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Accessing Cassandra data with CassandraRDD

This section describes how to access data from Cassandra table with Spark.

Obtaining a Cassandra table as an RDD

To get a Spark RDD that represents a Cassandra table, call the cassandraTable method on the SparkContext object.

import com.datastax.spark.connector._ //Loads implicit functions
sc.cassandraTable("keyspace name", "table name")

If no explicit type is given to cassandraTable, the result of this expression is CassandraRDD[CassandraRow]. This can be thought of as a container of CassandraRow objects. For information on mapping to other objects please see the documentation's Mapping section

Example of loading an RDD from Cassandra

Create this keyspace and table in Cassandra using cqlsh:

CREATE KEYSPACE test WITH REPLICATION = {'class': 'SimpleStrategy', 'replication_factor': 1 };
CREATE TABLE test.words (word text PRIMARY KEY, count int);

Load data into the table:

INSERT INTO test.words (word, count) VALUES ('foo', 20);
INSERT INTO test.words (word, count) VALUES ('bar', 20);

Now you can read that table as RDD:

val rdd = sc.cassandraTable("test", "words")
// rdd: com.datastax.spark.connector.rdd.CassandraRDD[com.datastax.spark.connector.rdd.reader.CassandraRow] = CassandraRDD[0] at RDD at CassandraRDD.scala:41

// CassandraRow{word: bar, count: 20}
// CassandraRow{word: foo, count: 20}   

Using emptyCassandraRDD implementation

To create an instance of CassandraRDD for a table which does not exist use the emptyCassandraRDD method. emptyCassandraRDDs do not perform validation or create partitions so they can be used to represent absent tables. To create one, either initialize a CassandraRDD as usual and then call toEmptyCassandraRDD method on it or call emptyCassandraTable method on Spark context.

Example of using an EmptyCassandraRDD

// validation is deferred, so it is not triggered during rdd creation
val rdd = sc.cassandraTable[SomeType]("ks", "not_existing_table")
val emptyRDD = rdd.toEmptyCassandraRDD

val emptyRDD2 = sc.emptyCassandraTable[SomeType]("ks", "not_existing_table"))

Reading primitive column values

You can read columns in a Cassandra table using the get methods of the CassandraRow object. The get methods access individual column values by column name or column index. Type conversions are applied on the fly. Use getOption variants when you expect to receive Cassandra null values.

Continuing with the previous example, follow these steps to access individual column values.

Example Accessing the values within a CassandraRow

Store the first item of the rdd in the firstRow value.

val firstRow = rdd.first
// firstRow: com.datastax.spark.connector.rdd.reader.CassandraRow = CassandraRow{word: bar, count: 20}

Get the number of columns and column names:

firstRow.columnNames    // Stream(word, count) 
firstRow.size           // 2 

Use one of getXXX getters to obtain a column value converted to desired type:

firstRow.getInt("count")       // 20       
firstRow.getLong("count")      // 20L  

Or use a generic get to query the table by passing the return type directly:

firstRow.get[Int]("count")                   // 20       
firstRow.get[Long]("count")                  // 20L
firstRow.get[BigInt]("count")                // BigInt(20)
firstRow.get[java.math.BigInteger]("count")  // BigInteger(20)

Working with nullable data

When reading potentially null data, use the Option type on the Scala side to prevent getting a NullPointerException.

Example accessing values in a CassandraRow that may be null

firstRow.getIntOption("count")        // Some(20)
firstRow.get[Option[Int]]("count")    // Some(20)    

Reading collections

You can read collection columns in a Cassandra table using the getList, getSet, getMap or generic get methods of the CassandraRow object. The get methods access the collection column and return a corresponding Scala collection. The generic get method lets you specify the precise type of the returned collection.

Assuming you set up the test keyspace earlier, follow these steps to access a Cassandra collection.

In the test keyspace, set up a collection set using cqlsh:

CREATE TABLE test.users (username text PRIMARY KEY, emails SET<text>);
INSERT INTO test.users (username, emails) 
     VALUES ('someone', {'', ''});

Then in your application, retrieve the first row:

val row = sc.cassandraTable("test", "users").first
// row: com.datastax.spark.connector.rdd.reader.CassandraRow = CassandraRow{username: someone, emails: [,]}

Query the collection set in Cassandra from Spark:

row.getList[String]("emails")            // Vector(,
row.get[List[String]]("emails")          // List(,    
row.get[Seq[String]]("emails")           // List(,   :Seq[String]
row.get[IndexedSeq[String]]("emails")    // Vector(, :IndexedSeq[String]
row.get[Set[String]]("emails")           // Set(,

It is also possible to convert a collection to CQL String representation:

row.get[String]("emails")               // "[,]"

A null collection is equivalent to an empty collection, therefore you don't need to use get[Option[...]] with collections.

