Azure API Management: Replace Backend Service URLs in Response Body

In this post, let's see how we can replace backend API URLs in the response body from an Azure API Management (APIM) policy.

Say, we have a backend API endpoint that has the following code.

WebApplicationBuilder builder = WebApplication.CreateBuilder(args);
 
WebApplication app = builder.Build();
 
app.UseHttpsRedirection();
 
app.MapGet("/endpoints", (HttpContext httpContext) =>
{
    string baseUrl = $"{httpContext.Request.Scheme}://{httpContext.Request.Host}";
 
    return new
    {
        StatusQueryUri = $"{baseUrl}/status",
        HealthQueryUri = $"{baseUrl}/health",
    };
})
.WithName("GetEndpoints")
.WithOpenApi();

// other endpoints

And it would work as follows.

Response from Backend API

Now if we are exposing this API via Azure APIM, we can't be returning internal endpoints. We need to replace the Base URL with the corresponding APIM API endpoints.

To achieve that we can use set-body policy and do something like the following.

<policies>
  <inbound>
    <base />
  </inbound>
  <backend>
    <base />
  </backend>
  <outbound>
    <base />
    <set-body>
    @{
        string urlToReplace = context.Request.Url.Scheme + "://" + context.Request.Url.Host;
        string urlToReplaceWith = context.Request.OriginalUrl.Scheme
          + "://" + context.Request.OriginalUrl.Host 
          + context.Api.Path;
          
        string response = context.Response.Body.As<string>();
        return response.Replace(urlToReplace, urlToReplaceWith);
    }
    </set-body>
  </outbound>
  <on-error>
    <base />
  </on-error>
</policies>

Here,

• context.Request.Url: Backend URL
• context.Request.OriginalUrl: URL of the incoming request
• context.Api.Path: API URL Suffix

The output is as follows:

Response from APIM

Hope this helps.

Happy Coding.

Regards,

Jaliya