Data Push using Server Sent Events

Server-sent events (SSE) is a technology where a browser receives automatic updates from a server via HTTP connection - Wikipedia.

• Server-Sent Events (SSE) is an HTML5 technology that allows the server to push fresh data to clients (commonly called data push).
• Data push is where the server chooses to send new data to the clients.
• SSE excels when you need to update part of a web application with fresh data, without requiring any action on the part of the user.
• Anything you can do with WebSockets can be done with SSE, and vice versa, but each is better suited to certain tasks.
• The EventSource JavaScript object is used to receive server-sent event notifications.
• Each time an update is received, the onmessage event occurs.
• In Apache, each SSE connection is not just using a socket, but it is also using up a thread or process in Apache.
• In PHP, it’s starting a new PHP instance specific for the connection.
• Apache and PHP will be using a chunk of memory, that limits the number of simultaneous connections can be supported.
• Event handler also referred to as a callback. In JavaScript, objects generate events, and each object has its own set of events we might want to listen for.

Question and Answer

1. How often are server-side events going to happen?
   • The higher this is the better data push (whether SSE or WebSockets) will be.
2. How often are client-side events going to happen?
   • If such events occur less than once every five seconds, and especially if there is less than one event every second, WebSockets is going to be a better choice than SSE. If such events occur less than once every 5 to 10 seconds, this becomes a minor factor in the decision-making process.
3. Are the server-side events not just fairly infrequent but also happening at predictable times?
   • When such events are less frequent than once a minute, data pull has the advantage that it won’t be holding open a socket. Be aware of the issues with lots of clients trying to all connect at the same time.

Reference :

1. Server-sent events Web APIs MDN

2. Data Push Apps With HTML5 SSE: Pragmatic Solutions for Real-World Clients, Darren Cook, O’Reilly Media, Mar 2014. isbn:9781430264484, isbn:9781449371937, amazon:1449371930, google:7gYiAwAAQBAJ

3. Wikipedia

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