How does Discourse implement live update notifications?

Dev
1

Hi,

I have written a plugin that has a news feed with a lists of latest topics in a sidebar. It updates regularly by making an ajax call to /latest.json every few seconds. The existing latest page in the forum has the same kind of functionality but the update is instant when something is posted. Is it safe to turn down the ajax calls to a very short interval or is this implemented using something like websockets, etc.

Thanx

2

Discourse does this via its messagebus.

The messagebus the Discourse team built has its own separate repo here.

It looks like the ‘latest’ topics are subscribed to here.

So rather than making ajax calls on a timeout, just subscribe to the messagebus.

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3

If you’re curious, MessageBus is implemented using long polling (it does not support Websockets), which means the ‘live’ updating is done through requests which are held waiting on the server until something needs to update, at which point it responds with some data, and then initiates another ‘long’ request.

Sam’s exhaustive explanation here.

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4

Thanx guys!

Things cleared a lot. :slight_smile:

5

Is it necessary or even possible to stop subscribing if I switch to another page or for some reason don’t want my feed to update? That is without stoping any subscriptions in discovery.

6

Is there an answer to this? :slight_smile:

7

Yep, it’s certainly possible. Babble’s implementation:

const messageBus = Discourse.__container__.lookup('message-bus:main')
messageBus.subscribe('/babble/topics/1/posts', (data) => { performSomeAction() })

Later, when we switch chat channels and don’t want to listen for topic 1’s events anymore:

messageBus.unsubscribe('/babble/topics/1/posts')
messageBus.subscribe('/babble/topics/2/posts', (data) => { performSomeOtherAction() })
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