public interface LoadBalancingPolicy extends Host.StateListener
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.
LoadBalancingPolicy is a
and is thus informed of hosts up/down events. For efficiency purposes, the
policy is expected to exclude down hosts from query plans.
|Modifier and Type||Method and Description|
Returns the distance assigned by this policy to the provided host.
Initialize this load balancing policy.
Returns the hosts to use for a new query.
void init(Cluster cluster, Collection<Host> hosts)
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.
Clusterinstance for which the policy is created.
hosts- the initial hosts to use.
HostDistance distance(Host host)
The distance of an host influence how much connections are kept to the
HostDistance). A policy should assign a
LOCAL distance to nodes that are susceptible to be returned first by
newQueryPlan and it is useless for
return hosts to which it assigns an
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.
host- the host of which to return the distance of.
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
loggedKeyspace- the currently logged keyspace (the one set through either
Cluster.connect(String)or by manually doing a
USEquery) for the session on which this plan need to be built. This can be
nullif the corresponding session has no keyspace logged in.
statement- the query for which to build a plan.
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