Spark: How to Add Multiple Columns in Dataframes (and How Not to)

There are generally two ways to dynamically add columns to a dataframe in Spark. A foldLeft or a map (passing a RowEncoder). The foldLeft way is quite popular (and elegant) but recently I came across an issue regarding its performance when the number of columns to add is not trivial. I think it’s worth to share the lesson learned: a map solution offers substantial better performance when the number of columns to be added tends to increase.

Let’s start with an example dataframe with four columns: “id”, “author”, “title”, “incipit” (the opening line of the novel). A simple dataframe with just one row might look something like:

+---+-----------+-------------+-----------------------------+
| id|     author|        title|                      incipit|
+---+-----------+-------------+-----------------------------+
|  6|Leo Tolstoy|Anna Karenina|Happy families are all alike |
+---+-----------+-------------+-----------------------------+

You are given an arbitrary list of words and, for each of them, you would like to add a column (named after the word) to the original dataframe and flag with a boolean whether or not that word appear at least once in the opening line (incipit).
For a list of two words List("families", "giraffe"), the above dataframe will be transformed into the following:

+---+-----------+-------------+-----------------------------+--------+-------+
| id|     author|        title|                      incipit|families|giraffe|
+---+-----------+-------------+-----------------------------+--------+-------+
|  6|Leo Tolstoy|Anna Karenina|Happy families are all alike |    true|  false|
+---+-----------+-------------+-----------------------------+--------+-------+

As the list of columns is arbitrary, there are two possible approaches to this problem. I wrapper both in a method to make testing easier. First approach would be the foldLeft way:

def addColumnsViaFold(df: DataFrame, columns: List[String]): DataFrame = {
  import df.sparkSession.implicits._
  columns.foldLeft(df)((acc, col) => {
    acc.withColumn(col, acc("incipit").as[String].contains(col))
  })
}

And the second one (which involves a bit more coding) is the map way:

def addColumnsViaMap(df: DataFrame, words: List[String]): DataFrame = {
   val encoder = RowEncoder.apply(getSchema(df, words))
   df.map(mappingRows(df.schema)(words))(encoder)
}

private val mappingRows: StructType => List[String] => Row => Row =
  (schema) => (words) => (row) => {
    val addedCols: List[Boolean] = words.map {
      word => row.getString(schema.fieldIndex("incipit")).contains(word)
    }
    Row.merge(row, Row.fromSeq(addedCols))
  }

private def getSchema(df: DataFrame, words: List[String]): StructType = {
  var schema: StructType = df.schema
  words.foreach(word => schema = schema.add(word, DataTypes.BooleanType, false))
  schema
}

The code (including all tests) is available here
When we run the scala meter tests to get some idea of how the two approaches behave when dealing with 100 new columns, we get the following results1. For foldLeft (addColumnsViaFold method):
adding-column-via-fold-100
Whereas those one are the results for map (addColumnsViaMap method):
adding_columns_via_map-100
When the number of columns increases, foldLeft is taking considerably more time. If we take the number of columns to 500 the result is similar (and more dramatic)
Once again, for foldLeft :
adding-column-via-fold
And for map:
adding_columns_via_map-500
With 1000 columns, foldLeft job aborts:

[error] Error running separate JVM: java.lang.OutOfMemoryError: GC overhead limit exceeded

Probably sign that the heap is running low and the CG can’t free much memory. Still, the map based solution seems to cope much better even with 1000 row:
adding_columns_via_map
When it comes to the reason behind this different behavior, my guess would be that somehow Catalyst is not able to optimize the foldLeft operation. The below plans (explain(true)) show how the Parsed and Analyzed Logical Plans end up in a sort of nested structure.

