A Fairy Tale of F# and Durable Functions

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The post is a part of F# Advent Calendar 2018. It’s Christmas time!

This summer I was hired by the office of Santa Claus. Santa is not just a fairy tale character on his own—he leads a large organization that supplies gifts and happiness to millions of children around the globe. Like any large organization, Santa’s office employs an impressive number of IT systems.

As part of its IT modernization effort, North Pole HQ restructured the whole supply chain of Christmas gifts. Many legacy components were moved from a self-managed data center at the North Pole—although the cooling is quite cheap there—to Azure cloud. Azure was an easy sell since Santa’s techy elves use Office 365, SharePoint and the .NET development stack.

One of the goals of the redesign was to leverage managed cloud services and serverless architecture wherever possible. Santa has no spare elves to keep reinventing IT wheels.

Wish Fulfillment Service

My assignment was to redesign the Wish Fulfillment service. The service receives wish lists from clients (they call children “clients”):

Christmas Card with a Wish List (c) my son Tim

Christmas Card with a Wish List (c) my son Tim

Luckily, the list is already parsed by some other service, and also contains the metadata about the kid’s background (age, gender, and so on) and preferences.

For each item in the list, our service calls the Matching service, which uses machine learning, Azure Cognitive services, and a bit of magic to determine the actual products (they call gifts “products”) that best fit the client’s expressed desire and profile. For instance, my son’s wish for “LEGO Draak” matches to “LEGO NINJAGO Masters of Spinjitzu Firstbourne Red Dragon”. You get the point.

There might be several matches for each desired item, and each result has an estimate of how likely it is to fulfill the original request and make the child happy.

All the matching products are combined and sent over to the Gift Picking service. Gift Picking selects one of the options based on its price, demand, confidence level, and the Naughty-or-Nice score of the client.

The last step of the workflow is to Reserve the selected gift in the warehouse and shipping system called “Santa’s Archive of Products”, also referred to as SAP.

Here is the whole flow in one picture:

Gift Fulfillment Workflow

Gift Fulfillment Workflow

How should we implement this service?

Original Design

The Wish Fulfillment service should run in the cloud and integrate with other services. It should be able to process millions of requests in December and stay very cheap to run during the rest of the year. We decided to leverage serverless architecture with Azure Functions on the Consumption Plan. Serverless Functions are:

  • Fully Managed: the cloud provider provisions resources, scales them based on the load, takes care of uptime and reliability;

  • Event-Driven: for each serverless Function you have to define a specific trigger—the event type which causes it to run, be it an HTTP endpoint or a queue message;

  • Changed per Execution: it costs nothing to run the application if there is no usage, and the cost of busy applications is proportional to the actual resource utilization.

Here is the diagram of the original design:

Workflow Design with Azure Functions and Storage Queues

Workflow Design with Azure Functions and Storage Queues

We used Azure Storage Queues to keep the whole flow asynchronous and more resilient to failures and load fluctuation.

This design would mostly work, but we found a couple of problems with it:

  • The Functions were manually wired via storage queues and corresponding bindings. The workflow was spread over infrastructure definition and thus was hard to grasp.

  • We had to pass all items of each wish list into a single invocation of Matching Function, otherwise combining the matching results from multiple queue messages would be tricky.

  • Although not in scope for the initial release, there were plans to add manual elf intervention for poorly matched items. This feature would require a change in the flow design: it’s not trivial to fit long-running processes into the pipeline.

To improve on these points, we decided to try Durable Functions—a library that brings workflow orchestration to Azure Functions. It introduces several tools to define stateful, potentially long-running operations, and handles a lot of the mechanics of reliable communication and state management behind the scenes.

If you want to know more about what Durable Functions are and why they might be a good idea, I invite you to read my article Making Sense of Azure Durable Functions (20 minutes read).

For the rest of this post, I will walk you through the implementation of the Wish Fulfillment workflow with Azure Durable Functions.

Domain Model

A good design starts with a decent domain model. Luckily, the project was built with F#—the language with the richest domain modeling capabilities in the .NET ecosystem.


