Stories - posts that delete themselves in 24 hours

Why this design

Stories - Instagram, Snapchat, WhatsApp Status, LinkedIn and many others shipped them - look like a feed with a timer. The timer is the interesting part. "Disappears after 24 hours" sounds like a TTL on a row, but no database deletes on time, and a story visible at 24 hours and 4 minutes is a broken promise that users notice and screenshot. The design answer is that expiry is two separate mechanisms with two separate guarantees: a filter that makes expired stories invisible exactly on time, and a sweep that makes them gone some time later. Then there is the part nobody expects to be expensive: the list of who watched each story.

What you will learn

• Why visibility at 24 hours is enforced by WHERE expires_at > now() on every read, and deletion by a background sweep that may lag.
• Why the tray ("who has a story for me") is assembled at read time from a story-heads cache rather than pushed into every follower's inbox.
• Why the viewer list is 25 times the write volume of the stories, and how idempotent inserts and hourly partitions keep it bounded.
• How signed CDN URLs that expire with the story stop new fetches, and why they do not recall bytes already cached.
• Why highlights keep a story past 24 hours by reference, and what that does to the sweep.

How to read this board

1. Press Play and follow one story from POST through STORE, then a viewer opening the tray in WATCH.
2. Then VIEWER LIST, which is where the writes go, and THE SWEEP at the bottom.
3. 1 · Requirements, 2 · Capacity, 3 · Storage - the views and the media are the two big numbers.
4. 4 · Availability - what a late sweep or a slow cache shard costs.
5. 5 · How it works, 6 · Deployment plan, then the DATA MODEL.

Ask these before drawing anything

• Exactly 24 hours, or "about a day"? The answer decides whether a TTL alone is acceptable. It almost never is.
• Who sees a story - followers, close friends, a custom list? Audience is checked at read time, so changing a list after posting changes who can see it.
• Does the author see who viewed, and for how long after expiry? Instagram keeps the list visible for 48 hours.
• Can a story be kept (archive, highlights)? If yes, "deleted at 24 hours" becomes "removed from the public tray at 24 hours", and the legal wording matters.
• What does delete mean for compliance - must bytes be gone from backups and caches, and by when?

Functional

• Post a photo or a video of up to 15 seconds; it is visible to its audience for exactly 24 hours.
• Open a tray of accounts with live stories, unseen ones first.
• Record each view once per viewer per story and show the author a viewer list with a count.
• Optionally pin a story to a highlight, which keeps it beyond 24 hours on the author's profile.

Non-functional

• Expired stories never returned by any API after expires_at, to the millisecond.
• Tray open at p99 under 200 ms; first frame of a story under 400 ms.
• Media bytes of an expired, unpinned story deleted from origin within 6 hours, and CDN copies unreachable within 1 hour.
• A lost view beacon is acceptable; a doubled view is not (counts are shown to authors).