PresenceChannel configuration is cached using redis. That cache is used, and sometimes repopulated, during normal GET requests. When the primary redis server was readonly, that `redis.set` call would raise an error and cause the entire request to fail. Instead, we should ignore the failure and continue without populating the cache.
These calls were originally introduced to ensure that any stale users were cleaned up regularly. This is quite an expensive process to run on every `GET /presence/get` call, and will also cause errors during readonly mode.
Since the original introduction of this logic, we added the `Jobs::PresenceChannelAutoLeave` which runs every minute. That should be enough to clean up any stale users.
Note that users which explicitly `leave` a channel are still removed immediately. This auto_leave logic just takes care of clients which have disappeared without leaving.
This removes all custom controllers and redis/messagebus logic from discourse-presence, and replaces it with core's new PresenceChannel system.
All functionality should be retained. This implementation should scale much better to large numbers of users, reduce the number of HTTP requests made by clients, and reduce the volume of messages on the MessageBus.
For more information on PresenceChannel, see 31db8352
- Allow the `/presence/get` endpoint to return multiple channels in a single request (limited to 50)
- When multiple presence channels are initialized in a single Ember runloop, batch them into a single GET request
- Introduce the `presence-pretender` to allow easy testing of PresenceChannel-related features
- Introduce a `use_cache` boolean (default true) on the the server-side PresenceChannel initializer. Useful during testing.
PresenceChannel aims to be a generic system for allow the server, and end-users, to track the number and identity of users performing a specific task on the site. For example, it might be used to track who is currently 'replying' to a specific topic, editing a specific wiki post, etc.
A few key pieces of information about the system:
- PresenceChannels are identified by a name of the format `/prefix/blah`, where `prefix` has been configured by some core/plugin implementation, and `blah` can be any string the implementation wants to use.
- Presence is a boolean thing - each user is either present, or not present. If a user has multiple clients 'present' in a channel, they will be deduplicated so that the user is only counted once
- Developers can configure the existence and configuration of channels 'just in time' using a callback. The result of this is cached for 2 minutes.
- Configuration of a channel can specify permissions in a similar way to MessageBus (public boolean, a list of allowed_user_ids, and a list of allowed_group_ids). A channel can also be placed in 'count_only' mode, where the identity of present users is not revealed to end-users.
- The backend implementation uses redis lua scripts, and is designed to scale well. In the future, hard limits may be introduced on the maximum number of users that can be present in a channel.
- Clients can enter/leave at will. If a client has not marked itself 'present' in the last 60 seconds, they will automatically 'leave' the channel. The JS implementation takes care of this regular check-in.
- On the client-side, PresenceChannel instances can be fetched from the `presence` ember service. Each PresenceChannel can be used entered/left/subscribed/unsubscribed, and the service will automatically deduplicate information before interacting with the server.
- When a client joins a PresenceChannel, the JS implementation will automatically make a GET request for the current channel state. To avoid this, the channel state can be serialized into one of your existing endpoints, and then passed to the `subscribe` method on the channel.
- The PresenceChannel JS object is an ember object. The `users` and `count` property can be used directly in ember templates, and in computed properties.
- It is important to make sure that you `unsubscribe()` and `leave()` any PresenceChannel objects after use
An example implementation may look something like this. On the server:
```ruby
register_presence_channel_prefix("site") do |channel|
next nil unless channel == "/site/online"
PresenceChannel::Config.new(public: true)
end
```
And on the client, a component could be implemented like this:
```javascript
import Component from "@ember/component";
import { inject as service } from "@ember/service";
export default Component.extend({
presence: service(),
init() {
this._super(...arguments);
this.set("presenceChannel", this.presence.getChannel("/site/online"));
},
didInsertElement() {
this.presenceChannel.enter();
this.presenceChannel.subscribe();
},
willDestroyElement() {
this.presenceChannel.leave();
this.presenceChannel.unsubscribe();
},
});
```
With this template:
```handlebars
Online: {{presenceChannel.count}}
<ul>
{{#each presenceChannel.users as |user|}}
<li>{{avatar user imageSize="tiny"}} {{user.username}}</li>
{{/each}}
</ul>
```
