NoteDb is the next generation of Gerrit storage backend, which replaces the traditional SQL backend for change and account metadata with storing data in the same repository as code changes.

Advantages
  • Simplicity: All data is stored in one location in the site directory, rather than being split between the site directory and a possibly external database server.

  • Consistency: Replication and backups can use a snapshot of the Git repository refs, which will include both the branch and patch set refs, and the change metadata that points to them.

  • Auditability: Rather than storing mutable rows in a database, modifications to changes are stored as a sequence of Git commits, automatically preserving history of the metadata.
    There are no strict guarantees, and meta refs may be rewritten, but the default assumption is that all operations are logged.

  • Extensibility: Plugin developers can add new fields to metadata without the core database schema having to know about them.

  • New features: Enables simple federation between Gerrit servers, as well as offline code review and interoperation with other tools.

Current Status

  • Storing change metadata is fully implemented in master, and is live on the servers behind googlesource.com. In other words, if you use gerrit-review, you’re already using NoteDb.
    Specifically, gerrit-review is running with noteDb.changes.write=true, noteDb.changes.read=true, noteDb.changes.primaryStorage=NOTE_DB, and all old changes have been migrated to NoteDb primary.

  • Storing some account data, e.g. user preferences, is implemented in releases back to 2.13.

  • Storing the rest of account data is a work in progress.

  • Storing group data is a work in progress.

For an example NoteDb change, poke around at this one:

  git fetch https://gerrit.googlesource.com/gerrit refs/changes/70/98070/meta \
      && git log -p FETCH_HEAD

Configuration

Account and group data is migrated to NoteDb automatically using the normal schema upgrade process during updates. The remainder of this section details the configuration options that control migration of the change data, which is mostly but not fully implemented.

Change migration state is configured in gerrit.config with options like noteDb.changes.*. These options are undocumented outside of this file, and the general approach has been to add one new option for each phase of the migration. Assume that each config option in the following list requires all of the previous options, unless otherwise noted.

  • noteDb.changes.write=true: During a ReviewDb write, the state of the change in NoteDb is written to the note_db_state field in the Change entity. After the ReviewDb write, this state is written into NoteDb, resulting in effectively double the time for write operations. NoteDb write errors are dropped on the floor, and no attempt is made to read from ReviewDb or correct errors (without additional configuration, below).
    This state allows for a rolling update in a multi-master setting, where some servers can start reading from NoteDb, but older servers are still reading only from ReviewDb.

  • noteDb.changes.read=true: Change data is written to and read from NoteDb, but ReviewDb is still the source of truth. During reads, first read the change from ReviewDb, and compare its note_db_state with what is in NoteDb. If it doesn’t match, immediately "auto-rebuild" the change, copying data from ReviewDb to NoteDb and returning the result.

  • noteDb.changes.primaryStorage=NOTE_DB: New changes are written only to NoteDb, but changes whose primary storage is ReviewDb are still supported. Continues to read from ReviewDb first as in the previous stage, but if the change is not in ReviewDb, falls back to reading from NoteDb.
    Migration of existing changes is described in the Migration section below.
    Due to an implementation detail, writes to Changes or related tables still result in write calls to the database layer, but they are inside a transaction that is always rolled back.

  • noteDb.changes.disableReviewDb=true: All access to Changes or related tables is disabled; reads return no results, and writes are no-ops. Assumes the state of all changes in NoteDb is accurate, and so is only safe once all changes are NoteDb primary. Otherwise, reading changes only from NoteDb might result in inaccurate results, and writing to NoteDb would compound the problem.
    Thus it is up to an admin of a previously-ReviewDb site to ensure MigratePrimaryStorage has been run for all changes. Note that the current implementation of the rebuild-note-db program does not do this.
    In this phase, it would be possible to delete the Changes tables out from under a running server with no effect.

Migration

Once configuration options are set, migration to NoteDb is primarily accomplished by running the rebuild-note-db program. Currently, this program bulk copies ReviewDb data into NoteDb, but leaves primary storage of these changes in ReviewDb, so the site is runnable with noteDb.changes.{write,read}=true, but ReviewDb is still required.

Eventually, rebuild-note-db will set primary storage to NoteDb for all changes by default, so a site will be able to stop using ReviewDb for changes immediately after a successful run.

There is code in PrimaryStorageMigrator.java to migrate individual changes from NoteDb primary to ReviewDb primary. This code is not intended to be used except in the event of a critical bug in NoteDb primary changes in production. It will likely never be used by rebuild-note-db, and in fact it’s not recommended to run rebuild-note-db until the code is stable enough that the reverse migration won’t be necessary.

Zero-Downtime Multi-Master Migration

Single-master Gerrit sites can use rebuild-note-db on an offline site to rebuild NoteDb, but this doesn’t work in a zero-downtime environment like googlesource.com.

Here, the migration process looks like:

  • Turn on noteDb.changes.write=true to start writing to NoteDb.

  • Run a parallel FlumeJava pipeline to write NoteDb data for all changes, and update all note_db_state fields. (Sorry, this implementation is entirely closed-source.)

  • Turn on noteDb.changes.read=true to start reading from NoteDb.

  • Turn on noteDb.changes.primaryStorage=NOTE_DB to start writing new changes to NoteDb only.

  • Run a Flume to migrate all existing changes to NoteDb primary. (Also closed-source, but basically just a wrapper around PrimaryStorageMigrator.)

  • Turn off access to ReviewDb changes tables.