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Re-reading ASP.Net Core request bodies with EnableBuffering()

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Jeremy

In some scenarios there’s a need to read the request body multiple times. Some examples include

  • Logging the raw requests to replay in load test environment
  • Middleware that read the request body multiple times to process it

Usually Request.Body does not support rewinding, so it can only be read once. A straightforward solution is to save a copy of the stream in another stream that supports seeking so the content can be read multiple times from the copy.

In ASP.NET framework it was possible to read the body of an HTTP request multiple times using HttpRequest.GetBufferedInputStream method. However, in ASP.NET Core a different approach must be used.

In ASP.NET Core 2.1 we added an extension method EnableBuffering() for HttpRequest. This is the suggested way to enable request body for multiple reads. Here is an example usage in the InvokeAsync() method of a custom ASP.NET middleware:

public async Task InvokeAsync(HttpContext context, RequestDelegate next)
{ context.Request.EnableBuffering(); // Leave the body open so the next middleware can read it. using (var reader = new StreamReader( context.Request.Body, encoding: Encoding.UTF8, detectEncodingFromByteOrderMarks: false, bufferSize: bufferSize, leaveOpen: true)) { var body = await reader.ReadToEndAsync(); // Do some processing with body… // Reset the request body stream position so the next middleware can read it context.Request.Body.Position = 0; } // Call the next delegate/middleware in the pipeline await next(context);
}

The backing FileBufferingReadStream uses memory stream of a certain size first then falls back to a temporary file stream. By default the size of the memory stream is 30KB. There are also other EnableBuffering() overloads that allow specifying a different threshold, and/or a limit for the total size:

public static void EnableBuffering(this HttpRequest request, int bufferThreshold) public static void EnableBuffering(this HttpRequest request, long bufferLimit) public static void EnableBuffering(this HttpRequest request, int bufferThreshold, long bufferLimit)

For example, a call of

context.Request.EnableBuffering(bufferThreshold: 1024 * 45, bufferLimit: 1024 * 100);

enables a read buffer with limit of 100KB. Data is buffered in memory until the content exceeds 45KB, then it’s moved to a temporary file. By default there’s no limit on the buffer size but if there’s one specified and the content of request body exceeds the limit, an System.IOException will be thrown.

These overloads offer flexibility if there’s a need to fine-tune the buffering behaviors. Just keep in mind that:

  • Even though the memory stream is rented from a pool, it still has memory cost associated with it.
  • After the read is over the bufferThreshold the performance will be slower since a file stream will be used.
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Jeremy Meng

Software Development Engineer

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