Delivering messages to offline users in a multi-user chat (ejabberd)

Question

Actually we are using ejabberd server for one of our client's Chat application. Everything is working well except for Group chat.

We are using MUC for Group chat but it is not sending Messages to the member whenever uses is offline. Is there any alternative plugin or something where we can make that working?

Or any one can suggest about how to receive offline messages for that user from Group chat history.

Thanks in advance

Answer 1

That's because there's no such concept for multi-user chat rooms. In fact, if you'll think about this a bit more you'll understand why:

• Potentially unbound number of participants might be present in a room at any given time.

  So exactly for which users not currently present in the MUC room should the server store the messages in the offline storage? I mean, in the generic case, the server does not know all the users who could ever possibly chat in a given room it hosts.

  (Well, if this would be the only problem, it could possibly work for members-only rooms, I must admit.)

• MUC rooms are not "local server only": a potentially unbound number of users from any number of other servers might join the room, and messages to those users will be delivered by routing them via their respective servers.

  Obviously, this is another reason why such an idea of "MUC room offline storage" has no sense.

• MUC rooms are by definition transient: when a user is offline, they're not in any room— (re-)joninig a room is an explicit action.

  This is in fact the most important reason for not supporting offline storage.

As you can see, XMPP MUC rooms are much like IRC chats on steroids.

So what you really want is "room history"—a part of the XMPP-0045 extension which allows the client to explicitly ask the room for the message history they missed. In a sense, instead of storing offline message for each user, the room might be configured to store just a certain number of the most recent messages sent to it (or all such messages for a given period of time). Then the room supports querying these messages by the joined users.

There's another possibility which you might explore: "multicast addressing" of XEP-0033 ("Extended stanza addressing"). Basically it allows a client to use a special multicast service to send their message to multiple recipients at once. The upside is that offline storage is there again. The downside is that I doubt such a multicast service is supported out of the box in ejabberd, and it seems like that extension leaves much details about how it could be implemented unspecified.

Answer 2

I faced your issue as I sought to implement groupchats for my chatting app. I faced the same problem of MUC not storing offline messages for each recipient. And I did not want to retrieve MUC history which requires the user to rejoin every MUC to update his messages database. What I wanted is for the server to save offline messages by recipient, and for the recipient to get all MUC messages when he gets online (without having to join each MUC).

The way I did it is through pubsub. Using pubsub will force the server to store offline message per recipient. When the user reconnects, he gets all the offline messages including the pubsub messages which are sent as normal messages - that is it. One issue I had with pubsub over MUC though is that it is hard to get the list of subscribers. So when my app creates a groupchat, it creates a pubsub node for messages, invite all participants to subscribe (including self) to the pubsub and my app also creates a MUC and makes every participant an owner of that MUC. This way the list of the groupchat participants can be retrieved by checking the list of owners of the MUC. The only purposes of the MUC are to hold the list of participants as well as the name of the groupchat. Everything else is handled by the pubsub node.

Anything unclear please let me know.

ADDITIONAL DETAILS: Essentially when the user wants to create a groupchat, our app creates a pubsub node as well as a MUC. You need to be familiar with both concepts. For the pubsub node, you need to set an option to allow any subscriber to post. When a user sends a message, he actually publishes on the node, and ejabberd will send the message to all subscribers as if it were a regular message (except it comes from pubsub.yourdomain.com). Therefore if a recipient is offline, ejabberd will store this message as any other regular message.

This is not how ejabberd handles MUC messages. Those are only sent to people CURRENTLY in the chatroom. History of messages can be stored by ejabberd however, but for a recipient to get the history he will need to join the MUC. Which means that everytime the app reconnects, it would have to join all the user's existing MUCs. We found this was not practical.

We also use a MUC for the same groupchat, but this is only to store participants so that a user can get the list at any time (no way to do it with pubsub).

An additional benefit of using pubsub over MUC is that the way ejabberd stores pubsub data is way more efficient. I have not studied this in depth, but I expect much better performance from pubsub.

Answer 3

New ejabberd server at 16.09 version have improvements for multi-user chat - MUC Sub:

The goal of MUC Sub is to try to rely as much as possible on existing MUC specification, while making the smallest possible change that make mobile group conversation client easy.

The feature is enabled by default. To use it, just make sure you set the new parameter “Allow subscription” in the room on which you want to use it.

Here is link to documentation: https://docs.ejabberd.im/developer/proposed-extensions/muc-sub/

More info here: https://blog.process-one.net/xmpp-mobile-groupchat-introducing-muc-subscription/