Pyspark: applying kmeans on different groups of a dataframe

Question

Using Pyspark I would like to apply kmeans separately on groups of a dataframe and not to the whole dataframe at once. For the moment I use a for loop which iterates on each group, applies kmeans and appends the result to another table. But having a lot of groups makes it time consuming. Anyone could help me please?? Thanks a lot!

for customer in customer_list:
    temp_df = togroup.filter(col("customer_id")==customer)
    df = assembler.transform(temp_df)
    k = 1
    while (k < 5 & mtrc < width):
        k += 1
        kmeans = KMeans(k=k,seed=5,maxIter=20,initSteps=5)
        model = kmeans.fit(df)
        mtric = 1 - model.computeCost(df)/ttvar
        a = model.transform(df)select(cols)
        allcustomers = allcustomers .union(a)

Answer 1

I came up with a solution using pandas_udf. A pure spark or scala solution is preferred and yet to be offered. Assume my data is

import pandas as pd
df_pd = pd.DataFrame([['cat1',10.],['cat1',20.],['cat1',11.],['cat1',21.],['cat1',22.],['cat1',9.],['cat2',101.],['cat2',201.],['cat2',111.],['cat2',214.],['cat2',224.],['cat2',99.]],columns=['cat','val'])
df_sprk = spark.createDataFrame(df_pd)

First solve the problem in pandas:

from sklearn.cluster import KMeans
kmeans = KMeans(n_clusters=2,random_state=0)

def skmean(kmeans,x):
    X = np.array(x)
    kmeans.fit(X)
    return(kmeans.predict(X))

You can apply skmean() to a panda data frame (to make sure it works properly):

df_pd.groupby('cat').apply(lambda x:skmean(kmeans,x)).reset_index()

To apply the function to pyspark data frame, we use pandas_udf. But first define a schema for the output data frame:

from pyspark.sql.types import *
schema = StructType(
       [StructField('cat',StringType(),True),
        StructField('clusters',ArrayType(IntegerType()))])

Convert the function above to a pandas_udf:

from pyspark.sql.functions import pandas_udf
from pyspark.sql.functions import PandasUDFType  

@pandas_udf(schema, functionType=PandasUDFType.GROUPED_MAP)
def skmean_udf(df):
    result = pd.DataFrame(
             df.groupby('cat').apply(lambda x: skmean(kmeans,x))
    result.reset_index(inplace=True, drop=False)
    return(result)

You can use the function as follows:

df_spark.groupby('cat').apply(skmean_udf).show()