how the server send message SSE worked in multiple server instance environments

Question

I have a question on how to make SSE worked in multiple server environments.

In UI, there are two steps:

1. source = new EventSource('http://localhost:3000/stream');
   source.addEventListener('open', function(e) {
      $("#state").text("Connected")
    }, false);

2. user in UI can post to api to update data

3. after user post to api, server is sending event to UI to udpate UI

In one server environement, this worked perfect fine, no problem at all.

But in multi server instance environments, this won't be working. For example, I have two server instance, and UI subscribed to server 1, then server 1 is remembering the connection, but data update is from server 2, when data is changed, there is no connection for SSE in server 2. Then in this senario, how can server 2 send SSE to UI?

In order to make SSE working in multiple server environments, do we need to adopt any saving solution to save the connection information so that any server instance can send SSE accurately to UI?

Let me clarify this more: yes, both service 1 and service 2 are behind load balancer, they do not have to have same URL. UI is pure frontend end application, can even be mobile app. So, if UI is sending a eventSource request to LB of server1, then only this instance can use this connection to send event back to UI, right? But if we have multiple instance of server 1, that means any server 1 instance other than current one can NOT send event back to UI. I believe this is the limitation of SSE unless the connection can be shared among all the instances. But how.

Thanks

Answer 1

If you have two servers, with different URLs, make one SSE connection (from each client) to each server.

Be aware of CORS restrictions, i.e. the same origin policy. (It works identically to xhr2 CORS, so fairly easy to google; my book also covers it in detail, chapter 9.)

If you have two servers behind a load balancer, which is presenting a single URL to the clients, then you just have to make sure the load balancer is configured correctly. I.e. to always pass through that socket to the correct server. If a back-end server dies, and needs replacing, the load balancer should close the SSE socket; the client will then auto-reconnect, and get a new back-end server.

The multiple servers behind a load balancer, should either be having their own data push socket connections to a master data source, or should all be polling the master data source.