Spark机器学习

Spark机器学习

Pipelines中的主要概念

MLlib 提供的API可以通过Pipelines将多个复杂的机器学习算法结合成单个pipeline或者单个工作流。这个概念和scikit-learn里的概念类似，根据官方的说法是，此抽象概念的设计灵感来自于scikit-learn。

· DataFrame:通过Spark SQL 组件里的DataFrame作为机器学习的数据集。支持多种数据类型.比如 DataFrame 可以将文本，数据库等外部数据源划分为不同的列,包含特征向量, 特征值等。

· Transformer: 一个 Transformer可以将一个DataFrame 转换成另一个DataFrame. 比如, 一个机器学习模型可以将带有特征值的DataFrame转换为一个带有模型预测结果数据的DataFrame.

· Estimator:通过 DataFrame数据集进行训练 产生一个机器学习模型的算法。

· Pipeline:联合多个 Transformer和 Estimator构成一个机器学习工作流。

· Parameter: 所有Transformer和 Estimator指定参数的共享API。

DataFrame

DataFrame里广泛运用的数据结构，可以包含向量，文本，图片，以及结构化数据。DataFrame通过Spark SQL支持多种数据源。

工作流程如图所示：

机器学习中Pipleline流程图

正如图中所示，Pipeline有三个阶段，每个阶段要么是Transformer ，要么就是Estimator，这些阶段按照一定的顺序执行，执行的过程中，通过圆柱体代表的DataFrame类型的Raw text产生一个新的Words(DataFrame类型)，最后建立了一个LogisticRegressionModel。图中的Tokenizer,HashingTF都是Transformer,而LogisticRegressionModel是Estimator 。

在Transformer 阶段，主要调用transform()方法进行计算。

在Estimator阶段，主要调用fit()方法进行计算。

DAG Pipelines：多个阶段形成一个pipeline,同理，DAG Pipelines就是多个pipeline组成的一个有向无环图。

运行时检查：数据结构DataFrame中可以有各种各样的数据，但是在编译的时候不会检查数据的数据类型，而是在运行的时候才根据DataFrame的Schema来检查数据类型。

唯一ID标识:Pipeline的每一个阶段（stage）都通过id来进行唯一的标识，同一个相同的实列，比如HashingTF不会插入到同一个Pipeline俩次，因为每一个stage都有自身的唯一的ID来进行标识。

保存和读取pipeline

代码案例：

Estimator, Transformer, 以及 Param综合案例

importorg.apache.spark.ml.classification.LogisticRegression

importorg.apache.spark.ml.linalg.{Vector,Vectors}

importorg.apache.spark.ml.param.ParamMap

importorg.apache.spark.sql.Row

// Prepare training data from a list of (label, features)tuples.

valtraining=spark.createDataFrame(Seq(

(1.0,Vectors.dense(0.0,1.1,0.1)),

(0.0,Vectors.dense(2.0,1.0,-1.0)),

(0.0,Vectors.dense(2.0,1.3,1.0)),

(1.0,Vectors.dense(0.0,1.2,-0.5))

)).toDF("label","features")

// Create a LogisticRegression instance. This instance is anEstimator.

vallr=newLogisticRegression()

// Print out the parameters, documentation, and any defaultvalues.

println("LogisticRegressionparameters:\n"+lr.explainParams()+"\n")

// We may set parameters using setter methods.

lr.setMaxIter(10)

.setRegParam(0.01)

// Learn a LogisticRegression model. This uses the parametersstored in lr.

valmodel1=lr.fit(training)

// Since model1 is a Model (i.e., a Transformer produced byan Estimator),

// we can view the parameters it used during fit().

// This prints the parameter (name: value) pairs, where namesare unique IDs for this

// LogisticRegression instance.

println("Model 1 was fit usingparameters: "+model1.parent.extractParamMap)

// We may alternatively specify parameters using a ParamMap,

// which supports several methods for specifying parameters.

valparamMap=ParamMap(lr.maxIter->20)

.put(lr.maxIter,30) // Specify 1 Param. This overwrites the original maxIter.

.put(lr.regParam->0.1,lr.threshold->0.55) // Specify multiple Params.

// One can also combine ParamMaps.

valparamMap2=ParamMap(lr.probabilityCol->"myProbability") // Change output column name.

valparamMapCombined=paramMap++paramMap2

// Now learn a new model using the paramMapCombinedparameters.

// paramMapCombined overrides all parameters set earlier vialr.set* methods.

valmodel2=lr.fit(training,paramMapCombined)

println("Model 2 was fit usingparameters: "+model2.parent.extractParamMap)

// Prepare test data.

valtest=spark.createDataFrame(Seq(

(1.0,Vectors.dense(-1.0,1.5,1.3)),

(0.0,Vectors.dense(3.0,2.0,-0.1)),

(1.0,Vectors.dense(0.0,2.2,-1.5))

)).toDF("label","features")

// Make predictions on test data using theTransformer.transform() method.

// LogisticRegression.transform will only use the 'features'column.

// Note that model2.transform() outputs a 'myProbability'column instead of the usual

// 'probability' column since we renamed thelr.probabilityCol parameter previously.

model2.transform(test)

.select("features","label","myProbability","prediction")

.collect()

.foreach{caseRow(features:Vector,label:Double,prob:Vector,prediction:Double)=>

println(s"($features, $label) -> prob=$prob, prediction=$prediction")

}

Pipeline单独的案例代码

importorg.apache.spark.ml.{Pipeline,PipelineModel}

importorg.apache.spark.ml.classification.LogisticRegression

importorg.apache.spark.ml.feature.{HashingTF,Tokenizer}

importorg.apache.spark.ml.linalg.Vector

importorg.apache.spark.sql.Row

// Prepare training documents from a list of (id, text, label) tuples.

val training = spark.createDataFrame(Seq(

  (0L,"a b c d e spark",1.0),

  (1L,"b d",0.0),

  (2L,"spark f g h",1.0),

  (3L,"hadoop mapreduce",0.0)

)).toDF("id","text","label")

// Configure an ML pipeline, which consists of three stages: tokenizer, hashingTF, and lr.

val tokenizer =newTokenizer()

  .setInputCol("text")

  .setOutputCol("words")

val hashingTF =newHashingTF()

  .setNumFeatures(1000)

  .setInputCol(tokenizer.getOutputCol)

  .setOutputCol("features")

val lr =newLogisticRegression()

  .setMaxIter(10)

  .setRegParam(0.001)

val pipeline =newPipeline()

  .setStages(Array(tokenizer, hashingTF, lr))

// Fit the pipeline to training documents.

val model = pipeline.fit(training)

// Now we can optionally save the fitted pipeline to disk

model.write.overwrite().save("/tmp/spark-logistic-regression-model")

// We can also save this unfit pipeline to disk

pipeline.write.overwrite().save("/tmp/unfit-lr-model")

// And load it back in during production

val sameModel =PipelineModel.load("/tmp/spark-logistic-regression-model")

// Prepare test documents, which are unlabeled (id, text) tuples.

val test = spark.createDataFrame(Seq(

  (4L,"spark i j k"),

  (5L,"l m n"),

  (6L,"spark hadoop spark"),

  (7L,"apache hadoop")

)).toDF("id","text")

// Make predictions on test documents.

model.transform(test)

  .select("id","text","probability","prediction")

  .collect()

  .foreach{caseRow(id:Long, text:String, prob:Vector, prediction:Double)=>

    println(s"($id, $text) --> prob=$prob, prediction=$prediction")

  }