @Override
            protected Row applyToRow(Row row)
            {
                addSize(row.unsharedHeapSize());
                return row;
            }

            @Override
            protected RangeTombstoneMarker applyToMarker(RangeTombstoneMarker marker)
            {
                addSize(marker.unsharedHeapSize());
                return marker;
            }

            @Override
            protected DeletionTime applyToDeletion(DeletionTime deletionTime)
            {
                addSize(deletionTime.unsharedHeapSize());
                return deletionTime;
            }

            private void addSize(long size)
            {
                this.sizeInBytes += size;
                if (failBytes != -1 && this.sizeInBytes >= failBytes)
                {
                    String msg = String.format("Query %s attempted to read %d bytes but max allowed is %s; query aborted  (see local_read_size_fail_threshold)",
                                               ReadCommand.this.toCQLString(), this.sizeInBytes, failThreshold);
                    Tracing.trace(msg);
                    MessageParams.remove(ParamType.LOCAL_READ_SIZE_WARN);
                    MessageParams.add(ParamType.LOCAL_READ_SIZE_FAIL, this.sizeInBytes);
                    throw new LocalReadSizeTooLargeException(msg);
                }
                else if (warnBytes != -1 && this.sizeInBytes >= warnBytes)
                {
                    MessageParams.add(ParamType.LOCAL_READ_SIZE_WARN, this.sizeInBytes);
                }
            }

            @Override
            protected void onClose()
            {
                ColumnFamilyStore cfs = Schema.instance.getColumnFamilyStoreInstance(metadata().id);
                if (cfs != null)
                    cfs.metric.localReadSize.update(sizeInBytes);
            }
        }

        iterator = Transformation.apply(iterator, new QuerySizeTracking());
        return iterator;
    }

    private class QueryCancellationChecker extends StoppingTransformation<UnfilteredRowIterator>
    {
        long lastCheckedAt = 0;

        @Override
        protected UnfilteredRowIterator applyToPartition(UnfilteredRowIterator partition)
        {
            maybeCancel();
            return Transformation.apply(partition, this);
        }

        @Override
        protected Row applyToRow(Row row)
        {
            maybeCancel();
            return row;
        }

        private void maybeCancel()
        {
            /*
             * The value returned by approxTime.now() is updated only every
             * {@link org.apache.cassandra.utils.MonotonicClock.SampledClock.CHECK_INTERVAL_MS}, by default 2 millis.
             * Since MonitorableImpl relies on approxTime, we don't need to check unless the approximate time has elapsed.
             */
            if (lastCheckedAt == approxTime.now())
                return;
            lastCheckedAt = approxTime.now();

            if (isAborted())
            {
                stop();
                throw new QueryCancelledException(ReadCommand.this);
            }
        }
    }

    private UnfilteredPartitionIterator withQueryCancellation(UnfilteredPartitionIterator iter)
    {
        return Transformation.apply(iter, new QueryCancellationChecker());
    }

    /**
     *  A transformation used for simulating slow queries by tests.
     */
    private static class DelayInjector extends Transformation<UnfilteredRowIterator>
    {
        @Override
        protected UnfilteredRowIterator applyToPartition(UnfilteredRowIterator partition)
        {
            FBUtilities.sleepQuietly(TEST_ITERATION_DELAY_MILLIS);
            return Transformation.apply(partition, this);
        }

        @Override
        protected Row applyToRow(Row row)
        {
            FBUtilities.sleepQuietly(TEST_ITERATION_DELAY_MILLIS);
            return row;
        }
    }

    private UnfilteredPartitionIterator maybeSlowDownForTesting(UnfilteredPartitionIterator iter)
    {
        if (TEST_ITERATION_DELAY_MILLIS > 0 && !SchemaConstants.isSystemKeyspace(metadata().keyspace))
            return Transformation.apply(iter, new DelayInjector());
        else
            return iter;
    }

    /**
     * Creates a message for this command.
     */
    public Message<ReadCommand> createMessage(boolean trackRepairedData)
    {
        Message<ReadCommand> msg = trackRepairedData
                                   ? Message.outWithFlags(verb(), this, MessageFlag.CALL_BACK_ON_FAILURE, MessageFlag.TRACK_REPAIRED_DATA)
                                   : Message.outWithFlag(verb(), this, MessageFlag.CALL_BACK_ON_FAILURE);
        if (trackWarnings)
            msg = msg.withFlag(MessageFlag.TRACK_WARNINGS);
        return msg;
    }

    protected abstract boolean intersects(SSTableReader sstable);

    protected boolean hasRequiredStatics(SSTableReader sstable) {
        // If some static columns are queried, we should always include the sstable: the clustering values stats of the sstable
        // don't tell us if the sstable contains static values in particular.
        return !columnFilter().fetchedColumns().statics.isEmpty() && sstable.header.hasStatic();
    }

    protected boolean hasPartitionLevelDeletions(SSTableReader sstable)
    {
        return sstable.getSSTableMetadata().hasPartitionLevelDeletions;
    }

    public abstract Verb verb();

    protected abstract void appendCQLWhereClause(StringBuilder sb);