== Parsed Logical Plan ==
Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 20 more fields]
+- AnalysisBarrier
      +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 19 more fields]
         +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 18 more fields]
            +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 17 more fields]
               +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 16 more fields]
                  +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 15 more fields]
                     +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 14 more fields]
                        +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 13 more fields]
                           +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 12 more fields]
                              +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 11 more fields]
                                 +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 10 more fields]
                                    +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 9 more fields]
                                       +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 8 more fields]
                                          +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 7 more fields]
                                             +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 6 more fields]
                                                +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 5 more fields]
                                                   +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 4 more fields]
                                                      +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 3 more fields]
                                                         +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 2 more fields]
                                                            +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, Contains(incipit#12, toing) AS toing#368]
                                                               +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, Contains(incipit#12, hence) AS hence#342]
                                                                  +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, Contains(incipit#12, four) AS four#317]
                                                                     +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, Contains(incipit#12, John) AS John#293]
                                                                        +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, Contains(incipit#12, pop) AS pop#270]
                                                                           +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, Contains(incipit#12, drunk) AS drunk#248]
                                                                              +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, Contains(incipit#12, two) AS two#227]
                                                                                 +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, Contains(incipit#12, when) AS when#207]
                                                                                    +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, Contains(incipit#12, city) AS city#188]
                                                                                       +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, Contains(incipit#12, morning) AS morning#170]
                                                                                          +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, Contains(incipit#12, way) AS way#153]
                                                                                             +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, Contains(incipit#12, pig) AS pig#137]
                                                                                                +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, Contains(incipit#12, happy) AS happy#122]
                                                                                                   +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, Contains(incipit#12, was) AS was#108]
                                                                                                      +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, Contains(incipit#12, anna) AS anna#95]
                                                                                                         +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, Contains(incipit#12, in) AS in#83]
                                                                                                            +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, Contains(incipit#12, man) AS man#72]
                                                                                                               +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, Contains(incipit#12, with) AS with#62]
                                                                                                                  +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, Contains(incipit#12, alike) AS alike#53]
                                                                                                                     +- Project [id#9, author#10, title#11, incipit#12, to#38, Contains(incipit#12, get) AS get#45]
                                                                                                                        +- Project [id#9, author#10, title#11, incipit#12, Contains(incipit#12, to) AS to#38]
                                                                                                                           +- Project [_1#4 AS id#9, _2#5 AS author#10, _3#6 AS title#11, _4#7 AS incipit#12]
                                                                                                                              +- LocalRelation [_1#4, _2#5, _3#6, _4#7]

== Analyzed Logical Plan ==
id: int, author: string, title: string, incipit: string, to: boolean, get: boolean, alike: boolean, with: boolean, man: boolean, in: boolean, anna: boolean, was: boolean, happy: boolean, pig: boolean, way: boolean, morning: boolean, city: boolean, when: boolean, two: boolean, drunk: boolean, pop: boolean, John: boolean, four: boolean, hence: boolean, ... 20 more fields
Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 20 more fields]
+- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 19 more fields]
   +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 18 more fields]
      +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 17 more fields]
         +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 16 more fields]
            +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 15 more fields]
               +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 14 more fields]
                  +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 13 more fields]
                     +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 12 more fields]
                        +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 11 more fields]
                           +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 10 more fields]
                              +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 9 more fields]
                                 +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 8 more fields]
                                    +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 7 more fields]
                                       +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 6 more fields]
                                          +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 5 more fields]
                                             +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 4 more fields]
                                                +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 3 more fields]
                                                   +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, ... 2 more fields]
                                                      +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, hence#342, Contains(incipit#12, toing) AS toing#368]
                                                         +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, four#317, Contains(incipit#12, hence) AS hence#342]
                                                            +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, John#293, Contains(incipit#12, four) AS four#317]
                                                               +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, pop#270, Contains(incipit#12, John) AS John#293]
                                                                  +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, drunk#248, Contains(incipit#12, pop) AS pop#270]
                                                                     +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, two#227, Contains(incipit#12, drunk) AS drunk#248]
                                                                        +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, when#207, Contains(incipit#12, two) AS two#227]
                                                                           +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, city#188, Contains(incipit#12, when) AS when#207]
                                                                              +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, morning#170, Contains(incipit#12, city) AS city#188]
                                                                                 +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, way#153, Contains(incipit#12, morning) AS morning#170]
                                                                                    +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, pig#137, Contains(incipit#12, way) AS way#153]
                                                                                       +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, happy#122, Contains(incipit#12, pig) AS pig#137]
                                                                                          +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, was#108, Contains(incipit#12, happy) AS happy#122]
                                                                                             +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, anna#95, Contains(incipit#12, was) AS was#108]
                                                                                                +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, in#83, Contains(incipit#12, anna) AS anna#95]
                                                                                                   +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, man#72, Contains(incipit#12, in) AS in#83]
                                                                                                      +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, with#62, Contains(incipit#12, man) AS man#72]
                                                                                                         +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, alike#53, Contains(incipit#12, with) AS with#62]
                                                                                                            +- Project [id#9, author#10, title#11, incipit#12, to#38, get#45, Contains(incipit#12, alike) AS alike#53]
                                                                                                               +- Project [id#9, author#10, title#11, incipit#12, to#38, Contains(incipit#12, get) AS get#45]
                                                                                                                  +- Project [id#9, author#10, title#11, incipit#12, Contains(incipit#12, to) AS to#38]
                                                                                                                     +- Project [_1#4 AS id#9, _2#5 AS author#10, _3#6 AS title#11, _4#7 AS incipit#12]
                                                                                                                        +- LocalRelation [_1#4, _2#5, _3#6, _4#7]

