com.datastax.driver.core.policies.RoundRobinPolicy Maven / Gradle / Ivy
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/*
* Copyright DataStax, Inc.
*
* Licensed 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 com.datastax.driver.core.policies;
import com.datastax.driver.core.Cluster;
import com.datastax.driver.core.Configuration;
import com.datastax.driver.core.ConsistencyLevel;
import com.datastax.driver.core.Host;
import com.datastax.driver.core.HostDistance;
import com.datastax.driver.core.Statement;
import com.google.common.collect.AbstractIterator;
import java.util.Collection;
import java.util.Iterator;
import java.util.List;
import java.util.Random;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.atomic.AtomicInteger;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
/**
* A Round-robin load balancing policy.
*
* This policy queries nodes in a round-robin fashion. For a given query, if an host fail, the
* next one (following the round-robin order) is tried, until all hosts have been tried.
*
*
This policy is not datacenter aware and will include every known Cassandra host in its round
* robin algorithm. If you use multiple datacenter this will be inefficient and you will want to use
* the {@link DCAwareRoundRobinPolicy} load balancing policy instead.
*/
public class RoundRobinPolicy implements LoadBalancingPolicy {
private static final Logger logger = LoggerFactory.getLogger(RoundRobinPolicy.class);
private final CopyOnWriteArrayList liveHosts = new CopyOnWriteArrayList();
private final AtomicInteger index = new AtomicInteger();
private volatile Configuration configuration;
private volatile boolean hasLoggedLocalCLUse;
/**
* Creates a load balancing policy that picks host to query in a round robin fashion (on all the
* hosts of the Cassandra cluster).
*/
public RoundRobinPolicy() {}
@Override
public void init(Cluster cluster, Collection hosts) {
this.liveHosts.addAll(hosts);
this.configuration = cluster.getConfiguration();
this.index.set(new Random().nextInt(Math.max(hosts.size(), 1)));
}
/**
* Return the HostDistance for the provided host.
*
* This policy consider all nodes as local. This is generally the right thing to do in a single
* datacenter deployment. If you use multiple datacenter, see {@link DCAwareRoundRobinPolicy}
* instead.
*
* @param host the host of which to return the distance of.
* @return the HostDistance to {@code host}.
*/
@Override
public HostDistance distance(Host host) {
return HostDistance.LOCAL;
}
/**
* Returns the hosts to use for a new query.
*
*
The returned plan will try each known host of the cluster. Upon each call to this method,
* the {@code i}th host of the plans returned will cycle over all the hosts of the cluster in a
* round-robin fashion.
*
* @param loggedKeyspace the keyspace currently logged in on for this query.
* @param statement the query for which to build the plan.
* @return a new query plan, i.e. an iterator indicating which host to try first for querying,
* which one to use as failover, etc...
*/
@Override
public Iterator newQueryPlan(String loggedKeyspace, Statement statement) {
if (!hasLoggedLocalCLUse) {
ConsistencyLevel cl =
statement.getConsistencyLevel() == null
? configuration.getQueryOptions().getConsistencyLevel()
: statement.getConsistencyLevel();
if (cl.isDCLocal()) {
hasLoggedLocalCLUse = true;
logger.warn(
"Detected request at Consistency Level {} but the non-DC aware RoundRobinPolicy is in use. "
+ "It is strongly advised to use DCAwareRoundRobinPolicy if you have multiple DCs/use DC-aware consistency levels "
+ "(note: this message will only be logged once)",
cl);
}
}
// We clone liveHosts because we want a version of the list that
// cannot change concurrently of the query plan iterator (this
// would be racy). We use clone() as it don't involve a copy of the
// underlying array (and thus we rely on liveHosts being a CopyOnWriteArrayList).
@SuppressWarnings("unchecked")
final List hosts = (List) liveHosts.clone();
final int startIdx = index.getAndIncrement();
// Overflow protection; not theoretically thread safe but should be good enough
if (startIdx > Integer.MAX_VALUE - 10000) index.set(0);
return new AbstractIterator() {
private int idx = startIdx;
private int remaining = hosts.size();
@Override
protected Host computeNext() {
if (remaining <= 0) return endOfData();
remaining--;
int c = idx++ % hosts.size();
if (c < 0) c += hosts.size();
return hosts.get(c);
}
};
}
@Override
public void onUp(Host host) {
liveHosts.addIfAbsent(host);
}
@Override
public void onDown(Host host) {
liveHosts.remove(host);
}
@Override
public void onAdd(Host host) {
onUp(host);
}
@Override
public void onRemove(Host host) {
onDown(host);
}
@Override
public void close() {
// nothing to do
}
}