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I have a Kafka stream sending me data of heartbeat for cyclist on circuit. I need to able to do AVG of heartbeat for each lap he did. I tried to use the session but it only works on time and in my case the time could be different each lap.

I found with foreachBatch I can create a Window on any column.

.foreachBatch(calculate_heartbeat)

and in this function:

def calculate_heartbeat(df, batch_id):
    lap_window = Window.partitionBy("lap")
    df = df.withColumn("avg", avg("heartbeat").over(lap_window))
    df.show(truncate=False)
    df.groupBy("lap").agg(avg("heartbeat")).show()

    return df

but when using the foreachBatch I a not able to accumulate the whole data of the lap, is there a way to do it ?

I tried different approach to create and empty dataframe and add each batch I received to it I am expecting to get accumulate the whole lap in the dataframe, or any better approach to do my calculation ? or windows the lap

MrBigData
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1 Answers1

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I'm using Spark 3.4.0 for this test.

I have generated some CSVs in /content/input directory with the following content which I understand will be present in your event stream:

lapId,heartbeat,timestamp
1,122,2023-05-23 10:01:00
1,132,2023-05-23 10:02:00
2,137,2023-05-23 10:03:00
2,122,2023-05-23 10:04:00
2,132,2023-05-23 10:05:00
3,137,2023-05-23 10:06:00
3,122,2023-05-23 10:07:00
3,132,2023-05-23 10:08:00
4,137,2023-05-23 10:09:00

Using the session window aggregation functionality of Spark Structured Streaming where in this case it is assumed that one event will be reported at least every 5 minutes (otherwise the lap is considered as over)

from pyspark.sql import SparkSession
from pyspark.sql.types import StructType,StructField, TimestampType,StringType,LongType
from pyspark.sql.functions import session_window, avg

spark = SparkSession.builder.master("local[*]").getOrCreate()

schema = StructType([
  StructField('lapId', StringType(), True),
  StructField('heartbeat', LongType(), True),
  StructField('timestamp', TimestampType(), True)
])

df = spark.readStream.format("csv").schema(schema).option("header",True).load("/content/input")

# This is the part that interests you
avg_heartbeat_rate_per_lap = df \
    .withWatermark("timestamp", "10 minutes") \
    .groupBy(
        session_window(df.timestamp, "5 minutes"),
        df.lapId) \
    .agg(avg("heartbeat"))

query = avg_heartbeat_rate_per_lap \
    .writeStream \
    .outputMode("complete") \
    .queryName("aggregates") \
    .format("memory") \
    .start()

spark.sql("select * from aggregates").show(truncate=False)

The results are correct as per the inputs:

+------------------------------------------+-----+------------------+
|session_window                            |lapId|avg(heartbeat)    |
+------------------------------------------+-----+------------------+
|{2023-05-23 10:06:00, 2023-05-23 10:13:00}|3    |130.33333333333334|
|{2023-05-23 10:01:00, 2023-05-23 10:07:00}|1    |127.0             |
|{2023-05-23 10:09:00, 2023-05-23 10:14:00}|4    |137.0             |
|{2023-05-23 10:03:00, 2023-05-23 10:10:00}|2    |130.33333333333334|
+------------------------------------------+-----+------------------+
Alexandre Juma
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  • Thank you, I didn't know we can use the session windows in this way. I thought we can only use it when we have an empty gap between 2 sessions. – MrBigData May 23 '23 at 12:47
  • is there a way on stream to use the avg calculated value to filter data from the main stream ? df = df.where(df["heartbeat"] > avg_heartbeat_rate_per_lap.tail(1)["avg(heartbeat)"]) this did not work, but something similar – MrBigData May 25 '23 at 15:22
  • You should open a new question and reference this one to get additional help – Alexandre Juma May 25 '23 at 18:44