Integration for Cascading and Apache Hive
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README.md

cascading-hive

Cascading-Hive is an integration between Apache Hive and Cascading. It currently has the following major features:

  • running Hive queries within a Cascade
  • reading from Hive tables within a Cascading Flow (including transactional tables)
  • writing/creating Hive tables from a Cascading Flow
  • writing/creating partitioned Hive tables from a Cascading Flow
  • deconstructing a Hive view into Taps
  • reading/writing taps from HCatalog

The demo sub-directory contains applications that demonstrate those features.

The code will pick up the configuration for a remote Hive MetaStore automatically, as long as it is present in your hive or hadoop configuration files.

Hive dependency

Cascading-Hive works Hive 1.x, 2.x is not yet supported. When using the cascading-hive in your project you have to specify the version of Hive you are using as a runtime dependency yourself. This is done to avoid classpath issues with the various Hadoop and Hive distributions in existence. See the demo project for an example.

Installing

To install cascading-hive into your local maven repository do this:

> gradle install

Maven, Ivy, Gradle

Cascading-Hive is also available on http://conjars.org.

Limitations

Views

Please note that the support for Hive views is currently limited since views pretend to be a resource or a Tap in Cascading terms, but are actually computation. It is currently not possible to read from a Hive View within a Cascading Flow. To work around this, you can create a table in Hive instead, and read from that within your Cascading Flow. This restriction might change in the future.

Transactional Tables

Also note that it is not yet possible to write to transactional tables and that the HiveTap will prevent any attempt to do so.

HCatTap

Using HCatTap to sink data to a Blob Storage service may lead to issues that can be hard to deal with due to Blob Storage not being a real file system. If you are planning to do so, use it at your own risk.

Users encountered various issues when using HCatTap to sync tables in S3 but there is a known workaround (tested on EMR 4.7.0):

	mapred.output.direct.EmrFileSystem = false
	mapred.output.direct.NativeS3FileSystem = false

These settings are required in order be able to commit dynamic partitions. This also implies that direct commits in EMR will be disabled and the job may take longer during the commit phase of tasks and jobs since the underlying FileSystem will have to copy the files to their final locations and delete the temporary copies. Depending on your use case this waiting time and relying on eventually consistent data may or may not be an issue.

Note that even though direct commits won't be available, EMR consistent views can still be used.

Environment

Finally note that Hive relies on the hadoop command being present in your PATH when it executes the queries on the Hadoop cluster.