How to use named values with JSON content in Azure API Management Policies

When you want to use a Named Value in Azure API Management that contains a JSON string in an APIM policy expression, you might run into some problems. In this blogpost, I’m going to cover how I worked around them and finally fixed them.

What do I want to achieve ?

I have an API defined in API Management, and I have defined multiple APIM Products defined for this API. When a consumer sends requests to the API, I want to pass in some additional information to the backend. The content of this additional information can vary depending on the APIM Product to which the consumer has subscribed to. Azure APIM can determine -based on the subscription key that is used- in what Product context the API is being called and based on that, I can decide what additional information must be send to the backend.

My first approach was to define a named value in APIM for every APIM product that is defined. The name of each named value followed a specific convention: some-value-<productid>.

Suppose I have 2 APIM products defined with the ID’s product-a and product-b, I would have these named values: some-value-product-a, some-value-product-b.

I then created an APIM policy that would use this named value, like this:

<set-header name="myheader" exist-action="override">
    <value>{{some-value-@(context.Product.Id)}}</value>
</set-header>

And by that, I thought I was done as I would add an additional header to the backend request where the value of that header would be the content of the named value that corresponds with the product context. Unfortunately it is not possible to dynamically construct and use named values. I learned that named values are not evaluated at run-time when executing a policy. Instead, the named value reference is replaced by it’s value when the APIM policy is saved, and therefore it is obvious that you cannot dynamically reference named value instances.

JSON to the rescue ?

To work around this, I thought of creating one single named value that contains a JSON object which contains a property for every defined APIM product, like this:

{
    "product-a": "somevalue",
    "product-b": "anothervalue",
    "product-c": "foobar"
}

The idea was to retrieve and parse this named value in the set-header policy:

<set-header name="myheader" exist-action="override">
    <value>
    @{
        var contents = "";

        var parsedJson = (Newtonsoft.Json.Linq.JObject)JsonConvert.DeserializeObject(contents);

        return (string)parsedJson[context.Product.Id];
    }
    </value>
</set-header>

But, when saving this policy the following error was given:

Error in element ‘set-header’ on line xx, column yy: Unterminated string literal. Strings that start with a quotation mark (“) must be terminated before the end of the line. However, strings that start with @ and a quotation mark (@”) can span multiple lines.

It looks like APIM gets confused because there are quotes inside the contents of the named value.

Work around using a context variable

The final solution is to make use of the set-variable policy. For some reason, it is possible to assign the named value that contains a JSON object to a context variable, and use that context variable inside a set-header policy.

The final solution looks like this:

<set-variable name="myvariable" value="" />

<set-header name="myheader" exists-action="override">
    <value>
    @{
        var contents = (string)context.Variables["myvariable"];
        var minifiedJson = contents.Replace("\n", "").Replace("\r", "").Replace("\\", "");

        var parsedJson = JObject.Parse(minifiedJson);

        if( parsedJson.ContainsKey(context.Product.Id) == false )
        {
            return "some-fallback-value";
        }
        return parsedJson[context.Product.Id].Value<string>();
    }
    </value>
</set-header>

Pay attention to the fact that you need to do some string replacements before you’re able to parse the JSON contents of the named value.

When deploying this API Management Policy via the Set-AzApiManagementPolicy Powershell Cmdlet, be sure to specify the -Format application/vnd.ms-azure-apim.policy.raw+xml parameter, otherwise the validation of the policy will fail.

Hope this helps! Frederik

Written on November 15, 2022