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spark-instrumented-optimizer/docs/ml-guide.md
Liang-Chi Hsieh 8bfea86b1c
[SPARK-26133][ML] Remove deprecated OneHotEncoder and rename OneHotEncoderEstimator to OneHotEncoder 
## What changes were proposed in this pull request?

We have deprecated `OneHotEncoder` at Spark 2.3.0 and introduced `OneHotEncoderEstimator`. At 3.0.0, we remove deprecated `OneHotEncoder` and rename `OneHotEncoderEstimator` to `OneHotEncoder`.

TODO: According to ML migration guide, we need to keep `OneHotEncoderEstimator` as an alias after renaming. This is not done at this patch in order to facilitate review.

## How was this patch tested?

Existing tests.

Closes #23100 from viirya/remove_one_hot_encoder.

Authored-by: Liang-Chi Hsieh <viirya@gmail.com>
Signed-off-by: DB Tsai <d_tsai@apple.com>
2018-11-29 01:54:06 +00:00

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---
layout: global
title: "MLlib: Main Guide"
displayTitle: "Machine Learning Library (MLlib) Guide"
---
MLlib is Spark's machine learning (ML) library.
Its goal is to make practical machine learning scalable and easy.
At a high level, it provides tools such as:
* ML Algorithms: common learning algorithms such as classification, regression, clustering, and collaborative filtering
* Featurization: feature extraction, transformation, dimensionality reduction, and selection
* Pipelines: tools for constructing, evaluating, and tuning ML Pipelines
* Persistence: saving and load algorithms, models, and Pipelines
* Utilities: linear algebra, statistics, data handling, etc.
# Announcement: DataFrame-based API is primary API
**The MLlib RDD-based API is now in maintenance mode.**
As of Spark 2.0, the [RDD](rdd-programming-guide.html#resilient-distributed-datasets-rdds)-based APIs in the `spark.mllib` package have entered maintenance mode.
The primary Machine Learning API for Spark is now the [DataFrame](sql-programming-guide.html)-based API in the `spark.ml` package.
*What are the implications?*
* MLlib will still support the RDD-based API in `spark.mllib` with bug fixes.
* MLlib will not add new features to the RDD-based API.
* In the Spark 2.x releases, MLlib will add features to the DataFrames-based API to reach feature parity with the RDD-based API.
* After reaching feature parity (roughly estimated for Spark 2.3), the RDD-based API will be deprecated.
* The RDD-based API is expected to be removed in Spark 3.0.
*Why is MLlib switching to the DataFrame-based API?*
* DataFrames provide a more user-friendly API than RDDs. The many benefits of DataFrames include Spark Datasources, SQL/DataFrame queries, Tungsten and Catalyst optimizations, and uniform APIs across languages.
* The DataFrame-based API for MLlib provides a uniform API across ML algorithms and across multiple languages.
* DataFrames facilitate practical ML Pipelines, particularly feature transformations. See the [Pipelines guide](ml-pipeline.html) for details.
*What is "Spark ML"?*
* "Spark ML" is not an official name but occasionally used to refer to the MLlib DataFrame-based API.
This is majorly due to the `org.apache.spark.ml` Scala package name used by the DataFrame-based API,
and the "Spark ML Pipelines" term we used initially to emphasize the pipeline concept.
*Is MLlib deprecated?*
* No. MLlib includes both the RDD-based API and the DataFrame-based API.
The RDD-based API is now in maintenance mode.
But neither API is deprecated, nor MLlib as a whole.
# Dependencies
MLlib uses the linear algebra package [Breeze](http://www.scalanlp.org/), which depends on
[netlib-java](https://github.com/fommil/netlib-java) for optimised numerical processing.
If native libraries[^1] are not available at runtime, you will see a warning message and a pure JVM
implementation will be used instead.
Due to licensing issues with runtime proprietary binaries, we do not include `netlib-java`'s native
proxies by default.
To configure `netlib-java` / Breeze to use system optimised binaries, include
`com.github.fommil.netlib:all:1.1.2` (or build Spark with `-Pnetlib-lgpl`) as a dependency of your
project and read the [netlib-java](https://github.com/fommil/netlib-java) documentation for your
platform's additional installation instructions.
The most popular native BLAS such as [Intel MKL](https://software.intel.com/en-us/mkl), [OpenBLAS](http://www.openblas.net), can use multiple threads in a single operation, which can conflict with Spark's execution model.
Configuring these BLAS implementations to use a single thread for operations may actually improve performance (see [SPARK-21305](https://issues.apache.org/jira/browse/SPARK-21305)). It is usually optimal to match this to the number of cores each Spark task is configured to use, which is 1 by default and typically left at 1.
