This post demonstrates how to explicitly validate the schema of a DataFrame in custom transformations so your code is easier to read and provides better error messages.

Spark’s lazy evaluation and execution plan optimizations yield amazingly fast results, but can also create cryptic error messages.

This post will demonstrate how schema validations create code that’s easier to read, maintain, and debug.

Custom Transformations Refresher

A custom transformation is a function that takes a DataFrame as an argument and returns a DataFrame. This blog post provides great background information on custom transformations.

Let’s look at an example of a custom transformation that makes an assumption.

The following transformation appends an is_senior_citizen column to a DataFrame.

def withIsSeniorCitizen()(df: DataFrame): DataFrame = {

df.withColumn("is_senior_citizen", df("age") >= 65)

}

Suppose we have the following peopleDF :

+------+---+

| name|age|

+------+---+

|miguel| 80|

| liz| 10|

+------+---+

Let’s run the withIsSeniorCitizen transformation.

val actualDF = peopleDF.transform(withIsSeniorCitizen())

actualDF will have the following data.

+------+---+-----------------+

| name|age|is_senior_citizen|

+------+---+-----------------+

|miguel| 80| true|

| liz| 10| false|

+------+---+-----------------+

withIsSeniorCitizen assumes that the DataFrame has an age column with the IntegerType . In this case, the withIsSeniorCitizen transformation’s assumption was correct and the code worked perfectly ;)

A Custom Transformation Making a Bad Assumption

Let’s use the following withFullName transformation to illustrate how making incorrect assumptions yields bad error messages.

def withFullName()(df: DataFrame): DataFrame = {

df.withColumn(

"full_name",

concat_ws(" ", col("first_name"), col("last_name"))

)

}

Suppose we have the following animalDF .

+---+

|pet|

+---+

|cat|

|dog|

+---+

Let’s run the withFullName transformation.

animalDF.transform(withFullName())

The code will error out with this message.

org.apache.spark.sql.AnalysisException: cannot resolve ‘`first_name`’ given input columns: [pet]

The withFullName transformation assumes the DataFrame has first_name and last_name columns. The assumption isn’t met, so the code errors out.

The default error message isn’t terrible, but it’s not complete. We would like to have an error message that specifies both the first_name and last_name columns are required to run the withFullName transformation.

Column Presence Validation

Let’s use the spark-daria DataFrameValidator to specify the column assumptions within the withFullName transformation.

import com.github.mrpowers.spark.daria.sql.DataFrameValidator def withFullName()(df: DataFrame): DataFrame = {

validatePresenceOfColumns(df, Seq("first_name", "last_name"))

df.withColumn(

"full_name",

concat_ws(" ", col("first_name"), col("last_name"))

)

}

Let’s run the code again.

animalDF.transform(withFullName())

This is the new error message.

com.github.mrpowers.spark.daria.sql.MissingDataFrameColumnsException: The [first_name, last_name] columns are not included in the DataFrame with the following columns [pet]

validatePresenceOfColumns makes the withFullName transformation better in two important ways.

withFullName will be easier to maintain and use because the transformation requirements are explicitly documented in the code When the withFullName assumptions aren’t met, the error message is more descriptive

Full Schema Validation

We can also use the spark-daria DataFrameValidator to validate the presence of StructFields in DataFrames (i.e. validate the presence of the name, data type, and nullable property for each column that’s required).

Let’s look at a withSum transformation that adds the num1 and num2 columns in a DataFrame.

def withSum()(df: DataFrame): DataFrame = {

df.withColumn(

"sum",

col("num1") + col("num2")

)

}

When the num1 and num2 columns contain numerical data, the withSum transformation works as expected.

val numsDF = Seq(

(1, 3),

(7, 8)

).toDF("num1", "num2")



numsDF.transform(withSum()).show() +----+----+---+

|num1|num2|sum|

+----+----+---+

| 1| 3| 4|

| 7| 8| 15|

+----+----+---+

withSum doesn’t work well when the num1 and num2 columns contain strings.

val wordsDF = Seq(

("one", "three"),

("seven", "eight")

).toDF("num1", "num2")



wordsDF.transform(withSum()).show() +-----+-----+----+

| num1| num2| sum|

+-----+-----+----+

| one|three|null|

|seven|eight|null|

+-----+-----+----+

withSum should error out if the num1 and num2 columns aren’t numeric. Let’s refactor the function to error out with a descriptive error message.

def withSum()(df: DataFrame): DataFrame = {

val requiredSchema = StructType(

List(

StructField("num1", IntegerType, true),

StructField("num2", IntegerType, true)

)

)

validateSchema(df, requiredSchema)

df.withColumn(

"sum",

col("num1") + col("num2")

)

}

Let’s run the code again.

wordsDF.transform(withSum()).show()

Now we get a more descriptive error message.

com.github.mrpowers.spark.daria.sql.InvalidDataFrameSchemaException: The [StructField(num1,IntegerType,true), StructField(num2,IntegerType,true)] StructFields are not included in the DataFrame with the following StructFields [StructType(StructField(num1,StringType,true), StructField(num2,StringType,true))]

Documenting DataFrame Assumptions is Especially Important for Chained DataFrame Transformations

Production applications will define several standalone transformations and chain them together for the final result.

val resultDF = df

.transform(myFirstTransform()) // one set of assumptions

.transform(mySecondTransform()) // more assumptions

.transform(myThirdTransform()) // even more assumptions

Debugging order dependent transformations, each with a different set of assumptions, is a nightmare! Don’t torture yourself!

Conclusion

DataFrame schema assumptions should be explicitly documented in the code with validations.

Code that doesn’t make assumptions is easier to read, better to maintain, and returns more descriptive error message.

spark-daria contains the DataFrame validation functions you’ll need in your projects. Follow these setup instructions and write DataFrame transformations like this:

import com.github.mrpowers.spark.daria.sql.DataFrameValidator



object MyTransformations extends DataFrameValidator {



def withStandardizedPersonInfo(df: DataFrame): DataFrame = {

val requiredColNames = Seq("name", "age")

validatePresenceOfColumns(df, requiredColNames)

// some transformation code

}



}

Applications with proper DataFrame schema validations are significantly easier to debug, especially when complex transformations are chained.

Alternate Path Forward?

Schema independent DataFrame transformations are another way to remove schema assumptions from your code. I’ll discuss these in detail in another blog post ;)