Simple SSE subscription implementation

[tdraier]

Hi,

I don't know if it can be useful since you're working on subscription rework, but I needed a simple SSE implementation of subscriptions on 4.7.0 and implemented this. I think some part can still be integrated :

• SSE support itself, handled in doQuery when ExecutionResult is a subscription result
• Default subscription strategy was not set
• readOnlySchema must include subscriptions ( I'm using SSE with GET requests )
• OSGi servlet register subscriptions with new "SubscriptionsProvider"
• OSGi servlet support async

Also needed to upgrade graphql-java because of graphql-java/graphql-java#844

Feel free to take anything from here or drop it ..

Regards

[oliemansm]

I'm not familiar with SSE myself, but after quickly scanning this article https://www.html5rocks.com/en/tutorials/eventsource/basics/ I understand it's an alternative to WebSockets with some differences in approach that could be relevant depending on the use case.

The current approach is based on WebSockets so integrating this would add the alternative of using SSE instead. Sounds like a pretty good addition to me.

@apottere What do you think?

[apottere]

Yeah, it looks like a good thing to include. The PR will need to be updated to current master first, though.

[wleroux]

@apottere I am looking into server-sent events for graphql-java and I stumbled on this issue. It appears that the current version does not support server-sent events.

Would you like me to take a stab at updating this PR to master?

[wleroux]

I've taken a stab at porting the code here: #145.

[oliemansm]

Ported by @wleroux in #145