Create rule S7194: PySpark broadcasting should be used when joining a small DataFrame to a larger DataFrame.

rules/S7194/metadata.json

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+{
+}

rules/S7194/python/metadata.json

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+{
+  "title": "PySpark broadcasting should be used when joining a small DataFrame to a larger DataFrame",
+  "type": "CODE_SMELL",
+  "status": "ready",
+  "remediation": {
+    "func": "Constant\/Issue",
+    "constantCost": "5min"
+  },
+  "tags": [
+    "data-science",
+    "pyspark"
+  ],
+  "defaultSeverity": "Major",
+  "ruleSpecification": "RSPEC-7194",
+  "sqKey": "S7194",
+  "scope": "All",
+  "defaultQualityProfiles": ["Sonar way"],
+  "quickfix": "unknown",
+  "code": {
+    "impacts": {
+      "RELIABILITY": "LOW"
+    },
+    "attribute": "EFFICIENT"
+  }
+}

rules/S7194/python/rule.adoc

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+This rule raises an issue when a small DataFrame is joined to another DataFrame without the use of the broadcast operation.
+
+== Why is this an issue?
+
+In PySpark, shuffling refers to the process of transferring data between worker nodes within a cluster. 
+This operation, while necessary for tasks such as join and aggregation on DataFrames, can be resource-intensive. 
+Although Spark handles shuffling automatically, there are strategies to minimize it, thereby enhancing the performance of these operations.
+
+When performing join operations with multiple DataFrames in PySpark, it is crucial to consider the size of the DataFrames involved. 
+If a small DataFrame is being joined to a larger one, utilizing the `broadcast` function to distribute the small DataFrame across all worker nodes can be beneficial. 
+This approach significantly reduces the volume of data shuffled between nodes, thereby improving the efficiency of the join operation.
+
+== How to fix it
+
+To fix this issue, use the `broadcast` function on the small DataFrame before performing the join operation.
+
+=== Code examples
+
+==== Noncompliant code example
+
+[source,python,diff-id=1,diff-type=noncompliant]
+----
+from pyspark.sql import SparkSession
+
+spark = SparkSession.builder.appName('myspark').getOrCreate()
+
+data = [
+    (1, "Alice"),
+    (2, "Bob"),
+    (2, "Charlie"),
+    (1, "Dan"),
+    (2, "Elsa")
+]
+large_df = spark.createDataFrame(data, ["department_id", "name"])
+small_df = spark.createDataFrame([(1, 'HR'), (2, 'Finance')], ["department_id", "department"])
+
+joined_df = large_df.join(small_df, on="department_id", how="left") # NonCompliant: the small DataFrame is not broadcasted
+----
+
+==== Compliant solution
+
+[source,python,diff-id=1,diff-type=compliant]
+----
+from pyspark.sql import SparkSession
+from pyspark.sql.functions import broadcast
+
+spark = SparkSession.builder.appName('myspark').getOrCreate()
+
+data = [
+    (1, "Alice"),
+    (2, "Bob"),
+    (2, "Charlie"),
+    (1, "Dan"),
+    (2, "Elsa")
+]
+large_df = spark.createDataFrame(data, ["department_id", "name"])
+small_df = spark.createDataFrame([(1, 'HR'), (2, 'Finance')], ["department_id", "department"])
+
+joined_df = large_df.join(broadcast(small_df), on="department_id", how="left") # Compliant
+----
+
+== Resources
+=== Documentation
+
+* PySpark Documentation - https://spark.apache.org/docs/latest/api/python/reference/pyspark.sql/api/pyspark.sql.functions.broadcast.html[pyspark.sql.functions.broadcast]
+
+=== Articles & blog posts
+
+* Medium Article - https://aspinfo.medium.com/what-is-broadcast-join-how-to-perform-broadcast-in-pyspark-699aef2eff5a[What is broadcast join, how to perform broadcast in pyspark?]
+