I have a large DataFrame (Spark 1.6 Scala) which looks like this:
Type,Value1,Value2,Value3,...
--------------------------
A,11.4,2,3
A,82.0,1,2
A,53.8,3,4
B,31.0,4,5
B,22.6,5,6
B,43.1,6,7
B,11.0,7,8
C,22.1,8,9
C,3.2,9,1
C,13.1,2,3
From this I want to group by Type and apply machine learning algorithms and/or perform complex functions on each group.
My objective is perform complex functions on each group in parallel.
I have tried the following approaches:
Approach 1) Convert Dataframe to Dataset and then use ds.mapGroups() api. But this is giving me an Iterator of each group values.
If i want to perform RandomForestClassificationModel.transform(dataset: DataFrame), i need a DataFrame with only a particular group values.
I was not sure converting Iterator to a Dataframe within mapGroups is a good idea.
Approach 2) Distinct on Type, then map on them and then filter for each Type with in the map loop:
val types = df.select("Type").distinct()
val ff = types.map(row => {
val type = row.getString(0)
val thisGroupDF = df.filter(col("Type") == type)
// Apply complex functions on thisGroupDF
(type, predictedValue)
})
For some reason, the above is never completing (seems to be getting into some kind of infinite loop)
Approach 3) Exploring Window functions, but did not find a method which can provide dataframe of particular group values.
Please help.
Related
I have a Spark Dataset, and I would like to group the data and process the groups, yielding zero or one element per each group. Something like:
val resulDataset = inputDataset
.groupBy('x, 'y)
.flatMap(...)
I didn't find a way to apply a function after a groupBy, but it appears I can use groupByKey instead (is it a good idea? is there a better way?):
val resulDataset = inputDataset
.groupByKey(v => (v.x, v.y))
.flatMap(...)
This works, but here is a thing: I would like process the groups as Datasets. The reason is that I already have convenient functions to use on Datasets and would like to reuse them when calculating the result for each group. But, the groupByKey.flatMap yields an Iterator over the grouped elements, not the Dataset.
The question: is there a way in Spark to group an input Dataset and map a custom function to each group, while treating the grouped elements as a Dataset ? E.g.:
val inputDataset: Dataset[T] = ...
val resulDataset: Dataset[U] = inputDataset
.groupBy(...)
.flatMap(group: Dataset[T] => {
// using Dataset API to calculate resulting value, e.g.:
group.withColumn(row_number().over(...))....as[U]
})
Note, that grouped data is bounded, and it is OK to process it on a single node. But the number of groups can be very high, so the resulting Dataset needs to be distributed. The point of using the Dataset API to process a group is purely a question of using a convenient API.
What I tried so far:
creating a Dataset from an Iterator in the mapped function - it fails with an NPE from a SparkSession (my understanding is that it boils down to the fact that one cannot create a Dataset within the functions which process a Dataset; see this and this)
tried to overcome the issues in the first solution, attempted to create new SparkSession to create the Dataset within a new session; fails with NPE from SparkSession.newSession
(ab)using repartition('x, 'y).mapPartitions(...), but this also yields an Iterator[T] for each partition, not a Dataset[T]
finally, (ab)using filter: I can collect all distinct values of the grouping criteria into an Array (select.distinct.collect), and iterate this array to filter the source Dataset, yielding one Dataset for each group (sort of joins the idea of multiplexing from this article); although this works, my understanding is that it collects all the data on a single node, so it doesn't scale and will eventually have memory issues
Suppose I have a Spark dataframe called trades which has in its schema a few columns, some dimensions (let's say Product and Type) and some facts (let's say Price and Volume).
Rows in the dataframe which have the same dimension columns belong logically to the same group.
What I need is to map each dimension set (Product, Type) to a numeric value, so to obtain in the end a dataframe stats which has as many rows as the distinct number of dimensions and a value - this is the critical part - which is obtained from all the rows in trades of that (Product, Type) and which must be computed sequentially in order, because the function applied row by row is neither associative nor commutative, and it cannot be parallelized.
I managed to handle the sequential function I need to apply to each subset by repartitioning to 1 single chunk each dataframe and sorting the rows, so to get exactly what I need.
The thing I am struggling with is how to do the map from trades to stats as a Spark job: in my scenario master is remote and can leverage multiple executors, while the deploy mode is local and local machine is poorly equipped.
So I don't want to do looping over the driver, but push it down to the cluster.
