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:documentationPath: /pipeline/transforms/
:language: en_US
:description: The Reservoir Sampling transform allows you to sample a fixed number of rows from an incoming data stream when the total number of incoming rows is not known in advance.

= image:transforms/icons/reservoirsampling.svg[Reservoir Sampling transform Icon, role="image-doc-icon"] Reservoir Sampling

[%noheader,cols="3a,1a", role="table-no-borders" ]
|===
|
== Description

The Reservoir Sampling transform allows you to sample a fixed number of rows from an incoming data stream when the total number of incoming rows is not known in advance.

The transform uses uniform sampling; all incoming rows have an equal chance of being selected.

This transform is particularly useful when used in conjunction with the ARFF output transform in order to generate a suitable sized data set to be used by WEKA.

The reservoir sampling transform uses link:https://en.wikipedia.org/wiki/Reservoir_sampling[Algorithm R] by Jeffery Vitter.

|
== Supported Engines
[%noheader,cols="2,1a",frame=none, role="table-supported-engines"]
!===
!Hop Engine! image:check_mark.svg[Supported, 24]
!Spark! image:question_mark.svg[Maybe Supported, 24]
!Flink! image:question_mark.svg[Maybe Supported, 24]
!Dataflow! image:question_mark.svg[Maybe Supported, 24]
!===
|===

== Options

[options="header"]
|===
|Option|Description
|Transform name|Name of the transform this name has to be unique in a single pipeline.
|Sample size|Select how many rows to sample from an incoming stream.
Setting a value of 0 will cause all rows to be sampled; setting a negative value will block all rows.
|Random seed|Choose a seed for the random number generator.
Repeating a pipeline with a different value for the seed will result in a different random sample being chosen.
|===