== Optimized Logical Plan ==
Project [id#9, author#10, title#11, incipit#12, Contains(incipit#12, to) AS to#38, Contains(incipit#12, get) AS get#45, Contains(incipit#12, alike) AS alike#53, Contains(incipit#12, with) AS with#62, Contains(incipit#12, man) AS man#72, Contains(incipit#12, in) AS in#83, Contains(incipit#12, anna) AS anna#95, Contains(incipit#12, was) AS was#108, Contains(incipit#12, happy) AS happy#122, Contains(incipit#12, pig) AS pig#137, Contains(incipit#12, way) AS way#153, Contains(incipit#12, morning) AS morning#170, Contains(incipit#12, city) AS city#188, Contains(incipit#12, when) AS when#207, Contains(incipit#12, two) AS two#227, Contains(incipit#12, drunk) AS drunk#248, Contains(incipit#12, pop) AS pop#270, Contains(incipit#12, John) AS John#293, Contains(incipit#12, four) AS four#317, Contains(incipit#12, hence) AS hence#342, ... 20 more fields]
+- InMemoryRelation [id#9, author#10, title#11, incipit#12], true, 10000, StorageLevel(disk, memory, deserialized, 1 replicas)
      +- LocalTableScan [id#9, author#10, title#11, incipit#12]

== Physical Plan ==
*(1) Project [id#9, author#10, title#11, incipit#12, Contains(incipit#12, to) AS to#38, Contains(incipit#12, get) AS get#45, Contains(incipit#12, alike) AS alike#53, Contains(incipit#12, with) AS with#62, Contains(incipit#12, man) AS man#72, Contains(incipit#12, in) AS in#83, Contains(incipit#12, anna) AS anna#95, Contains(incipit#12, was) AS was#108, Contains(incipit#12, happy) AS happy#122, Contains(incipit#12, pig) AS pig#137, Contains(incipit#12, way) AS way#153, Contains(incipit#12, morning) AS morning#170, Contains(incipit#12, city) AS city#188, Contains(incipit#12, when) AS when#207, Contains(incipit#12, two) AS two#227, Contains(incipit#12, drunk) AS drunk#248, Contains(incipit#12, pop) AS pop#270, Contains(incipit#12, John) AS John#293, Contains(incipit#12, four) AS four#317, Contains(incipit#12, hence) AS hence#342, ... 20 more fields]
+- InMemoryTableScan [author#10, id#9, incipit#12, title#11]
      +- InMemoryRelation [id#9, author#10, title#11, incipit#12], true, 10000, StorageLevel(disk, memory, deserialized, 1 replicas)
            +- LocalTableScan [id#9, author#10, title#11, incipit#12]

