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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
* 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.service;
import java.net.InetAddress;
import java.util.Collection;
import java.util.List;
import java.util.concurrent.TimeUnit;
import com.google.common.collect.Iterables;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.apache.cassandra.concurrent.Stage;
import org.apache.cassandra.concurrent.StageManager;
import org.apache.cassandra.config.ReadRepairDecision;
import org.apache.cassandra.db.ColumnFamilyStore;
import org.apache.cassandra.db.ConsistencyLevel;
import org.apache.cassandra.db.ReadCommand;
import org.apache.cassandra.db.SinglePartitionReadCommand;
import org.apache.cassandra.db.Keyspace;
import org.apache.cassandra.db.partitions.PartitionIterator;
import org.apache.cassandra.exceptions.ReadFailureException;
import org.apache.cassandra.exceptions.ReadTimeoutException;
import org.apache.cassandra.exceptions.UnavailableException;
import org.apache.cassandra.metrics.ReadRepairMetrics;
import org.apache.cassandra.net.MessageOut;
import org.apache.cassandra.net.MessagingService;
import org.apache.cassandra.schema.SpeculativeRetryParam;
import org.apache.cassandra.service.StorageProxy.LocalReadRunnable;
import org.apache.cassandra.tracing.TraceState;
import org.apache.cassandra.tracing.Tracing;
/**
* Sends a read request to the replicas needed to satisfy a given ConsistencyLevel.
*
* Optionally, may perform additional requests to provide redundancy against replica failure:
* AlwaysSpeculatingReadExecutor will always send a request to one extra replica, while
* SpeculatingReadExecutor will wait until it looks like the original request is in danger
* of timing out before performing extra reads.
*/
public abstract class AbstractReadExecutor
{
private static final Logger logger = LoggerFactory.getLogger(AbstractReadExecutor.class);
protected final ReadCommand command;
protected final List targetReplicas;
protected final ReadCallback handler;
protected final TraceState traceState;
AbstractReadExecutor(Keyspace keyspace, ReadCommand command, ConsistencyLevel consistencyLevel, List targetReplicas, long queryStartNanoTime)
{
this.command = command;
this.targetReplicas = targetReplicas;
this.handler = new ReadCallback(new DigestResolver(keyspace, command, consistencyLevel, targetReplicas.size()), consistencyLevel, command, targetReplicas, queryStartNanoTime);
this.traceState = Tracing.instance.get();
// Set the digest version (if we request some digests). This is the smallest version amongst all our target replicas since new nodes
// knows how to produce older digest but the reverse is not true.
// TODO: we need this when talking with pre-3.0 nodes. So if we preserve the digest format moving forward, we can get rid of this once
// we stop being compatible with pre-3.0 nodes.
int digestVersion = MessagingService.current_version;
for (InetAddress replica : targetReplicas)
digestVersion = Math.min(digestVersion, MessagingService.instance().getVersion(replica));
command.setDigestVersion(digestVersion);
}
protected void makeDataRequests(Iterable endpoints)
{
makeRequests(command, endpoints);
}
protected void makeDigestRequests(Iterable endpoints)
{
makeRequests(command.copyAsDigestQuery(), endpoints);
}
private void makeRequests(ReadCommand readCommand, Iterable endpoints)
{
boolean hasLocalEndpoint = false;
for (InetAddress endpoint : endpoints)
{
if (StorageProxy.canDoLocalRequest(endpoint))
{
hasLocalEndpoint = true;
continue;
}
if (traceState != null)
traceState.trace("reading {} from {}", readCommand.isDigestQuery() ? "digest" : "data", endpoint);
logger.trace("reading {} from {}", readCommand.isDigestQuery() ? "digest" : "data", endpoint);
MessageOut message = readCommand.createMessage(MessagingService.instance().getVersion(endpoint));
MessagingService.instance().sendRRWithFailure(message, endpoint, handler);
}
// We delay the local (potentially blocking) read till the end to avoid stalling remote requests.
if (hasLocalEndpoint)
{
logger.trace("reading {} locally", readCommand.isDigestQuery() ? "digest" : "data");
StageManager.getStage(Stage.READ).maybeExecuteImmediately(new LocalReadRunnable(command, handler));
}
}
/**
* Perform additional requests if it looks like the original will time out. May block while it waits
* to see if the original requests are answered first.
