moonfire-nvr/guide/schema.md

# Moonfire NVR Schema Guide

This document has notes about the Moonfire NVR storage schema. As described in
[README.md](../README.md), this consists of two kinds of state:

   * a SQLite database, typically <1 GiB. It should be stored on flash if
     available.
   * the "sample file directory", which holds the actual samples/frames of
     H.264 video. This should be quite large and typically is stored on a hard
     drive.

## Upgrading

The database schema includes a version number to quickly identify if a
the database is compatible with a particular version of the software. Some
software upgrades will require you to upgrade the database.

Note that in general upgrades are one-way and backward-incompatible. That is,
you can't downgrade the database to the old version, and you can't run the old
software on the new database. To minimize the corresponding risk, you should
save a backup of the old SQLite database and verify the new software works in
read-only mode prior to deleting the old database.

### Procedure

First ensure there is sufficient space available for four copies of the
SQLite database:

  1. the primary copy, which will be upgraded
  2. a copy you create manually as a backup so that you can restore if you
     discover a problem while running the new software against the upgraded
     database in read-only mode. If disk space is tight, you can save this
     to a different filesystem than the primary copy.
  3. internal copies made and destroyed by Moonfire NVR and SQLite during the
     upgrade:
        * during earlier steps, possibly duplicate copies of tables, which
          may occupy space both in the main database and the journal
        * during the final vacuum step, a complete database copy
     If disk space is tight, and you are _very careful_, you can skip these
     copies with the `--preset-journal=off --no-vacuum` arguments to
     the updater. If you aren't confident in your ability to do this, *don't
     do it*. If you are confident, take additional safety precautions anyway:
        * double-check you have the full backup described above. Without the
          journal any problems during the upgrade will corrupt your database
          and you will need to restore.
        * ensure you re-enable journalling via `pragma journal_mode = wal;`
          before using the upgraded database, or any problems after the
          upgrade will corrupt your database. The upgrade procedure should do
          this automatically, but you will want to verify by hand that you are
          no longer in the dangerous mode.

Next ensure Moonfire NVR is not running and does not automatically restart if
the system is rebooted during the upgrade. If you are using systemd with the
service name `moonfire-nvr`, you can do this as follows:

    $ sudo systemctl stop moonfire-nvr
    $ sudo systemctl disable moonfire-nvr

The service takes a moment to shut down; wait until the following command
reports that it is not running:

    $ sudo systemctl status moonfire-nvr

Then back up your SQLite database. If you are using the default path, you can
do so as follows:

    $ sudo -u moonfire-nvr cp /var/lib/moonfire-nvr/db/db{,.pre-upgrade}

By default, the upgrade command will reset the SQLite `journal_mode` to
`delete` prior to the upgrade. This works around a problem with
`journal_mode = wal` in older SQLite versions, as documented in [the SQLite
manual for write-ahead logging](https://www.sqlite.org/wal.html):

> WAL works best with smaller transactions. WAL does not work well for very
> large transactions. For transactions larger than about 100 megabytes,
> traditional rollback journal modes will likely be faster. For transactions
> in excess of a gigabyte, WAL mode may fail with an I/O or disk-full error.
> It is recommended that one of the rollback journal modes be used for
> transactions larger than a few dozen megabytes. Beginning with version
> 3.11.0 (2016-02-15), WAL mode works as efficiently with large transactions
> as does rollback mode.

Run the upgrade procedure using the new software binary (here referred to as
`new-moonfire-nvr`; if you are installing from source, you may find it as
`target/release/moonfire-nvr`).

    $ sudo -u moonfire-nvr new-moonfire-nvr --upgrade

Then run the system in read-only mode to verify correct operation:

    $ sudo -u moonfire-nvr new-moonfire-nvr --read-only

Go to the web interface and ensure the system is operating correctly. If
you detect a problem now, you can copy the old database back over the new one.
If you detect a problem after enabling read-write operation, a restore will be
more complicated.

Then install the new software to the path expected by your systemd
configuration and start it up:

    $ sudo install -m 755 new-moonfire-nvr /usr/local/bin/moonfire-nvr
    $ sudo systemctl enable moonfire-nvr
    $ sudo systemctl start moonfire-nvr

Hopefully your system is functioning correctly. If not, there are two options
for restore; neither are easy:

   * go back to your old database. There will be two classes of problems:
        * If the new system deleted any recordings, the old system will
          incorrectly believe they are still present. You could wait until all
          existing files are rotated away, or you could try to delete them
          manually from the database.
        * if the new system created any recordings, the old system will not
          know about them and will not delete them. Your disk may become full.
          You should find some way to discover these files and manually delete
          them.
   * undo the changes by hand. There's no documentation on this; you'll need
     to read the code and come up with a reverse transformation.

Once you're confident of correct operation, delete the unneeded backup:

    $ sudo systemctl rm /var/lib/moonfire-nvr/db/db.pre-upgrade

### Unversioned to version 0

Early versions of Moonfire NVR (prior to 2016-12-20) did not include the
version information in the schema. You can manually add this information to
your schema using the `sqlite3` commandline. This process is backward
compatible, meaning that software versions that accept an unversioned database
will also accept a version 0 database.

Version 0 makes two changes:

   * it adds schema versioning, as described above.
   * it adds a column (`video_sync_samples`) to a database index to speed up
     certain operations.

There's a special procedure for this upgrade. The good news is that a backup
is unnecessary; there's no risk with this procedure.

First ensure Moonfire NVR is not running as described in the general procedure
above.

Then use `sqlite3` to manually edit the database. The default
path is `/var/lib/moonfire-nvr/db/db`; if you've specified a different
`--db_dir`, use that directory with a suffix of `/db`.

    $ sudo -u moonfire-nvr sqlite3 /var/lib/moonfire-nvr/db/db
    sqlite3>

At the prompt, run the following commands:

```sql
begin transaction;

create table version (
  id integer primary key,
  unix_time integer not null,
  notes text
);

insert into version values (0, cast(strftime('%s', 'now') as int),
                            'manual upgrade to version 0');

drop index recording_cover;

create index recording_cover on recording (
  camera_id,
  start_time_90k,
  duration_90k,
  video_samples,
  video_sample_entry_id,
  sample_file_bytes
);

commit transaction;
```

When you are done, you can restart the service via `systemctl` and continue
using it with your existing or new version of Moonfire NVR.

### Version 0 to version 1

Version 1 makes several changes to the recording tables and indices. These
changes allow overlapping recordings to be unambiguously listed and viewed.
They also reduce the amount of I/O; in one test of retrieving playback
indexes, the number of (mostly 1024-byte) read syscalls on the database
dropped from 605 to 39.

The general upgrade procedure applies to this upgrade.