Serving synthetic JSON responses from the edge.

Why have your user go ALLLLLLL the way to your origin just to be served back a JSON response? Great question, I don’t have a great answer either. What I do have is a way to serve JSON responses from the edge, using Fastly, VCL, and some interesting syntax.

You can't create a simple vcl_error synthetic response like you would with html. However by using VCL with the right syntax you can serve up that synthetic JSON response. Unfortunately, VCL and JSON are not the best of friends but will play nicely if you can get them to talk together.

For a quick review a typical HTML response looks like:

if (obj.status == <some custom status>) {

  set obj.status = <some HTML response code>;

  set obj.response = “<Some message>”;

  synthetic {"

    <html>

      <head>

      </head>

      <body>

        <h1>Custom Response</h1>

      </body>

    </html>

  "};

  return(deliver);

}

Pretty easy. However, if you tried to hack together a solution for JSON by simply changing the HTML document, you’d hit a few road bumps.

Instead we are going to tell the requesting client what kind of document it is receiving. That’s where set obj.http.Content-Type = "application/json” comes in. Then in the synthetic potion we need to have synthetic {json”{<your JSON response>}"json}.

This way VCL knows what, exactly, we are sending out and how to handle it. Then simply return(deliver); and the requesting client is getting back well formed, fast JSON responses. To give you a quick idea of what it can look like is this ode sample below, along with a Fiddle

https://fiddle.fastlydemo.net/fiddle/7abef350.

if (obj.status == <some custom status>) {

  set obj.status = 200;

  set obj.http.Content-Type = "application/json";

  synthetic {json"{"Key1": "Value1", "Key2": "Value2"}"json};

  return(deliver);

}

One other thing that you may want to do is have a lookup table that holds a set JSON responses. This may be useful if you know you have different contact info for your support teams and want to be able to send that info from the requesting client without holding that data on our origin servers. The below fiddle will give you. Quick idea on how to incorporate lookup tables with synthetic JSON responses.

https://fiddle.fastlydemo.net/fiddle/ac2f3c67

As always, if you have questions or need assistance, feel free to Submit a request and our support engineers will be happy to assist you.

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