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Client/Server communication

One of the most powerful features of SAFE is the ability to seamlessly share data, types and code across client and server.

Sharing Types

Sharing your domain types and contracts between client and server is extremely simple. Thanks to Fable's excellent F# transpilation into Javascript, you can use all standard F# language features such as Records, Tuples and Discriminated Unions without worry. To share types across both your client and server project, first create a file in your repository called e.g Shared.fs.

Then, create types in the file as needed e.g

type Customer = { Name : string }

Reference this file to your server project. You can now reference those types in the server.

<Project Sdk="Microsoft.NET.Sdk">
        <Compile Include="../Shared/Shared.fs" />

Finally, reference this file to your client project (as above). You can now reference those types in the client; types will be compiled into Javascript types and seamlessly translated for you.

Sharing Code

You can also share code using the same mechanism. This is extremely useful for e.g shared validation or business logic that needs to occur on both client and server.

Fable will translate your functions into native Javascript, and will even translate many calls to the .NET base class library into corresponding Javascript! This allows you to compile your domain model and domain logic to many many different targets including:

  • ASP.NET Core (via Saturn)
  • Azure Functions
  • Javascript that runs in the browser
  • Javascript that runs on mobile devices with React Native.
  • Raspberry Pi (via .NET Core)

You can read more about this on the Fable website.

Sharing Data

Sharing data can be achieved in two main ways in SAFE: through the Saturn API directly, or via the Fable.Remoting library.

Sharing data with Saturn

Sharing data using Saturn is very simple. Start by creating a function in your server that returns some data:

let loadCustomersFromDb() =
    [ { Name = "Joe Bloggs" } ]

Next, create a method which returns the data as JSON within Giraffe's HTTP context.

/// Returns the results of loadCustomersFromDb as JSON.
let getCustomers next ctx =
    json (loadCustomersFromDb()) next ctx

You can opt to combine both of the above functions into one, depending on your preferences, but it's often good practice to separate your data access from serving data in HTTP endpoints.

Also note the next and ctx arguments. These are used by Giraffe as part of its HTTP pipeline and are required by the json function.

Now expose the api method using Saturn's scope construct and add the scope to your overall application scope:

let myApis = scope {
    get "/api/customers/" getCustomers

For simple endpoints you may elect to embed the API call directly in the scope (and use partial application to omit the next and ctx arguments):

let myApis = scope {
    get "/api/customers/" (json (loadCustomersFromDb()))

Finally, call the endpoint from your client application.

promise {    
    let! customers = Fetch.fetchAs<Customer array> (sprintf "api/customers") []
    // do more with customers here...

Note the use of the promise { } computation expression. This behaves similarly to async { } blocks that you might already know, whilst the fetchAs function automatically deserializes the JSON back into a Customer array.

Turning on Fable's JSON Converter

By default, serialization between Fable and Giraffe is not compatible. In order to fix this, you must replace the JSON converter in Giraffe with Fable's own IJsonSerializer.

If you are using the SAFE Template, this will automatically be done for you - see the config function in Server.fs.

Sharing data using Fable.Remoting

As an alternative to raw HTTP, you can also use the Fable.Remoting library, which provides an RPC-style mechanism for calling server endpoints.

In our case, instead of creating a scope { } on the server and using fetch on the client, you create a simple protocol which contains methods exposed by the server:

type ICustomer = {
    customers : unit -> Async<Customer array>

let server : ICustomer = {
    customers = fun () -> async { return loadCustomersFromDb() }

On the client, you need only create a proxy for the protocol and then can call methods on it directly.

async {
    let server = Proxy.remoting<ICustomer> {()}
    let! customers = server.customers()
    /// ...

Notice here, there is no need to create routes, or worry about HTTP verbs, or even involve yourself with the Giraffe pipeline.

Sharing State

With the addition of the Elmish.Bridge library, it's now possible to maintain a stateful server and send notifications to clients to maintain state.

Which technology should I use?

Fable Remoting provides an excellent way to quickly get up and running with the SAFE stack. You can rapidly create contracts between client / server and have guaranteed contracts between both client and server. However, note that Fable Remoting is not designed for use as an "open" API for consumption by multiple client. It also forces all HTTP traffic to be delivered as a POST, which cannot be cached by the browser. If you're using a "closed" app without exposing an API to other consumers, and do not need close control of the HTTP channel, consider using Fable.Remoting.

Alternatively, the raw HTTP model provided by Saturn with scope { } requires you to construct routes manually and does not guarantee that the client and endpoint have the same contract (you have to specify the same type on both sides yourself). However, Saturn gives you total control over the routing and verbs used. If you have a public API that is exposed not just to your own application but to third-parties, or you need more fine grained control over your routes and data, you should use this approach.

Lastly, Elmish Bridge provides an alternative way of modelling client/server communication. Unlike Fable Remoting, Elmish Bridge provides the same Elmish model on the server as well as the client, as well as the ability to send notificatiosn from the server back to the client via websockets.

Alternatively, consider using a combination of both Remoting and Saturn endpoints - Remoting for those that are used "internally" by your application, and Saturn for those exposed to external callers.

Fable.Remoting Raw HTTP Elmish.Bridge
Client / Server support Very easy Easy Very Easy
State model Stateless Stateless Stateful
"Open" API? No Yes No
HTTP Verbs? POST Fully Configurable None
Push messages? No No Yes
Pipeline Control? Limited Full Limited