Not sure whether that is somehow related to the Catalyst inability to optimize foldLeft but the explained map plan doesn’t show the same nested structure:

== Parsed Logical Plan ==
'SerializeFromObject [validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 0, id), IntegerType) AS id#83, if (assertnotnull(input[0, org.apache.spark.sql.Row, true]).isNullAt) null else staticinvoke(class org.apache.spark.unsafe.types.UTF8String, StringType, fromString, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 1, author), StringType), true, false) AS author#84, if (assertnotnull(input[0, org.apache.spark.sql.Row, true]).isNullAt) null else staticinvoke(class org.apache.spark.unsafe.types.UTF8String, StringType, fromString, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 2, title), StringType), true, false) AS title#85, if (assertnotnull(input[0, org.apache.spark.sql.Row, true]).isNullAt) null else staticinvoke(class org.apache.spark.unsafe.types.UTF8String, StringType, fromString, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 3, incipit), StringType), true, false) AS incipit#86, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 4, to), BooleanType) AS to#87, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 5, get), BooleanType) AS get#88, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 6, alike), BooleanType) AS alike#89, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 7, with), BooleanType) AS with#90, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 8, man), BooleanType) AS man#91, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 9, in), BooleanType) AS in#92, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 10, anna), BooleanType) AS anna#93, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 11, was), BooleanType) AS was#94, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 12, happy), BooleanType) AS happy#95, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 13, pig), BooleanType) AS pig#96, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 14, way), BooleanType) AS way#97, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 15, morning), BooleanType) AS morning#98, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 16, city), BooleanType) AS city#99, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 17, when), BooleanType) AS when#100, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 18, two), BooleanType) AS two#101, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 19, drunk), BooleanType) AS drunk#102, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 20, pop), BooleanType) AS pop#103, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 21, John), BooleanType) AS John#104, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 22, four), BooleanType) AS four#105, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 23, hence), BooleanType) AS hence#106, ... 20 more fields]
+- 'MapElements , interface org.apache.spark.sql.Row, [StructField(id,IntegerType,false), StructField(author,StringType,true), StructField(title,StringType,true), StructField(incipit,StringType,true)], obj#82: org.apache.spark.sql.Row
   +- 'DeserializeToObject unresolveddeserializer(createexternalrow(getcolumnbyordinal(0, IntegerType), getcolumnbyordinal(1, StringType).toString, getcolumnbyordinal(2, StringType).toString, getcolumnbyordinal(3, StringType).toString, StructField(id,IntegerType,false), StructField(author,StringType,true), StructField(title,StringType,true), StructField(incipit,StringType,true))), obj#81: org.apache.spark.sql.Row
      +- AnalysisBarrier
            +- Project [_1#4 AS id#9, _2#5 AS author#10, _3#6 AS title#11, _4#7 AS incipit#12]
               +- LocalRelation [_1#4, _2#5, _3#6, _4#7]

