Klaxon — page alerts, to phone by Acid Alchamy

Klaxon watches a web page and sends the alert to a hub you run. There is no Klaxon
account, no sign-in, no server of ours in the path, and no analytics of any kind. You
give it a hub address and a hook token, and that is the only place anything it reads
ever goes.

Alerts arrive in the Klaxon app's feed. Whether they also light up your phone's lock
screen depends on your own hub having a delivery path configured — a hub without one
records every event faithfully and delivers none of them.

WHAT IT WATCHES ON A SCHEDULE

• The page changes. Klaxon fetches it, reduces it to readable text, and alerts when
that text moves. The alert says what changed; the whole before-and-after is attached
to the event.
• A phrase appears or disappears. "In stock", "Sold out", "Appointment available". It
fires on the edge, not while the words sit there.
• A number moves. Point the picker at a price, a stock count, a queue depth or a rank,
then pick a condition: drops below, rises above, goes up at all, goes down at all,
moves by a percentage, or changes at all.
• The site goes down or comes back. Reachability and status code, not content. It needs
several consecutive failures before it says anything, so one blip is not an outage —
and it tells you when the site recovers.
• A value in a JSON answer moves. Point it at any URL that returns JSON, pull a value
out with a dotted path like data.items.0.price, and apply the same conditions plus
"becomes exactly" and "starts containing".
• A feed publishes something new. RSS, Atom or JSON Feed. New items only, once,
optionally filtered by a word.
• A button changes state. "Add to cart" becoming clickable, "Notify me" becoming
"Buy now".

WHAT IT NOTICES WHILE YOU BROWSE

• A keyword on a page you open. An opt-in list of words, each scoped to a site you name —
one host, or a host and its subdomains. The scope is required; there is no blank that
quietly means everywhere. "Every site" is available if you want it: you type * and
confirm it, and that is the point at which Firefox asks for access to all websites. You
get the sentence around the match, not just the word. It never crawls and never opens a
page by itself.
• A page returned a server error, on the sites you list, at the status floor you choose.
• A tab stopped responding — the page's process died and stopped answering.

WHAT YOU SEND BY HAND

• Right-click a selection, a link, an image or the page.
• Send a page as a reading item, with the article's text captured into the alert so it
opens with the words in it instead of a link to fetch later.
• Watch a feed in one click when the page you are on advertises one.
• Downloads, when one finishes or fails. Optional, off by default, and the browser's
download permission is requested only when you switch it on. The alert carries the
filename, the size and the address it came from with the query string removed — a
download link routinely carries a signature or a one-time token there, and that is not
something you chose to send anywhere.

INSTEAD OF ALL OF THEM AT ONCE

• The daily check-in: one alert at a time you choose, listing what was sent, what a
quiet period held back, and what broke.
• The roll-up: when the hourly ceiling is reached Klaxon does not go quiet. It sends one
alert saying how many it held back and which watches were loudest.

HOW IT BEHAVES

Every trigger has its own interval, its own quiet period and its own hourly cap, with
one ceiling across the whole extension on top of those. One minute is the polling floor,
because browser alarms will not fire faster than that in every browser this supports,
and an interval that gets silently rounded up is a lie told to the person relying on it.

A failure is never reported as a change. An HTTP 500, an empty body, a redirect to a
login page and a selector that stopped matching each get their own outcome, and none of
them move the stored baseline.

PERMISSIONS

Klaxon asks for access to no website when you install it. Access is requested at the
moment you create a watch, a keyword rule or a page-problem entry, and it asks for that
one site. Every site is available as a deliberate choice — you type * and confirm it, and
Firefox then asks for all websites in those words — but nothing here picks that for you.
You can withdraw any of it from about:addons at any time. There is no declared content
script, no remote code and no telemetry. Nothing in the package is minified or bundled:
what you install is the source.

You need a Klaxon hub. It is a small self-hosted server; klaxonalerts.com explains how
to run one.