moonfire-nvr/design/api.md

# Moonfire NVR API

Status: **unstable**. This is an early draft; the API may change without
warning.

## Objective

Allow a JavaScript-based web interface to list cameras and view recordings.

In the future, this is likely to be expanded:

*   configuration support
*   commandline tool over a UNIX-domain socket
    (at least for bootstrapping web authentication)
*   mobile interface

## Detailed design

All requests for JSON data should be sent with the header `Accept:
application/json` (exactly). Without this header, replies will generally be in
HTML rather than JSON.

TODO(slamb): authentication.

### `/cameras/`

A `GET` request on this URL returns basic information about all cameras. The
`application/json` response will have a top-level `cameras` with a list of
attributes about each camera:

*   `uuid`: in text format
*   `short_name`: a short name (typically one or two words)
*   `description`: a longer description (typically a phrase or paragraph)
*   `retain_bytes`: the configured total number of bytes of completed
    recordings to retain.
*   `min_start_time_90k`: the start time of the earliest recording for this
    camera, in 90kHz units since 1970-01-01 00:00:00 UTC.
*   `max_end_time_90k`: the end time of the latest recording for this
    camera, in 90kHz units since 1970-01-01 00:00:00 UTC.
*   `total_duration_90k`: the total duration recorded, in 90 kHz units.
    This is no greater than `max_end_time_90k - max_start_time_90k`; it
    will be lesser if there are gaps in the recorded data.
*   `total_sample_file_bytes`: the total number of bytes of sample data (the
    `mdat` portion of a `.mp4` file).

Example response:

```json
{
  "cameras": [
    {
      "uuid": "fd20f7a2-9d69-4cb3-94ed-d51a20c3edfe",
      "short_name": "driveway",
      "description": "Hikvision DS-2CD2032 overlooking the driveway from east",
      "retain_bytes": 536870912000,
      "min_start_time_90k": 130888729442361,
      "max_end_time_90k": 130985466591817,
      "total_duration_90k": 96736169725,
      "total_sample_file_bytes": 446774393937,
    },
    ...
  ],
}
```

### `/cameras/<uuid>/`

A GET returns information for the camera with the given URL. The information
returned is a superset of that returned by the camera list. It also includes a
list of calendar days (in the server's time zone) with data in the server's
time zone. The `days` entry is a object mapping `YYYY-mm-dd` to a day object
with the following attributes:

*   `total_duration_90k` is the total duration recorded during that day.
    If a recording spans a day boundary, some portion of it is accounted to
    each day.
*   `start_time_90k` is the start of that calendar day in the server's time
    zone.
*   `end_time_90k` is the end of that calendar day in the server's time zone.
    It is usually 24 hours after the start time. It might be 23 hours or 25
    hours during spring forward or fall back, respectively.

A calendar day will be present in the `days` object iff there is a non-zero
total duration of recordings for that day.

Example response:

```json
{
  "days": {
    "2016-05-01": {
      "end_time_90k": 131595516000000,
      "start_time_90k": 131587740000000,
      "total_duration_90k": 52617609
    },
    "2016-05-02": {
      "end_time_90k": 131603292000000,
      "start_time_90k": 131595516000000,
      "total_duration_90k": 20946022
    }
  },
  "description":"",
  "max_end_time_90k": 131598273666690,
  "min_start_time_90k": 131590386129355,
  "retain_bytes": 104857600,
  "short_name": "driveway",
  "total_duration_90k": 73563631,
  "total_sample_file_bytes": 98901406,
}
```

### `/camera/<uuid>/recordings`

A GET returns information about recordings, in descending order.

Valid request parameters:

*   `start_time_90k` and and `end_time_90k` limit the data returned to only
    recordings which overlap with the given half-open interval. Either or both
    may be absent; they default to the beginning and end of time, respectively.
*   TODO(slamb): `continue` to support paging. (If data is too large, the
    server should return a `continue` key which is expected to be returned on
    following requests.)

TODO(slamb): once we support annotations, should they be included in the same
URI or as a separate `/annotations`?

In the property `recordings`, returns a list of recordings in arbitrary order.
Each recording object has the following properties:

*   `start_id`. The id of this recording, which can be used with `/view.mp4`
    to retrieve its content.
*   `end_id` (optional). If absent, this object describes a single recording.
    If present, this indicates that recordings `start_id-end_id` (inclusive)
    together are as described. Adjacent recordings from the same RTSP session
    may be coalesced in this fashion to reduce the amount of redundant data
    transferred.
*   `start_time_90k`: the start time of the given recording. Note this may be
    less than the requested `start_time_90k` if this recording was ongoing
    at the requested time.
*   `end_time_90k`: the end time of the given recording. Note this may be
    greater than the requested `end_time_90k` if this recording was ongoing at
    the requested time.
*   `sample_file_bytes`
*   `video_sample_entry_sha1`
*   `video_sample_entry_width`
*   `video_sample_entry_height`
*   `video_samples`: the number of samples (aka frames) of video in this
    recording.

