Applying a function in each row of a big PySpark dataframe?

Question

I have a big dataframe (~30M rows). I have a function f. The business of f is to run through each row, check some logics and feed the outputs into a dictionary. The function needs to be performed row by row.

I tried:

dic = dict() for row in df.rdd.collect(): f(row, dic)

But I always meet the error OOM. I set the memory of Docker to 8GB.

How can I effectively perform the business?

Answer 1

from pyspark.sql.functions import udf, struct
from pyspark.sql.types import StringType, MapType

#sample data
df = sc.parallelize([
    ['a', 'b'],
    ['c', 'd'],
    ['e', 'f']
]).toDF(('col1', 'col2'))

#add logic to create dictionary element using rows of the dataframe    
def add_to_dict(l):
    d = {}
    d[l[0]] = l[1]
    return d
add_to_dict_udf = udf(add_to_dict, MapType(StringType(), StringType()))
#struct is used to pass rows of dataframe
df = df.withColumn("dictionary_item", add_to_dict_udf(struct([df[x] for x in df.columns])))
df.show()

#list of dictionary elements
dictionary_list = [i[0] for i in df.select('dictionary_item').collect()]
print dictionary_list

Output is:

[{u'a': u'b'}, {u'c': u'd'}, {u'e': u'f'}]

Answer 2

By using collect you pull all the data out of the Spark Executors into your Driver. You really should avoid this, as it makes using Spark pointless (you could just use plain python in that case).

What could you do:

• reimplement your logic using functions already available: pyspark.sql.functions doc

• if you cannot do the first, because there is functionality missing, you can define a User Defined Function