Fable.Remoting

Fable.Remoting is a RPC communication layer for Fable and .NET apps, it abstracts away Http and Json and lets you think of your client-server interactions only in terms of pure stateless functions that are statically checked at compile-time:

Define a shared interface

This interface is a record type where each field is either Async<'T> or a function that returns Async<'T>

type IGreetingApi = {
  greet : string -> Async<string>
}

Implement the interface on the server

let greetingApi = {
  greet = fun name ->
    async {
      let greeting = sprintf "Hello, %s" name
      return greeting
    }
}

// Expose the implementation as a HTTP service
let webApp =
  Remoting.createApi()
  |> Remoting.fromValue greetingApi

Call the functions from the client

// get a typed-proxy for the service
let greetingApi =
  Remoting.createApi()
  |> Remoting.buildProxy<IGreetingApi>

// Start using the service
async {
  let! message = greetingApi.greet "World"
  printfn "%s" message // Hello, World
}

That's it, no HTTP, no JSON and it is all type-safe.

Applications using Remoting

• SAFE-TodoList A simple full-stack Todo list application (beginner)
• tabula-rasa a real-world-ish blogging platform (intermediate)
• Yobo Yoga Class Booking System implemented with Event Sourcing (advanced)

Full Documentation

The library runs everywhere on the backend: As Suave WebPart, as Giraffe/Saturn HttpHandler or any other framework as Asp.NET Core middleware. Clients can be Fable or .NET application.

"Fable.Remoting solves the age-old problem of keeping your front-end code in sync with your backend code at compile time, and in a language as enjoyable to use as F#" - David Falkner

Quick Start

Use the SAFE Simplified template where Fable.Remoting is already set up and ready to go

Available Packages:

Library	Version
Fable.Remoting.MsgPack
Fable.Remoting.Client
Fable.Remoting.Json
Fable.Remoting.Server
Fable.Remoting.Suave
Fable.Remoting.Giraffe
Fable.Remoting.AspNetCore
Fable.Remoting.DotnetClient
Fable.Remoting.AzureFunctions.Worker
Fable.Remoting.AwsLambda

Scaffold from scratch - Suave

Create a new F# console app:

dotnet new console -lang F#

Define the types you want to share between client and server:

// SharedTypes.fs
module SharedTypes

type Student = {
    Name : string
    Age : int
}

// Shared specs between Server and Client
type IStudentApi = {
    studentByName : string -> Async<Student option>
    allStudents : unit -> Async<list<Student>>
}

The type IStudentApi is very important, this is the specification of the protocol between your server and client. Fable.Remoting expects such type to only have functions returning Async on the final result:

Async<A>
A -> Async<B>
A -> B -> Async<C>
// etc...

Try to put such types in seperate files to reference these files later from the Client

Then provide an implementation for IStudentApi on the server:

open SharedTypes

let getStudents() =
  async {
    return [
        { Name = "Mike";  Age = 23; }
        { Name = "John";  Age = 22; }
        { Name = "Diana"; Age = 22; }
    ]
  }

let findStudentByName name =
  async {
    let! students = getStudents()
    let student = List.tryFind (fun student -> student.Name = name) students
    return student
  }

let studentApi : IStudentApi = {
    studentByName = findStudentByName
    allStudents = getStudents
}

Now that we have the implementation studentApi, you can expose it as a web service from different web frameworks. We start with Suave

Install the library from Nuget using Paket:

paket add Fable.Remoting.Suave --project /path/to/Project.fsproj

Create a WebPart from the value studentApi

open Suave
open Fable.Remoting.Server
open Fable.Remoting.Suave

let webApp : WebPart =
    Remoting.createApi()
    |> Remoting.fromValue studentApi
    |> Remoting.buildWebPart

// start the web server
startWebServer defaultConfig webApp

Yes, it is that simple. You can think of the webApp value as if it was the following in pseudo-code:

let webApp =
 choose [
  POST
   >=> path "/IStudentApi/studentByName"
   >=> (* deserialize request body (from json) *)
   >=> (* invoke studentApi.getStudentByName with the deserialized input *)
   >=> (* give client the output back serialized (to json) *)

 // other routes
 ]

You can enable diagnostic logging from Fable.Remoting.Server (recommended) to see how the library is doing it's magic behind the scenes :)

let webApp =
    Remoting.createApi()
    |> Remoting.fromValue studentApi
    |> Remoting.withDiagnosticsLogger (printfn "%s")
    |> Remoting.buildWebPart

AspNetCore Middleware

Install the package from Nuget using paket

paket add Fable.Remoting.AspNetCore --project /path/to/Project.fsproj

Now you can configure your remote handler as AspNetCore middleware

let webApp =
    Remoting.createApi()
    |> Remoting.fromValue studentApi

let configureApp (app : IApplicationBuilder) =
    // Add Remoting handler to the ASP.NET Core pipeline
    app.UseRemoting webApp

[<EntryPoint>]
let main _ =
    WebHostBuilder()
        .UseKestrel()
        .Configure(Action<IApplicationBuilder> configureApp)
        .Build()
        .Run()
    0

Giraffe

You can follow the Suave part up to the library installation, where it will become:

paket add Fable.Remoting.Giraffe --project /path/to/Project.fsproj

Now instead of a WebPart, by opening the Fable.Remoting.Giraffe namespace, you will get a HttpHandler from the value server:

open Giraffe
open Fable.Remoting.Server
open Fable.Remoting.Giraffe

let webApp : HttpHandler =
    Remoting.createApi()
    |> Remoting.fromValue studentApi
    |> Remoting.buildHttpHandler

let configureApp (app : IApplicationBuilder) =
    // Add Giraffe to the ASP.NET Core pipeline
    app.UseGiraffe webApp

let configureServices (services : IServiceCollection) =
    // Add Giraffe dependencies
    services.AddGiraffe() |> ignore

