this is how to really communicate on matrix.

this post kind of requires prerequisite matrix knowledge, but i’ll try to explain things.

basics

EDUs and PDUs

if you don’t know about matrix you probably thought, “what’s an EDU?”

well, matrix has two basic units of data. the Persistent Data Unit, which carries timeline events like your messages, or room state like the room name and members. basically, the things that persist.

then there’s the Ephemeral Data Unit. things like presence, read receipts and typing notifications are EDUs. they’re not persisted by servers and as such aren’t stored anywhere.

even if don’t know much about matrix, EDUs are far easier to craft than PDUs.

transactions

how do these get across the federation? they’re batched in transactions. when a server wants to send stuff somewhere¹. up to 50 PDUs and 100 EDUs (TODO: bullshit numbers please fill them in) get sent to a server per transaction.

great, now you know the key concepts behind this. not everything, but two useful things.

synapse logging EDUs

so let’s say we send a transaction to synapse with a typing EDU (m.typing), with a bogus room ID that the server definitely doesn’t recognise:

{ txn here }

what happens is this:

synapse logs here

it went, “wait, this isn’t even for a room i’m in” and logged it. isn’t that cool?

what if we send multiple EDUs, each with a more funny choice of room ID… like lines of ASCII art?:

logs

making the server

this is pretty easy because i use [mautrix/go]. its federation package includes a key server, federation client.

you can find the code here: https://git.squarebowl.club/plate/m3