Implementing the HTTP Prefer Header with an ASP.NET Core Filter

15 nov 2017

Have you heard about Prefer Header for HTTP? The RFC-7240 defines how this header can be used by a client, in order to request that certain behaviors be employed by a server while processing a request.

Why does it matter? You probably have consumed API's where you use the HTTP POST or PUT method to modify a resource and the server returns a complete representation of the resource. In other cases the API only returns the Identifier of the resource created, for example.

What if you want to defer to the client to specify which optional behavior is preferred? That's why you need to know about the HTTP Prefer Header.

The RFC specifies a group of Preference Definitions like a preference to indicate that the client prefers the server to respond asynchronously to a response for example, but this post will be focused on the "return=representation" and "return=minimal" preferences.

"return=representation"

The "return=representation" preference indicates that the client prefers that the server include an entity representing the current state of the resource in the response to a successful request.

"return=minimal"

The "return=minimal" preference, on the other hand, indicates that the client wishes the server to return only a minimal response to a successful request.

This preference is really useful when dealing with replies to a POST, PUT or PATCH request.

The selection of which type of response to return to client have impact on what a client must do after receiving the response. Example: by returning a representation of the resource in the response the client won't need to do an additional GET request.

How to implement in ASP.NET Core?

One of the first questions that I faced when trying to implement the prefer header was "how to implement it in ASP.NET Core?".

The easy answer would be:

  • Do it in your Controller Action. Easy peasy, right?

Not so sure my friend!

Following that approach will be difficult to manage when you face yourself with dozens of Controllers each one with a POST and a PUT. Now you see it... the beauty of the duplicated code :)

So, what if we write an ASP.NET Core filter?

If you don't know what is a Filter I recommend you to go read here.

If you find yourself wondering why don't create a Middleware, please see this awesome video.

Setup

In my implementation, I defined that DTOs that can be controlled by the "return=minimal" preference should implement an interface "IIdentifiable". That interface specifies the properties that I need to return in the "minimal" version (an Id for instance).

So, go ahead and create an IIdentifiable interface and add it to your DTO.

The filter

I implemented an Action Filter that will only return the full object when the client sends the "return=representation" preference.

So the flow is something like:

  1. POST a new Resource
  2. Handle the request
  3. Return an Action Result with a DTO in it
  4. Catch the response in a filter

    1. The Request doesn't have the Prefer header?
    2. The Request has the Prefer header without the value "return=representation"?
    3. If the answer to any of the previous questions is "Yes", I update the result to a new object with the minimal information.

The filter:

As you can see I am creating a new IdentifiableDto. That class is just a simple implementation of the IIdentifiable interface. If you prefer, you can just return a dynamic object.

Now you can use it in your Controllers like this:

Note: If you need to do an extra effort to load the resource from the database, probably the Filter approach isn't the best fit for you.

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