Couldn't get GraphQL subscriptions to work

[zbhavyai]

Hello,

I am following the quickstart given at https://quarkus.io/guides/smallrye-graphql, and was trying to get the GraphQL subscriptions to work. I understand from the note that Subscription is currently still considered experimental.

I followed the quickstart guide exactly, and made some changes to the FilmResource class by adding the subscription code given in the guide. Modified FilmResource can be found here https://gist.github.com/zbhavyai/ab1b0818524270a53177743d762e419d. The modifications are done in a local copy of https://github.com/quarkusio/quarkus-quickstarts/tree/main/microprofile-graphql-quickstart.

Upon making ws connection with graphql-ui, it prints the message "Connection initialized (protocol=graphql-transport-ws), requested a subscription...", but doesn't output any added Hero details (screenshot attached below). I tried this with Quarkus versions 2.8.0.Final and 2.8.1.Final.

I came across the issue #21434, and was wondering if this concerns the problem I am facing.

So, I would really like to know if I am doing it the right way, and if subscriptions really work?

Thank you.

[quarkus-bot[bot]]

/cc @jmartisk, @phillip-kruger

[phillip-kruger]

Hi @zbhavyai - do you have a small reproducer I can look at ? Thanks

[zbhavyai]

Hi @phillip-kruger
Here is code I tried - https://github.com/zbhavyai/quarkus-subscription-code-share.

Thanks

[phillip-kruger]

Hi @zbhavyai - Thanks. Have you tried with older version of Quarkus ? (Just to test). This should work, but it does not. @jmartisk can you have a look too ?

[zbhavyai]

@jmartisk Unfortunately, the suggested alternative statement doesn't work as intended. If I use what you suggested, it will print the details only once, and the ws connection seems to end after printing; it's not really listening to future events. What I want is the ws connection to remain open, and data (which is Hero details in our case) should keep printing when new Hero is added.

I had used BroadcastProcessor as it was mentioned in the quickstart guide.

So, do you think this is a bug?

[jmartisk]

So, I dug deeper into it and now I know what the problem is!
The usage of the BroadcastProcessor itself is correct, but the problem is that your @GraphQLApi endpoint is request-scoped. That means that each time you invoke a mutation, it will execute in a new instance of the FilmResource class, and therefore with a different instance of the BroadcastProcessor than what was originally returned from the subscription method, therefore the onNext call to the processor won't be properly matched to the running subscription. You need to annotate the FilmResource with @ApplicationScoped for this to work.
Let me know if this fixes your issue.
I think I will submit a PR to add @ApplicationScoped to the FilmResource class in the quickstart to prevent this confusion from happening again in the future.

[phillip-kruger]

Good catch ! Nice @jmartisk ! Thanks

[jmartisk]

I've submitted a PR that incorporates the whole subscription-related part of the guide into the quickstart: quarkusio/quarkus-quickstarts#1105

[zbhavyai]

@jmartisk Many thanks! Now it works.