Precompiled Azure Functions in F#

This post is giving a start to F# Advent Calendar in English 2017. Please follow the calendar for all the great posts to come.

Azure Functions is a “serverless” cloud offering from Microsoft. It allows you to run your custom code as response to events in the cloud. Functions are very easy to start with; and you only pay per execution - with free allowance sufficient for any proof-of-concept, hobby project or even low-usage production loads. And when you need more, Azure will scale your project up automatically.

F# is one of the officially supported languages for Azure Functions. Originally, F# support started with F# Script files (authored directly in Azure portal or copied from local editor), so you can find many articles online to get started, e.g. Creating an Azure Function in F# from the ground up and Part 2 by Mathias Brandewinder.

However, I find script-based model a bit limited. In today’s article I will focus on creating Azure Functions as precompiled .NET libraries. Along the way, I’ll use cross-platform tools like .NET Core and VS Code, and I’ll show how to integrate Functions with some popular tools like Suave and Paket.

Create a Project

You can follow this walkthrough on Windows or Mac, just make sure that you have .NET Core 2 and Node.js 8.x with npm installed. My editor of choice is Visual Studio Code with Ionide plugin.

I’ll show you how to create a new F# Function App from scratch. If you want to jump to runnable project, you can get it from my github.

We start with creating a new F# library project for .NET Standard 2. Run in your command line:

dotnet new classlib --language F# --name HelloFunctions

This command creates a folder with two files: HelloFunctions.fsproj project file and Library.fs source code file.

Now, add a reference to Azure Functions NuGet package:

dotnet add package Microsoft.NET.Sdk.Functions

Define a Function

Open Library.fs code file and change it to the following code:

namespace HelloFunctions

open System
open Microsoft.Azure.WebJobs
open Microsoft.Azure.WebJobs.Host

module Say =
  let private daysUntil (d: DateTime) =
    (d - DateTime.Now).TotalDays |> int

  let hello (timer: TimerInfo, log: TraceWriter) =
    let christmas = new DateTime(2017, 12, 25)

    daysUntil christmas
    |> sprintf "%d days until Christmas"
    |> log.Info

We defined a function hello which should be triggered by Functions runtime based on time intervals. Every time the function is called, we log how many days we still need to wait before Christmas 2017.

To convert this simple F# function to an Azure Function, create a folder called Hello (or choose any other name) next to the project file and add function.json file in there:

  "bindings": [
      "name": "timer",
      "type": "timerTrigger",
      "schedule": "0 * * * * *"
  "scriptFile": "../bin/HelloFunctions.dll",
  "entryPoint": "HelloFunctions.Say.hello"

We defined that:

  • Our function is triggered by timer
  • It runs every minute at 0 seconds
  • The entry point is our hello function in the compiled assembly

Prepare Local Runtime

There are a couple more configuration files needed to be able to run the Function App locally. host.json defines hosting parameters; empty file will do for now:


Most triggers need to connect to a Storage Account. For examples, timer trigger uses it to hold leases to define which running instance will actually execute the action every minute. Copy a connection string to your Storage Account (local Storage emulator is fine too) and put it into local.settings.json file:

  "IsEncrypted": false,
  "Values": {
    "AzureWebJobsStorage": "...your connection string..."

Note that this file is only used for local development and is not published to Azure by default.

Finally, we need to modify fsproj file to make the build tool copy those files into bin folder. Add the following section in there:

  <Content Include="Hello\function.json">
  <Content Include="host.json">
  <Content Include="local.settings.json">

Run App Locally

The first step is to build and publish our Function App with dotnet commands:

dotnet build
dotnet publish

The first line produces the dll file and the second line copies it and all of its dependencies to publish folder.

The nice thing about Azure Functions is that you can easily run them locally on a development machine. Execute the following command to install the runtime and all the required libraries:

npm install -g azure-functions-core-tools@core

This will add a func CLI to your system which is the tool to use for all Function related operations.

Navigate to bin\Debug\netstandard2.0\publish folder and run func start from there. You should see that your app is now running, and your timer function is scheduled for execution:

Function App Start

Once the next minute comes, the timer will trigger and you will see messages in the log:

Timer Trigger Working

Integrate into VS Code

You are free to use full Visual Studio or any editor to develop Function Apps in F#. I’ve been mostly using VS Code for this purpose, and I believe it’s quite popular among F# community.

If you use VS Code, be sure to setup the tasks that you can use from within the editor. I usually have at least 3 tasks: “build” (dotnet build), “publish” (dotnet publish) and “run” (func start --script-root bin\\debug\\netstandard2.0\\publish), with shortcuts configured to all of them.

You can find an example of tasks.json file here.

Also, check out Azure Functions Extension.

Deploy to Azure

You can deploy the exact same application binaries to Azure. Start by creating an empty Function App in the portal, or via Azure CLI (func CLI does not support that).

Then run the following command to deploy your precompiled function to this app:

func azure functionapp publish <FunctionAppName>

At the first run, it will verify your Azure credentials.

In real-life production scenarios your workflow is probably going to be similar to this:

  • Change Function App code
  • Run it locally to test the change
  • Push the code changes to the source control repository
  • Have your CI/CD pipeline build it, run the tests and then push the binaries to Azure Functions environment

HTTP Trigger

Timer-triggered functions are useful, but that’s just one limited use case. Several other event types can trigger Azure Functions, and for all of them you can create precompiled functions and run them locally.