    // Skip purgeable tombstones. We do this because it's safe to do (post-merge of the memtable and sstable at least), it
    // can save us some bandwith, and avoid making us throw a TombstoneOverwhelmingException for purgeable tombstones (which
    // are to some extend an artefact of compaction lagging behind and hence counting them is somewhat unintuitive).
    protected UnfilteredPartitionIterator withoutPurgeableTombstones(UnfilteredPartitionIterator iterator, 
                                                                     ColumnFamilyStore cfs,
                                                                     ReadExecutionController controller)
    {
        class WithoutPurgeableTombstones extends PurgeFunction
        {
            public WithoutPurgeableTombstones()
            {
                super(nowInSec(), cfs.gcBefore(nowInSec()), controller.oldestUnrepairedTombstone(),
                      cfs.getCompactionStrategyManager().onlyPurgeRepairedTombstones(),
                      iterator.metadata().enforceStrictLiveness());
            }

            protected LongPredicate getPurgeEvaluator()
            {
                return time -> true;
            }
        }
        return Transformation.apply(iterator, new WithoutPurgeableTombstones());
    }

    /**
     * Return the queried token(s) for logging
     */
    public abstract String loggableTokens();

    // Monitorable interface
    public String name()
    {
        return toCQLString();
    }

    @SuppressWarnings("resource") // resultant iterators are closed by their callers
    InputCollector<UnfilteredRowIterator> iteratorsForPartition(ColumnFamilyStore.ViewFragment view, ReadExecutionController controller)
    {
        final BiFunction<List<UnfilteredRowIterator>, RepairedDataInfo, UnfilteredRowIterator> merge =
            (unfilteredRowIterators, repairedDataInfo) -> {
                UnfilteredRowIterator repaired = UnfilteredRowIterators.merge(unfilteredRowIterators);
                return repairedDataInfo.withRepairedDataInfo(repaired);
            };

        // For single partition reads, after reading up to the command's DataLimit nothing extra is required.
        // The merged & repaired row iterator will be consumed until it's exhausted or the RepairedDataInfo's
        // internal counter is satisfied
        final Function<UnfilteredRowIterator, UnfilteredPartitionIterator> postLimitPartitions =
            (rows) -> EmptyIterators.unfilteredPartition(metadata());
        return new InputCollector<>(view, controller, merge, postLimitPartitions);
    }

    @SuppressWarnings("resource") // resultant iterators are closed by their callers
    InputCollector<UnfilteredPartitionIterator> iteratorsForRange(ColumnFamilyStore.ViewFragment view, ReadExecutionController controller)
    {
        final BiFunction<List<UnfilteredPartitionIterator>, RepairedDataInfo, UnfilteredPartitionIterator> merge =
            (unfilteredPartitionIterators, repairedDataInfo) -> {
                UnfilteredPartitionIterator repaired = UnfilteredPartitionIterators.merge(unfilteredPartitionIterators,
                                                                                          NOOP);
                return repairedDataInfo.withRepairedDataInfo(repaired);
            };

        // Uses identity function to provide additional partitions to be consumed after the command's
        // DataLimits are satisfied. The input to the function will be the iterator of merged, repaired partitions
        // which we'll keep reading until the RepairedDataInfo's internal counter is satisfied.
        return new InputCollector<>(view, controller, merge, Function.identity());
    }

    /**
     * Handles the collation of unfiltered row or partition iterators that comprise the
     * input for a query. Separates them according to repaired status and of repaired
     * status is being tracked, handles the merge and wrapping in a digest generator of
     * the repaired iterators.
     *
     * Intentionally not AutoCloseable so we don't mistakenly use this in ARM blocks
     * as this prematurely closes the underlying iterators
     */
    static class InputCollector<T extends AutoCloseable>
    {
        final RepairedDataInfo repairedDataInfo;
        private final boolean isTrackingRepairedStatus;
        Set<SSTableReader> repairedSSTables;
        BiFunction<List<T>, RepairedDataInfo, T> repairedMerger;
        Function<T, UnfilteredPartitionIterator> postLimitAdditionalPartitions;
        List<T> repairedIters;
        List<T> unrepairedIters;