Reading columns of Cassandra User Defined Types from a CassandraRow

UDT column values are represented by com.datastax.spark.connector.UDTValue type. The same set of getters is available on UDTValue as on CassandraRow. UDT's can also be mapped to Scala classes see Mapping section

Assume the following table definition:

CREATE TYPE test.address (city text, street text, number int);
CREATE TABLE test.companies (name text PRIMARY KEY, address FROZEN<address>);

You can read the address field of the company in the following way:

val address: UDTValue = row.getUDTValue("address")
val city = address.getString("city")
val street = address.getString("street")
val number = address.getInt("number")

Data type conversions

The following table shows recommended Scala types corresponding to Cassandra column types.

Cassandra type Scala types
ascii, text String
bigint Long
blob ByteBuffer, Array[Byte]
boolean Boolean, Int
counter Long
date Int, String (YYYY-MM-DD), java.util.Date, java.sql.Date, org.joda.time.DateTime
decimal BigDecimal, java.math.BigDecimal
double Double
float Float
int Int
list Vector, List, Iterable, Seq, IndexedSeq, java.util.List
map Map, TreeMap, java.util.HashMap
set Set, TreeSet, java.util.HashSet
smallint Short
text String
time Long, Do Not Read this Column as a Date*
timestamp Long, java.util.Date, java.sql.Date, org.joda.time.DateTime
timeuuid java.util.UUID
tinyint Byte
uuid java.util.UUID
varchar String
varint BigInt, java.math.BigInteger
frozen<tuple<>> TupleValue, scala.Product, org.apache.commons.lang3.tuple.Pair, org.apache.commons.lang3.tuple.Triple
user defined UDTValue

Since time is encoded in nanoseconds from epoch rather than milliseconds there will be Scale error with an automatic conversion to java.util.Date

Other conversions might work, but may cause loss of precision or may not work for all values. All types are convertible to strings. Converting strings to numbers, dates, addresses or UUIDs is possible as long as the string has proper contents, defined by the CQL3 standard. Maps can be implicitly converted to/from sequences of key-value tuples.

Performing Efficient Joins With Cassandra Tables (since 1.2)

Repartitioning RDDs based on a Cassandra Table's Replication

The method repartitionByCassandraReplica can be used to relocate data in an RDD to match the replication strategy of a given table and keyspace. The method will look for partition key information in the given RDD and then use those values to determine which nodes in the Cluster would be responsible for that data. You can control the resultant number of partitions with the parameter partitionsPerHost.

//CREATE TABLE test.shopping_history ( cust_id INT, date TIMESTAMP,  product TEXT, quantity INT, PRIMARY KEY (cust_id, date, product));
case class CustomerID(cust_id: Int) // Defines partition key
val idsOfInterest = sc.parallelize(1 to 1000).map(CustomerID(_))
val repartitioned =  idsOfInterest.repartitionByCassandraReplica("test", "shopping_history", 10)
//res0: Array[org.apache.spark.Partition] = Array(ReplicaPartition(0,Set(/, ...)
//res1: Option[org.apache.spark.Partitioner] = Some(com.datastax.spark.connector.rdd.partitioner.ReplicaPartitioner@4484d6c2)
scala> repartitioned
//res2: com.datastax.spark.connector.rdd.partitioner.CassandraPartitionedRDD[CustomerID] = CassandraPartitionedRDD[5] at RDD at CassandraPartitionedRDD.scala:12

Using joinWithCassandraTable

The connector supports using any RDD as a source of a direct join with a Cassandra Table through joinWithCassandraTable. Any RDD which is writable to a Cassandra table via the saveToCassandra method can be used with this procedure as long as the full partition key is specified.

joinWithCassandraTable utilizes the java drive to execute a single query for every partition required by the source RDD so no un-needed data will be requested or serialized. This means a join between any RDD and a Cassandra Table can be performed without doing a full table scan. When performed between two Cassandra Tables which share the same partition key this will not require movement of data between machines. In all cases this method will use the source RDD's partitioning and placement for data locality.

joinWithCassandraTable is not affected by cassandra.input.split.size_in_mb since partitions are automatically inherited from the source RDD. The other input properties have their normal effects.

