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Coding is not the only path by which a person can become a committer - contributions in bug finding and triage, testing, and documentation are all welcome and are paths by which a contributor can gain committership.
A note about technical feedback: Impala depends on committers to act as a gate ensuring that docs, bug reports, code, and releases are high quality. To become a committer, a contributor should demonstrate that they are willing and able to review these artifacts and make sure they meet Impala's quality standards. For instance, a contributor elected to committership by way of code contributions will also have provided numerous careful code reviews of patches from other contributors.