onModuleInit called twice for singleton / DEFAULT scope provider

[fresh moon]

Hi everyone,

I have created a module that exports a provider registered with the DEFAULT scope. In my app module I import this module, and also inject it into the ThrottleModule using the forRootAsync method. When I start up my app i see that the provider itself is created only once ( the constructor is hit only once ), yet the onModuleInit method and the onModuleDestroy method of the provider are invoked twice!

This is very confusing - I would assume that I could use the lifecycle methods to perform resource management, yet this doesn't seem to be the case. Please could someone shed some light on the issue? I have a minimal repo here: https://github.com/magoogli/nest-js

Any help would be most appreciated.

[trim whale]

Singleton (instance management) and lifecycle are separate mechanisms.
The lifecycle methods of a service created as a singleton are invoked in each module when the module is initialized.

https://docs.nestjs.com/fundamentals/lifecycle-events

[fresh moon]

@trim whale What is best practice around managing a singleton instance then? If I have a singleton instance that say creates a connection or some resource, I need to clean that up. I Assumed ( apparently incorrectly ) that I could create the resource in the onModuleInit method and release it in the onModuleDestroy. If I cannot rely on these and I now create the resource in the constructor ( which NestJS invokes ) - how can I release the resource?

[trim whale]

Instances injected into NestJS are managed by the framework.
External resources such as connections can be handled in the way you’re thinking.

https://docs.nestjs.com/fundamentals/lifecycle-events#application-shutdown
e.g.

• https://github.com/nestjs/typeorm/blob/0c4ac547195cb0b966900ddd81d366d0934badc5/lib/typeorm-core.module.ts#L141-L152
• https://github.com/nestjs/schedule/blob/67b0ac9740e71c6b46cfc2cdce0fd07fca2572d9/lib/scheduler.orchestrator.ts#L38-L42