* The ELK stack is upgraded to 6.3.2.
* `elasticsearch6`, `logstash6` and `kibana6` now come with X-Pack which is
a suite of additional features. These are however licensed under the unfree
"Elastic License".
* Fortunately they also provide OSS versions which are now packaged
under: `elasticsearch6-oss`, `logstash6-oss` and `kibana6-oss`.
Note that the naming of the attributes is consistent with upstream.
* The test `nix-build nixos/tests/elk.nix -A ELK-6` will test the OSS
version by default. You can also run the test on the unfree ELK using:
`NIXPKGS_ALLOW_UNFREE=1 nix-build nixos/tests/elk.nix -A ELK-6 --arg enableUnfree true`
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