Spark.ML之PipeLine学习笔记

地址：

http://spark.apache.org/docs/2.0.0/ml-pipeline.html

Spark PipeLine

是基于DataFrames的高层的API，可以方便用户构建和调试机器学习流水线

可以使得多个机器学习算法顺序执行，达到高效的数据处理的目的

DataFrame是来自Spark SQL的ML DataSet 可以存储一系列的数据类型，text,特征向量，Label和预测结果

Transformer:将DataFrame转化为另外一个DataFrame的算法，通过实现transform()方法

Estimator：将DataFrame转化为一个Transformer的算法，通过实现fit()方法

PipeLine:将多个Transformer和Estimator串成一个特定的ML Wolkflow

Parameter:Tansformer和Estimator共用同一个声明参数的API

Transformer和Estimator是PipeLine的Stage

Pipeline是一系列的Stage按照声明的顺序排列成的工作流

Transformer.transform()和Estimator.fit()都是无状态的

每一个Transformer和Estimator的实例都有唯一的ID在声明参数的时候非常有用

下面是一个线性的PipeLine的流程

上面创建的是线性的PipeLine,每一步都依赖上一步的结果

如果数据流可以组成有向不循环图（Directed Acyclic Graph DAG）

那么可以创建Non-Linear Pipeline

RuntimeCheching:因为PipeLine可以操作多种类型的DataFrame

所以不能使用编译时检测

那么PipeLine或者PipeLine Model使用运行时检测

这种检测使用了DataFrame Schema这个Schema是DataFrame列的数据类型的描述

Unique PipeLine Stage:PipeLine Stage应当都是唯一的实例，都拥有唯一的ID

Param是一个命名参数，带有自包含文档

ParamMap是一个参数与值的对（parameter,value）

将参数传递给算法主要有下面两种方式：

1. 为实例设置参数，若Ir是LogisticRegression的实例，调用Ir.SetMaxIter(10)意味着Ir.fit()做多调用10次

2. 传递一个ParamMap给fit()或者transform()那么位于map中的所有的parameter都会通过setter方法override以前的参数

很多时候将PipeLine保存到disk方便以后的使用是值得的

Spark 1.6时候，model Import/Export函数被添加到PipeLine API

大部分transformer和一些ML Model支持I/O

下面是基本组件的一些操作的例子:

1. #导入向量和模型
2. from pyspark.ml.linalg importVectors
3. from pyspark.ml.classification importLogisticRegression
5. #准备训练数据
6. # Prepare training data from a list of (label, features) tuples.
7. training = spark.createDataFrame([
8. (1.0,Vectors.dense([0.0,1.1,0.1])),
9. (0.0,Vectors.dense([2.0,1.0,-1.0])),
10. (0.0,Vectors.dense([2.0,1.3,1.0])),
11. (1.0,Vectors.dense([0.0,1.2,-0.5]))],["label","features"])
13. #创建回归实例，这个实例是Estimator
14. # Create a LogisticRegression instance. This instance is an Estimator.
15. lr =LogisticRegression(maxIter=10, regParam=0.01)
16. #打印出参数和文档
17. # Print out the parameters, documentation, and any default values.
18. print"LogisticRegression parameters:\n"+ lr.explainParams()+"\n"
20. #使用Ir中的参数训练出Model1
21. # Learn a LogisticRegression model. This uses the parameters stored in lr.
22. model1 = lr.fit(training)
24. # Since model1 is a Model (i.e., a transformer produced by an Estimator),
25. # we can view the parameters it used during fit().
26. # This prints the parameter (name: value) pairs, where names are unique IDs for this
27. # LogisticRegression instance.
28. #查看model1在fit()中使用的参数
29. print"Model 1 was fit using parameters: "
30. print model1.extractParamMap()
32. #修改其中的一个参数
33. # We may alternatively specify parameters using a Python dictionary as a paramMap
34. paramMap ={lr.maxIter:20}
35. #覆盖掉
36. paramMap[lr.maxIter]=30# Specify 1 Param, overwriting the original maxIter.
37. #更新参数对
38. paramMap.update({lr.regParam:0.1, lr.threshold:0.55})# Specify multiple Params.
40. # You can combine paramMaps, which are python dictionaries.
41. #新的参数，合并为两组参数对
42. paramMap2 ={lr.probabilityCol:"myProbability"}# Change output column name
43. paramMapCombined = paramMap.copy()
44. paramMapCombined.update(paramMap2)
46. #重新得到model2并拿出来参数看看
47. # Now learn a new model using the paramMapCombined parameters.
48. # paramMapCombined overrides all parameters set earlier via lr.set* methods.
49. model2 = lr.fit(training, paramMapCombined)
50. print"Model 2 was fit using parameters: "
51. print model2.extractParamMap()
53. #准备测试的数据
54. # Prepare test data
55. test = spark.createDataFrame([
56. (1.0,Vectors.dense([-1.0,1.5,1.3])),
57. (0.0,Vectors.dense([3.0,2.0,-0.1])),
58. (1.0,Vectors.dense([0.0,2.2,-1.5]))],["label","features"])
60. # Make predictions on test data using the Transformer.transform() method.
61. # LogisticRegression.transform will only use the 'features' column.
62. # Note that model2.transform() outputs a "myProbability" column instead of the usual
63. # 'probability' column since we renamed the lr.probabilityCol parameter previously.
64. prediction = model2.transform(test)
65. #得到预测的DataFrame打印出预测中的选中列
66. selected = prediction.select("features","label","myProbability","prediction")
67. for row in selected.collect():
68. print row

下面是一个PipeLine的实例：

1. from pyspark.ml importPipeline
2. from pyspark.ml.classification importLogisticRegression
3. from pyspark.ml.feature importHashingTF,Tokenizer
5. #准备测试数据
6. # Prepare training documents from a list of (id, text, label) tuples.
7. training = spark.createDataFrame([
8. (0L,"a b c d e spark",1.0),
9. (1L,"b d",0.0),
10. (2L,"spark f g h",1.0),
11. (3L,"hadoop mapreduce",0.0)],["id","text","label"])
13. #构建机器学习流水线
14. # Configure an ML pipeline, which consists of three stages: tokenizer, hashingTF, and lr.
15. tokenizer =Tokenizer(inputCol="text", outputCol="words")
16. hashingTF =HashingTF(inputCol=tokenizer.getOutputCol(), outputCol="features")
17. lr =LogisticRegression(maxIter=10, regParam=0.01)
18. pipeline =Pipeline(stages=[tokenizer, hashingTF, lr])
20. #训练出model
21. # Fit the pipeline to training documents.
22. model = pipeline.fit(training)
24. #测试数据
25. # Prepare test documents, which are unlabeled (id, text) tuples.
26. test = spark.createDataFrame([
27. (4L,"spark i j k"),
28. (5L,"l m n"),
29. (6L,"mapreduce spark"),
30. (7L,"apache hadoop")],["id","text"])
32. #预测，打印出想要的结果
33. # Make predictions on test documents and print columns of interest.
34. prediction = model.transform(test)
35. selected = prediction.select("id","text","prediction")
36. for row in selected.collect():
37. print(row)

来自为知笔记(Wiz)