How to config policy to call backend with body x-www-form-urlencoded ?

Question

Hi, we have a backend API whose body is like these
x-www-form-urlencoded

We hope API consumer can call api through APIM like this

Not sure how to config the policy to meet this requirement.

We've tried to config like this but with not luck.

<inbound>
        <set-body>@{ 
            var body = context.Request.Body.AsFormUrlEncodedContent();
            return body.ToFormUrlEncodedContent(); 
        }</set-body>
        <base />
        <set-backend-service base-url="https://xxx.com/portal.php" />
    </inbound>

The error is like this

"messages": [
        {
            "message": "Expression evaluation failed.",
            "expression": " \n            var body = context.Request.Body.AsFormUrlEncodedContent();\n            return body.ToFormUrlEncodedContent(); \n        ",
            "details": "The form-url-encoded-content is not correctly formatted.Last read content {\"k1\":\"v1\",\"k2\":\"v2\",\"k3\":\"v3\"}\r\n   at Gateway.Pipeline.IO.FormUrlEncodedConverter.ThrowException(String errorMessage)\r\n   at Gateway.Pipeline.IO.FormUrlEncodedConverter.Convert[T](Stream stream, Encoding encoding, ILog log, Object conversionSettings)\r\n   at Microsoft.WindowsAzure.ApiManagement.Proxy.Gateway.MessageBody.Convert[T](Boolean preserveContent, JsonSerializerSettings settings)"

Need your guidance. Thank you

Answer 1

Amy Zhang Thank you for posting this question in Microsoft Q&A. Based on my understanding, you have a backend API which expects content-type as x-www-form-urlencoded but you need to pass the request body as JSON to APIM. Is that correct?

The above code snippet based on doc: Access and return body as URL-encoded form data is expecting the request body as x-www-form-urlencoded but the actual request body was JSON and hence it failed. Looking at the Postman request, it formed key/pair value, and you can achieve the same by writing a custom policy with set-body.

Sample code snippet:

<set-header name="Content-Type" exists-action="override">
            <value>application/x-www-form-urlencoded</value>
        </set-header>
        <set-body>@{
            JObject varBody = context.Request.Body.As<JObject>(preserveContent:true);
            return $"k1={varBody["k1"]}&k2={varBody["k2"]}&k3={varBody["k3"]}";
        }</set-body>

The above code snippet is just for reference, and you can write generic policy expressions based on your need. Refer API Management policy expressions and for more samples, check out Azure API Management Policy Snippets.

I hope this helps with your question and let me know if you have any other. Would be happy to answer any.

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