Our service is invoked with a wish list as the input parameter, so let’s start with the type WishList:

type WishList =
    Kid: Customer
    Wishes: string list

It contains information about the author of the list and recognized “order” items. Customer is a custom type; for now, it’s not important what’s in it.

For each wish we want to produce a list of possible matches:

type Match =
    Product: Product
    Confidence: Probability

The product is a specific gift option from Santa’s catalog, and the confidence is a number from 0.0 to 1.0 of how strong the match is.

The end goal of our service is to produce a Reservation:

type Reservation =
    Kid: Customer
    Product: Product

It represents the exact product selection for the specific kid.


The Wish Fulfillment service needs to perform three actions, which can be modeled with three strongly-typed asynchronous functions.

Note: I use lowercase “function” for F# functions and capitalize “Function” for Azure Functions throughout the article to minimize confusion.

The first action finds matches for each wish:

// string -> Async<Match list>
let findMatchingGift (wish: string) = async {
    // Call a custom machine learning model
    // The real implementation uses the Customer profile to adjust decisions by age, etc.
    // but we'll keep the model simple for now.

The first line of all my function snippets shows the function type. In this case, it’s a mapping from the text of the child’s wish (string) to a list of matches (Match list).

The second action takes the combined list of all matches of all wishes and picks one. Its real implementation is Santa’s secret sauce, but my model just picks the one with the highest confidence level:

// Match list -> Product
let pickGift (candidates: Match list) =
    |> List.sortByDescending (fun x -> x.Confidence)
    |> List.head
    |> (fun x -> x.Product)

Given the picked gift, the reservation is merely { Kid = wishlist.Kid; Product = gift }, not worthy of a separate action.

The third action registers a reservation in the SAP system:

// Reservation -> Async<unit>
let reserve (reservation: Reservation) = async {
    // Call Santa's Archive of Products


The fulfillment service combines the three actions into one workflow:

// WishList -> Async<Reservation>
let workflow (wishlist: WishList) = async {

    // 1. Find matches for each wish
    let! matches =
        |> List.map findMatchingGift
        |> Async.Parallel

    // 2. Pick one product from the combined list of matches
    let gift = pickGift (List.concat matches)

    // 3. Register and return the reservation
    let reservation = { Kid = wishlist.Kid; Product = gift }
    do! reserve reservation
    return reservation

The workflow implementation is a nice and concise summary of the actual domain flow.

Note that the Matching service is called multiple times in parallel, and then the results are easily combined by virtue of the Async.Parallel F# function.

So how do we translate the domain model to the actual implementation on top of serverless Durable Functions?

Classic Durable Functions API

C# was the first target language for Durable Functions; Javascript is now fully supported too.

F# wasn’t initially declared as officially supported, but since F# runs on top of the same .NET runtime as C#, it has always worked. I have a blog post about Azure Durable Functions in F# and have added F# samples to the official repository.

Here are two examples from that old F# code of mine (they have nothing to do with our gift fulfillment domain):

// 1. Simple sequencing of activities
let Run([<OrchestrationTrigger>] context: DurableOrchestrationContext) = task {
  let! hello1 = context.CallActivityAsync<string>("E1_SayHello", "Tokyo")
  let! hello2 = context.CallActivityAsync<string>("E1_SayHello", "Seattle")
  let! hello3 = context.CallActivityAsync<string>("E1_SayHello", "London")
  return [hello1; hello2; hello3]

// 2. Parallel calls snippet
let tasks = Array.map (fun f -> context.CallActivityAsync<int64>("E2_CopyFileToBlob", f)) files
let! results = Task.WhenAll tasks

This code works and does its job, but doesn’t look like idiomatic F# code:

  • No strong typing: Activity Functions are called by name and with types manually specified
  • Functions are not curried, so partial application is hard
  • The need to pass the context object around for any Durable operation

Although not shown here, the other samples read input parameters, handle errors, and enforce timeouts—all look too C#-y.