        InputCollector(ColumnFamilyStore.ViewFragment view,
                       ReadExecutionController controller,
                       BiFunction<List<T>, RepairedDataInfo, T> repairedMerger,
                       Function<T, UnfilteredPartitionIterator> postLimitAdditionalPartitions)
        {
            this.repairedDataInfo = controller.getRepairedDataInfo();
            this.isTrackingRepairedStatus = controller.isTrackingRepairedStatus();
            
            if (isTrackingRepairedStatus)
            {
                for (SSTableReader sstable : view.sstables)
                {
                    if (considerRepairedForTracking(sstable))
                    {
                        if (repairedSSTables == null)
                            repairedSSTables = Sets.newHashSetWithExpectedSize(view.sstables.size());
                        repairedSSTables.add(sstable);
                    }
                }
            }
            if (repairedSSTables == null)
            {
                repairedIters = Collections.emptyList();
                unrepairedIters = new ArrayList<>(view.sstables.size());
            }
            else
            {
                repairedIters = new ArrayList<>(repairedSSTables.size());
                // when we're done collating, we'll merge the repaired iters and add the
                // result to the unrepaired list, so size that list accordingly
                unrepairedIters = new ArrayList<>((view.sstables.size() - repairedSSTables.size()) + Iterables.size(view.memtables) + 1);
            }
            this.repairedMerger = repairedMerger;
            this.postLimitAdditionalPartitions = postLimitAdditionalPartitions;
        }

        void addMemtableIterator(T iter)
        {
            unrepairedIters.add(iter);
        }

        void addSSTableIterator(SSTableReader sstable, T iter)
        {
            if (repairedSSTables != null && repairedSSTables.contains(sstable))
                repairedIters.add(iter);
            else
                unrepairedIters.add(iter);
        }

        @SuppressWarnings("resource") // the returned iterators are closed by the caller
        List<T> finalizeIterators(ColumnFamilyStore cfs, long nowInSec, long oldestUnrepairedTombstone)
        {
            if (repairedIters.isEmpty())
                return unrepairedIters;

            // merge the repaired data before returning, wrapping in a digest generator
            repairedDataInfo.prepare(cfs, nowInSec, oldestUnrepairedTombstone);
            T repairedIter = repairedMerger.apply(repairedIters, repairedDataInfo);
            repairedDataInfo.finalize(postLimitAdditionalPartitions.apply(repairedIter));
            unrepairedIters.add(repairedIter);
            return unrepairedIters;
        }

        boolean isEmpty()
        {
            return repairedIters.isEmpty() && unrepairedIters.isEmpty();
        }

        // For tracking purposes we consider data repaired if the sstable is either:
        // * marked repaired
        // * marked pending, but the local session has been committed. This reduces the window
        //   whereby the tracking is affected by compaction backlog causing repaired sstables to
        //   remain in the pending state
        // If an sstable is involved in a pending repair which is not yet committed, we mark the
        // repaired data info inconclusive, as the same data on other replicas may be in a
        // slightly different state.
        private boolean considerRepairedForTracking(SSTableReader sstable)
        {
            if (!isTrackingRepairedStatus)
                return false;

            TimeUUID pendingRepair = sstable.getPendingRepair();
            if (pendingRepair != ActiveRepairService.NO_PENDING_REPAIR)
            {
                if (ActiveRepairService.instance.consistent.local.isSessionFinalized(pendingRepair))
                    return true;

                // In the edge case where compaction is backed up long enough for the session to
                // timeout and be purged by LocalSessions::cleanup, consider the sstable unrepaired
                // as it will be marked unrepaired when compaction catches up
                if (!ActiveRepairService.instance.consistent.local.sessionExists(pendingRepair))
                    return false;

                repairedDataInfo.markInconclusive();
            }

            return sstable.isRepaired();
        }

        void markInconclusive()
        {
            repairedDataInfo.markInconclusive();
        }

        public void close() throws Exception
        {
            FBUtilities.closeAll(unrepairedIters);
            FBUtilities.closeAll(repairedIters);
        }
    }

    @VisibleForTesting
    public static class Serializer implements IVersionedSerializer<ReadCommand>
    {
        private final SchemaProvider schema;

        public Serializer()
        {
            this(Schema.instance);
        }

        @VisibleForTesting
        public Serializer(SchemaProvider schema)
        {
            this.schema = Objects.requireNonNull(schema, "schema");
        }

        private static int digestFlag(boolean isDigest)
        {
            return isDigest ? 0x01 : 0;
        }

        private static boolean isDigest(int flags)
        {
            return (flags & 0x01) != 0;
        }

        private static boolean acceptsTransient(int flags)
        {
            return (flags & 0x08) != 0;
        }

        private static int acceptsTransientFlag(boolean acceptsTransient)
        {
            return acceptsTransient ? 0x08 : 0;
        }

