main.fsx

let now () = System.DateTime.Now.ToString("s")

type CacheCommand<'id, 'result> =
    | Get of 'id * AsyncReplyChannel<Result<'result, string>>
    | Clear


type Cache(getter) =
    let mailbox =
        MailboxProcessor.Start(fun inbox ->
            let rec loop cache =
                async {
                    let! cmd = inbox.Receive()

                    match cmd with
                    | Get (i, repl) ->
                        let (item, cache') =
                            match Map.tryFind i cache with
                            | None ->
                                match getter i with
                                | None -> (Error "нет данных", cache)
                                | Some it -> (Ok it, Map.add i it cache)
                            | Some v -> (Ok v, cache)

                        repl.Reply(item)
                        return! loop cache'
                    | Clear -> return! loop Map.empty
                }

            loop (Map [ 1, "Привет"; 2, "Как" ]))

    member this.GetItem i =
        async {
            let! answer = mailbox.PostAndAsyncReply(fun res -> Get(i, res))
            let nw = now ()
            match answer with
            | Ok v -> printfn "%s [INFO] id = %i , value = %s" nw i v
            | Error e -> printfn "%s [ERROR] %s" nw e
        }
        |> Async.RunSynchronously

    member this.Cleanup() = mailbox.Post Clear

let getFromDb =
    fun i -> if i < 10 then Some(i.ToString()) else None

let cache = Cache getFromDb

async {

    while true do
        for i = 1 to 12 do
            cache.GetItem i
            do! Async.Sleep 1000
        cache.Cleanup()
        do! Async.Sleep 1000
} |> Async.RunSynchronously