com.datastax.driver.core.policies

Class TokenAwarePolicy

java.lang.Object

com.datastax.driver.core.policies.TokenAwarePolicy

All Implemented Interfaces:

ChainableLoadBalancingPolicy, LoadBalancingPolicy

public class TokenAwarePolicy
extends Object
implements ChainableLoadBalancingPolicy

A wrapper load balancing policy that add token awareness to a child policy.

This policy encapsulates another policy. The resulting policy works in the following way:

• the distance method is inherited from the child policy.
• the iterator return by the newQueryPlan method will first return the LOCAL replicas for the query (based on Statement.getRoutingKey(com.datastax.driver.core.ProtocolVersion, com.datastax.driver.core.CodecRegistry)) if possible (i.e. if the query getRoutingKey method doesn't return null and if Metadata.getReplicas(java.lang.String, java.nio.ByteBuffer) returns a non empty set of replicas for that partition key). If no local replica can be either found or successfully contacted, the rest of the query plan will fallback to one of the child policy.

Do note that only replica for which the child policy distance method returns HostDistance.LOCAL will be considered having priority. For example, if you wrap DCAwareRoundRobinPolicy with this token aware policy, replicas from remote data centers may only be returned after all the host of the local data center.

Constructor Summary

Constructors

Constructor and Description
TokenAwarePolicy(LoadBalancingPolicy childPolicy) Creates a new TokenAware policy with shuffling of replicas.
TokenAwarePolicy(LoadBalancingPolicy childPolicy, boolean shuffleReplicas) Creates a new TokenAware policy.

Method Summary

Modifier and Type	Method and Description
void	close() Gets invoked at cluster shutdown.
HostDistance	distance(Host host) Return the HostDistance for the provided host.
LoadBalancingPolicy	getChildPolicy() Returns the child policy.
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.

Methods inherited from class java.lang.Object

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

Constructor Detail

TokenAwarePolicy

public TokenAwarePolicy(LoadBalancingPolicy childPolicy,
                        boolean shuffleReplicas)

Creates a new TokenAware policy.

Parameters:

childPolicy - the load balancing policy to wrap with token awareness.

shuffleReplicas - whether to shuffle the replicas returned by getRoutingKey. Note that setting this parameter to true might decrease the effectiveness of caching (especially at consistency level ONE), since the same row will be retrieved from any replica (instead of only the "primary" replica without shuffling). On the other hand, shuffling will better distribute writes, and can alleviate hotspots caused by "fat" partitions.

TokenAwarePolicy

public TokenAwarePolicy(LoadBalancingPolicy childPolicy)

Creates a new TokenAware policy with shuffling of replicas.

Parameters:

childPolicy - the load balancing policy to wrap with token awareness.

See Also:

TokenAwarePolicy(LoadBalancingPolicy, boolean)

Method Detail

getChildPolicy

public LoadBalancingPolicy getChildPolicy()

Description copied from interface: ChainableLoadBalancingPolicy

Returns the child policy.

Specified by:

getChildPolicy in interface ChainableLoadBalancingPolicy

Returns:

the child policy.

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.

Specified by:

distance in interface LoadBalancingPolicy

Parameters:

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

Returns:

the HostDistance to host as returned by the wrapped policy.

newQueryPlan

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

Returns the hosts to use for a new query.

The returned plan will first return replicas (whose HostDistance for the child policy is LOCAL) for the query if it can determine them (i.e. mainly if statement.getRoutingKey() is not null). Following what it will return the plan of the child policy.

Specified by:

newQueryPlan in interface LoadBalancingPolicy

Parameters:

statement - the query for which to build the plan.

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.

Returns:

the new query plan.

onUp

public void onUp(Host host)

Description copied from interface: LoadBalancingPolicy

Called when a node is determined to be up.

Specified by:

onUp in interface LoadBalancingPolicy

Parameters:

host - the host that has been detected up.

onDown

public void onDown(Host host)

Description copied from interface: LoadBalancingPolicy

Called when a node is determined to be down.

Specified by:

onDown in interface LoadBalancingPolicy

Parameters:

host - the host that has been detected down.

onAdd

public void onAdd(Host host)

Description copied from interface: LoadBalancingPolicy

Called when a new node is added to the cluster.

The newly added node should be considered up.

Specified by:

onAdd in interface LoadBalancingPolicy

Parameters:

host - the host that has been newly added.

onRemove

public void onRemove(Host host)

Description copied from interface: LoadBalancingPolicy

Called when a node is removed from the cluster.

Specified by:

onRemove in interface LoadBalancingPolicy

Parameters:

host - the removed host.

close

public void close()

Description copied from interface: LoadBalancingPolicy

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 LoadBalancingPolicy