        // We don't set this flag anymore, but still look if we receive a
        // command with it set in case someone is using thrift a mixed 3.0/4.0+
        // cluster (which is unsupported). This is also a reminder for not
        // re-using this flag until we drop 3.0/3.X compatibility (since it's
        // used by these release for thrift and would thus confuse things)
        private static boolean isForThrift(int flags)
        {
            return (flags & 0x02) != 0;
        }

        private static int indexFlag(boolean hasIndex)
        {
            return hasIndex ? 0x04 : 0;
        }

        private static boolean hasIndex(int flags)
        {
            return (flags & 0x04) != 0;
        }

        public void serialize(ReadCommand command, DataOutputPlus out, int version) throws IOException
        {
            out.writeByte(command.kind.ordinal());
            out.writeByte(
                    digestFlag(command.isDigestQuery())
                    | indexFlag(null != command.indexMetadata())
                    | acceptsTransientFlag(command.acceptsTransient())
            );
            if (command.isDigestQuery())
                out.writeUnsignedVInt32(command.digestVersion());
            command.metadata().id.serialize(out);
            out.writeInt(version >= MessagingService.VERSION_50 ? CassandraUInt.fromLong(command.nowInSec()) : (int) command.nowInSec());
            ColumnFilter.serializer.serialize(command.columnFilter(), out, version);
            RowFilter.serializer.serialize(command.rowFilter(), out, version);
            DataLimits.serializer.serialize(command.limits(), out, version, command.metadata().comparator);
            if (null != command.index)
                IndexMetadata.serializer.serialize(command.index, out, version);

            command.serializeSelection(out, version);
        }

        public ReadCommand deserialize(DataInputPlus in, int version) throws IOException
        {
            Kind kind = Kind.values()[in.readByte()];
            int flags = in.readByte();
            boolean isDigest = isDigest(flags);
            boolean acceptsTransient = acceptsTransient(flags);
            // Shouldn't happen or it's a user error (see comment above) but
            // better complain loudly than doing the wrong thing.
            if (isForThrift(flags))
                throw new IllegalStateException("Received a command with the thrift flag set. "
                                              + "This means thrift is in use in a mixed 3.0/3.X and 4.0+ cluster, "
                                              + "which is unsupported. Make sure to stop using thrift before "
                                              + "upgrading to 4.0");

            boolean hasIndex = hasIndex(flags);
            int digestVersion = isDigest ? in.readUnsignedVInt32() : 0;
            TableMetadata metadata = schema.getExistingTableMetadata(TableId.deserialize(in));
            long nowInSec = version >= MessagingService.VERSION_50 ? CassandraUInt.toLong(in.readInt()) : in.readInt();
            ColumnFilter columnFilter = ColumnFilter.serializer.deserialize(in, version, metadata);
            RowFilter rowFilter = RowFilter.serializer.deserialize(in, version, metadata);
            DataLimits limits = DataLimits.serializer.deserialize(in, version,  metadata);
            IndexMetadata index = hasIndex ? deserializeIndexMetadata(in, version, metadata) : null;

            return kind.selectionDeserializer.deserialize(in, version, isDigest, digestVersion, acceptsTransient, metadata, nowInSec, columnFilter, rowFilter, limits, index);
        }

        private IndexMetadata deserializeIndexMetadata(DataInputPlus in, int version, TableMetadata metadata) throws IOException
        {
            try
            {
                return IndexMetadata.serializer.deserialize(in, version, metadata);
            }
            catch (UnknownIndexException e)
            {
                logger.info("Couldn't find a defined index on {}.{} with the id {}. " +
                            "If an index was just created, this is likely due to the schema not " +
                            "being fully propagated. Local read will proceed without using the " +
                            "index. Please wait for schema agreement after index creation.",
                            metadata.keyspace, metadata.name, e.indexId);
                return null;
            }
        }

        public long serializedSize(ReadCommand command, int version)
        {
            return 2 // kind + flags
                   + (command.isDigestQuery() ? TypeSizes.sizeofUnsignedVInt(command.digestVersion()) : 0)
                   + command.metadata().id.serializedSize()
                   + TypeSizes.INT_SIZE // command.nowInSec() is serialized as uint
                   + ColumnFilter.serializer.serializedSize(command.columnFilter(), version)
                   + RowFilter.serializer.serializedSize(command.rowFilter(), version)
                   + DataLimits.serializer.serializedSize(command.limits(), version, command.metadata().comparator)
                   + command.selectionSerializedSize(version)
                   + command.indexSerializedSize(version);
        }
    }
}