== Analyzed Logical Plan ==
id: int, author: string, title: string, incipit: string, to: boolean, get: boolean, alike: boolean, with: boolean, man: boolean, in: boolean, anna: boolean, was: boolean, happy: boolean, pig: boolean, way: boolean, morning: boolean, city: boolean, when: boolean, two: boolean, drunk: boolean, pop: boolean, John: boolean, four: boolean, hence: boolean, ... 20 more fields
SerializeFromObject [validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 0, id), IntegerType) AS id#83, if (assertnotnull(input[0, org.apache.spark.sql.Row, true]).isNullAt) null else staticinvoke(class org.apache.spark.unsafe.types.UTF8String, StringType, fromString, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 1, author), StringType), true, false) AS author#84, if (assertnotnull(input[0, org.apache.spark.sql.Row, true]).isNullAt) null else staticinvoke(class org.apache.spark.unsafe.types.UTF8String, StringType, fromString, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 2, title), StringType), true, false) AS title#85, if (assertnotnull(input[0, org.apache.spark.sql.Row, true]).isNullAt) null else staticinvoke(class org.apache.spark.unsafe.types.UTF8String, StringType, fromString, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 3, incipit), StringType), true, false) AS incipit#86, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 4, to), BooleanType) AS to#87, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 5, get), BooleanType) AS get#88, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 6, alike), BooleanType) AS alike#89, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 7, with), BooleanType) AS with#90, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 8, man), BooleanType) AS man#91, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 9, in), BooleanType) AS in#92, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 10, anna), BooleanType) AS anna#93, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 11, was), BooleanType) AS was#94, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 12, happy), BooleanType) AS happy#95, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 13, pig), BooleanType) AS pig#96, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 14, way), BooleanType) AS way#97, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 15, morning), BooleanType) AS morning#98, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 16, city), BooleanType) AS city#99, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 17, when), BooleanType) AS when#100, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 18, two), BooleanType) AS two#101, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 19, drunk), BooleanType) AS drunk#102, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 20, pop), BooleanType) AS pop#103, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 21, John), BooleanType) AS John#104, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 22, four), BooleanType) AS four#105, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 23, hence), BooleanType) AS hence#106, ... 20 more fields]
+- MapElements , interface org.apache.spark.sql.Row, [StructField(id,IntegerType,false), StructField(author,StringType,true), StructField(title,StringType,true), StructField(incipit,StringType,true)], obj#82: org.apache.spark.sql.Row
   +- DeserializeToObject createexternalrow(id#9, author#10.toString, title#11.toString, incipit#12.toString, StructField(id,IntegerType,false), StructField(author,StringType,true), StructField(title,StringType,true), StructField(incipit,StringType,true)), obj#81: org.apache.spark.sql.Row
      +- Project [_1#4 AS id#9, _2#5 AS author#10, _3#6 AS title#11, _4#7 AS incipit#12]
         +- LocalRelation [_1#4, _2#5, _3#6, _4#7]

== Optimized Logical Plan ==
SerializeFromObject [validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 0, id), IntegerType) AS id#83, if (assertnotnull(input[0, org.apache.spark.sql.Row, true]).isNullAt) null else staticinvoke(class org.apache.spark.unsafe.types.UTF8String, StringType, fromString, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 1, author), StringType), true, false) AS author#84, if (assertnotnull(input[0, org.apache.spark.sql.Row, true]).isNullAt) null else staticinvoke(class org.apache.spark.unsafe.types.UTF8String, StringType, fromString, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 2, title), StringType), true, false) AS title#85, if (assertnotnull(input[0, org.apache.spark.sql.Row, true]).isNullAt) null else staticinvoke(class org.apache.spark.unsafe.types.UTF8String, StringType, fromString, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 3, incipit), StringType), true, false) AS incipit#86, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 4, to), BooleanType) AS to#87, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 5, get), BooleanType) AS get#88, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 6, alike), BooleanType) AS alike#89, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 7, with), BooleanType) AS with#90, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 8, man), BooleanType) AS man#91, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 9, in), BooleanType) AS in#92, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 10, anna), BooleanType) AS anna#93, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 11, was), BooleanType) AS was#94, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 12, happy), BooleanType) AS happy#95, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 13, pig), BooleanType) AS pig#96, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 14, way), BooleanType) AS way#97, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 15, morning), BooleanType) AS morning#98, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 16, city), BooleanType) AS city#99, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 17, when), BooleanType) AS when#100, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 18, two), BooleanType) AS two#101, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 19, drunk), BooleanType) AS drunk#102, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 20, pop), BooleanType) AS pop#103, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 21, John), BooleanType) AS John#104, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 22, four), BooleanType) AS four#105, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 23, hence), BooleanType) AS hence#106, ... 20 more fields]
+- MapElements , interface org.apache.spark.sql.Row, [StructField(id,IntegerType,false), StructField(author,StringType,true), StructField(title,StringType,true), StructField(incipit,StringType,true)], obj#82: org.apache.spark.sql.Row
   +- DeserializeToObject createexternalrow(id#9, author#10.toString, title#11.toString, incipit#12.toString, StructField(id,IntegerType,false), StructField(author,StringType,true), StructField(title,StringType,true), StructField(incipit,StringType,true)), obj#81: org.apache.spark.sql.Row
      +- InMemoryRelation [id#9, author#10, title#11, incipit#12], true, 10000, StorageLevel(disk, memory, deserialized, 1 replicas)
            +- LocalTableScan [id#9, author#10, title#11, incipit#12]