The most ubiquotous trigger for any serverless app is probably HTTP. So, for the rest of the article I will focus on several approaches to implement HTTP functions. Nonetheless, the same techique can be applied to other triggers too.

F# code for the simplest HTTP Function can look like this:

namespace PrecompiledApp

open Microsoft.AspNetCore.Mvc
open Microsoft.AspNetCore.Http
open Microsoft.Azure.WebJobs.Host

module PrecompiledHttp =

  let run(req: HttpRequest, log: TraceWriter) =
    log.Info("F# HTTP trigger function processed a request.")
    ContentResult(Content = "HO HO HO Merry Christmas", ContentType = "text/html")

You can find a full example of HTTP Function App here.

This code is using ASP.NET Core classes for request and response. It’s still just an F# function, so we need to bind it to a trigger in function.json:

  "bindings": [
      "type": "httpTrigger",
      "methods": ["get"],
      "authLevel": "anonymous",
      "name": "req",
      "route": "hellosanta"
  "scriptFile": "../bin/PrecompiledApp.dll",
  "entryPoint": ""

If you run the app, the function will be hosted at localhost

HTTP Trigger Working

And a request to http://localhost:7071/api/hellosanta will get responded with our “HO HO HO” message.

This function is of “Hello World” level, but the fact that it’s inside a normal F# library gives you lots of power.

Let’s see at some examples of how to use it.

Suave Function

What can we do to enhance developer experience? We can use our favourite F# libraries.

Suave is one of the most popular libraries to implement Web API’s with. And we can use it in Azure Functions too!

Let’s first make a small twist to HTTP trigger definition in function.json:

"bindings": [
    "type": "httpTrigger",
    "methods": ["get"],
    "authLevel": "anonymous",
    "name": "req",
    "route": "{*anything}"

Binding now defines a wildcard route to redirect all requests to this function. That’s because we want Suave to take care of routing for us.

The definition of such routing will look familiar to all Suave users:

module App =
  open Suave
  open Suave.Successful
  open Suave.Operators
  open Suave.Filters

  let app =
    GET >=> choose
      [ path "/api/what" >=> OK "Every time we love, every time we give, it's Christmas."
        path "/api/when" >=> OK "Christmas isn't a season. It's a feeling."
        path "/api/how" >=> OK "For it is in giving that we receive." ]

Azure Function is just a one-liner wiring Suave app into the pipeline:

module Http =
  open Suave.Azure.Functions.Context

  let run req =
    req |> runWebPart  |> Async.StartAsTask

The heavy lifting is done by runWebPart function, which is a utility function defined in the same application. You can see the full code of this wiring in my repo.

Run the application and request the URL http://localhost:7071/api/what to see the function in action.

This example is very simple, but you can do lots of powerful stuff with Suave! Most probably, you shouldn’t go over the root and try to fit whole mulpti-resource REST API into a single Azure Function. But it might still make sense to keep related HTTP calls together, and Suave can help to keep it cleaner.

Managing Dependencies with Paket

Once your Function App becomes bigger and you start using multiple F# projects, it makes sense to switch to Paket package manager.

It is totally possible to use Paket with Azure Functions. There isn’t much specific to Azure Functions, really. Here is an example of paket.dependecies file


framework: >= netstandard2.0
nuget FSharp.Core
nuget Microsoft.NET.Sdk.Functions
nuget Microsoft.AspNetCore.Mvc.Core

that I used in example which demonstrates Paket + Functions combination.

Attribute-Based Functions

Up until now, we were writing function.json files manually for each function. This is not very tedious, but it is error prone. Microsoft offers an alternative programming model where these files are auto-generated by Functions SDK.

This programming model is based on attributes, which are similar to WebJobs SDK attributes. With this approach, there’s no function.json file in the project. Instead, the function declaration is decorated with attributes:

let run([<HttpTrigger>] req: HttpRequest, log: TraceWriter)

The same development flow still works. Once you run dotnet build, a new function.json file will be generated and placed into bin folder. Functions runtime will be able to use it to run the function as usual.

Note that the generated file looks a bit different from the manual equivalent:

  1. It manifests itself with

    "generatedBy": "Microsoft.NET.Sdk.Functions.Generator-1.0.6",
    "configurationSource": "attributes",
  2. In case you use input and output bindings, you won’t be able to see them in the generated file. Only trigger will be visible in json. Don’t worry, input and output bindings will still work.

You can find an example of HTTP function with attributes here.

There are pro’s and con’s in this model. Obviously, not having to write JSON files manually is beneficial. Some people find the binding attributes really ugly though, especially when you have 3 or 4 bindings and each has multiple parameters.

My preference is to use attributes, but don’t mix attribute decoration with real code. I.e. keep the Function’s body to a simple 1-liner, and delegate the call to a properly defined F# function with the actual domain logic.

Wrapping Up

Lots of F# users value the language for how quickly one can be productive with it: based on concise syntax, powerful libraries and tools like FSI.

In my opinion, Azure Functions fit nicely into the picture. It takes just several minutes before you can run your first Function App on developer machine, and then seamlessly transfer it into the cloud.

I’ve prepared a github repository where you can find more Examples of Azure Functions implemented in F#.

Merry Serverless Functional Christmas!

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

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