Request throttling agent

open Microsoft.Extensions.Options
open Microsoft.Extensions.Caching.Distributed
open Microsoft.Extensions.Caching.Memory
open MBrace.FsPickler

type IDistributedCache with
    member self.SetValue(serializer, key, value, options) =
        self.Set(key, serializer value, options)
    member self.GetValue<'T>(deserializer: byte[] -> 'T, key) =
        match self.Get(key) with
        | null -> None
        | obj -> Some (deserializer obj)
    member self.SetValueAsync(serializer, key, value, options, token) =
        self.SetAsync(key, serializer value, options, token)
    member self.GetValueAsync<'T>(deserializer: byte[] -> 'T, key, token) = task {
        match! self.GetAsync(key, token) with
        | null -> return None
        | obj -> return Some (deserializer(obj))
    }
    member self.SetAsyncValue(serializer, key, value, options) = async {
        let! token = Async.CancellationToken
        return! Async.AwaitTask (self.SetValueAsync(serializer, key, value, options, token))
    }
    member self.GetAsyncValue<'T>(deserializer, key) = async {
        let! token = Async.CancellationToken
        return! Async.AwaitTask (self.GetValueAsync<'T>(deserializer, key, token))
    }

type Throttler (cache: IDistributedCache, cacheDuration, maxRequests) =
    let serializer = FsPickler.CreateBinarySerializer()
    let agent = MailboxProcessor.Start(fun agent ->
        let rec loop () = async {
            let! (cacheKey, reply: AsyncReplyChannel<bool>) = agent.Receive()
            let! value = cache.GetAsyncValue(serializer.UnPickle, cacheKey)
            let count, start, expiration =
                match value with
                | Some (start: System.DateTimeOffset, count) ->
                    let count' = count + 1
                    let expiration = start.Add(cacheDuration)
                    count', start, expiration
                | None ->
                    let now = System.DateTimeOffset.UtcNow
                    let expiration = now.Add(cacheDuration)
                    1, now, expiration
            do! cache.SetAsyncValue(
                serializer.Pickle,
                cacheKey,
                (start, count),
                DistributedCacheEntryOptions(
                    AbsoluteExpiration=expiration
                )
            )
            reply.Reply(not (count > maxRequests))
            return! loop ()
        }
        loop ()
    )
    member _.Get(cacheKey) =
        agent.PostAndReply(fun channel -> (cacheKey, channel))
    member _.GetAsync(cacheKey) =
        agent.PostAndAsyncReply(fun channel -> (cacheKey, channel))

(* Example *)

// The cache is normally provided by DI in ASP.NET Core
let options = Options.Create(MemoryDistributedCacheOptions())
let cache = new MemoryDistributedCache(options)

let cacheDuration = System.TimeSpan.FromSeconds(1)
let maxRequests = 4

let throttler = Throttler(cache, cacheDuration, maxRequests)

// The cache key could be the IP address or the username or anything you want to provide as the limiting factor for the request.
for _ in 0..10 do
    throttler.Get("hello")
    |> printfn "%A"
    System.Threading.Thread.Sleep 120

// Result:
//true
//true
//true
//true
//false
//false
//false
//false
//true
//true
//true