Mahout
In HDP-2.3.x and 2.4.x, instead of shipping a specific Apache release of Mahout, we synchronized to a particular revision point on Apache Mahout trunk. This revision point is after the 0.9.0 release, but before the 0.10.0 release. This provides a large number of bug fixes and functional enhancements over the 0.9.0 release, but provides a stable release of the Mahout functionality before the complete conversion to new Spark-based Mahout in 0.10.0.
The revision point chosen for Mahout in HDP 2.3.x and 2.4.x is from the "mahout-0.10.x" branch of Apache Mahout, as of 19 December 2014, revision 0f037cb03e77c096 in GitHub.
In HDP-2.5.x and 2.6.x, we removed the "commons-httpclient" library from Mahout because we view it as an obsolete library with possible security issues., and upgraded the Hadoop-Client in Mahout to version 2.7.3, the same version used in HDP-2.5. As a result:
Previously compiled Mahout jobs will need to be recompiled in the HDP-2.5 or 2.6 environment.
There is a small possibility that some Mahout jobs may encounter "ClassNotFoundException" or "could not load class" errors related to "org.apache.commons.httpclient", "net.java.dev.jets3t", or related class name prefixes. If these errors happen, you may consider whether to manually install the needed jars in your classpath for the job, if the risk of security issues in the obsolete library is acceptable in your environment.
There is an even smaller possibility that some Mahout jobs may encounter crashes in Mahout's hbase-client code calls to the hadoop-common libraries, due to binary compatibility problems. Regrettably, there is no way to resolve this issue except revert to the HDP-2.4.2 version of Mahout, which may have security issues. Again, this should be very unusual, and is unlikely to occur in any given Mahout job suite.