SparkR (R on Spark)
- SparkR DataFrames
SparkR is an R package that provides a light-weight frontend to use Apache Spark from R. In Spark 1.4.0, SparkR provides a distributed data frame implementation that supports operations like selection, filtering, aggregation etc. (similar to R data frames, dplyr) but on large datasets.
A DataFrame is a distributed collection of data organized into named columns. It is conceptually equivalent to a table in a relational database or a data frame in R, but with richer optimizations under the hood. DataFrames can be constructed from a wide array of sources such as: structured data files, tables in Hive, external databases, or existing local R data frames.
All of the examples on this page use sample data included in R or the Spark distribution and can be run using the
Starting Up: SparkContext, SQLContext
The entry point into SparkR is the
SparkContext which connects your R program to a Spark cluster.
You can create a
sparkR.init and pass in options such as the application name
etc. Further, to work with DataFrames we will need a
SQLContext, which can be created from the
SparkContext. If you are working from the SparkR shell, the
already be created for you.
sc <- sparkR.init() sqlContext <- sparkRSQL.init(sc)
From local data frames
The simplest way to create a data frame is to convert a local R data frame into a SparkR DataFrame. Specifically we can use
createDataFrame and pass in the local R data frame to create a SparkR DataFrame. As an example, the following creates a
DataFrame based using the
faithful dataset from R.
df <- createDataFrame(sqlContext, faithful) # Displays the content of the DataFrame to stdout head(df) ## eruptions waiting ##1 3.600 79 ##2 1.800 54 ##3 3.333 74
From Data Sources
SparkR supports operating on a variety of data sources through the
DataFrame interface. This section describes the general methods for loading and saving data using Data Sources. You can check the Spark SQL programming guide for more specific options that are available for the built-in data sources.
The general method for creating DataFrames from data sources is
read.df. This method takes in the
SQLContext, the path for the file to load and the type of data source. SparkR supports reading JSON and Parquet files natively and through Spark Packages you can find data source connectors for popular file formats like CSV and Avro.
We can see how to use data sources using an example JSON input file. Note that the file that is used here is not a typical JSON file. Each line in the file must contain a separate, self-contained valid JSON object. As a consequence, a regular multi-line JSON file will most often fail.
people <- read.df(sqlContext, "./examples/src/main/resources/people.json", "json") head(people) ## age name ##1 NA Michael ##2 30 Andy ##3 19 Justin # SparkR automatically infers the schema from the JSON file printSchema(people) # root # |-- age: integer (nullable = true) # |-- name: string (nullable = true)
The data sources API can also be used to save out DataFrames into multiple file formats. For example we can save the DataFrame from the previous example
to a Parquet file using
write.df(people, path="people.parquet", source="parquet", mode="overwrite")
From Hive tables
You can also create SparkR DataFrames from Hive tables. To do this we will need to create a HiveContext which can access tables in the Hive MetaStore. Note that Spark should have been built with Hive support and more details on the difference between SQLContext and HiveContext can be found in the SQL programming guide.
# sc is an existing SparkContext. hiveContext <- sparkRHive.init(sc) sql(hiveContext, "CREATE TABLE IF NOT EXISTS src (key INT, value STRING)") sql(hiveContext, "LOAD DATA LOCAL INPATH 'examples/src/main/resources/kv1.txt' INTO TABLE src") # Queries can be expressed in HiveQL. results <- hiveContext.sql("FROM src SELECT key, value") # results is now a DataFrame head(results) ## key value ## 1 238 val_238 ## 2 86 val_86 ## 3 311 val_311
SparkR DataFrames support a number of functions to do structured data processing. Here we include some basic examples and a complete list can be found in the API docs:
Selecting rows, columns
# Create the DataFrame df <- createDataFrame(sqlContext, faithful) # Get basic information about the DataFrame df ## DataFrame[eruptions:double, waiting:double] # Select only the "eruptions" column head(select(df, df$eruptions)) ## eruptions ##1 3.600 ##2 1.800 ##3 3.333 # You can also pass in column name as strings head(select(df, "eruptions")) # Filter the DataFrame to only retain rows with wait times shorter than 50 mins head(filter(df, df$waiting < 50)) ## eruptions waiting ##1 1.750 47 ##2 1.750 47 ##3 1.867 48
SparkR data frames support a number of commonly used functions to aggregate data after grouping. For example we can compute a histogram of the
waiting time in the
faithful dataset as shown below
# We use the `n` operator to count the number of times each waiting time appears head(summarize(groupBy(df, df$waiting), count = n(df$waiting))) ## waiting count ##1 81 13 ##2 60 6 ##3 68 1 # We can also sort the output from the aggregation to get the most common waiting times waiting_counts <- summarize(groupBy(df, df$waiting), count = n(df$waiting)) head(arrange(waiting_counts, desc(waiting_counts$count))) ## waiting count ##1 78 15 ##2 83 14 ##3 81 13
Operating on Columns
SparkR also provides a number of functions that can directly applied to columns for data processing and during aggregation. The example below shows the use of basic arithmetic functions.
# Convert waiting time from hours to seconds. # Note that we can assign this to a new column in the same DataFrame df$waiting_secs <- df$waiting * 60 head(df) ## eruptions waiting waiting_secs ##1 3.600 79 4740 ##2 1.800 54 3240 ##3 3.333 74 4440
Running SQL Queries from SparkR
A SparkR DataFrame can also be registered as a temporary table in Spark SQL and registering a DataFrame as a table allows you to run SQL queries over its data.
sql function enables applications to run SQL queries programmatically and returns the result as a
# Load a JSON file people <- read.df(sqlContext, "./examples/src/main/resources/people.json", "json") # Register this DataFrame as a table. registerTempTable(people, "people") # SQL statements can be run by using the sql method teenagers <- sql(sqlContext, "SELECT name FROM people WHERE age >= 13 AND age <= 19") head(teenagers) ## name ##1 Justin