Example request URI (with added whitespace between parameters):

```
/camera/fd20f7a2-9d69-4cb3-94ed-d51a20c3edfe/recordings
    ?start_time_90k=130888729442361
    &end_time_90k=130985466591817
```

Example response:

```json
{
  "recordings": [
    {
      "start_id": 1,
      "start_time_90k": 130985461191810,
      "end_time_90k": 130985466591817,
      "sample_file_bytes": 8405564,
      "video_sample_entry_sha1": "81710c9c51a02cc95439caa8dd3bc12b77ffe767",
      "video_sample_entry_width": 1280,
      "video_sample_entry_height": 720,
    },
    {
      "end_time_90k": 130985461191810,
      ...
    },
    ...
  ],
  "continue": "<opaque blob>",
}
```

### `/camera/<uuid>/view.mp4`

A GET returns a `.mp4` file, with an etag and support for range requests.

Expected query parameters:

*   `s` (one or more): a string of the form
    `START_ID[-END_ID][.[REL_START_TIME]-[REL_END_TIME]]`. This specifies
    recording segments to include. The produced `.mp4` file will be a
    concatenation of the segments indicated by all `s` parameters.  The ids to
    retrieve are as returned by the `/recordings` URL. The optional start and
    end times are in 90k units and relative to the start of the first specified
    id. These can be used to clip the returned segments. Note they can be used
    to skip over some ids entirely; this is allowed so that the caller doesn't
    need to know the start time of each interior id. If there is no key frame
    at the desired relative start time, frames back to the last key frame will
    be included in the returned data, and an edit list will instruct the
    viewer to skip to the desired start time.
*   `ts` (optional): should be set to `true` to request a subtitle track be
    added with human-readable recording timestamps.

Example request URI to retrieve all of recording id 1 from the given camera:

```
    /camera/fd20f7a2-9d69-4cb3-94ed-d51a20c3edfe/view.mp4?s=1
```

Example request URI to retrieve all of recording ids 1–5 from the given camera,
with timestamp subtitles:

```
    /camera/fd20f7a2-9d69-4cb3-94ed-d51a20c3edfe/view.mp4?s=1-5&ts=true
```

Example request URI to retrieve recording id 1, skipping its first 26
90,000ths of a second:

```
    /camera/fd20f7a2-9d69-4cb3-94ed-d51a20c3edfe/view.mp4?s=1.26
```

TODO: error behavior on missing segment. It should be a 404, likely with an
`application/json` body describing what portion if any (still) exists.

### `/camera/<uuid>/view.m4s`

A GET returns a `.mp4` suitable for use as a [HTML5 Media Source Extensions
media segment][media-segment].

Expected query parameters:

*   `s` (one or more): as with the `.mp4` URL, except that media segments
    can't contain edit lists so none will be generated. TODO: maybe add a
    `Leading-Time:` header to indicate how many leading 90,000ths of a second
    are present, so that the caller can trim it in some other way.

It's recommended that each `.m4s` retrieval be for at most one Moonfire NVR
recording segment for several reasons:

*   The Media Source Extension API appears structured for adding a complete
    segment at a time. Large media segments thus impose significant latency on
    seeking.
*   There is currently a hard limit of 4 GiB of data because the `.m4s` uses a
    single `moof` followed by a single `mdat`; the former references the
    latter with 32-bit offsets.
*   There's currently no way to generate an initialization segment for more
    than one video sample entry, so a `.m4s` that uses more than one video
    sample entry can't be used.

### `/init/<sha1>.mp4`

A GET returns a `.mp4` suitable for use as a [HTML5 Media Source Extensions
initialization segment][init-segment].

TODO: return a MIME type with a `Codecs` parameter as in [RFC 6381](rfc-6381). Web
browsers expect this parameter when initializing a `SourceBuffer`; currently
the user agent must divine this information.

[media-segment]: https://w3c.github.io/media-source/isobmff-byte-stream-format.html#iso-media-segments
[init-segment]: https://w3c.github.io/media-source/isobmff-byte-stream-format.html#iso-init-segments
[rfc-6381]: https://tools.ietf.org/html/rfc6381