== Physical Plan ==
*(1) SerializeFromObject [validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 0, id), IntegerType) AS id#83, if (assertnotnull(input[0, org.apache.spark.sql.Row, true]).isNullAt) null else staticinvoke(class org.apache.spark.unsafe.types.UTF8String, StringType, fromString, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 1, author), StringType), true, false) AS author#84, if (assertnotnull(input[0, org.apache.spark.sql.Row, true]).isNullAt) null else staticinvoke(class org.apache.spark.unsafe.types.UTF8String, StringType, fromString, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 2, title), StringType), true, false) AS title#85, if (assertnotnull(input[0, org.apache.spark.sql.Row, true]).isNullAt) null else staticinvoke(class org.apache.spark.unsafe.types.UTF8String, StringType, fromString, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 3, incipit), StringType), true, false) AS incipit#86, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 4, to), BooleanType) AS to#87, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 5, get), BooleanType) AS get#88, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 6, alike), BooleanType) AS alike#89, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 7, with), BooleanType) AS with#90, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 8, man), BooleanType) AS man#91, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 9, in), BooleanType) AS in#92, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 10, anna), BooleanType) AS anna#93, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 11, was), BooleanType) AS was#94, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 12, happy), BooleanType) AS happy#95, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 13, pig), BooleanType) AS pig#96, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 14, way), BooleanType) AS way#97, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 15, morning), BooleanType) AS morning#98, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 16, city), BooleanType) AS city#99, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 17, when), BooleanType) AS when#100, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 18, two), BooleanType) AS two#101, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 19, drunk), BooleanType) AS drunk#102, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 20, pop), BooleanType) AS pop#103, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 21, John), BooleanType) AS John#104, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 22, four), BooleanType) AS four#105, validateexternaltype(getexternalrowfield(assertnotnull(input[0, org.apache.spark.sql.Row, true]), 23, hence), BooleanType) AS hence#106, ... 20 more fields]
+- *(1) MapElements , obj#82: org.apache.spark.sql.Row
   +- *(1) DeserializeToObject createexternalrow(id#9, author#10.toString, title#11.toString, incipit#12.toString, StructField(id,IntegerType,false), StructField(author,StringType,true), StructField(title,StringType,true), StructField(incipit,StringType,true)), obj#81: org.apache.spark.sql.Row
      +- InMemoryTableScan [id#9, author#10, title#11, incipit#12]
            +- InMemoryRelation [id#9, author#10, title#11, incipit#12], true, 10000, StorageLevel(disk, memory, deserialized, 1 replicas)
                  +- LocalTableScan [id#9, author#10, title#11, incipit#12]

Whatever the root cause is, the conclusion is clear. As per Spark 2.3.0 (and probably previous versions) adding (dynamically) a congruous number of columns to a dataframe should be done via a map operation and not foldLeft for the reasons we’ve seen.

—————————————-

1. [I run the tests on a virtual box with three cores running Spark 2.3.0. Time in milliseconds reflects the underline infrastructure and I would expect different performance on a proper cluster. Nevertheless, that a foldLeft solution performs significantly worse that a map based solution should be an outcome independent of the underline hardware.]

