Thord: worked example, ActivityPub delivery and thin reach

STUDY MAP: ActivityPub delivery and thin reach

Topic: how ActivityPub actually delivers a post to a stranger's timeline, and why reach stays thin even when a blog federates fine.

CORE IDEA

ActivityPub is a delivery list, not a web page that gets found. When you publish, your server posts a copy of that activity into the inbox of each account that already follows you. No followers means nothing is delivered anywhere. A stranger sees your post only if someone on their server follows you, or the post was boosted or mentioned into a place they read, or their server pulled it from a relay. A green "delivered" on federation proves the pipe accepted the packet. Reach is a second, separate fact.

THE QUESTIONS THAT ACTUALLY GET ASKED

- Why is my post on my own instance but not on mastodon.social?

- Federation says delivered, impressions are zero. What broke?

- Can I submit my blog to a big instance's search?

- Do relays fix thin reach?

- Why does my post show up as a bare link with no preview?

TEN FLASHCARDS

1. Delivery model: push to the inboxes of explicit followers. No crawling, no pull.

2. What makes an instance show your post: it was notified of that specific post (a local account follows you, or a boost or mention carried it in).

3. Backfill of old posts: none. An instance starts syncing you when someone local follows, and only for posts after that.

4. Actor: your account or server, with inbox and outbox endpoints.

5. Completing a follow: the target answers the Follow with an Accept; you then sit in the followers list and new posts get delivered.

6. Relay: a server that subscribes to instances' public posts and forwards them to subscriber instances. Helps discovery; many big instances allowlist or restrict relays.

7. Authorized Fetch (secure mode): an instance can require signed requests to fetch a post. Unsigned fetches are refused.

8. Instance-level cut: defederation or domain blocks, one-way or two-way.

9. Bare link with no preview: the fetching instance could not retrieve your post object (signature, format, or timeout), so only the URL survived.

10. Reach with zero followers: zero. The delivery list is the audience.

TEN PRACTICE QUESTIONS WITH ANSWERS

1. Post federated to 40 servers, 3 impressions. Broken? No. Delivery succeeded; reach is a separate count, and zero followers there means zero views.

2. Can an instance show a post it was never sent? No. Mastodon does not pull posts it was not notified about.

3. A big-instance user follows you. Do your older posts show there? No. Only new ones from the follow onward.

4. What does a 200 OK from an inbox prove? Accepted for processing. Not shown, not read.

5. Name two discovery paths in the fediverse. Boosts and mentions (also hashtags, relays, directories).

6. Why can Authorized Fetch cut your reach? Servers and crawlers that cannot sign requests cannot fetch your objects, so no previews, no indexing.

7. Fastest reliable way to get one post in front of strangers? Get a person who has a following to boost it. Delivery follows the follow graph.

8. Instance A blocks instance B. Can B's users still reach A's? Generally no. Defederation removes content and interactions both ways.

9. A friend on another instance sees a bare URL. First check? Fetch your object as their instance would, signed; usually signature verification or a domain block.

10. Biggest single reason small-instance reach is thin? An empty graph. Nobody on the big servers follows you, so nothing routes your post there.

WHAT MOST PEOPLE GET WRONG

They treat federation success as reach. They are different machines: delivery is push to a known list; reach is whether a human had it in view. Fixing reach means fixing the graph (follows, boosts, mentions, relays, directories), not sending harder.

SOURCES

- W3C ActivityPub, Recommendation, January 2018 (actors, inboxes, outboxes, Follow/Accept).

- SocialHub forum thread: "Issue with Mastodon pulling posts & followers from new ActivityPub integration": Mastodon never pulls a post it was not notified about.

- Tildes thread on fediverse search (instance syncs only after a local follow, no backfill).

- Fedi.Tips, "Authorized Fetch on Mastodon" (secure mode, signed fetches).

- EFF, "Is Mastodon Private and Secure?" (defederation).

- brodi1/activitypub-relays (relay list and allowlist caveat).

Map by Thord. One topic in, one study map back. First map free. thord@ilands.app