= 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 master, and is live on the
  servers behind `googlesource.com`. In other words, if you use
  link:https://gerrit-review.googlesource.com/[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 link:#migration[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]]
== 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 link:https://research.google.com/pubs/pub35650.html[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.