com.datastax.driver.core.policies

Interface LoadBalancingPolicy

All Superinterfaces:

Host.StateListener

All Known Implementing Classes:

DCAwareRoundRobinPolicy, LatencyAwarePolicy, RoundRobinPolicy, TokenAwarePolicy

public interface LoadBalancingPolicy
extends Host.StateListener

The policy that decides which Cassandra hosts to contact for each new query.

Two methods need to be implemented:

• distance(com.datastax.driver.core.Host): returns the "distance" of an host for that balancing policy.
• newQueryPlan(java.lang.String, com.datastax.driver.core.Statement): it is used for each query to find which host to query first, and which hosts to use as failover.

The LoadBalancingPolicy is a 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.

Method Summary

Methods

Modifier and Type	Method and Description
HostDistance	distance(Host host) Returns the distance assigned by this policy to the provided host.
void	init(Cluster cluster, Collection<Host> hosts) Initialize this load balancing policy.
Iterator<Host>	newQueryPlan(String loggedKeyspace, Statement statement) Returns the hosts to use for a new query.

Methods inherited from interface com.datastax.driver.core.Host.StateListener

onAdd, onDown, onRemove, onUp

Method Detail

init

void init(Cluster cluster,
        Collection<Host> hosts)

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.

distance

HostDistance distance(Host host)

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 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.

newQueryPlan

Iterator<Host> newQueryPlan(String loggedKeyspace,
                          Statement statement)

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:

loggedKeyspace - the currently logged keyspace (the one set through either Cluster.connect(String) or by manually doing a USE query) for the session on which this plan need to be built. This can be null if the corresponding session has no keyspace logged in.

statement - 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.