Flow return type in Subscription causes an exception when booting up Spring Server

[serandel]

I'm trying to use the support for Flow in subscriptions, to avoid having to convert them to publisher as a last step in my functions. (Which seems to work perfectly fine, btw.)

If I write my subscription like

class ProductSubscription(private val productRepository: ProductRepository) : Subscription {
    fun getAllLive(code: String? = null): Flow<List<Product>> = // whatever...
}

then I get this when executing bootRun:

[com/expediagroup/graphql/server/spring/NonFederatedSchemaAutoConfiguration.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [graphql.schema.GraphQLSchema]: Factory method 'schema' threw exception; nested exception is com.expediagroup.graphql.generator.exceptions.InvalidSubscriptionTypeException: Schema requires all subscriptions to be public and return a valid type from the hooks. ProductSubscription has PUBLIC visibility modifier. **The function return type is Flow**

After reading the documentation, I reckon that the schema generation is using NoopSchemaGeneratorHooks. So I'm trying to enable FlowSubscriptionSchemaGeneratorHooks by creating this class:

package com.example

class ProductSchemaHooksProvider : SchemaGeneratorHooksProvider {
    override fun hooks(): SchemaGeneratorHooks = FlowSubscriptionSchemaGeneratorHooks()
}

and creating a src/main/resources/META-INF/services/com.expediagroup.graphql.plugin.schema.hooks.SchemaGeneratorHooksProvider file with this content:

com.example.ProductSchemaHooksProvider

Nevertheless, nothing seems to change, and I still get the same error. Am I even going the right way about enabling this? The documentation just says that:

graphql-kotlin-spring-server provides automatic support for Kotlin Flow through FlowSubscriptionExecutionStrategy that supports existing Publishers and relies on Kotlin reactive-streams interop to convert Flow to a Publisher.

Any help would be very appreciated! :)

GraphQL-Kotlin: 6.0.0-alpha5
Spring Boot: 2.7.1
Java: 17

[serandel]

I got it working by providing this bean in my app:

class ProductSchemaGeneratorConfig(@Value("\${graphql.packages[0]}") supportedPackages: String) : SchemaGeneratorConfig(
    supportedPackages = listOf(supportedPackages),
    hooks=FlowSubscriptionSchemaGeneratorHooks())

Should this perhaps be the default?

[serandel]

The graphqlGenerateSDL Gradle task doesn't work though :(

[samuelAndalon]

Hello we will take a look at this, in the meantime have you looked at
https://opensource.expediagroup.com/graphql-kotlin/docs/schema-generator/execution/subscriptions ?

TLTR;

you just have to have the return type wrapped in an implementation of a reactive-streams Publisher

so for now -- if you data fetcher / resolver returns a flow you can just simply interop to Flux

[serandel]

Yes, with Reactive Streams it works without any problem. I would be very keen on having Flow as the return type of my bean functions and be able to test them with Turbine, though.

Thanks for your help!