I'm trying to make a mobile application and I found ionic framework which seems cool.
It seems that REST is a common way to do things and I don't understand one thing with this protocol. How does the server notify that he has data to send. If we have a text application, how does the server notify the application (without push notification, or maybe they're mandatory ?) that another client sent you a message ?
It seems like REST is only good for the requests coming from the client. Does it means that I have to send request for update every X milliseconds to have a responsive application ? Do I have to use websockets for such purpose ? I also saw Comet, is it a good way to use rest ?
The REST architecture doesn't really address "Publication/Subscription" (pub/sub) paradigms. It's much more coarse than that.
The major issues with pub/sub are technical, especially today. In a fully connected internet where everything is online at known locations all the time, then the basic REST architecture just works. Simply, the roles of Client and Server swap back and forth (i.e. the Server becomes a Client when it needs to send an notification).
But that's not the real world.
The real world is while we have a lot of connected devices, we do not have a lot of known locations. Your phone moves around all the time, and who knows what IP address it's at at any one time. DNS doesn't help because your phone is likely not registered under any particular known name. Then there's infrastructure issues where the vast majority of clients are locked away to where they can send messages, but can not receive them, even if we did know who and where they were.
So, REST doesn't explore that area simply because it typically violates a few key attributes.
One is that the URL goes away. Since we don't know where or who you are, we can't get to you by name. So, one way folks get around it is through long lived connections. Your client hooks up to the server and retains its connection so that the server can talk back. But this is an implicitly stateful connection. REST drives towards statelessness, and is effectively connectionless at the architecture level.
So, in that environment, REST is not an appropriate architecture, since the mechanics can not support some of the fundamental precepts.
