Fix issue with custom response ".NET core API gateway ocelot - middleware"

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Good day,
I have an issue with a custom response in API Gateway Ocelot with Middleware.
inside FormatResponse(context.Response) I change response for specific endpoint and I see the new response on debug but I receive the original Response in final result on postman.
ex :
original Response

JavaScript

{
"name":"mo"
}

after a change will be

JavaScript

{
"name":"mo123"
}

my code

C#

 // .NET Core 3.1
 public async Task InvokeAsync(HttpContext context)
 {
     context.Request.EnableBuffering();

     var builder = new StringBuilder();

     var request = await FormatRequest(context.Request);

     builder.Append("Request: ").AppendLine(request);
     builder.AppendLine("Request headers:");
     foreach (var header in context.Request.Headers)
     {
         builder.Append(header.Key).Append(':').AppendLine(header.Value);
     }

     //Copy a pointer to the original response body stream
     var originalBodyStream = context.Response.Body;

     //Create a new memory stream...
     using var responseBody = new MemoryStream();
     //...and use that for the temporary response body
     context.Response.Body = responseBody;

     //Continue down the Middleware pipeline, eventually returning to this class
     await _next(context);

     //Format the response from the server
     var response = await FormatResponse(context.Response); // here ,i see new respose
     builder.Append("Response: ").AppendLine(response);
     builder.AppendLine("Response headers: ");
     foreach (var header in context.Response.Headers)
     {
         builder.Append(header.Key).Append(':').AppendLine(header.Value);
     }

     //Save log to chosen datastore
     _logger.LogInformation(builder.ToString());

     //Copy the contents of the new memory stream (which contains the response) to the original
// stream, which is then returned to the client.
     await responseBody.CopyToAsync(originalBodyStream);
 }

 private async Task<string> FormatResponse(HttpResponse response)
 {
     //We need to read the response stream from the beginning...
     response.Body.Seek(0, SeekOrigin.Begin);
     //...and copy it into a string
     string text = await new StreamReader(response.Body).ReadToEndAsync();
     text = CustomRes(text); //************************ here, I change response
     //We need to reset the reader for the response so that the client can read it.
     response.Body.Seek(0, SeekOrigin.Begin);
     //Return the string for the response, including the status code (e.g. 200, 404, 401, etc.)
     return $"{response.StatusCode}: {text}";
 }

Reference: https://www.abhith.net/blog/dotnet-core-api-gateway-ocelot-logging-http-requests-response-including-headers-body/

What I have tried:

I tried to make changes inside InvokeAsync function

Posted 14-Feb-21 20:12pm

Dev Mo

Updated 15-Feb-21 6:39am

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Comments

RickZeeland 15-Feb-21 7:36am

Seems to be an Async problem, what happens if you make the FormatResponse() a normal method?

Dev Mo 15-Feb-21 9:27am

I make it a normal method, nothing happens
if you can help me I will be thankful

2 solutions

Solution 1

Here is a good example of how to use Async and Tasks: Asynchronous programming in C# | Microsoft Docs[^]

Also see: Best~model-for-async-programming-in-net[^]

Posted 15-Feb-21 1:47am

RickZeeland

Updated 15-Feb-21 6:10am

v2

Comments

Dev Mo 15-Feb-21 9:37am

thank you I will read your example, for now just I need to return a new response what I can do, I believe that will be a simple tip

RickZeeland 15-Feb-21 10:04am

Calling Async "simple" would be the understatement of the year :)
Take a good look at how the example uses a combination of Tasks and Async.

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Richard Deeming · Accepted Answer · 2021-02-15T06:39:00

C#

//We need to read the response stream from the beginning...
response.Body.Seek(0, SeekOrigin.Begin);
//...and copy it into a string
string text = await new StreamReader(response.Body).ReadToEndAsync();
text = CustomRes(text); //************************ here, I change response
//We need to reset the reader for the response so that the client can read it.
response.Body.Seek(0, SeekOrigin.Begin);

The problem is, you are changing a copy of the response. The data in the underlying stream is never changed.

If you want to send your modified response back to the client, then you need to change the data that you send back:

C#

public async Task InvokeAsync(HttpContext context)
{
    context.Request.EnableBuffering();

    var builder = new StringBuilder();
    var request = await FormatRequest(context.Request);
    builder.Append("Request: ").AppendLine(request);
    builder.AppendLine("Request headers:");
    foreach (var header in context.Request.Headers)
    {
        builder.Append(header.Key).Append(':').AppendLine(header.Value);
    }
    
    var originalBodyStream = context.Response.Body;
    
    using var cachedResponseBody = new MemoryStream();
    context.Response.Body = cachedResponseBody;
    await _next(context);
    
    using var modifiedResponseBody = new MemoryStream();
    string response = await ModifyResponse(context.Response, modifiedResponseBody);
    builder.Append("Response: ").AppendLine(response);
    builder.AppendLine("Response headers: ");
    foreach (var header in context.Response.Headers)
    {
        builder.Append(header.Key).Append(':').AppendLine(header.Value);
    }

    // Save log to chosen datastore
    _logger.LogInformation(builder.ToString());

    // Copy the contents of the modified memory stream (which contains the response) 
    // to the original stream, which is then returned to the client.
    await modifiedResponseBody.CopyToAsync(originalBodyStream);
}

private async Task<string> ModifyResponse(HttpResponse response, Stream modifiedResponseBody)
{
    response.Body.Seek(0, SeekOrigin.Begin);
    string text = await new StreamReader(response.Body).ReadToEndAsync();
    
    // Modify the response body:
    text = CustomRes(text);
    
    using (var writer = new StreamWriter(modifiedResponseBody))
    {
        writer.Write(text);
        writer.Flush();
    }

    modifiedResponseBody.Seek(0, SeekOrigin.Begin);
    return $"{response.StatusCode}: {text}";
}