com.datastax.driver.core.policies

Interface LoadBalancingPolicy

All Known Subinterfaces:

ChainableLoadBalancingPolicy

All Known Implementing Classes:

DCAwareRoundRobinPolicy, ErrorAwarePolicy, HostFilterPolicy, LatencyAwarePolicy, RoundRobinPolicy, TokenAwarePolicy, WhiteListPolicy

public interface LoadBalancingPolicy

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 informed of hosts up/down events. For efficiency purposes, the policy is expected to exclude down hosts from query plans.

Method Summary

Modifier and Type	Method and Description
void	close() Gets invoked at cluster shutdown.
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.
void	onAdd(Host host) Called when a new node is added to the cluster.
void	onDown(Host host) Called when a node is determined to be down.
void	onRemove(Host host) Called when a node is removed from the cluster.
void	onUp(Host host) Called when a node is determined to be up.

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.

onAdd

void onAdd(Host host)

Called when a new node is added to the cluster.

The newly added node should be considered up.

Parameters:

host - the host that has been newly added.

onUp

void onUp(Host host)

Called when a node is determined to be up.

Parameters:

host - the host that has been detected up.

onDown

void onDown(Host host)

Called when a node is determined to be down.

Parameters:

host - the host that has been detected down.

onRemove

void onRemove(Host host)

Called when a node is removed from the cluster.

Parameters:

host - the removed host.

close

void close()

Gets invoked at cluster shutdown.

This gives the policy the opportunity to perform some cleanup, for instance stop threads that it might have started.