com.datastax.driver.core.policies

Class RoundRobinPolicy

java.lang.Object

com.datastax.driver.core.policies.RoundRobinPolicy

All Implemented Interfaces:

Host.StateListener, CloseableLoadBalancingPolicy, LoadBalancingPolicy

public class RoundRobinPolicy
extends Object
implements LoadBalancingPolicy, CloseableLoadBalancingPolicy

A Round-robin load balancing policy.

This policy queries nodes in a round-robin fashion. For a given query, if an host fail, the next one (following the round-robin order) is tried, until all hosts have been tried.

This policy is not datacenter aware and will include every known Cassandra host in its round robin algorithm. If you use multiple datacenter this will be inefficient and you will want to use the DCAwareRoundRobinPolicy load balancing policy instead.

Constructor Summary

Constructors

Constructor and Description
RoundRobinPolicy() Creates a load balancing policy that picks host to query in a round robin fashion (on all the hosts of the Cassandra cluster).

Method Summary

Methods

Modifier and Type	Method and Description
void	close() Gets invoked at cluster shutdown.
HostDistance	distance(Host host) Return the HostDistance for 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	onSuspected(Host host) Called when a node is suspected to be dead.
void	onUp(Host host) Called when a node is determined to be up.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

RoundRobinPolicy

public RoundRobinPolicy()

Creates a load balancing policy that picks host to query in a round robin fashion (on all the hosts of the Cassandra cluster).

Method Detail

init

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

Description copied from interface: LoadBalancingPolicy

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.

Specified by:

init in interface LoadBalancingPolicy

Parameters:

cluster - the Cluster instance for which the policy is created.

hosts - the initial hosts to use.

distance

public HostDistance distance(Host host)

Return the HostDistance for the provided host.

This policy consider all nodes as local. This is generally the right thing to do in a single datacenter deployment. If you use multiple datacenter, see DCAwareRoundRobinPolicy instead.

Specified by:

distance in interface LoadBalancingPolicy

Parameters:

host - the host of which to return the distance of.

Returns:

the HostDistance to host.

newQueryPlan

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

Returns the hosts to use for a new query.

The returned plan will try each known host of the cluster. Upon each call to this method, the ith host of the plans returned will cycle over all the hosts of the cluster in a round-robin fashion.

Specified by:

newQueryPlan in interface LoadBalancingPolicy

Parameters:

loggedKeyspace - the keyspace currently logged in on for this query.

statement - the query for which to build the plan.

Returns:

a new query plan, i.e. an iterator indicating which host to try first for querying, which one to use as failover, etc...

onUp

public void onUp(Host host)

Description copied from interface: Host.StateListener

Called when a node is determined to be up.

Specified by:

onUp in interface Host.StateListener

Parameters:

host - the host that has been detected up.

onSuspected

public void onSuspected(Host host)

Description copied from interface: Host.StateListener

Called when a node is suspected to be dead.

A node is suspected to be dead when an error occurs on one of it's opened connection. As soon as an host is suspected, a connection attempt to that host is immediately tried. If this succeed, then it means that the connection was disfunctional but that the node was not really down. If this fails however, this means the node is truly dead, onDown() is called and further reconnection attempts are scheduled according to the ReconnectionPolicy in place.

When this event is triggered, it is possible to call the host getInitialReconnectionAttemptFuture method to wait until the initial and immediate reconnection attempt succeed or fail.

Note that some StateListener may ignore that event. If a node that that is suspected down turns out to be truly down (that is, the driver cannot successfully connect to it right away), then onDown will be called.

Specified by:

onSuspected in interface Host.StateListener

onDown

public void onDown(Host host)

Description copied from interface: Host.StateListener

Called when a node is determined to be down.

Specified by:

onDown in interface Host.StateListener

Parameters:

host - the host that has been detected down.

onAdd

public void onAdd(Host host)

Description copied from interface: Host.StateListener

Called when a new node is added to the cluster.

The newly added node should be considered up.

Specified by:

onAdd in interface Host.StateListener

Parameters:

host - the host that has been newly added.

onRemove

public void onRemove(Host host)

Description copied from interface: Host.StateListener

Called when a node is removed from the cluster.

Specified by:

onRemove in interface Host.StateListener

Parameters:

host - the removed host.

close

public void close()

Description copied from interface: CloseableLoadBalancingPolicy

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

Specified by:

close in interface CloseableLoadBalancingPolicy