The community has put together a comprehensive set of coding guidelines here: https://cwiki.apache.org/confluence/display/IGNITE/Coding+Guidelines.
We need a checkstyle definition that we can enforce automatically via the Maven Checkstyle plugin.
We can choose to output violations as warnings, or to fail the build if violations exist. At this point I would prefer the former to avoid "stopping the world". With such a large codebase it is likely that checkstyle violations are already present.
This will help contributors adequate their style to the community's standard while they are coding, and will reduce frustration otherwise caused by submitting pull requests that do not satisfy guidelines, and having to rectify them at a later stage.