This is only the database schema, which I'm adding now in the hopes of
freezing schema version 3. There's no way yet to create users, much less
actually authenticate.
These are not actually populated by the code yet. I'm trying to get the
v3 schema frozen as soon as possible; actually using the fields can come
later.
Add some explanation of their value in time.md, along with some general
musing on leap seconds, and a correction on the frequency error of my cameras.
This improves the practicality of having many streams (including the doubling
of streams by having main + sub streams for each camera). With these tuned
properly, extra streams don't cause any extra write cycles in normal or error
cases. Consider the worst case in which each RTSP session immediately sends a
single frame and then fails. Moonfire retries every second, so this would
formerly cause one commit per second per stream. (flush_if_sec=0 preserves
this behavior.) Now the commits can be arbitrarily infrequent by setting
higher values of flush_if_sec.
WARNING: this isn't production-ready! I hacked up dir.rs to make tests pass
and "moonfire-nvr run" work in the best-case scenario, but it doesn't handle
errors gracefully. I've been debating what to do when writing a recording
fails. I considered "abandoning" the recording then either reusing or skipping
its id. (in the latter case, marking the file as garbage if it can't be
unlinked immediately). I think now there's no point in abandoning a recording.
If I can't write to that file, there's no reason to believe another will work
better. It's better to retry that recording forever, and perhaps put the whole
directory into an error state that stops recording until those writes go
through. I'm planning to redesign dir.rs to make this happen.
The filenames now represent composite ids (stream id + recording id) rather
than a separate uuid system with its own reservation for a few benefits:
* This provides more information when there are inconsistencies.
* This avoids the need for managing the reservations during recording. I
expect this to simplify delaying flushing of newly written sample files.
Now the directory has to be scanned at startup for files that never got
written to the database, but that's acceptably fast even with millions of
files.
* Less information to keep in memory and in the recording_playback table.
I'd considered using one directory per stream, which might help if the
filesystem has trouble coping with huge directories. But that would mean each
dir has to be fsync()ed separately (more latency and/or more multithreading).
So I'll stick with this until I see concrete evidence of a problem that would
solve.
Test coverage of the error conditions is poor. I plan to do some restructuring
of the db/dir code, hopefully making steps toward testability along the way.
The idea is to avoid the problems described in src/schema.proto; those
possibilities have bothered me for a while. A bonus is that (in a future
commit) it can replace the sample file uuid scheme in favor of using
<camera_uuid>-<stream_type>/<recording_id> for several advantages:
* on data integrity problems (specifically, extra sample files), more
information to use to understand what happened.
* no more reserving sample files prior to using them. This avoids some extra
database transactions on startup (now there's an extra two total rather
than an extra one per stream). It also simplifies an upcoming change I
want to make in which some streams are not flushed immediately, reducing
the write load significantly (maybe one per minute total rather than one
per stream per minute).
* get rid of eight bytes per playback cache entry in RAM (and nine bytes
per recording_playback row on flash).
The implementation is still pretty rough in places:
* Lack of tests.
* Poor ode organization. In particular, SampleFileDirectory::write_meta
shouldn't be exposed beyond db. I'm thinking about moving db.rs and
SampleFileDirectory to a new crate, moonfire_nvr_db. This would improve
compile times as well.
* No tooling for renaming a sample file directory.
* Config subcommand still panics in conditions that can be reasonably
expected to happen.
This is still pretty basic support. There's no config UI support for
renaming/moving the sample file directories after they are created, and no
error checking that the files are still in the expected place. I can imagine
sysadmins getting into trouble trying to change things. I hope to address at
least some of that in a follow-up change to introduce a versioning/locking
scheme that ensures databases and sample file dirs match in some way.
A bonus change that kinda got pulled along for the ride: a dialog pops up in
the config UI while a stream is being tested. The experience was pretty bad
before; there was no indication the button worked at all until it was done,
sometimes many seconds later.
This avoids having codec-specific logic to synthesize it in db.rs. It's not
too much of a problem now with only H.264 support, but it'd be a pain when
supporting H.265 and other codecs.
The advantages of the new schema are:
* overlapping recordings can be unambiguously described and viewed.
This is a significant problem right now; the clock on my cameras appears to
run faster than the (NTP-synchronized) clock on my NVR. Thus, if an
RTSP session drops and is quickly reconnected, there's likely to be
overlap.
* less I/O is required to view mp4s when there are multiple cameras.
This is a pretty dramatic difference in the number of database read
syscalls with pragma page_size = 1024 (605 -> 39 in one test),
although I'm not sure how much of that maps to actual I/O wait time.
That's probably as dramatic as it is due to overflow page chaining.
But even with larger page sizes, there's an improvement. It helps to
stop interleaving the video_index fields from different cameras.
There are changes to the JSON API to take advantage of this, described
in design/api.md.
There's an upgrade procedure, described in guide/schema.md.
Scott Lamb