Auto-configuration and ownership boundaries

Contract scope: 2.0.0 current contract on develop. The stable rollback reference remains 2.0.0.

What the auto-configuration does

The module’s AutoConfiguration.imports registers only ReactiveMongoAutoConfiguration. It runs after Spring Boot’s DataMongoReactiveAutoConfiguration, requires ReactiveMongoOperations on the classpath, and backs off when an operations bean already exists.

@AutoConfiguration(
    afterName = [
        "org.springframework.boot.data.mongodb.autoconfigure.DataMongoReactiveAutoConfiguration",
    ],
)
@ConditionalOnClass(ReactiveMongoOperations::class)
@ConditionalOnMissingBean(ReactiveMongoOperations::class)
class ReactiveMongoAutoConfiguration : EnvironmentAware {
    override fun setEnvironment(environment: Environment) {
        if (environment.containsProperty("spring.data.mongodb.uri") &&
            !environment.containsProperty("spring.mongodb.uri")
        ) {
            throw IllegalStateException(
                "Unsupported legacy MongoDB property 'spring.data.mongodb.uri'; " +
                    "use 'spring.mongodb.uri' on Spring Boot 4.1+",
            )
        }
    }

    @Bean
    fun reactiveMongoTemplate(
        databaseFactory: ReactiveMongoDatabaseFactory,
        mongoConverter: MongoConverter,
    ): ReactiveMongoTemplate =
        ReactiveMongoTemplate(databaseFactory, mongoConverter)
}

It does not create ReactiveMongoDatabaseFactory or MongoConverter. The fallback can be completed only when both beans are already available.

Property migration and precedence

Spring Boot 4.1 binds MongoDB connection settings under spring.mongodb.*. Use the current URI key:

spring:
  mongodb:
    uri: mongodb://127.0.0.1:27018/synthetic

Properties present	Result when the library fallback participates
spring.mongodb.uri only	Use the current Spring Boot 4.1 namespace
spring.data.mongodb.uri only	Fail at startup instead of silently using localhost
Both keys	spring.mongodb.uri takes precedence

The legacy-only failure is:

IllegalStateException: Unsupported legacy MongoDB property 'spring.data.mongodb.uri'; use 'spring.mongodb.uri' on Spring Boot 4.1+

Use synthetic URIs in tests and keep credentials out of logs and diagnostic artifacts.

Back-off and ownership

Spring Boot’s reactive MongoDB auto-configuration normally supplies ReactiveMongoOperations. The class-level @ConditionalOnMissingBean then backs off the entire bluetape4k configuration, including the legacy-property guard. The same rule applies when the application provides its own operations bean. Adding this artifact therefore does not create a second client or pool. Use the condition evaluation report to inspect the actual bean graph.

Responsibility map

Layer	Owned responsibilities
bluetape4k module	Coroutine extensions, query DSL, template fallback
Spring Boot and Spring Data	Property binding, factory, mapping converter, template, lifecycle integration
Application	Custom conversions, auditing, index policy, transactions, domain repositories
MongoDB driver	Connection pool, server selection, protocol, sessions, concerns, timeouts

The presence of MongoDB Kotlin driver dependencies in the build does not move driver configuration into bluetape4k auto-configuration.

Custom conversion and auditing

The current module does not create a MongoCustomConversions bean or enable MongoDB auditing. Applications that need either feature should use a separate configuration.

@Configuration(proxyBeanMethods = false)
@EnableMongoAuditing
class MongoDomainConfiguration {
    @Bean
    fun mongoCustomConversions(): MongoCustomConversions =
        MongoCustomConversions.create { adapter ->
            adapter.registerConverter(MoneyWriteConverter())
        }
}

A converter changes the stored wire format. Verify that old and new application versions can read the same documents before a rolling deployment.

Source and tests

• ReactiveMongoAutoConfiguration.kt
• AutoConfiguration.imports
• ReactiveMongoAutoConfigurationTest.kt on develop
• Module build

Next chapter

Use the configured template in Coroutine reads and cardinality.