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Documentation

Server-side data selection, filtering and grouping

In this section, you'll learn how to reduce the amount of data transferred from Cassandra to Spark to speed up processing.

Selecting columns

For performance reasons, you should not fetch columns you don't need. You can achieve this with the select method.

sc.cassandraTable("test", "users").select("username").toArray.foreach(println)
// CassandraRow{username: noemail} 
// CassandraRow{username: someone}

The select method can be chained. Every next call can be used to select a subset of columns already selected. Selecting a non-existing column would result in throwing an exception.

The select method allows querying for TTL and timestamp of the table cell.

val row = rdd.select("column", "column".ttl, "column".writeTime).first
val ttl = row.getLong("ttl(column)")
val timestamp = row.getLong("writetime(column)")

The selected columns can be given aliases by calling as on the column selector, which is particularly handy when fetching TTLs and timestamps.

rdd.select("column".ttl as "column_ttl").first
val ttl = row.getLong("column_ttl")

Filtering rows

To filter rows, you can use the filter transformation provided by Spark. However, this approach causes all rows to be fetched from Cassandra and then filtered by Spark. Also, some CPU cycles are wasted serializing and deserializing objects that wouldn't be included in the result. To avoid this overhead, CassandraRDD offers the where method, which lets you pass arbitrary CQL condition(s) to filter the row set on the server.

sc.cassandraTable("test", "cars").select("id", "model").where("color = ?", "black").toArray.foreach(println)
// CassandraRow[id: KF-334L, model: Ford Mondeo]
// CassandraRow[id: MT-8787, model: Hyundai x35]

sc.cassandraTable("test", "cars").select("id", "model").where("color = ?", "silver").toArray.foreach(println)
// CassandraRow[id: WX-2234, model: Toyota Yaris]

Note: Although the ALLOW FILTERING clause is implicitly added to the generated CQL query, not all predicates are currently allowed by the Cassandra engine. This limitation is going to be addressed in the future Cassandra releases. Currently, ALLOW FILTERING works well with columns indexed by secondary indexes or clustering columns.

Ordering rows

CQL allows for requesting ascending or descending order of rows within a single Cassandra partition. It is allowed to pass the ordering direction by withAscOrder or withDescOrder methods of CassandraRDD. Note that it will work only if there is at least one clustering column in the table and a partition key predicate is specified by where clause.

Limiting rows

When a table is designed so that it include clustering keys and the use case is that only the first n rows from an explicitly specified Cassandra partition are supposed to be fetched, one can find useful the limit method. It allows to add the LIMIT clause to each CQL executed for a particular RDD.

Note that, when you specify the limit without specifying a partition key predicate for the where clause, you will get unpredictable amount of rows because the limit will be applied on each Spark partition which is created for the RDD.

Grouping rows by partition key

Physically, Cassandra stores data already grouped by partition key and ordered by clustering column(s) within each partition. As a single Cassandra partition never spans multiple Spark partitions, it is possible to very efficiently group data by partition key without shuffling data around. Call spanBy or spanByKey methods instead of groupBy or groupByKey:

CREATE TABLE events (year int, month int, ts timestamp, data varchar, PRIMARY KEY (year,month,ts));
sc.cassandraTable("test", "events")
  .spanBy(row => (row.getInt("year"), row.getInt("month")))

sc.cassandraTable("test", "events")
  .keyBy(row => (row.getInt("year"), row.getInt("month")))
  .spanByKey

Note: This only works for sequentially ordered data. Because data is ordered in Cassandra by the clustering keys, all viable spans must follow the natural clustering key order.

This means in the above example that spanBy will be possible on (year), (year,month), (year,month,ts) but not (month), (ts), or (month,ts).

The methods spanBy and spanByKey iterate every Spark partition locally and put every RDD item into the same group as long as the key doesn't change. Whenever the key changes, a new group is started. You need enough memory to store the biggest group.

Counting rows

Although Spark provides count() method, it requires all the rows to be fetched from Cassandra, which adds significant memory and network overhead. Instead, cassandraCount() method can be used on any Cassandra based RDD to push down selection of count(*) and fetching the number of rows directly.

Note: Until release 1.2.4, Spark count was overridden by native Cassandra count in all Cassandra based RDDs. Since 1.2.4, count() is for Spark count and cassandraCount() is for Cassandra native count.

Next - Working with user-defined case classes and tuples