If this was not Spark, I'd have done something like:
val dimensions = trades.select("Product", "Type").distinct()
val stats = dimensions.map( row =>
val product = row.getAs[String]("Product")
val type = row.getAs[String]("Type")
val inScope = col("Product") === product and col("Type") === type
val tradesInScope = trades.filter(inScope)
Row(product, type, callSequentialFunction(tradesInScope))
)
This seemed fine to me, but it's absolutely not working: I am trying to do a nested call on trades, and it seem they are not supported. Indeed, when running this the spark job compile but when actually performing an action I get a NullPointerException because the dataframe trades is null within the map
I am new to Spark, and I don't know any other way of achieving the same intent in a valid way. Could you help me?
you get a NullpointerExecptionbecause you cannot use dataframes within executor-side code, they only live on the driver.Also, your code would not ensure thatcallSequentialFunction will be called sequentially, because map on a dataframe will run in parallel (if you have more than 1 partition). What you can do is something like this:
val dimensions = trades.select("Product", "Type").distinct().as[(String,String)].collect()
val stats = dimensions.map{case (product,type) =>
val inScope = col("Product") === product and col("Type") === type
val tradesInScope = trades.filter(inScope)
(product, type, callSequentialFunction(tradesInScope))
}
But note that the order in dimensionsis somewhat arbitrary, so you should sort dimensionsaccording to your needs
I have a Dataset/Dataframe with a mllib.linalg.Vector (of Doubles) as one of the columns. I would like to add another column to this dataset of type ml.linalg.Vector to this data set (so I will have both types of Vectors). The reason is I am evaluating few algorithms and some of those expect mllib vector and some expect ml vector. Also, I have to feed o/p of one algorithm to another and each use different types.
Can someone please help me convert mllib.linalg.Vector to ml.linalg.Vector and append a new column to the data set in hand. I tried using MLUtils.convertVectorColumnsToML() inside an UDF and regular functions but not able to get it to working. I am trying to avoid creating a new dataset and then doing inner join and dropping the columns as the data set will be huge eventually and joins are expensive.
You can use the method toML to convert from mllib to ml vector. An UDF and usage example can look like this:
val convertToML = udf((mllibVec: org.apache.spark.mllib.linalg.Vector) = > {
mllibVec.asML
})
val df2 = df.withColumn("mlVector", convertToML($"mllibVector"))
Assuming df to be the original dataframe and the column with the mllib vector to be named mllibVector.
I am reading a csv as a Data Frame by below:
val df = sqlContext.read.format("com.databricks.spark.csv").option("header", "true").load("D:/ModelData.csv")
Then I group by three columns as below which returns a RelationalGroupedDataset
df.groupBy("col1", "col2","col3")
And I want each grouped data frame to be send through the below function
def ModelFunction(daf: DataFrame) = {
//do some calculation
}
For example if I have col1 having 2 unique (0,1) values and col2 having 2 unique values(1,2) and col3 having 3 unique values(1,2,3) Then i would like to pass each combination grouping to the Model function Like for col1=0 ,col2=1,col3=1 I will havea dataframe and I want to pass that to the ModelFunction and so on for each combination of the three columns.
I tried
df.groupBy("col1", "col2","col3").ModelFunction();
But it throw an error.
.
Any help is appreciated.
The short answer is that you cannot do that. You can only do aggregate functions on RelationalGroupedDataset (either ones you write as UDAF or built in ones in org.apache.spark.sql.functions)
The way I see it you have several options:
Option 1: The amount of data for each unique combination is small enough and not skewed too much compared to other combinations.
In this case you can do:
val grouped = df.groupBy("col1", "col2","col3").agg(collect_list(struct(all other columns)))
grouped.as[some case class to represent the data including the combination].map[your own logistic regression function).
Option 2: If the total number of combinations is small enough you can do:
val values: df.select("col1", "col2", "col3").distinct().collect()
and then loop through them creating a new dataframe from each combination by doing a filter.
Option 3: Write your own UDAF
This would probably not be good enough as the data comes in a stream without the ability to do iteration, however, if you have an implemenation of logistic regression which matches you can try to write a UDAF to do this. See for example: How to define and use a User-Defined Aggregate Function in Spark SQL?
Coming from R, I am used to easily doing operations on columns. Is there any easy way to take this function that I've written in scala
def round_tenths_place( un_rounded:Double ) : Double = {
val rounded = BigDecimal(un_rounded).setScale(1, BigDecimal.RoundingMode.HALF_UP).toDouble
return rounded
}
And apply it to a one column of a dataframe - kind of what I hoped this would do:
bid_results.withColumn("bid_price_bucket", round_tenths_place(bid_results("bid_price")) )
I haven't found any easy way and am struggling to figure out how to do this. There's got to be an easier way than converting the dataframe to and RDD and then selecting from rdd of rows to get the right field and mapping the function across all of the values, yeah? And also something more succinct creating a SQL table and then doing this with a sparkSQL UDF?
You can define an UDF as follows:
val round_tenths_place_udf = udf(round_tenths_place _)
bid_results.withColumn(
"bid_price_bucket", round_tenths_place_udf($"bid_price"))
although built-in Round expression is using exactly the same logic as your function and should be more than enough, not to mention much more efficient:
import org.apache.spark.sql.functions.round
bid_results.withColumn("bid_price_bucket", round($"bid_price", 1))
See also following:
Updating a dataframe column in spark
How to apply a function to a column of a Spark DataFrame?