Aug 28, 2020

YouTube feed parser, ewma and state logs

Recently Google stopped sending email notifications for YouTube subscription feeds, as apparently conversion of these to page views was 0.1% or something.

And even though I've used these to track updates exclusively, guess it's fair, as I also had xdg-open script jury-rigged to just open any youtube links in mpv instead of bothering with much inferior ad-ridden in-browser player.

One alternative workaround is to grab OPML of subscription Atom feeds and only use those from now on, converting these to familiar to-watch notification emails, which I kinda like for this purpose because they are persistent and have convenient state tracking (via read/unread and tags) and info text without the need to click through, collected in a dedicated mailbox dir.

Fetching/parsing feeds and sending emails are long-solved problems (feedparser in python does great job on former, MTAs on the latter), while check intervals and state tracking usually need to be custom, and can have a few tricks that I haven't seen used too widely.


Trick One - use moving average for an estimate of when events (feed updates) happen.

Some feeds can have daily activity, some have updates once per month, and checking both every couple hours would be incredibly wasteful to both client and server, yet it seem to be common practice in this type of scenario.

Obvious fix is to get some "average update interval" and space-out checks in time based on that, but using simple mean value ("sum / n") has significant drawbacks for this:

  • You have to keep a history of old timestamps/intervals to calculate it.
  • It treats recent intervals same as old ones, even though they are more relevant.

Weighted moving average value fixes both of these elegantly:

interval_ewma = w * last_interval + (1 - w) * interval_ewma

Where "w" is a weight for latest interval vs all previous ones, e.g. 0.3 to have new value be ~30% determined by last interval, ~30% of the remainder by pre-last, and so on.

Allows to keep only one "interval_ewma" float in state (for each individual feed) instead of a list of values needed for mean and works better for prediction due to higher weights for more recent values.

For checking feeds in particular, it can also be updated on "no new items" attempts, to have backoff interval increase (up to some max value), instead of using last interval ad infinitum, long past the point when it was relevant.

Trick Two - keep a state log.

Very useful thing for debugging automated stuff like this, where instead of keeping only last "feed last_timestamp interval_ewma ..." you append every new one to a log file.

When such log file grows to be too long (e.g. couple megs), rename it to .old and seed new one with last states for each distinct thing (feed) from there.

Add some timestamp and basic-info prefix before json-line there and it'd allow to trivially check when script was run, which feeds did it check, what change(s) it did detect there (affecting that state value), as well as e.g. easily remove last line for some feed to test-run processing last update there again.

When something goes wrong, this kind of log is invaluable, as not only you can re-trace what happened, but also repeat last state transition with e.g. some --debug option and see what exactly happened there.

Keeping only one last state instead doesn't allow for any of that, and you'd have to either keep separate log of operations for that anyway, and manually re-construct older state from there to retrace last script steps properly, tweak the inputs to re-construct state that way, or maybe just drop old state and hope that re-running script with latest inputs without it hits same bug(s).

I.e. there's basically no substitute for that, as text log is pretty much same thing, describing state changes in non-machine-readable and often-incomplete text, which can be added to such state-log instead as an extra metadata anyway.

With youtube-rss-tracker script with multiple feeds, it's a single log file, storing mostly-json lines like these (wrapped for readability):

2020-08-28 05:59 :: UC2C_jShtL725hvbm1arSV9w 'CGP Grey' ::
  {"chan": "UC2C_jShtL725hvbm1arSV9w", "delay_ewma": 400150.7167069313,
    "ts_last_check": 1598576342.064458, "last_entry_id": "FUV-dyMpi8K_"}
2020-08-28 05:59 :: UCUcyEsEjhPEDf69RRVhRh4A 'The Great War' ::
  {"chan": "UCUcyEsEjhPEDf69RRVhRh4A", "delay_ewma": 398816.38761319243,
    "ts_last_check": 1598576342.064458, "last_entry_id": "UGomntKCjFJL"}

Text prefix there is useful when reading the log, while script itself only cares about json bit after that.

Anything doesn't work well - notifications missing, formatted badly, errors, etc - you just remove last state and tweak/re-run the script (maybe in --dry-run mode too), and get pretty much exactly same thing as happened last, aside from any input (feed) changes, which should be very predictable in this particular case.

Not sure what this concept is called in CS, there gotta be some fancy name for it.


Link to YouTube feed email-notification script used as an example here:
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