Matrix Bridges Working Together

I am testing SmsMatrix, an SMS-to-Matrix bridge on Android. It is one of many Matrix bridges. But this post is not mainly about SmsMatrix, it is mainly the start of a train of thought about how we can manage communications with one person bridged over different channels.

This is a write-up of thoughts I first posted to the SmsMatrix discussion room, which would be better continued on

Let’s say my friend has telephone number 123456 and is in my phone’s address book as “Ray”.

When my phone receives an SMS from Ray, SMSMatrix sees that message arrive in the Android SMS subsystem, and (if it hasn’t previously done so) it creates a new Matrix direct-chat room which it will associate with the SMS sender’s phone number, and invites me to join the room.

The other participant in this room is the bridge bot. Its username is @sms-bot:my_hs, and it sets its own display name to “Ray”. I assume that is a per-room display name, so it will show as a different name in each SMS conversation. The bot does not set a room name, and so Riot displays it as “Ray” (the other participant’s display name), and the bot sets the room “topic” field to “123456”, which Riot displays next to the room name. The idea is that in the user interface it looks like I am having a chat with “Ray” and it works like I am having a chat with Ray.

In Riot-web’s message view, the sender of this first SMS message shows as (precisely) “sms-bot”. The display name of @sms-bot does seem to be correctly set to “Ray” — I can see that in the “users” panel. Perhaps that’s a Riot bug that it doesn’t always display the user’s display name.

This bot works without admin privileges. It can do so because it doesn’t try to create and control a new Matrix user for each SMS sender, nor does it try to find and puppet an existing “real” matrix user account corresponding to the sender.

I wonder if it would be nice to upgrade it to a puppeting bridge if I one day have enough time to devote to it. I’m wondering exactly what we could achieve if we did so.

I also have some other bridges, and let’s say Ray also has her own Matrix account on her own homeserver, where she calls herself by her full name “Radiator”. Now I have Ray’s messages coming in to me in these Matrix rooms/users:

  • @sms-bot:my_hs “Ray”
    • in room (unnamed) displayed as “Ray 123456”;
  • @whatsapp_123456:my_hs “Ray (WA)”
    • in room (unnamed) displayed as “Ray (WA) WhatsApp private chat”;
  • @telegram_123456:my_hs “Ray (TG)”
    • in room (unnamed) displayed as “Ray (TG)”;
  • @ray:rays_hs “Radiator”, Ray’s real Matrix user account
    • in room named “Ray”

The mautrix-telegram and mautrix-whatsapp bridges each create a new user id for Ray in their own namespace of matrix user-ids, which is different from the SMS bridge.

I am interested in researching how we could improve this situation. Would we want to make all bridges to the same person somehow bridge to the same Matrix user?

@pwr22 asked, “How would the bridge know which method to use when sending a message?” That’s a good UX question. The first thought coming to my mind is that’s easy if we dedicate a separate room for each bridge for that user. (Ray’s SMS chat room, Ray’s Telegram chat room, …).

“In that case what do we gain from having one puppet user instead of separate ones per service?” We gain things like,

  • “search for all messages from @ray [containing the word ‘zoo’]”,
  • “search for the rooms that @ray is in”.

I expect users would want to choose, by their own preferences and perhaps per contact, whether a contact’s messages are all grouped in the same room or a separate room per bridge.

If I want to group all Ray’s messages into one room, then the sending UX issue must be solved a different way. Perhaps the client would allow me to set a preference. (That could be stored as client-managed metadata, like how Riot stores some settings in “account data” under “” keys.) And/or there could be bot commands so I can tell a bridge "!sms-bot: enable [or disable] sending outgoing messages in this room through SMS".

In response to my thought about making all bridges bridge to the same Matrix user, @swedneck replied, “i’m not sure that’d make sense. i would probably have all those bridges in the same room if possible. you’d probably want his bots to ignore each other though.” So:

  • one room (“My direct chat room with Ray”) containing
    • myself,
    • the virtual user @sms-bot:my_hs (“Ray”),
    • the virtual user @telegram_123456:my_hs (“Ray”),
    • …, and
    • the Matrix user @ray:rays_hs (“Radiator”).

I see how that would work well for incoming messages: I would be able to see which bridge each message came through, if I wanted, while keeping all the display names the same if I mostly didn’t care to see the difference.

We would need a way to make the bots know about each other. When Ray sends me a message through one bridge, we (probably) don’t want all the other bridges to send him copies of it. They’re all “my” bots running against my Matrix home server (HS), so I could add something to their configs easily enough. Alternatively, we could define common metadata that they could each set, so they could detect the situation automatically. The latter would be more flexible, in case some of them are not configured directly alongside my HS.

What about when I send messages to Ray? If I just type without a mention, then the same applies as I said before: some way of configuring in advance which bridge(s) is/are to send it.

When I Mention Ray, there are different matrix usernames I might mention, depending how I do it: if I click on one of the received msgs, I might select a bridge username that isn’t the currently configured preferred send method; how should that all work?

Maybe bridged usernames should be specially treated: all treated as something like aliases, and all resolved to the same single “real” matrix username?

Needs more thought.

MSC2346: Bridge information state event

As part of a solution, we will need a mechanism to allow multiple bridges in a room to communicate with each other. This proposal may achieve it:

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