Example Join between two Cassandra Tables Sharing a Partition Key

//CREATE TABLE test.customer_info ( cust_id INT, name TEXT, address TEXT, PRIMARY KEY (cust_id));
val internalJoin = sc.cassandraTable("test","customer_info").joinWithCassandraTable("test","shopping_history")
//res4: String = (1) CassandraJoinRDD[9] at RDD at CassandraRDD.scala:14 []
//(CassandraRow{cust_id: 3, address: Poland, name: Jacek},CassandraRow{cust_id: 3, date: 2015-03-09 13:59:25-0700, product: Guacamole, quantity: 2})
//(CassandraRow{cust_id: 0, address: West Coast, name: Russ},CassandraRow{cust_id: 0, date: 2015-03-09 13:58:14-0700, product: Scala is Fun, quantity: 1})
//(CassandraRow{cust_id: 0, address: West Coast, name: Russ},CassandraRow{cust_id: 0, date: 2015-03-09 13:59:04-0700, product: Candy, quantity: 3})

Example Join with Generic RDD

val joinWithRDD = sc.parallelize(0 to 5).filter(_%2==0).map(CustomerID(_)).joinWithCassandraTable("test","customer_info")
//(CustomerID(0),CassandraRow{cust_id: 0, address: West Coast, name: Russ})
//(CustomerID(2),CassandraRow{cust_id: 2, address: Poland, name: Piotr})

The repartitionByCassandraReplica method can be used prior to calling joinWithCassandraTable to obtain data locality, such that each spark partition will only require queries to their local node. This method can also be used with two Cassandra Tables which have partitioned with different partition keys.

Example Join with a generic RDD after repartitioning

val oddIds = sc.parallelize(0 to 5).filter(_%2==1).map(CustomerID(_))
val localQueryRDD = oddIds.repartitionByCassandraReplica("test","customer_info").joinWithCassandraTable("test","customer_info")
//(CustomerID(1),CassandraRow{cust_id: 1, address: East Coast, name: Helena})
//(CustomerID(3),CassandraRow{cust_id: 3, address: Poland, name: Jacek})

###Compatibility of joinWithCassandraTable and other CassandraRDD APIs The result of a joinWithCassandraRDD is compatible with all of the standard CassandraRDD api options with one additional function, .on. Use .on(ColumnSelector) for specifying which columns to join on. Since .on only applies to CassandraJoinRDDs it must immediately follow the joinWithCassandraTable call.

Joining on any column or columns in the primary key is supported as long as it can be made into a valid CQL query. This means the entire partition key must be specified and if any clustering key is specified all previous clustering keys must be supplied as well.

Cassandra Operations on a CassandraJoinRDD

val recentOrders = internalJoin.where("date > '2015-03-09'") // Where applied to every partition
val someOrders = internalJoin.limit(1) // Returns at most 1 CQL Row per Spark Partition
val numOrders = internalJoin.count() // Sums the total number of cql Rows
val orderQuantities ="quantity") // Returns only the amount column as the right side of the join
val specifiedJoin = internalJoin.on(SomeColumns("cust_id")) // Joins on the cust_id column
val emptyJoin = internalJoin.toEmptyCassandraRDD // Makes an EmptyRDD

Configuration Options for Adjusting Reads

The following options can be specified in the SparkConf object or as --conf flag to spark-submit to adjust the read parameters of a Cassandra table.

See Reference Section

Using Implicits for Configuration

In addition you are able to set these parameters on a per table basis by using implicit vals. This allows a user to define a set of parameters in a separate object and import them into a block of code rather than repeatedly passing the same ReadConf object.

Example Using Implicits for Read Configuration

object ReadConfigurationOne {
  implicit val readConf = ReadConf(100,100)
import ReadConfigurationOne._
val rdd = sc.cassandraTable("write_test","collections")
//com.datastax.spark.connector.rdd.ReadConf = ReadConf(100,100,LOCAL_ONE,true)

Or you can define them implicitly in the same block as the cassandraTable call

implicit val anotherConf = ReadConf(200,200)
val rddWithADifferentConf = sc.cassandraTable("write_test","collections")
//com.datastax.spark.connector.rdd.ReadConf = ReadConf(200,200,LOCAL_ONE,true)

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