F# Singleton and disposing internal resources - f#

Question

In my AWS lambda function, I need to access secret values from the AWS Secret manager. I am using the AWSSDK.SecretsManager.Caching package to achieve this. To get the most out of the provided caching mechanism, I have decided to create a simple singleton wrapper around the SecretsManagerCache:
namespace MyProject
open Amazon.SecretsManager.Extensions.Caching
type SecretsProvider private () =
let secretsCache = new SecretsManagerCache()
static let instance =
SecretsProvider()
static member Instance = instance
member this.GetSecretString key =
secretsCache.GetSecretString key
This seems to be fine, but there is one thing that concerns me. The SecretsManagerCache implements IDisposable. I have tried writing
use secretsCache = new SecretsManagerCache()
but that gives me a compiler error saying 'use' bindings are not permitted in primary constructors. Is it OK to simply use the let keyword? Or should I implement a finalizer?
override this.Finalize() =
secretsCache.Dispose()
Or is there another way how to safely dispose an internal resource from a singleton?

Answer 1

You can wrap your SecretsManager in a SecretsProvider like you have done but implementing IDisposable to dispose of the SecretsManager object.
type ISecretsProvider =
abstract member GetSecretString : key:string -> string
type SecretsProvider () =
let secretsCache = new SecretsManagerCache()
interface IDisposable with
member x.Dispose() = ()
secretsCache.Dispose()
interface ISecretsProvider with
member this.GetSecretString key =
secretsCache.GetSecretString key
You can then declare your dependencies in your functions e.g:
f : ISecretsProvider->int->'T
g : 'T->unit
h : ISecretsProvider->unit
For a console application you can provide the only instance of this dependency in your entry point function:
[<EntryPoint>]
let main _ =
use sp = new SecretsProvider()
let a = f sp 2
g a
h sp
If you're using ASP.NET core you can do so with configureServices:
let configureServices (services : IServiceCollection) =
services.AddSingleton<ISecretsProvider, SecretsProvider>() |> ignore
Update: As Discussed with #Panagiotis Kanavos the SecretsProvider may not be necessary. You may use SecretsManager instead of ISecretsProvider an SecretsProvider in the above code. However, to achieve loose coupling interfaces are good practice, this will depend more on the complexity and goals of your overall solution.