# Gerrit Code Review - NoteDb Backend

version v3.4.4

NoteDb is the storage backend for code review metadata. It is based on Git, so code reviews are stored together with the code under review. NoteDb replaced the traditional SQL backend for change, account and group metadata that was used in the 2.x series.

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.

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

## Current Status

NoteDb is the only database format supported by Gerrit 3.0+. The change data migration tools are only included in Gerrit 2.16; they are not available in 3.0, so any upgrade from Gerrit 2.x to 3.x must go through 2.16 to effect the NoteDb upgrade.

## Migration

Migrating change metadata can take a long time for large sites, so administrators choose whether to do the migration offline or online, depending on their available resources and tolerance for downtime.

Only change metadata requires manual steps to migrate it from ReviewDb; account and group data is migrated automatically by `gerrit.war init`.

### Online

Note that online migration is only available in 2.x. To do the online migration from 2.14.x or 2.15.x to 3.0, it is necessary to first upgrade to 2.16.x.

To start the online migration, set the `noteDb.changes.autoMigrate` option in `gerrit.config` and restart Gerrit:

    [noteDb "changes"]
      autoMigrate = true

Alternatively, pass the `--migrate-to-note-db` flag to `gerrit.war daemon`:

      java -jar gerrit.war daemon -d /path/to/site --migrate-to-note-db

Both ways of starting the online migration are equivalent. Once started, it is safe to restart the server at any time; the migration will pick up where it left off. Migration progress will be reported to the Gerrit logs.

Advantages

- No downtime required.

Disadvantages

- Only available in 2.x; not available in Gerrit 3.0.

- Much slower than offline; uses only a single thread, to leave resources available for serving traffic.

- Performance may be degraded, particularly of updates; data needs to be written to both ReviewDb and NoteDb while the migration is in progress.

### Offline

To run the offline migration, run the `migrate-to-note-db` program:

      java -jar gerrit.war migrate-to-note-db -d /path/to/site

Once started, it is safe to cancel and restart the migration process, or to switch to the online process.

Note | Migration requires a heap size comparable to running a Gerrit server. If you normally run `gerrit.war daemon` with an `-Xmx` flag, pass that to the migration tool as well.

Note | Note that by appending `--reindex false` to the above command, you can skip the lengthy, implicit reindexing step of the migration. This is useful if you plan to perform further Gerrit upgrades while the server is offline and have to reindex later anyway (E.g.: a follow-up upgrade to Gerrit 3.2 or newer, which requires to reindex changes anyway).

Advantages

- Much faster than online; can use all available CPUs, since no live traffic needs to be served.

- No degraded performance of live servers due to writing data to 2 locations.

Disadvantages

- Available in Gerrit 2.15 and 2.16 only.

- May require substantial downtime; takes about twice as long as an offline reindex. (In fact, one of the migration steps is a full reindex, so it can’t possibly take less time.)

#### Trial mode

The migration tool also supports "trial mode", where changes are migrated to NoteDb and read from NoteDb at runtime, but their primary storage location is still ReviewDb, and data is kept in sync between the two locations.

To run the migration in trial mode, add `--trial` to `migrate-to-note-db` or `daemon`:

      java -jar gerrit.war migrate-to-note-db --trial -d /path/to/site
      # OR
      java -jar gerrit.war daemon -d /path/to/site --migrate-to-note-db --trial

Or, set `noteDb.changes.trial=true` in `gerrit.config`.

There are several use cases for trial mode:

- Help test early releases of the migration tool for bugs with lower risk.

- Try out new NoteDb-only features like hashtags without running the full migration.

To continue with the full migration after running the trial migration, use either the online or offline migration steps as normal. To revert to ReviewDb-only, remove `noteDb.changes.read` and `noteDb.changes.write` from `notedb.config` and restart Gerrit.

## Configuration

The migration process works by setting a configuration option in `notedb.config` for each step in the process, then performing the corresponding data migration.

Config options are read from `notedb.config` first, falling back to `gerrit.config`. If editing config manually, you may edit either file, but the migration process itself only touches `notedb.config`. This means if your `gerrit.config` is managed with Puppet or a similar tool, it can overwrite `gerrit.config` without affecting the migration process. You should not manage `notedb.config` with Puppet, but you may copy values back into `gerrit.config` and delete `notedb.config` at some later point after completing the migration.

In general, users should not set the options described below manually; this section serves primarily as a reference.

- `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).

- `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 above.
  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.

## NoteDB to ReviewDB rollback

In case of rollback from NoteDB to ReviewDB, all the meta refs and the sequence ref need to be removed. The remove-notedb-refs.sh script has been written to automate this process.