Manage mutable state using actors with F# Mailbox processor

Manage mutable state using actors with F# Mailbox processor

Today I would like to share a technique that I use to maintain mutable shared state in my F# applications. This technique uses the MailboxProcesser, a simplified implementation of the actor model built into F#.

Here’s the issue:

I have a dataframe shared accross my whole application. The dataframe is constructed at bootup of the server which makes the application not scalable as to reconstruct the dataframe, a reboot is needed. The solution would be to provide a way to mutate the dataframe any time I need to. But mutable state involves concurrency issues and that’s where the MailboxProcesser comes to the rescue.

This is what the MailboxProcesser gives a thread safe way to perform operations on a shared object.

This post is composed by two parts:

  1. Introduction of the MailboxProcessor
  2. Usage in my scenario

1. Introduction of the MailboxProcessor

MailboxProcessor can be seen a mailbox. It contains an inbox available to the recipient. We post letters (I call those messages) to the mailbox and the recipient receives it via her inbox. Each messages are placed in a queue and dequeued at the rhythm of the recepient. Therefore the whole interactions with the recepient is completely concurrent safe as each message is guaranteed to be delivered one by one in the order by which they were received.

Althought the mailboxprocessor is a simplified version as it does not cross applications boundaries and does not survives reboot.

This is how the overall structure of the mailbox processor looks like:

let actor =
    MailboxProcessor.Start(fun inbox ->
        let rec processMessage state =
            async {
                let! msg = inbox.Receive()
                let newState = ``do something based on message`` msg
                return! processMessage newState
    loop initialState)

... somewhere else ...

actor.Post MessageA
actor.PostAndReply MessageB

The four important components are:

  • the inbox
  • the processMessage function
  • the do something based on message function
  • the state

As we described earlier, we can see the MailboxProcessor as a mailbox via which can post messages to. F# makes it very easy as a message can be represented as a discriminated union - we will see more later. The Start function of the MailboxProcessor takes a function as argument which represents the recipient execution:

  • receive a message - done using the inbox with the Receive function
  • do something with that message and become new state - done inside the function which handle the message and alter the state of the actor
  • ready to receive new message using new state - done by recursively looping back on processMessage with the new state

The actor returned exposes method to post messages to the mailbox, Post, PostAndReply and their async equivalent. The difference is that Post returns unit whereas PostAndReply gives a return channel that you can pass with the message itself to give the ability to the recepient to respond to the message.

Notice that the whole queuing process is completely abstracted from us. By using MailboxProcessor, we get thread safety on a function execution very easily.

2. Usage in my scenario

I have a dataframe shared accross my whole application. The dataframe used to be immutable therefore only built one time on server bootup but I now need to update it on the fly.

I defined the actor by stages:

  1. the messages
  2. the states
  3. the process
  4. the api

2.1 The messages

I need to Refresh the dataframe and Get it.

type ExpenseMessage =
    | Get of replyChannel: AsyncReplyChannel<ExpenseDataFrame>
    | Refresh 

Get takes a replyChannel argument, this will be provided by the mailbox processor. Refresh will be used to instruct the system to rebuild the dataframe.

2.2 The states

The benefit of an actor is that it is stateful. Since concurrency is abstracted away from the main function, it is easy to understand the flow and react properly to messages.

My actor will have two states, Ready and NotReady.

DataFrameState =
  | Ready of ExpenseDataFrame
  | NotReady

Depending on which states it is in, it will behave differently when it receives a message.

2.3 The process

The process contains three paths:

  • path 1: Get is received and the actor state is Ready, the expenses are returned through the replyChannel and the actor remains Ready and wait for the next message
  • path 2: Get is received and the actor is NotReady, it builds the frame and returns the result through the replyChannel and becomes Ready and wait for the next message
  • path 3: Refresh is received, not matter which state the actor is in, the frame is rebuilt and the actor becomes Ready with the new frame and wait for the next message
let buildFrame() =
    |> ExpenseDataFrame.FromFiles

let agent =
    let mailbox =
        MailboxProcessor.Start(fun inbox ->
            let rec loop state =
                async {
                    let! msg = inbox.Receive()

                    match msg with
                    | Get replyChannel ->

                        match msg with
                        | Ready expenses  ->
                            // path 1
                            replyChannel.Reply expenses
                            return! loop state

                        | NotReady ->
                            // path 2
                            let expenses = buildFrame() 
                            replyChannel.Reply expenses
                            return! Ready expenses

                    | Refresh ->
                        // path 3
                        let expenses = buildFrame()
                        return! Ready expenses
            loop State.Default)

2.4 The api

We could just use directly the MailboxProcessor and Post messages to it but it is best to not expose our infrastructure - the MailboxProcessor. Someone sending messages doesn’t need to know that a AsyncReplyChannel<_> is involved in a Get. To cater for that, we construct an Api which provides more abstract functions to interact with our agent.

type Api = {
    Get:     unit -> ExpenseDataFrame
    Refresh: unit -> unit

And here would be the instantiation of the Api:

let actor =
    let mailbox =
        ... mailbox code ...

    { Get     = fun () -> mailbox.PostAndReply Get
      Refresh = fun () -> mailbox.Post Refresh }

And we are done! We provided a completely thread safe solution for our issue.


The technique exposed in this tutorial is generic enough to be used in a lot of scenarios. A configuration global state is a very common one. MailboxProcessor being built into F#, it is one of the best way to handle shared mutable state in an F# application. Concurrency issues are always tricky so I rather leave it to the system to optimise for it as much as I can. I hope this tutorial was useful, let me know if you liked it. As always if you have any question leave it here or hit me on Twitter @Kimserey_Lam. See you next time!



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