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 /*
  *      Copyright (C) 2012 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;
The policy that decides which Cassandra hosts to contact for each new query.

Two methods need to be implemented:

The LoadBalancingPolicy is a com.datastax.driver.core.Host.StateListener and is thus informed of hosts up/down events. For efficiency purposes, the policy is expected to exclude down hosts from query plans.

public interface LoadBalancingPolicy extends Host.StateListener {

    
Initialize this load balancing policy.

Note that the driver guarantees that it will call this method exactly once per policy object and will do so before any call to another of the methods of the policy.

Parameters:
cluster the Cluster instance for which the policy is created.
hosts the initial hosts to use.
    public void init(Cluster clusterCollection<Hosthosts);

    
Returns the distance assigned by this policy to the provided host.

The distance of an host influence how much connections are kept to the node (see com.datastax.driver.core.HostDistance). A policy should assign a LOCAL distance to nodes that are susceptible to be returned first by newQueryPlan and it is useless for newQueryPlan to return hosts to which it assigns an IGNORED distance.

The host distance is primarily used to prevent keeping too many connections to host in remote datacenters when the policy itself always picks host in the local datacenter first.

Parameters:
host the host of which to return the distance of.
Returns:
the HostDistance to host.
    public HostDistance distance(Host host);

    
Returns the hosts to use for a new query.

Each new query will call this method. The first host in the result will then be used to perform the query. In the event of a connection problem (the queried host is down or appear to be so), the next host will be used. If all hosts of the returned Iterator are down, the query will fail.

Parameters:
query the query for which to build a plan.
Returns:
an iterator of Host. The query is tried against the hosts returned by this iterator in order, until the query has been sent successfully to one of the host.
    public Iterator<HostnewQueryPlan(Query query);
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