Class DCAwareRoundRobinPolicy

A data-center aware Round-robin load balancing policy.

This policy provides round-robin queries over the node of the local datacenter. Currently, it also includes in the query plans returned a configurable number of hosts in the remote datacenters (which are always tried after the local nodes) but this functionality will be removed in the next major version of the driver. See the comments on DCAwareRoundRobinPolicy(string, int) for more information.

Inheritance

object

DCAwareRoundRobinPolicy

Implements

ILoadBalancingPolicy

Inherited Members

object.Equals(object)

object.Equals(object, object)

object.GetHashCode()

object.GetType()

object.MemberwiseClone()

object.ReferenceEquals(object, object)

object.ToString()

Namespace: Cassandra

Assembly: Cassandra.dll

Syntax

public class DCAwareRoundRobinPolicy : ILoadBalancingPolicy

Constructors

DCAwareRoundRobinPolicy()

Creates a new datacenter aware round robin policy that auto-discover the local data-center.

If this constructor is used, the data-center used as local will the data-center of the first Cassandra node the driver connects to. This will always be ok if all the contact points use at Cluster creation are in the local data-center. If it's not the case, you should provide the local data-center name yourself by using one of the other constructor of this class.

Declaration

public DCAwareRoundRobinPolicy()

DCAwareRoundRobinPolicy(string)

Creates a new datacenter aware round robin policy given the name of the local datacenter.

The name of the local datacenter provided must be the local datacenter name as known by Cassandra.

The policy created will ignore all remote hosts. In other words, this is equivalent to new DCAwareRoundRobinPolicy(localDc, 0).

Declaration

public DCAwareRoundRobinPolicy(string localDc)

Parameters

Type	Name	Description
string	localDc	the name of the local datacenter (as known by Cassandra).

DCAwareRoundRobinPolicy(string, int)

Creates a new DCAwareRoundRobin policy given the name of the local datacenter and that uses the provided number of host per remote datacenter as failover for the local hosts.

The name of the local datacenter provided must be the local datacenter name as known by Cassandra.

Declaration

[Obsolete("The usedHostsPerRemoteDc parameter will be removed in the next major release of the driver. DC failover should not be done in the driver, which does not have the necessary context to know what makes sense considering application semantics. See https://datastax-oss.atlassian.net/browse/CSHARP-722")]
public DCAwareRoundRobinPolicy(string localDc, int usedHostsPerRemoteDc)

Parameters

Type	Name	Description
string	localDc	The name of the local datacenter (as known by Cassandra).
int	usedHostsPerRemoteDc	The number of host per remote datacenter that policies created by the returned factory should consider. Created policies distance method will return a HostDistance.Remote distance for only usedHostsPerRemoteDc hosts per remote datacenter. Other hosts of the remote datacenters will be ignored (and thus no connections to them will be maintained). Note that this parameter will be removed in the next major release of the driver.

Properties

LocalDc

Gets the Local Datacenter. This value is provided in the constructor.

Declaration

public string LocalDc { get; }

Property Value

Type	Description
string

UsedHostsPerRemoteDc

Gets the number of hosts per remote datacenter that should be considered. This value is provided in the constructor.

Declaration

[Obsolete("The usedHostsPerRemoteDc parameter will be removed in the next major release of the driver. DC failover should not be done in the driver, which does not have the necessary context to know what makes sense considering application semantics. See https://datastax-oss.atlassian.net/browse/CSHARP-722")]
public int UsedHostsPerRemoteDc { get; }

Property Value

Type	Description
int

Methods

Distance(Host)

Return the HostDistance for the provided host.

This policy consider nodes in the local datacenter as Local. For each remote datacenter, it considers a configurable number of hosts as Remote and the rest is Ignored.

To configure how many host in each remote datacenter is considered Remote.

Declaration

public 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

public 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.

The returned plan will always try each known host in the local datacenter first, and then, if none of the local host is reachable, will try up to a configurable number of other host per remote datacenter. The order of the local node in the returned query plan will follow a Round-robin algorithm.

Declaration

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

Parameters

Type	Name	Description
string	keyspace	Keyspace on which the query is going to be executed
IStatement	query	the query for which to build the plan.

Returns

Type	Description
IEnumerable<Host>	a new query plan, i.e. an iterator indicating which host to try first for querying, which one to use as failover, etc...

Implements

ILoadBalancingPolicy