NoteDb was introduced 5 years ago with the promise of simpifying the portability of reviews and repositories across Gerrit servers and, in the future, different code-review systems. The promise was that a JSON-based representation of the review meta-data would have allowed to rebuild a full state and history anywhere, without having to rely on a centralised DBMS.
With that promise in mind, the development of the Gerrit importer plugin developed by SAP stopped at version v2.15 and no more upgrades where planned.
From Gerrit v2.16 up to now, the Gerrit admins were unable to follow any phased-migration as they could have done with the use of the importer plugin, forcing companies to go through a painful release-by-release upgrade process or multi-year bing-bang migration projects.
Luca gives an overview of how the migration and consolidation is now possible with Gerrit v3.7, describing the challenges and limitations of what can be done.