*/
public abstract void maybeTryAdditionalReplicas();
/**
* Get the replicas involved in the [finished] request.
*
* @return target replicas + the extra replica, *IF* we speculated.
*/
public abstract Collection getContactedReplicas();
/**
* send the initial set of requests
*/
public abstract void executeAsync();
/**
* wait for an answer. Blocks until success or timeout, so it is caller's
* responsibility to call maybeTryAdditionalReplicas first.
*/
public PartitionIterator get() throws ReadFailureException, ReadTimeoutException, DigestMismatchException
{
return handler.get();
}
/**
* @return an executor appropriate for the configured speculative read policy
*/
public static AbstractReadExecutor getReadExecutor(SinglePartitionReadCommand command, ConsistencyLevel consistencyLevel, long queryStartNanoTime) throws UnavailableException
{
Keyspace keyspace = Keyspace.open(command.metadata().ksName);
List allReplicas = StorageProxy.getLiveSortedEndpoints(keyspace, command.partitionKey());
// 11980: Excluding EACH_QUORUM reads from potential RR, so that we do not miscount DC responses
ReadRepairDecision repairDecision = consistencyLevel == ConsistencyLevel.EACH_QUORUM
? ReadRepairDecision.NONE
: command.metadata().newReadRepairDecision();
List targetReplicas = consistencyLevel.filterForQuery(keyspace, allReplicas, repairDecision);
// Throw UAE early if we don't have enough replicas.
consistencyLevel.assureSufficientLiveNodes(keyspace, targetReplicas);
if (repairDecision != ReadRepairDecision.NONE)
{
Tracing.trace("Read-repair {}", repairDecision);
ReadRepairMetrics.attempted.mark();
}
ColumnFamilyStore cfs = keyspace.getColumnFamilyStore(command.metadata().cfId);
SpeculativeRetryParam retry = cfs.metadata.params.speculativeRetry;
// Speculative retry is disabled *OR* there are simply no extra replicas to speculate.
// 11980: Disable speculative retry if using EACH_QUORUM in order to prevent miscounting DC responses
if (retry.equals(SpeculativeRetryParam.NONE)
|| consistencyLevel == ConsistencyLevel.EACH_QUORUM
|| consistencyLevel.blockFor(keyspace) == allReplicas.size())
return new NeverSpeculatingReadExecutor(keyspace, command, consistencyLevel, targetReplicas, queryStartNanoTime);
if (targetReplicas.size() == allReplicas.size())
{
// CL.ALL, RRD.GLOBAL or RRD.DC_LOCAL and a single-DC.
// We are going to contact every node anyway, so ask for 2 full data requests instead of 1, for redundancy
// (same amount of requests in total, but we turn 1 digest request into a full blown data request).
return new AlwaysSpeculatingReadExecutor(keyspace, cfs, command, consistencyLevel, targetReplicas, queryStartNanoTime);
}
// RRD.NONE or RRD.DC_LOCAL w/ multiple DCs.
InetAddress extraReplica = allReplicas.get(targetReplicas.size());
// With repair decision DC_LOCAL all replicas/target replicas may be in different order, so
// we might have to find a replacement that's not already in targetReplicas.
if (repairDecision == ReadRepairDecision.DC_LOCAL && targetReplicas.contains(extraReplica))
{
for (InetAddress address : allReplicas)
{
if (!targetReplicas.contains(address))
{
extraReplica = address;
break;
}
}
}
targetReplicas.add(extraReplica);
if (retry.equals(SpeculativeRetryParam.ALWAYS))
return new AlwaysSpeculatingReadExecutor(keyspace, cfs, command, consistencyLevel, targetReplicas, queryStartNanoTime);
else // PERCENTILE or CUSTOM.
