The upgrade to Debian buster for the Hadoop cluster(s) might be a bit more complicated than what we thought, due to the fact that openjdk-8 is not available on Debian Buster. In T229347 Andrew was able to install it on stat1005 since the openjdk-8 was present in Buster before its final release, but not now (so if we reimage we'll not find it for example).
The above becomes problematic due to the following constraints:
- Spark 2.3 (our current version) doesn't support Java 11 (see also T229347#5394326). IIUC this is due to the Scala version used (2.11), that doesn't support Java 11 (https://docs.scala-lang.org/overviews/jdk-compatibility/overview.html)
- Support of scala 2.12+ for Java 11 is still incomplete - https://docs.scala-lang.org/overviews/jdk-compatibility/overview.html#jdk-11-compatibility-notes
- Spark 2.4 comes with scala 2.12 that offers experimental support for Java 11
Also, in stretch-backports we do have openjdk-11: https://packages.debian.org/stretch-backports/openjdk-11-jdk
Last but not the least, we'd also need to make sure that the HDFS/Yarn daemons work correctly on Buster and Java 11. CDH of course supports Java11 only from 6.3 onward: https://www.cloudera.com/documentation/enterprise/upgrade/topics/ug_jdk8.html
But it also true that CDH 6.3 ships with Spark 2.4, so either they support Java 11 as experimental feature or there is a way to make Spark 2.4 working: https://www.cloudera.com/documentation/enterprise/6/release-notes/topics/rg_cdh_63_packaging.html
- I am not a scala/spark expert so what I wrote above might not be true, please double check and in case correct me :)
- backporting openjdk-8 to buster is possible but it would require a big effort for the SRE team. The last backport of openjdk-8 for cassandra on Debian Jessie still needs to be maintained (application of patches for Debian Security Advisories, etc..), so it would be preferable not to go on that road again.