public interface LoadBalancingPolicy extends Host.StateListener
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 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.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.
|
onAdd, onDown, onRemove, onSuspected, onUp
void init(Cluster cluster, Collection<Host> hosts)
cluster
- the Cluster
instance for which the policy is created.hosts
- the initial hosts to use.HostDistance distance(Host host)
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.host
- the host of which to return the distance of.host
.Iterator<Host> newQueryPlan(String loggedKeyspace, Statement statement)
Iterator
are down, the query
will fail.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.