Apache Cassandra-Spark Bulk Reader [Beta]

This project provides a library for reading raw Cassandra SSTables into SparkSQL along the principles of ’streaming compaction’.

By reading the raw SSTables directly, the Cassandra-Spark Bulk Reader enables efficient and fast massive-scale analytics queries without impacting the performance of a production Cassandra cluster.

This is project is in Beta.

Requirements

1. Java >= 1.8 (OpenJDK or Oracle), or Java 11
2. Apache Cassandra 3.0+
3. Apache Spark 2.4

Overview

The primary interface for reading SSTables is through the DataLayer abstraction. A simple example 'LocalDataLayer' implementation is provided for reading SSTables from a local file system.

The role of the DataLayer is to:

• return a SchemaStruct, mapping the Cassandra CQL table schema to the SparkSQL schema.
• a list of sstables available for reading.
• a method to open an InputStream on any file component of an sstable (e.g. data, compression, summary etc).

The PartitionedDataLayer abstraction builds on the DataLayer interface for partitioning Spark workers across a Cassandra token ring - allowing the Spark job to scale linearly - and reading from sufficient Cassandra replicas to achieve a user-specified consistency level.

At the core, the Bulk Reader uses the Apache Cassandra CompactionIterator to perform the streaming compaction. The SparkRowIterator and SparkCellIterator iterate through the CompactionIterator, deserialize the ByteBuffers, convert into the appropriate SparkSQL data type, and finally pivot each cell into a SparkSQL row.

Features

The bulk reader supports all major Cassandra features:

• Cassandra 3.0 & 4.0.
• Murmur3 and Random Partitioner.
• Native CQL data types (ascii, bigint, blob, boolean, date, decimal, double, float, inet, int, smallint, text, time, timestamp, timeuuid, tinyint, uuid, varchar, varint).
• Tuples & collections (map, set, list).
• User-defined types.
• Frozen or nested data types (native types, collections, tuples or UDTs nested inside other data types).
• Any supported data-types for primary key fields.

Gotchas/unsupported features:

• Counters.
• Duration data type.
• The PartitionedDataLayer has been tested with Cassandra clusters that use 1 token per Cassandra instance. Any PartitionedDataLayer implementation that needs virtual nodes can extend the CassandraInstance class to allow multiple tokens per instance.
• Due to how Spark sets the precision and scale per Decimal data type, loss of precision can occur when using the BigDecimal data type.
• EACH_QUORUM consistency level has not been implemented yet.

Getting Started

For a basic local example see: SimpleExample.

By default the example expects the schema:

CREATE TABLE IF NOT EXISTS test.basic_test (a bigint PRIMARY KEY, b bigint, c bigint);

To run:

 ./gradlew example:build && ./gradlew example:run  --args="/path/to/cassandra/d1/data/dir,/path/to/cassandra/d2/data/dir"
   ....
   Row: [2,2,2]
   Row: [3,3,3]
   Row: [4,4,4]
   Row: [0,0,0]
   Row: [1,1,1]

Note, the core module pulls in Apache Spark as compileOnly, so you either need to depend on Spark as compile in your project or pull in the Spark jars at runtime.

To implement your own DataLayer, first take a look at the example local implementation: LocalDataSource, LocalDataLayer.

To implement a DataLayer that partitions the Spark workers and respects a given consistency level, extend the PartitionedDataLayer.

S3 Example

For an example implementation of the PartitionedDataLayer, see S3Example, S3DataLayer.

This example shows you how to read an entire C* cluster backed-up to cloud storage (e.g. for backups.), and how to build a PartitionedDataLayer that partitions the Spark workers and scales linearly across Spark workers.

Testing

The project is robustly tested using a bespoke property-based testing system that uses QuickTheories to enumerate many Cassandra CQL schemas, write random data using the Cassandra CQLSSTableWriter, read the SSTables into SparkSQL and verify the resulting SparkSQL rows match the expected.

For examples tests see org.apache.cassandra.spark.EndToEndTests.