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 adds 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 returned by the newQueryPlan method will first return the LOCAL replicas for the query if possible (i.e. if the query's routing key is not null and if the set of replicas for that partition key is not empty). If no local replica can be either found or successfully contacted, the rest of the query plan will fallback to the child policy's one.

The exact order in which local replicas are returned is dictated by the strategy provided at instantiation.

Do note that only replicas for which the child policy's distance method returns 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 hosts of the local data center.

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static class	TokenAwarePolicy.ReplicaOrdering Strategies for replica ordering.

Constructor Summary

Constructors

Constructor and Description
TokenAwarePolicy(LoadBalancingPolicy childPolicy) Creates a new TokenAware policy with RANDOM replica ordering.
TokenAwarePolicy(LoadBalancingPolicy childPolicy, boolean shuffleReplicas) Deprecated. Use TokenAwarePolicy(LoadBalancingPolicy, ReplicaOrdering) instead. This constructor will be removed in the next major release.
TokenAwarePolicy(LoadBalancingPolicy childPolicy, TokenAwarePolicy.ReplicaOrdering replicaOrdering) Creates a new TokenAware policy.

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.
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,
                        TokenAwarePolicy.ReplicaOrdering replicaOrdering)

Creates a new TokenAware policy.

Parameters:

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

replicaOrdering - the strategy to use to order replicas.

TokenAwarePolicy

@Deprecated
public TokenAwarePolicy(LoadBalancingPolicy childPolicy,
                                    boolean shuffleReplicas)

Deprecated. Use TokenAwarePolicy(LoadBalancingPolicy, ReplicaOrdering) instead. This constructor will be removed in the next major release.

Creates a new TokenAware policy.

Parameters:

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

shuffleReplicas - whether or not to shuffle the replicas. If true, then the RANDOM strategy will be used, otherwise the TOPOLOGICAL one will be used.

TokenAwarePolicy

public TokenAwarePolicy(LoadBalancingPolicy childPolicy)

Creates a new TokenAware policy with RANDOM replica ordering.

Parameters:

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

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)

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.

This implementation always returns distances as reported by the wrapped policy.

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.

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.

The returned plan will first return local replicas for the query (i.e. replicas whose distance according to the child policy is LOCAL), if it can determine them (i.e. mainly if the statement's routing key is not null), and ordered according to the ordering strategy specified at instantiation; following what it will return the rest of the child policy's original query plan.

Specified by:

newQueryPlan in interface LoadBalancingPolicy

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.

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