python - Choosing random items from a Spark GroupedData Object

Question

I'm new to using Spark in Python and have been unable to solve this problem: After running groupBy on a pyspark.sql.dataframe.DataFrame

df = sqlsc.read.json("data.json")
df.groupBy('teamId')

how can you choose N random samples from each resulting group (grouped by teamId) without replacement?

I'm basically trying to choose N random users from each team, maybe using groupBy is wrong to start with?

Answer 1

Well, it is kind of wrong. GroupedData is not really designed for a data access. It just describes grouping criteria and provides aggregation methods. See my answer to Using groupBy in Spark and getting back to a DataFrame for more details.

Another problem with this idea is selecting N random samples. It is a task which is really hard to achieve in parallel without psychical grouping of data and it is not something that happens when you call groupBy on a DataFrame:

There are at least two ways to handle this:

• convert to RDD, groupBy and perform local sampling

  import random
  
  n = 3
  
  def sample(iter, n): 
      rs = random.Random()  # We should probably use os.urandom as a seed
      return rs.sample(list(iter), n)    
  
  df = sqlContext.createDataFrame(
      [(x, y, random.random()) for x in (1, 2, 3) for y in "abcdefghi"], 
      ("teamId", "x1", "x2"))
  
  grouped = df.rdd.map(lambda row: (row.teamId, row)).groupByKey()
  
  sampled = sqlContext.createDataFrame(
      grouped.flatMap(lambda kv: sample(kv[1], n)))
  
  sampled.show()
  
  ## +------+---+-------------------+
  ## |teamId| x1|                 x2|
  ## +------+---+-------------------+
  ## |     1|  g|   0.81921738561455|
  ## |     1|  f| 0.8563875814036598|
  ## |     1|  a| 0.9010425238735935|
  ## |     2|  c| 0.3864428179837973|
  ## |     2|  g|0.06233470405822805|
  ## |     2|  d|0.37620872770129155|
  ## |     3|  f| 0.7518901502732027|
  ## |     3|  e| 0.5142305439671874|
  ## |     3|  d| 0.6250620479303716|
  ## +------+---+-------------------+
  

• use window functions

  from pyspark.sql import Window
  from pyspark.sql.functions import col, rand, rowNumber
  
  w = Window.partitionBy(col("teamId")).orderBy(col("rnd_"))
  
  sampled = (df
      .withColumn("rnd_", rand())  # Add random numbers column
      .withColumn("rn_", rowNumber().over(w))  # Add rowNumber over windw
      .where(col("rn_") <= n)  # Take n observations
      .drop("rn_")  # drop helper columns
      .drop("rnd_"))
  
  sampled.show()
  
  ## +------+---+--------------------+
  ## |teamId| x1|                  x2|
  ## +------+---+--------------------+
  ## |     1|  f|  0.8563875814036598|
  ## |     1|  g|    0.81921738561455|
  ## |     1|  i|  0.8173912535268248|
  ## |     2|  h| 0.10862995810038856|
  ## |     2|  c|  0.3864428179837973|
  ## |     2|  a|  0.6695356657072442|
  ## |     3|  b|0.012329360826023095|
  ## |     3|  a|  0.6450777858109182|
  ## |     3|  e|  0.5142305439671874|
  ## +------+---+--------------------+
  

but I am afraid both will be rather expensive. If size of the individual groups is balanced and relatively large I would simply use DataFrame.randomSplit.

If number of groups is relatively small it is possible to try something else:

from pyspark.sql.functions import count, udf
from pyspark.sql.types import BooleanType
from operator import truediv

counts = (df
    .groupBy(col("teamId"))
    .agg(count("*").alias("n"))
    .rdd.map(lambda r: (r.teamId, r.n))
    .collectAsMap()) 

# This defines fraction of observations from a group which should
# be taken to get n values 
counts_bd = sc.broadcast({k: truediv(n, v) for (k, v) in counts.items()})

to_take = udf(lambda k, rnd: rnd <= counts_bd.value.get(k), BooleanType())

sampled = (df
    .withColumn("rnd_", rand())
    .where(to_take(col("teamId"), col("rnd_")))
    .drop("rnd_"))

sampled.show()

## +------+---+--------------------+
## |teamId| x1|                  x2|
## +------+---+--------------------+
## |     1|  d| 0.14815204548854788|
## |     1|  f|  0.8563875814036598|
## |     1|  g|    0.81921738561455|
## |     2|  a|  0.6695356657072442|
## |     2|  d| 0.37620872770129155|
## |     2|  g| 0.06233470405822805|
## |     3|  b|0.012329360826023095|
## |     3|  h|  0.9022527556458557|
## +------+---+--------------------+

In Spark 1.5+ you can replace udf with a call to sampleBy method:

df.sampleBy("teamId", counts_bd.value)

It won't give you exact number of observations but should be good enough most of the time as long as a number of observations per group is large enough to get proper samples. You can also use sampleByKey on a RDD in a similar way.