return new SpeculatingReadExecutor(keyspace, cfs, command, consistencyLevel, targetReplicas, queryStartNanoTime);
}
public static class NeverSpeculatingReadExecutor extends AbstractReadExecutor
{
public NeverSpeculatingReadExecutor(Keyspace keyspace, ReadCommand command, ConsistencyLevel consistencyLevel, List targetReplicas, long queryStartNanoTime)
{
super(keyspace, command, consistencyLevel, targetReplicas, queryStartNanoTime);
}
public void executeAsync()
{
makeDataRequests(targetReplicas.subList(0, 1));
if (targetReplicas.size() > 1)
makeDigestRequests(targetReplicas.subList(1, targetReplicas.size()));
}
public void maybeTryAdditionalReplicas()
{
// no-op
}
public Collection getContactedReplicas()
{
return targetReplicas;
}
}
private static class SpeculatingReadExecutor extends AbstractReadExecutor
{
private final ColumnFamilyStore cfs;
private volatile boolean speculated = false;
public SpeculatingReadExecutor(Keyspace keyspace,
ColumnFamilyStore cfs,
ReadCommand command,
ConsistencyLevel consistencyLevel,
List targetReplicas,
long queryStartNanoTime)
{
super(keyspace, command, consistencyLevel, targetReplicas, queryStartNanoTime);
this.cfs = cfs;
}
public void executeAsync()
{
// if CL + RR result in covering all replicas, getReadExecutor forces AlwaysSpeculating. So we know
// that the last replica in our list is "extra."
List initialReplicas = targetReplicas.subList(0, targetReplicas.size() - 1);
if (handler.blockfor < initialReplicas.size())
{
// We're hitting additional targets for read repair. Since our "extra" replica is the least-
// preferred by the snitch, we do an extra data read to start with against a replica more
// likely to reply; better to let RR fail than the entire query.
makeDataRequests(initialReplicas.subList(0, 2));
if (initialReplicas.size() > 2)
makeDigestRequests(initialReplicas.subList(2, initialReplicas.size()));
}
else
{
// not doing read repair; all replies are important, so it doesn't matter which nodes we
// perform data reads against vs digest.
makeDataRequests(initialReplicas.subList(0, 1));
if (initialReplicas.size() > 1)
makeDigestRequests(initialReplicas.subList(1, initialReplicas.size()));
}
}
public void maybeTryAdditionalReplicas()
{
// no latency information, or we're overloaded
if (cfs.sampleLatencyNanos > TimeUnit.MILLISECONDS.toNanos(command.getTimeout()))
return;
if (!handler.await(cfs.sampleLatencyNanos, TimeUnit.NANOSECONDS))
{
// Could be waiting on the data, or on enough digests.
ReadCommand retryCommand = command;
if (handler.resolver.isDataPresent())
retryCommand = command.copyAsDigestQuery();
InetAddress extraReplica = Iterables.getLast(targetReplicas);
if (traceState != null)
traceState.trace("speculating read retry on {}", extraReplica);
logger.trace("speculating read retry on {}", extraReplica);
int version = MessagingService.instance().getVersion(extraReplica);
MessagingService.instance().sendRRWithFailure(retryCommand.createMessage(version), extraReplica, handler);
speculated = true;
cfs.metric.speculativeRetries.inc();
}
}
public Collection getContactedReplicas()
{
return speculated
? targetReplicas
: targetReplicas.subList(0, targetReplicas.size() - 1);
}
}
private static class AlwaysSpeculatingReadExecutor extends AbstractReadExecutor
{
private final ColumnFamilyStore cfs;
public AlwaysSpeculatingReadExecutor(Keyspace keyspace,
ColumnFamilyStore cfs,
ReadCommand command,
ConsistencyLevel consistencyLevel,
List targetReplicas,
long queryStartNanoTime)
{
super(keyspace, command, consistencyLevel, targetReplicas, queryStartNanoTime);
this.cfs = cfs;
}
public void maybeTryAdditionalReplicas()
{
// no-op
}
public Collection getContactedReplicas()
{
return targetReplicas;
}
@Override
public void executeAsync()
{
makeDataRequests(targetReplicas.subList(0, targetReplicas.size() > 1 ? 2 : 1));
if (targetReplicas.size() > 2)
makeDigestRequests(targetReplicas.subList(2, targetReplicas.size()));
cfs.metric.speculativeRetries.inc();
}
}
}