Please refer to resources like the following to understand how to configure the number of threads these BLAS implementations use: [Intel MKL](https://software.intel.com/en-us/articles/recommended-settings-for-calling-intel-mkl-routines-from-multi-threaded-applications) and [OpenBLAS](https://github.com/xianyi/OpenBLAS/wiki/faq#multi-threaded).
To use MLlib in Python, you will need [NumPy](http://www.numpy.org) version 1.4 or newer.
[^1]: To learn more about the benefits and background of system optimised natives, you may wish to
watch Sam Halliday's ScalaX talk on [High Performance Linear Algebra in Scala](http://fommil.github.io/scalax14/#/).
# Highlights in 2.3
The list below highlights some of the new features and enhancements added to MLlib in the `2.3`
release of Spark:
* Built-in support for reading images into a `DataFrame` was added
([SPARK-21866](https://issues.apache.org/jira/browse/SPARK-21866)).
* [`OneHotEncoderEstimator`](ml-features.html#onehotencoderestimator) was added, and should be
used instead of the existing `OneHotEncoder` transformer. The new estimator supports
transforming multiple columns.
* Multiple column support was also added to `QuantileDiscretizer` and `Bucketizer`
([SPARK-22397](https://issues.apache.org/jira/browse/SPARK-22397) and
[SPARK-20542](https://issues.apache.org/jira/browse/SPARK-20542))
* A new [`FeatureHasher`](ml-features.html#featurehasher) transformer was added
([SPARK-13969](https://issues.apache.org/jira/browse/SPARK-13969)).
* Added support for evaluating multiple models in parallel when performing cross-validation using
[`TrainValidationSplit` or `CrossValidator`](ml-tuning.html)
([SPARK-19357](https://issues.apache.org/jira/browse/SPARK-19357)).
* Improved support for custom pipeline components in Python (see
[SPARK-21633](https://issues.apache.org/jira/browse/SPARK-21633) and
[SPARK-21542](https://issues.apache.org/jira/browse/SPARK-21542)).
* `DataFrame` functions for descriptive summary statistics over vector columns
([SPARK-19634](https://issues.apache.org/jira/browse/SPARK-19634)).
* Robust linear regression with Huber loss
([SPARK-3181](https://issues.apache.org/jira/browse/SPARK-3181)).
# Migration guide
MLlib is under active development.
The APIs marked `Experimental`/`DeveloperApi` may change in future releases,
and the migration guide below will explain all changes between releases.
## From 2.4 to 3.0
### Breaking changes
* `OneHotEncoder` which is deprecated in 2.3, is removed in 3.0 and `OneHotEncoderEstimator` is now renamed to `OneHotEncoder`.
## From 2.2 to 2.3
### Breaking changes
* The class and trait hierarchy for logistic regression model summaries was changed to be cleaner
and better accommodate the addition of the multi-class summary. This is a breaking change for user
code that casts a `LogisticRegressionTrainingSummary` to a
`BinaryLogisticRegressionTrainingSummary`. Users should instead use the `model.binarySummary`
method. See [SPARK-17139](https://issues.apache.org/jira/browse/SPARK-17139) for more detail
(_note_ this is an `Experimental` API). This _does not_ affect the Python `summary` method, which
will still work correctly for both multinomial and binary cases.
### Deprecations and changes of behavior
**Deprecations**
* `OneHotEncoder` has been deprecated and will be removed in `3.0`. It has been replaced by the
new [`OneHotEncoderEstimator`](ml-features.html#onehotencoderestimator)
(see [SPARK-13030](https://issues.apache.org/jira/browse/SPARK-13030)). **Note** that
`OneHotEncoderEstimator` will be renamed to `OneHotEncoder` in `3.0` (but
`OneHotEncoderEstimator` will be kept as an alias).
**Changes of behavior**
* [SPARK-21027](https://issues.apache.org/jira/browse/SPARK-21027):
The default parallelism used in `OneVsRest` is now set to 1 (i.e. serial). In `2.2` and
earlier versions, the level of parallelism was set to the default threadpool size in Scala.
* [SPARK-22156](https://issues.apache.org/jira/browse/SPARK-22156):
The learning rate update for `Word2Vec` was incorrect when `numIterations` was set greater than
`1`. This will cause training results to be different between `2.3` and earlier versions.
* [SPARK-21681](https://issues.apache.org/jira/browse/SPARK-21681):
Fixed an edge case bug in multinomial logistic regression that resulted in incorrect coefficients
when some features had zero variance.
* [SPARK-16957](https://issues.apache.org/jira/browse/SPARK-16957):
Tree algorithms now use mid-points for split values. This may change results from model training.
* [SPARK-14657](https://issues.apache.org/jira/browse/SPARK-14657):
Fixed an issue where the features generated by `RFormula` without an intercept were inconsistent
with the output in R. This may change results from model training in this scenario.
## Previous Spark versions
Earlier migration guides are archived [on this page](ml-migration-guides.html).
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