Versions and Features
Hadoop has seen significant interest over the past few years. This has led to a proportional uptick in features and bug fixes. Some of these features were so significant or had such a sweeping impact that they were developed on branches. As you might expect, this in turn led to a somewhat dizzying array of releases and parallel lines of development.
Here is a whirlwind tour of the various lines of development and their status. This information is also depicted visually in Figure 4-1.
- 0.20.0–0.20.2
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The 0.20 branch of Hadoop is extremely stable and has seen quite a bit of production burn-in. This branch has been one of the longest-lived branches in Hadoop’s history since being at Apache, with the first release appearing in April 2009. CDH2 and CDH3 are both based off of this branch, albeit with many features and bug fixes from 0.21, 0.22, and 1.0 back-ported.
- 0.20-append
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One of the features missing from 0.20 was support for file appends in HDFS. Apache HBase relies on the ability to sync its write ahead log, (such as force file contents to disk) which under the hood, uses the same basic functionality as file append. Append was considered a potentially destabilizing feature and many disagreed on the implementation, so it was relegated to a branch. This branch was called 0.20-append. No official release was ever made from the 0.20-append branch.
- 0.20-security
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Yahoo!, one of the major contributors to Apache Hadoop, invested in adding full Kerberos support to core Hadoop. It later contributed this work back to Hadoop in the form of the 0.20-security branch, a version of Hadoop 0.20 with Kerberos authentication support. This branch would later be released as the 0.20.20X releases.
- 0.20.203–0.20.205
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There was a strong desire within the community to produce an official release of Hadoop that included the 0.20-security work. The 0.20.20X releases contained not only security features from 0.20-security, but also bug fixes and improvements on the 0.20 line of development. Generally, it no longer makes sense to deploy these releases as they’re superseded by 1.0.0.
- 0.21.0
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The 0.21 branch was cut from Hadoop trunk and released in August 2010. This was considered a developer preview or alpha quality release to highlight some of the features that were currently in development at the time. Despite the warning from the Hadoop developers, a small number of users deployed the 0.21 release anyway. This release does not include security, but does have append.
- 0.22.0
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Hold on, because this is where the story gets weird. In December 2011, the Hadoop community released version 0.22, which was based on trunk, like 0.21 was. This release includes security, but only for HDFS. Also a bit strange, 0.22 was released after 0.23 with less functionality. This was due to when the 0.22 branch was cut from trunk.
- 0.23.0
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In November 2011, version 0.23 of Hadoop was released. Also cut from trunk, 0.23 includes security, append, YARN, and HDFS federation. This release has been dubbed a developer preview or alpha-quality release. This line of development is superseded by 2.0.0.
- 1.0.0
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In a continuing theme of confusion, version 1.0.0 of Hadoop was released from the 0.20.205 line of development. This means that 1.0.0 does not contain all of the features and fixes found in the 0.21, 0.22, and 0.23 releases. That said, it does include security.
- 2.0.0
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In May 2012, version 2.0.0 was released from the 0.23.0 branch and like 0.23.0, is considered alpha-quality. Mainly, this is because it includes YARN and removes the traditional MRv1 jobtracker and tasktracker daemons. While YARN is API-compatible with MRv1, the underlying implementation is different enough for it to require more significant testing before being considered production-ready.