”We offer one of the very few email apps available without Google’s push notification service…
We’ve made a research on how others (Signal, Wire, Conversations, Riot, Facebook, Mastodon) have been solving similar problems. We had several options in mind, including WebSockets, MQTT, Server Sent Events and HTTP/2 Server Push…
We settled on the SSE (Server Sent Events) because it seemed like a simple solution…
SSE is a web API which allows a server to send events to the connected clients. It is a relatively old API which is, in my opinion, underused. I’ve never heard about SSE before looking at the federated network Mastodon: They use SSE for real-time timeline updates, and it is working great.
The protocol itself is very simple and resembles good old polling: The client opens a connection, and the server keeps it open. The difference from classical polling is that we keep this connection open for multiple events. The server can send events and data messages; they are just separated by new lines. So the only thing the client needs to do is to open a connection with big timeout and read the stream in a loop.
SSE fits our needs better than WebSocket would (it is cheaper and converges faster, because it’s not duplex). We’ve seen multiple chat apps trying to use WebSocket for push notifications and it didn’t seem power efficient.”