[<EntryPoint>]
let main _ =
    WebHostBuilder()
        .UseKestrel()
        .Configure(Action<IApplicationBuilder> configureApp)
        .ConfigureServices(configureServices)
        .Build()
        .Run()
    0

Saturn

You can use the same webApp generated by the Giraffe library.

open Saturn
open Fable.Remoting.Server
open Fable.Remoting.Giraffe

let webApp : HttpHandler =
    Remoting.createApi()
    |> Remoting.fromValue studentApi
    |> Remoting.buildHttpHandler

let app = application {
    url "http://127.0.0.1:8083/"
    use_router webApp
}

run app

Azure Functions (isolated)

To use Azure Functions in isolated mode with custom HttpTrigger as serverless remoting server, just install:

dotnet add package Fable.Remoting.AzureFunctions.Worker

or using paket

paket add Fable.Remoting.AzureFunctions.Worker --project /path/to/Project.fsproj

Since Azure Functions don't know anything about HttpHandler we need to use built-in HttpRequestData and HttpResponseData objects. Luckily we have Remoting.buildRequestHandler and HttpResponseData.fromRequestHandler functions to the rescue:

open Fable.Remoting.Server
open Fable.Remoting.AzureFunctions.Worker
open Microsoft.Azure.Functions.Worker
open Microsoft.Azure.Functions.Worker.Http
open Microsoft.Extensions.Logging

type Functions(log:ILogger<Functions>) =
    
    [<Function("Index")>]
    member _.Index ([<HttpTrigger(AuthorizationLevel.Anonymous, Route = "{*any}")>] req: HttpRequestData, ctx: FunctionContext) =
        Remoting.createApi()
        |> Remoting.withRouteBuilder FunctionsRouteBuilder.apiPrefix
        |> Remoting.fromValue myImplementation
        |> Remoting.buildRequestHandler
        |> HttpResponseData.fromRequestHandler req

Of course, having one implementation per Function App is not ideal, so HttpResponseData.fromRequestHandlers is here to the rescue:

type Functions(log:ILogger<Functions>) =
    
    [<Function("Index")>]
    member _.Index ([<HttpTrigger(AuthorizationLevel.Anonymous, Route = "{*any}")>] req: HttpRequestData, ctx: FunctionContext) =
        let handlerOne =
            Remoting.createApi()
            |> Remoting.withRouteBuilder FunctionsRouteBuilder.apiPrefix
            |> Remoting.fromValue myImplementationOne
            |> Remoting.buildRequestHandler
        
        let handlerTwo =
            Remoting.createApi()
            |> Remoting.withRouteBuilder FunctionsRouteBuilder.apiPrefix
            |> Remoting.fromValue myImplementationTwo
            |> Remoting.buildRequestHandler
        
        [ handlerOne; handlerTwo ] |> HttpResponseData.fromRequestHandlers req

Fable Client

Install Fable.Remoting.Client from nuget using Paket:

paket add Fable.Remoting.Client --project /path/to/Project.fsproj

Reference the shared types to your client project

<Compile Include="path/to/SharedTypes.fs" />

Start using the library:

open Fable.Remoting.Client
open SharedTypes

// studentApi : IStudentApi
let studentApi =
    Remoting.createApi()
    |> Remoting.buildProxy<IStudentApi>

async {
  // students : Student[]
  let! students = studentApi.allStudents()
  for student in students do
    // student : Student
    printfn "Student %s is %d years old" student.Name student.Age
}
|> Async.StartImmediate

Finally, when you are using webpack-dev-server, you have to change the config from this:

devServer: {
  contentBase: resolve('./public'),
  port: 8080
}

to this:

devServer: {
  contentBase: resolve('./public'),
  port: 8080,
  proxy: {
    '/*': { // tell webpack-dev-server to re-route all requests from client to the server
      target: "http://localhost:5000",// assuming the backend server is hosted on port 5000 during development
      changeOrigin: true
    }
}

That's it!

Dotnet Client

You can also use client functionality in non-fable projects, such as a console, desktop or mobile application.

Install Fable.Remoting.DotnetClient from nuget using Paket:

paket add Fable.Remoting.DotnetClient --project /path/to/Project.fsproj

Reference the shared types in your client project

<Compile Include="path/to/SharedTypes.fs" />

Start using the library:

open Fable.Remoting.DotnetClient
open SharedTypes

// studentApi : IStudentApi
let studentApi =
  Remoting.createApi "http://localhost:8085"
  |> Remoting.buildProxy<IStudentApi>

async {
  // students : Student[]
  let! students = studentApi.allStudents()
  for student in students do
    // student : Student
    printfn "Student %s is %d years old" student.Name student.Age
}
|> Async.StartImmediate

Notice here, that unlike the Fable client, you will need to provide the base Url of the backend because the dotnet client will be deployed separately from the backend.

Adding a new route

• Add another record field function to IStudentApi
• Implement that function
• Restart server and client

Done! You can now use that function from the client too.

See the following article if you are interested in how this library is implemented (a bit outdated but gives you an overview of the mechanism) Statically Typed Client-Server Communication with F#: Proof of Concept

In-depth Introduction (Blog)