Azure Durable Functions in F#

Azure Functions are designed for stateless, fast-to-execute, simple actions. Typically, they are triggered by an HTTP call or a queue message, then they read something from the storage or database and return the result to the caller or send it to another queue. All within several seconds at most.

However, there exists a preview of Durable Functions, an extension that lets you write stateful functions for long-running workflows. Here is a picture of one possible workflow from the docs:

Fan-out Fan-in Workflow

Such workflows might take arbitrary time to complete. Instead of blocking and waiting for all that period, Durable Functions use the combination of Storage Queues and Tables to do all the work asynchronously.

The code still feels like one continuous thing because it’s programmed as a single orchestrator function. So, it’s easier for a human to reason about the functionality without the complexities of low-level communication.

I won’t describe Durable Functions any further, just go read documentation, it’s nice and clean.

Language Support

As of February 2018, Durable Functions are still in preview. That also means that language support is limited:

Currently C# is the only supported language for Durable Functions. This includes orchestrator functions and activity functions. In the future, we will add support for all languages that Azure Functions supports.

I was a bit disappointed that F# is not an option. But actually, since Durable Functions support precompiled .NET assembly model, pretty much anything doable in C# can be done in F# too.

The goal of this post is to show that you can write Durable Functions in F#. I used precompiled .NET Standard 2.0 F# Function App running on 2.0 preview runtime.

Orchestration Functions

The stateful workflows are Azure Functions with a special OrchestrationTrigger. Since they are asynchronous, C# code is always based on Task and async-await. Here is a simple example of orchestrator in C#:

public static async Task<List<string>> Run([OrchestrationTrigger] DurableOrchestrationContext context)
    var outputs = new List<string>();

    outputs.Add(await context.CallActivityAsync<string>("E1_SayHello", "Tokyo"));
    outputs.Add(await context.CallActivityAsync<string>("E1_SayHello", "Seattle"));
    outputs.Add(await context.CallActivityAsync<string>("E1_SayHello", "London"));

    // returns ["Hello Tokyo!", "Hello Seattle!", "Hello London!"]
    return outputs;

F# has its own preferred way of doing asynchronous code based on async computation expression. The direct refactoring could look something like

let Run([<OrchestrationTrigger>] context: DurableOrchestrationContext) = async {
  let! hello1 = context.CallActivityAsync<string>("E1_SayHello", "Tokyo")   |> Async.AwaitTask
  let! hello2 = context.CallActivityAsync<string>("E1_SayHello", "Seattle") |> Async.AwaitTask
  let! hello3 = context.CallActivityAsync<string>("E1_SayHello", "London")  |> Async.AwaitTask
  return [hello1; hello2; hello3]
} |> Async.StartAsTask

That would work for a normal HTTP trigger, but it blows up for the Orchestrator trigger because multi-threading operations are not allowed:

Orchestrator code must never initiate any async operation except by using the DurableOrchestrationContext API. The Durable Task Framework executes orchestrator code on a single thread and cannot interact with any other threads that could be scheduled by other async APIs.

To solve this issue, we need to keep working with Task directly. This is not very handy with standard F# libraries. So, I pulled an extra NuGet package TaskBuilder.fs which provides a task computation expression.

The above function now looks very simple:

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]

And the best part is that it works just fine.

SayHello function is Activity trigger based, and no special effort is required to implement it in F#:

let SayHello([<ActivityTrigger>] name) =
  sprintf "Hello %s!" name

More Examples

Durable Functions repository comes with a set of 4 samples implemented in C#. I took all of those samples and ported them over to F#.

You’ve already seen the first Hello Sequence sample above: the orchestrator calls the activity function 3 times and combines the results. As simple as it looks, the function will actually run 3 times for each execution, saving state before each subsequent call.

The second Backup Site Content sample is using this persistence mechanism to run a potentially slow workflow of copying all files from a given directory to a backup location. It shows how multiple activities can be executed in parallel:

let tasks = (fun f -> backupContext.CallActivityAsync<int64>("E2_CopyFileToBlob", f)) files
let! results = Task.WhenAll tasks

The third Counter example demos a potentially infinite actor-like workflow, where state can exist and evolve for indefinite period of time. The key API calls are based on OrchestrationContext:

let counterState = counterContext.GetInput<int>()
let! command = counterContext.WaitForExternalEvent<string>("operation")

The final elaborate Phone Verification workflow has several twists, like output binding for activity (ICollector is required instead of C#’s out parameter), third-party integration (Twilio to send SMSs), recursive sub-function to loop through several attempts and context-based timers for reliable timeout implementation.

So, if you happen to be an F# fan, you can still give Durable Functions a try. Be sure to leave your feedback, so that the library could get even better before going GA.

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

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