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Using Haskell Servant to Power a Decoupled React Single Page Application

Recently I’ve been experimenting with different ways of building web applications. In particular, I’m interested to what extent it is feasible to start an application with a “pure” API, as distinct from a typical Ruby on Rails application. This approach would limit the backend server to only API endpoints, and restrict it from any kind of HTML generation. All HTML concerns would be pushed to a frontend using something like React.

I published an example application that demostrates this architecture using Servant and React. In this post, I’ll detail some of the issues I came across getting this working.

Authentication

One difficultly I came across was how to handle third-party authentication (via Google OAuth) in this scenario when running the backend and frontend as completely separate services. A typical OAuth flow requires server side calls and interactions that don’t work when the flow is split over two different services.

Google provides an OAuth flow for Web Applications that addresses the first issue. The hard part is how to verify that authentication in the backend.

This OAuth flow provides the client with a JWT containing information about the user, such as their email and granted scopes. This can be verified and trusted on the server using Google’s public key, which needs to be continually fetched from their endpoint to keep it current.

This verification can be done in Servant using a Generalized Authentication handler.

CORS

Requests between applications on different hosts have to negotiate CORS correctly. This could be mitigated by running a reverse proxy in front of both services and presenting them at a single domain, but I wanted to see if I could make it work without this.

A few things are required for correct CORS handling. First, appropriate Access-Control-Allow-Origin headers need to be set on requests. This is best handled with a middleware from the wai-cors package.

That would be sufficient for “simple” requests, but for since our API uses both a non-simple content type (application/json) and the Authorization header, they need to be added to the default policy:

1 corsPolicy = simpleCorsResourcePolicy
2                { corsRequestHeaders = [ "authorization", "content-type" ]
3                }

Also, these non-simple API requests will trigger a CORS preflight, which sends an OPTIONS request to our API. The API needs to be extended to handle these requests. servant-options provides a middleware to do this automatically from an API definition. Unfortunately, servant-options didn’t work out of the box with servant-auth. I needed to provide an instance of HasForeign for AuthProtect. A simple pass-through implementation looks like this:

1 instance (HasForeign lang ftype api) =>
2   HasForeign lang ftype (AuthProtect k :> api) where
3 
4   type Foreign ftype (AuthProtect k :> api) = Foreign ftype api
5 
6   foreignFor lang Proxy Proxy subR =
7     foreignFor lang Proxy (Proxy :: Proxy api) subR

I later extended this to include appropriate metadata so that I could use it to generate clients correctly.

JS Clients

A nice thing about Servant is the ability to auto-generate client wrappers for your API. servant-js provides a number of formats for this, though they weren’t as ergonomic as I was hoping. It doesn’t currently have support for servant-auth nor support for ES6-style exports. Rather than solve this generically, I wrote a custom generator. For fun, it outputs an API class that allows an authorization token to be supplied in the constructor, rather than as an argument to every function:

1 let api = new Api(jwt);
2 api.getEmail();

I’m not sure what the best way to distribute this API is. Currently, the example writes out a file in the frontend’s source tree. This works great for development, but for production I would consider either a dedicated build step in packaging, or serving the JS up directly from the API server.

Aside from this generated client, I didn’t do anything particularly interesting on the React front. The app included in the example is very simple.

Conclusion

This wasn’t a big enough project to draw any serious conclusions about the approach. It is evident however that Servant still has a couple of rough edges when you get outside of the common cases. It took a while to wrap my head around how Servant uses the type system. I found this post and exercise very helpful.

I hadn’t used JWTs before, and they strike me as a pretty neat way to thread authentication through a distributed application.

  • Posted on May 10, 2018
  • Tagged code
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