Better Durable Functions

Instead of following the sub-optimal route, we implemented the service with a more F#-idiomatic API. I’ll show the code first, and then I’ll explain its foundation.

The implementation consists of three parts:

  • Activity Functions—one per action from the domain model
  • Orchestrator Function defines the workflow
  • Azure Functions bindings to instruct how to run the application in the cloud

Activity Functions

Each Activity Function defines one step of the workflow: Matching, Picking, and Reserving. We simply reference the F# functions of those actions in one-line definitions:

let findMatchingGiftActivity = Activity.defineAsync "FindMatchingGift" findMatchingGift
let pickGiftActivity = Activity.define "PickGift" pickGift
let reserveActivity = Activity.defineAsync "Reserve" reserve

Each activity is defined by a name and a function.


The Orchestrator calls Activity Functions to produce the desired outcome of the service. The code uses a custom computation expression:

let workflow wishlist = orchestrator {
    let! matches =
        |> List.map (Activity.call findMatchingGiftActivity)
        |> Activity.all

    let! gift = Activity.call pickGiftActivity (List.concat matches)

    let reservation = { Kid = wishlist.Kid; Product = gift }
    do! Activity.call reserveActivity reservation
    return reservation

Notice how closely it matches the workflow definition from our domain model:

Async function vs. Durable Orchestrator

Async function vs. Durable Orchestrator

The only differences are:

  • orchestrator computation expression is used instead of async because multi-threading is not allowed in Orchestrators
  • Activity.call replaces of direct invocations of functions
  • Activity.all substitutes Async.Parallel

Hosting layer

An Azure Function trigger needs to be defined to host any piece of code as a cloud Function. This can be done manually in function.json, or via trigger generation from .NET attributes. In my case I added the following four definitions:

let FindMatchingGift([<ActivityTrigger>] wish) =
    Activity.run findMatchingGiftActivity wish

let PickGift([<ActivityTrigger>] matches) =
    Activity.run pickGiftActivity matches

let Reserve([<ActivityTrigger>] wish) =
    Activity.run reserveActivity wish

let Workflow ([<OrchestrationTrigger>] context: DurableOrchestrationContext) =
    Orchestrator.run (workflow, context)

The definitions are very mechanical and, again, strongly typed (apart from Functions' names).

Ship It!

These are all the bits required to get our Durable Wish Fulfillment service up and running. From this point, we can leverage all the existing tooling of Azure Functions:

There is a learning curve in the process of adopting the serverless architecture. However, a small project like ours is a great way to do the learning. It sets Santa’s IT department on the road to success, and children will get better gifts more reliably!


The above code was implemented with the library DurableFunctions.FSharp. I created this library as a thin F#-friendly wrapper around Durable Functions.

Frankly speaking, the whole purpose of this article is to introduce the library and make you curious enough to give it a try. DurableFunctions.FSharp has several pieces in the toolbox:

  • OrchestratorBuilder and orchestrator computation expression which encapsulates proper usage of Task-based API of DurableOrchestrationContext

  • Activity generic type to define activities as first-class values

  • Activity module with helper functions to call activities

  • Adapters for Azure Functions definition for Async and Orchestrator

  • API of the original Durable Extensions is still available, so you can fall back to them if needed

In my opinion, F# is a great language to develop serverless Functions. The simplicity of working with functions, immutability by default, strong type system, focus on data pipelines are all useful in the world of event-driven cloud applications.

Azure Durable Functions brings higher-level abstractions to compose workflows out of simple building blocks. The goal of DurableFunctions.FSharp is to make such composition natural and enjoyable for F# developers.

Getting Started is as easy as creating a new .NET Core project and referencing a NuGet package.

I’d love to get as much feedback as possible! Leave comments below, create issues on the GitHub repository, or open a PR. This would be super awesome!

Happy coding, and Merry Christmas!


Many thanks to Katy Shimizu, Devon Burriss, Dave Lowe, Chris Gillum for reviewing the draft of this article and their valuable contributions and suggestions.

Cloud developer and researcher.
Software engineer at Pulumi. Microsoft Azure MVP.

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