Interface ILoadBalancingPolicy

The policy that decides which Cassandra hosts to contact for each new query. For efficiency purposes, the policy is expected to exclude down hosts from query plans.

Namespace: Dse

Assembly: Dse.dll

Syntax

public interface ILoadBalancingPolicy

Methods

Distance(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.

Declaration

HostDistance Distance(Host host)

Parameters

Type	Name	Description
Host	host	the host of which to return the distance of.

Returns

Type	Description
HostDistance	the HostDistance to host.

Initialize(ICluster)

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.

Declaration

void Initialize(ICluster cluster)

Parameters

Type	Name	Description
ICluster	cluster	The information about the session instance for which the policy is created.

NewQueryPlan(String, IStatement)

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.

Declaration

IEnumerable<Host> NewQueryPlan(string keyspace, IStatement query)

Parameters

Type	Name	Description
System.String	keyspace	Keyspace on which the query is going to be executed, it can be null.
IStatement	query	The query for which to build a plan, it can be null.

Returns

Type	Description
System.Collections.Generic.IEnumerable<Host>	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.