public interface LoadBalancingPolicy
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 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|
Gets invoked at cluster shutdown.
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.
Called when a new node is added to the cluster.
Called when a node is determined to be down.
Called when a node is removed from the cluster.
Called when a node is determined to be up.
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 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
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.
Iterator<Host> newQueryPlan(String loggedKeyspace, Statement statement)
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.
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.
void onAdd(Host host)
The newly added node should be considered up.
host- the host that has been newly added.
void onUp(Host host)
host- the host that has been detected up.
void onDown(Host host)
host- the host that has been detected down.
void onRemove(Host host)
host- the removed host.
This gives the policy the opportunity to perform some cleanup, for instance stop threads that it might have started.
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