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# Interface IAffinityFunction

Represents a function that maps cache keys to cluster nodes.

Predefined implementations: RendezvousAffinityFunction.

###### Namespace: Apache.Ignite.Core.Cache.Affinity

###### Assembly: Apache.Ignite.Core.dll

##### Syntax

    public interface IAffinityFunction

### Properties

#### Partitions

Gets the total number of partitions.

All caches should always provide correct partition count which should be the same on all participating nodes. Note that partitions should always be numbered from 0 inclusively to N exclusively without any gaps.

##### Declaration

    int Partitions { get; }

##### Property Value

Type | Description
--- | ---
System.Int32 |

### Methods

#### AssignPartitions(AffinityFunctionContext)

Gets affinity nodes for a partition. In case of replicated cache, all returned nodes are updated in the same manner. In case of partitioned cache, the returned list should contain only the primary and back up nodes with primary node being always first. Note that partitioned affinity must obey the following contract: given that node

    N

is primary for some key

    K

, if any other node(s) leave

grid and no node joins grid, node

    N

will remain primary for key

    K

.

##### Declaration

    IEnumerable<IEnumerable<IClusterNode>> AssignPartitions(AffinityFunctionContext context)

##### Parameters

Type | Name | Description
--- | --- | ---
AffinityFunctionContext | context | The affinity function context.

##### Returns

Type | Description
--- | ---
IEnumerable<IEnumerable<IClusterNode>> | A collection of partitions, where each partition is a collection of nodes, where first node is a primary node, and other nodes are backup nodes.

#### GetPartition(Object)

Gets partition number for a given key starting from 0. Partitioned caches should make sure that keys are about evenly distributed across all partitions from 0 to Partitions for best performance.

Note that for fully replicated caches it is possible to segment key sets among different grid node groups. In that case each node group should return a unique partition number. However, unlike partitioned cache, mappings of keys to nodes in replicated caches are constant and a node cannot migrate from one partition to another.

##### Declaration

    int GetPartition(object key)

##### Parameters

Type | Name | Description
--- | --- | ---
System.Object | key | Key to get partition for.

##### Returns

Type | Description
--- | ---
System.Int32 | Partition number for a given key.

#### RemoveNode(Guid)

Removes node from affinity. This method is called when it is safe to remove disconnected node from affinity mapping.

##### Declaration

    void RemoveNode(Guid nodeId)

##### Parameters

Type | Name | Description
--- | --- | ---
Guid | nodeId | The node identifier.