Custom JSON serializer

In Scala there are quite a few frameworks that make the implementation of RESTful JSON services quite straightforward (Play, Scalatra, Akka HTTP are the main ones). They all follow the same general idea: a JSON payload deserialize into a Scala class (usually a case class) and serialize back into JSON. For example a JSON payload like:

{"name": "Paul", "age": 45, "gender": "male"}

Can be easily deserialized into something like:

final case class User(name: String, age: Int, gender: String)

This is pretty standard stuff and works almost out of the box in all the mentioned framework but in some cases you might want to take advantage of the type system for example by defining a type for “age”:

// A value class representing a person's age
final class Age(val age: Int) extends AnyVal

And also constant values for “gender” rather than a generic String type:

sealed trait Gender {
  val sex: String
}

final case object Male extends Gender {
  val sex = "male"
}

final case object Female extends Gender {
  val sex = "female"
}

final case object Other extends Gender {
  val sex = "other"
}

object GenderHelper {
  private val genders: Set[Gender] = Set(Male, Female, Other)
  def toGender(sex: String): Option[Gender] = genders.find(_.sex == sex)
}

We can now define User in a more robust way as:

final case class User(name: String, age: Age, gender: Gender)

Which looks better but doesn’t work out of the box anymore as, regardless of the JSON scala library we use, there is no way to tell how to serialize/deserialize types like Age or Gender. We need to write our own JSON custom serializer/deserializer. The following example uses Akka HTTP and Json4s (code is available here).

We first need to write two serializer/deserializer. One for Age:

import com.lansalo.model.Age
import org.json4s.CustomSerializer
import org.json4s.JsonAST.JInt

case object AgeSerializer extends CustomSerializer[Age](format => ( {
  case JInt(age) => new Age(age.intValue)
}, {
  case age: Int => JInt(BigInt(age))
}))

And one for Gender:

import com.lansalo.model.{Gender, GenderHelper}
import org.json4s.CustomSerializer
import org.json4s.JsonAST.JString

case object GenderSerializer extends CustomSerializer[Gender](format => ( {
  case JString(gender) => GenderHelper.toGender(gender).get
}, {
  case gender: Gender => JString(gender.sex)
}))

Then we just need to add them to the default json4s formats (in a Trait here):

import com.lansalo.json.serializer.{AgeSerializer, GenderSerializer}
import org.json4s.DefaultFormats

trait JsonSupport {
  implicit val formats = DefaultFormats + GenderSerializer + AgeSerializer
}

And finally bring the json formats in scope where we need it:

import akka.actor.ActorSystem
import akka.event.Logging
import akka.http.scaladsl.model.StatusCodes
import akka.http.scaladsl.server.Directives._
import akka.http.scaladsl.server.Route
import akka.http.scaladsl.server.directives.MethodDirectives.{get, post}
import akka.http.scaladsl.server.directives.PathDirectives.path
import akka.http.scaladsl.server.directives.RouteDirectives.complete
import akka.util.Timeout
import com.lansalo.json.JsonSupport
import com.lansalo.model.{Age, Female, User}
import de.heikoseeberger.akkahttpjson4s.Json4sSupport
import org.json4s.jackson

import scala.concurrent.duration._

trait UserRoutes extends JsonSupport {

  implicit def system: ActorSystem

  lazy val log = Logging(system, classOf[UserRoutes])

  implicit lazy val timeout = Timeout(5.seconds)
  import Json4sSupport._

  implicit val serialization = jackson.Serialization // or native.Serialization

  lazy val userRoutes: Route = pathPrefix("users") {
    pathEnd {
      post {
        entity(as[User]) { user =>
          log.info(s"Received user: $user")
          complete((StatusCodes.Created, "OK"))
        }
      }
    } ~
      path(Segment) { name =>
        get {
          complete(User(name, new Age(22), Female))
        }
      }
  }
}