= Gerrit Code Review - NoteDb Backend

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 the 2.15 release, and is the
  default for new sites.
- Admins may use an link:#offline-migration[offline] or
  link:#online-migration[online] tool to migrate change data in an existing
  site from ReviewDb.
- Storing link:config-accounts.html[account data] is fully implemented in the
  2.15 release. Account data is migrated automatically during the upgrade
  process by running `gerrit.war init`.
- Account and change metadata on the servers behind `googlesource.com` is fully
  migrated to NoteDb. In other words, if you use
  link:https://gerrit-review.googlesource.com/[gerrit-review], you're already
  using NoteDb.

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
----

== Future Work ("Gerrit 3.0")

- Storing group data is a work in progress. Like account data, it will be
  migrated automatically.
- NoteDb will be the only database format supported by Gerrit 3.0. The offline
  change data migration tool will be included in Gerrit 3.0, but online
  migration will only be available in the 2.x line.

[[migration]]
== 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-migration]]
=== Online

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; will not be 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-migration]]
=== 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.

*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.
* Available in both Gerrit 2.x and 3.0.

*Disadvantages*

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

[[trial-migration]]
==== 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
  link:rest-api-changes.txt#get-hashtags[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 link:#migration[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. +