kfaraz opened a new pull request, #18391:
URL: https://github.com/apache/druid/pull/18391
### Description
Druid MSQ engine offers a very convenient method of performing SQL-based
batch ingestion, export, etc.
Given the importance and heavy adoption of MSQ, this patch converts MSQ into
a core capability of Druid
rather than an extension, as was always intended.
### Changes
- Move all MSQ classes to `services` module without renaming the package
- Move the module loader file `org.apache.druid.guice.DruidModule` to
`services/src/main/resources/META-INF/services`
- This effectively means that the modules listed in this file will always
be loaded on all Druid services as desired
- Update dependencies as needed
- Remove mentions of extension from properties
### Alternative
The alternative was to simply move the `multi-stage-query` module into a
top-level module but that would
lead to cyclic dependencies between `services` and `multi-stage-query` since:
a) multi-stage-query depends on services for various classes
b) services is the entry point and must depend on all the core modules that
need to be on the classpath
### Pending
- Once initial feedback is received, the docs will also be updated to remove
mentions of the multi-stage-query extension.
### Future
- Over time, various MSQ classes may be moved out of `services` module into
more appropriate modules such as
`processing`, `server`, `indexing-service` or `sql` as applicable.
### Release note
- Multi-stage-query (MSQ engine) is now a core capability of Druid rather
than an extension.
- Remove `druid-multi-stage-query` from `druid.extensions.loadList` in
`common.runtime.properties`.
### Upgrade notes
While upgrading, the extension `druid-multi-stage-query` must be removed
from `druid.extensions.loadList`.
Otherwise, Druid services would fail to start as they would not be able to
locate this extension.
<hr>
This PR has:
- [ ] been self-reviewed.
- [ ] using the [concurrency
checklist](https://github.com/apache/druid/blob/master/dev/code-review/concurrency.md)
(Remove this item if the PR doesn't have any relation to concurrency.)
- [ ] added documentation for new or modified features or behaviors.
- [ ] a release note entry in the PR description.
- [ ] added Javadocs for most classes and all non-trivial methods. Linked
related entities via Javadoc links.
- [ ] added or updated version, license, or notice information in
[licenses.yaml](https://github.com/apache/druid/blob/master/dev/license.md)
- [ ] added comments explaining the "why" and the intent of the code
wherever would not be obvious for an unfamiliar reader.
- [ ] added unit tests or modified existing tests to cover new code paths,
ensuring the threshold for [code
coverage](https://github.com/apache/druid/blob/master/dev/code-review/code-coverage.md)
is met.
- [ ] added integration tests.
- [ ] been tested in a test Druid cluster.
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