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The Apache Cassandra Project develops a highly scalable second-generation distributed database, bringing together Dynamo's fully distributed design and Bigtable's ColumnFamily-based data model.
/*
* Licensed to the Apache Software Foundation (ASF) under one
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* to you under the Apache License, Version 2.0 (the
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*
* http://www.apache.org/licenses/LICENSE-2.0
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* Unless required by applicable law or agreed to in writing, software
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package org.apache.cassandra.service.reads;
import java.util.function.Function;
import org.apache.cassandra.db.Clustering;
import org.apache.cassandra.db.ColumnFamilyStore;
import org.apache.cassandra.db.DecoratedKey;
import org.apache.cassandra.db.ReadCommand;
import org.apache.cassandra.db.SinglePartitionReadCommand;
import org.apache.cassandra.db.filter.ClusteringIndexFilter;
import org.apache.cassandra.db.filter.DataLimits;
import org.apache.cassandra.db.partitions.UnfilteredPartitionIterator;
import org.apache.cassandra.db.partitions.UnfilteredPartitionIterators;
import org.apache.cassandra.db.rows.Row;
import org.apache.cassandra.db.rows.UnfilteredRowIterator;
import org.apache.cassandra.db.transform.MoreRows;
import org.apache.cassandra.db.transform.Transformation;
import org.apache.cassandra.locator.Replica;
import org.apache.cassandra.schema.TableMetadata;
import org.apache.cassandra.tracing.Tracing;
class ShortReadRowsProtection extends Transformation implements MoreRows
{
private final ReadCommand command;
private final Replica source;
private final DataLimits.Counter singleResultCounter; // unmerged per-source counter
private final DataLimits.Counter mergedResultCounter; // merged end-result counter
private final Function commandExecutor;
private final TableMetadata metadata;
private final DecoratedKey partitionKey;
private Clustering lastClustering; // clustering of the last observed row
private int lastCounted = 0; // last seen recorded # before attempting to fetch more rows
private int lastFetched = 0; // # rows returned by last attempt to get more (or by the original read command)
private int lastQueried = 0; // # extra rows requested from the replica last time
ShortReadRowsProtection(DecoratedKey partitionKey, ReadCommand command, Replica source,
Function commandExecutor,
DataLimits.Counter singleResultCounter, DataLimits.Counter mergedResultCounter)
{
this.command = command;
this.source = source;
this.commandExecutor = commandExecutor;
this.singleResultCounter = singleResultCounter;
this.mergedResultCounter = mergedResultCounter;
this.metadata = command.metadata();
this.partitionKey = partitionKey;
}
@Override
public Row applyToRow(Row row)
{
lastClustering = row.clustering();
return row;
}
/*
* We only get here once all the rows in this iterator have been iterated over, and so if the node
* had returned the requested number of rows but we still get here, then some results were skipped
* during reconciliation.
*/
public UnfilteredRowIterator moreContents()
{
// never try to request additional rows from replicas if our reconciled partition is already filled to the limit
assert !mergedResultCounter.isDoneForPartition();
// we do not apply short read protection when we have no limits at all
assert !command.limits().isUnlimited();
/*
* If the returned partition doesn't have enough rows to satisfy even the original limit, don't ask for more.
*
* Can only take the short cut if there is no per partition limit set. Otherwise it's possible to hit false
* positives due to some rows being uncounted for in certain scenarios (see CASSANDRA-13911).
*/
if (command.limits().isExhausted(singleResultCounter) && command.limits().perPartitionCount() == DataLimits.NO_LIMIT)
return null;
/*
* If the replica has no live rows in the partition, don't try to fetch more.
*
* Note that the previous branch [if (!singleResultCounter.isDoneForPartition()) return null] doesn't
* always cover this scenario:
* isDoneForPartition() is defined as [isDone() || rowInCurrentPartition >= perPartitionLimit],
* and will return true if isDone() returns true, even if there are 0 rows counted in the current partition.
*
* This can happen with a range read if after 1+ rounds of short read protection requests we managed to fetch
* enough extra rows for other partitions to satisfy the singleResultCounter's total row limit, but only
* have tombstones in the current partition.
*
* One other way we can hit this condition is when the partition only has a live static row and no regular
* rows. In that scenario the counter will remain at 0 until the partition is closed - which happens after
* the moreContents() call.
*/
if (singleResultCounter.rowsCountedInCurrentPartition() == 0)
return null;
/*
* This is a table with no clustering columns, and has at most one row per partition - with EMPTY clustering.
* We already have the row, so there is no point in asking for more from the partition.
*/
if (lastClustering != null && lastClustering.isEmpty())
return null;
lastFetched = singleResultCounter.rowsCountedInCurrentPartition() - lastCounted;
lastCounted = singleResultCounter.rowsCountedInCurrentPartition();
// getting back fewer rows than we asked for means the partition on the replica has been fully consumed
if (lastQueried > 0 && lastFetched < lastQueried)
return null;
/*
* At this point we know that:
* 1. the replica returned [repeatedly?] as many rows as we asked for and potentially has more
* rows in the partition
* 2. at least one of those returned rows was shadowed by a tombstone returned from another
* replica
* 3. we haven't satisfied the client's limits yet, and should attempt to query for more rows to
* avoid a short read
*
* In the ideal scenario, we would get exactly min(a, b) or fewer rows from the next request, where a and b
* are defined as follows:
* [a] limits.count() - mergedResultCounter.counted()
* [b] limits.perPartitionCount() - mergedResultCounter.countedInCurrentPartition()
*
* It would be naive to query for exactly that many rows, as it's possible and not unlikely
* that some of the returned rows would also be shadowed by tombstones from other hosts.
*
* Note: we don't know, nor do we care, how many rows from the replica made it into the reconciled result;
* we can only tell how many in total we queried for, and that [0, mrc.countedInCurrentPartition()) made it.
*
* In general, our goal should be to minimise the number of extra requests - *not* to minimise the number
* of rows fetched: there is a high transactional cost for every individual request, but a relatively low
* marginal cost for each extra row requested.
*
* As such it's better to overfetch than to underfetch extra rows from a host; but at the same
* time we want to respect paging limits and not blow up spectacularly.
*
* Note: it's ok to retrieve more rows that necessary since singleResultCounter is not stopping and only
* counts.
*
* With that in mind, we'll just request the minimum of (count(), perPartitionCount()) limits.
*
* See CASSANDRA-13794 for more details.
*/
lastQueried = Math.min(command.limits().count(), command.limits().perPartitionCount());
ColumnFamilyStore.metricsFor(metadata.id).shortReadProtectionRequests.mark();
Tracing.trace("Requesting {} extra rows from {} for short read protection", lastQueried, source);
SinglePartitionReadCommand cmd = makeFetchAdditionalRowsReadCommand(lastQueried);
return UnfilteredPartitionIterators.getOnlyElement(commandExecutor.apply(cmd), cmd);
}
private SinglePartitionReadCommand makeFetchAdditionalRowsReadCommand(int toQuery)
{
ClusteringIndexFilter filter = command.clusteringIndexFilter(partitionKey);
if (null != lastClustering)
filter = filter.forPaging(metadata.comparator, lastClustering, false);
return SinglePartitionReadCommand.create(command.metadata(),
command.nowInSec(),
command.columnFilter(),
command.rowFilter(),
command.limits().forShortReadRetry(toQuery),
partitionKey,
filter,
command.indexMetadata());
}
}