WARNING: This is still a Work In Progress

The final implementation may diverge. Currently, only the participants after a
configured threshold will be just viewers (visitors) and there is no promotion
mechanism to become a main participant yet.

TODO:
* Merge messaging between visitor nodes and main conference
* Polls
* Raise hand to be promoted to enter the main conference
* Make sure it works with tenants.


# Low-latency conference streaming to very large audiences

To have a low-latency conference with a very large audience, the media and
signaling load must be spread  beyond what can be handled by a typical Jitsi
installation. A call with 10k participants requires around 50 bridges on decent
vms (8+ cores). The main participants of a conference with a very large
audience will share a main prosody, like with normal conferences, and
additional prosody vms are needed to support signaling to the audience.

In the example configuration we use a 16 core machine. Eight of the cores are
used for the main prosody and other services (nginx, jicofo, etc) and the other
eight cores are used to run prosody services for visitors, i.e., "visitor
prosodies".

We consider 2000 participants per visitor node a safe value. So eight visitor
prosodies will be enough for one 10k participants meeting.

<img src="imgs/visitors-prosody.svg" alt="diagram of a central prosody connected to several visitor prosodies" width="500"/>

# Configuration
Use the `pre-configure.sh` script to configure your system, passing it the
number of visitor prosodies to set up.
`./pre-configure.sh 8`

The script will add for each visitor prosody:
- folders in `/etc/`
- a systemd unit file in `/lib/systemd/system/`
- a user for jicofo
- a config entry in jicofo.conf

Setting up configuration for the main prosody is a manual process:
- Add to the enabled modules list in the general part (e.g. [here](https://github.com/bjc/prosody/blob/76bf6d511f851c7cde8a81257afaaae0fb7a4160/prosody.cfg.lua.dist#L33)):
```
      "s2s_bidi";
      "certs_s2soutinjection";
      "s2soutinjection";
      "s2s_whitelist";
```

- Add the following config also in the general part (matching the number of prosodies you generated config for):
```
-- targets must be IPs, not hostnames
s2s_connect_overrides = {
    ["conference.v1.meet.jitsi"] = { "127.0.0.1", 52691 };
    ["conference.v2.meet.jitsi"] = { "127.0.0.1", 52692 };
    ["conference.v3.meet.jitsi"] = { "127.0.0.1", 52693 };
    ["conference.v4.meet.jitsi"] = { "127.0.0.1", 52694 };
    ["conference.v5.meet.jitsi"] = { "127.0.0.1", 52695 };
    ["conference.v6.meet.jitsi"] = { "127.0.0.1", 52696 };
    ["conference.v7.meet.jitsi"] = { "127.0.0.1", 52697 };
    ["conference.v8.meet.jitsi"] = { "127.0.0.1", 52698 };
}
-- allowed list of server-2-server connections
s2s_whitelist = {
    "conference.v1.meet.jitsi", "conference.v2.meet.jitsi", "conference.v3.meet.jitsi", "conference.v4.meet.jitsi",
    "conference.v5.meet.jitsi", "conference.v6.meet.jitsi", "conference.v7.meet.jitsi", "conference.v8.meet.jitsi"
};
```

- Make sure s2s is not in modules_disabled
- Enable `"xxl_conference";` module under the main virtual host (e.g. [here](https://github.com/jitsi/jitsi-meet/blob/f42772ec5bcc87ff6de17423d36df9bcad6e770d/doc/debian/jitsi-meet-prosody/prosody.cfg.lua-jvb.example#L57))

After configuring you can set the maximum number of main participants, before
redirecting to visitors.
```
hocon -f /etc/jitsi/jicofo/jicofo.conf set "jicofo.visitors.max-participants" 30
```
Now restart prosody and jicofo
```
service prosody restart
service jicofo restart
```

Now after the main 30 participants join, the rest will be visitors using the
visitor nodes.