What is the difference between Async Response and Server-Sent Events in Jersey? - rest

Question

What is the difference between Async Response and Server-Sent Events in Jersey and when to use them?

Answer 1

Both are for different usage, one allows to wait for a slow resource (long-polling), the other allows to send a stream of data on the same TCP-connection.
Here's more detail :
AsyncResponse was introduced in JAX-RS 2, in order to perform long-polling requests.
Client open connection
Client send request payload
Server receive payload, pause/suspend the connection and look for the resources
Then
If timeout has been reached the server can end the connection
Resource is ready, server resume the connection and send the resource payload.
Connection is closed
As this is part of the JAX-RS specification, so you can just use it with the default jersey dependencies. Note that on a too long connection where no data is transmitted network equipment like firewall can close the TCP connection.
Server-Sent Events is a specification that allows the server to send messages on the same TCP connection.
Client use javascript EventSource to get a resource
Then the server can send at some point in time a payload, a message.
Then another
And so on
The connection can be closed programmatically at any time by either the client or the server.
SSE is not part of JAX-RS, so you need to have the Jersey SSE module in your classpath (additionaly in earlier version of Jersey 2 you had to programmatically enable the SseFeature).
Other things to consider :
SSE does not allow to pass custom headers, so no Authorisation header. It's possible to use the URL query string, but if you're not on HTTPS this a security issue.
SSE does allow to POST data, so this might go in the URL query string
Connection can close due to network (equipment failing, firewall, phone not in covered area, etc.)
In my opinion websockets are more flexible than SSE, and they even allow the client to send multiple messages. But Jersey does not implement the JEE specification that support websocket (JSR 356).
But really you should read the documentation of their SSE implementation, their's additional info like what is polling and what web-sockets.

Answer 2

AsyncResponse is like an ajax polling with long waiting time. The client initiate a single AJAX request to check for updates that will not return until it receives data or a timeout occurs and trigger another request. It does create unnecessary checking loop (at the server side) and the load is equivalent to the number of client connected. More client, more loop initiated = more resources needed.
Server-Sent Events is somewhat similar to long-polling at the server side, both uses loop to check for update and trigger a response. The only difference is that long-polling will continuous send request (either after timeout or receive data) whereas SSE only need to initiate once. Thus SSE is more suitable for mobile application when you consider battery usage.
Websocket uses loop as well, but not only to check for updates; also to listen for new connections and upgrade the connections to WS/WSS after handshake. Unlike long-polling and SSE; where the load increases with the number of clients, websocket constantly running the loop like a daemon. In addition to the constant loop, the load adds on as more client are connected to the socket.
For example, if you are designing a web service for administrative purposes, server running on long-polling and SSE are allow to rest after office hour when no one is around, whereas websocket will continue to run, waiting for connection. And did I mention without proper authentication, anyone can create a client and connect to your websocket? Most of the time, authentication and refuse connection is not done at the handshaking part, but after the connection was made.
And should I continue on how to implement websocket on multiple tab?