Server-Sent Events (SSE) is an HTML5 standard that allows servers to push data to connected clients over HTTP. SSE connections only push data from server to client, unlike WebSockets which enable two-way communication. SSE is useful for notifications, continuous data streams, and news feeds. On the frontend, JavaScript can listen for messages from the server using EventSource objects. On the backend, Rails 4 supports live streaming via ActionController::Live. Servers must support long-running requests, live streaming, concurrency, and automatic reconnection for SSE to work properly. Sample code demonstrates setting up SSE connections and handling messages.

1. Rails 4 & Server-Sent Events
Piotr Karbownik

2. SSE - EventSource API
Server-Sent Events (SSE) is a HTML5 standard
describing how servers can initiate data
transmission towards clients once an initial
client connection has been established.
Browsers: Firefox, Chrome, Opera, Safari

3. SSE != Websockets
• SSE connections can only push data to the
browser.
• It’s still HTTP - no new protocol
• Automatic reconnection

4. SSE - applications
• Notifications
• Continuous data streams
• News feeds

5. SSE – on the frontend
function newStream(){
var source = new EventSource('/streaming');
source.addEventListener('update', function(e) {
//do stuff
});
}

6. SSE – on the frontend
function newStream(){
var source = new EventSource('/streaming');
source.onmessage (function(e) {
//do stuff
});
}

7. EventSource Object Methods
onopen -When a connection to the server is
opened
onmessage - When a message is received
onerror - When an error occurs

8. SSE – message format
Sample:
event: my_event
data: {"username": „Peter"}
event - The event's type. If this is specified, an event will be
dispatched on the browser to the listener for the specified event
name.
data - field for the message.
id - the event ID to set the EventSource object's last event ID value
retry - The reconnection time to use when attempting to send the
event.

9. Rails 4 - ActionController::Live
Live Streaming: the ability to send partial
responses to the client immediately.

10. What about the server?
Server must support:
• Long running requests
• Live streaming
• Concurrency
• Examples: Puma, Rainbows, Thin

11. Config
environments /development.rb:
• config.cache_classes = true
• config.eager_load = true
database.yml
• pool == server max threads
• for Puma default is 16

12. Sample code - 1

13. Sample code

14. Sample code - 2

16. Summary
• Easy to implement
• Real time
• Good for notifications
• Problems with client connections amount and
status.

17. …and the end
Sample apps:
https://github.com/cynamonium/srug-
streamapp-2/
https://github.com/cynamonium/srug-
streamapp-1