The library has a nice guide
and two working examples, so I tried the local_federation example. To build the example, you need Rust compiler, cargo package manager, and git:
$ git clone https://github.com/LemmyNet/activitypub-federation-rust
$ cd activitypub-federation-rust
$ cargo run --example local_federation axum
[INFO local_federation] Start with parameter `axum` or `actix-web` to select the webserver
[INFO local_federation::axum::http] Listening with axum on localhost:8001
[INFO local_federation::axum::http] Listening with axum on localhost:8002
[INFO local_federation] Local instances started
[INFO local_federation] Alpha user follows beta user via webfinger
[INFO activitypub_federation::fetch] Fetching remote object http://localhost:8002/.well-known/webfinger?resource=acct:beta@localhost:8002
[INFO activitypub_federation::fetch] Fetching remote object http://localhost:8002/beta
[INFO activitypub_federation::fetch] Fetching remote object http://localhost:8001/alpha
[INFO local_federation] Follow was successful
[INFO local_federation] Beta sends a post to its followers
[INFO local_federation] Alpha received post: Hello world!
[INFO local_federation] Test completed
You may want to use network analizyer (e.g, wireshark) to see how it works under the hood.
GET /.well-known/webfinger?resource=acct:beta@localhost:8002 HTTP/1.1
accept: application/jrd+json
digest: SHA-256=[redacted]
signature: keyId="http://localhost:8001/#main-key",algorithm="hs2019",[...]
host: localhost:8002
HTTP/1.1 200 OK
content-type: application/json
content-length: 269
date: Sat, 03 Feb 2024 23:05:19 GMT
{
"subject": "acct:beta@localhost:8002",
"links": [
{
"rel": "http://webfinger.net/rel/profile-page",
"type": "text/html",
"href": "http://localhost:8002/beta",
"template": null
},
{
"rel": "self",
"type": "application/activity+json",
"href": "http://localhost:8002/beta",
"template": null
}
]
}
[...]
One of the reasons is it makes moderation (including soft moderation by users like downvotes or reports) harder. Users not familiar with Japanese can't decide whether the post follows the rule and is on topic.