public async Task Invoke(HttpContext httpContext)
{
if (httpContext == null)
{
throw new ArgumentNullException(nameof(httpContext));
}
OverwriteTrackingContextHeader(httpContext, m_HeaderKeys, m_HeaderValueGenerator);
// In order to add http headers to the message response, reprocess the Tracking Context
// in a callback function. This is necessary because response headers cannot be set after
// the response body has been written.
httpContext.Response.OnStarting(state =>
{
var ctx = (HttpContext)state;
OverwriteTrackingContextHeader(httpContext, m_HeaderKeys, m_HeaderValueGenerator);
// Take the current Tracking Context (if it exists) and add it to the http response headers.
TrackingContextHelper.ProcessHttpHeaders(ctx?.Response?.Headers);
return(Task.FromResult(0));
}, httpContext);
await m_Next.Invoke(httpContext).ConfigureAwait(false);
}
private static void MergeTrackingContextHeader(HttpContext httpContext)
{
if (httpContext == null)
{
throw new ArgumentNullException(nameof(httpContext));
}
TrackingContext previousTrackingContext = TrackingContext.Current;
// Take the Tracking Context (if it exists) from the http request headers and make it current.
TrackingContextHelper.ProcessHttpHeaders(httpContext.Request?.Headers);
// Check to see how the new current Tracking Context compares to the previous one.
if (previousTrackingContext != null)
{
TrackingContext currentTrackingContext = TrackingContext.Current;
Debug.Assert(currentTrackingContext != null);
// Prepare the inputs for the new Tracking Context.
Guid callChainId = currentTrackingContext.CallChainId;
DateTime originatorUtcTimestamp = currentTrackingContext.OriginatorUtcTimestamp;
IDictionary <string, string> extraHeaders = currentTrackingContext
.ExtraHeaders
.ToDictionary(kvp => kvp.Key, kvp => kvp.Value);
// Extract the extra headers that were in the previous Tracking Context, but
// are not in the inputs for the new Tracking Context.
IList <string> nowMissingHeaderNames = previousTrackingContext
.ExtraHeaders.Keys
.Except(extraHeaders.Keys)
.ToList();
// Add the missing extra headers back to the inputs for the new Tracking Context.
foreach (string headerName in nowMissingHeaderNames)
{
extraHeaders.Add(headerName, previousTrackingContext.ExtraHeaders[headerName]);
}
// Extract the extra headers that were in the previous Tracking Context, and
// also in inputs for the new Tracking Context, but whose values are not empty.
IList <string> sharedHeaderNames = extraHeaders
.Keys
.Except(nowMissingHeaderNames)
.ToList();
// Add the missing extra header values back to the inputs for the new Tracking Context.
foreach (string headerName in sharedHeaderNames)
{
if (string.IsNullOrWhiteSpace(extraHeaders[headerName]))
{
extraHeaders[headerName] = previousTrackingContext.ExtraHeaders[headerName];
}
}
// Now recreate the current Tracking Context with the refilled extra headers.
var newTrackingContext = new TrackingContext(
callChainId,
originatorUtcTimestamp,
extraHeaders);
newTrackingContext.SetAsCurrent();
}
}
