From 55284bd0b0f0f14499417527b9a0f750e377969e Mon Sep 17 00:00:00 2001
From: Pranav K <prkrishn@hotmail.com>
Date: Thu, 18 Jun 2020 18:58:44 -0700
Subject: [PATCH] Add a note about output formatter buffering (#18849)

* Add a note about output formatter buffering

* Update 22-to-30.md

* Update 22-to-30.md (#18853)

* Update 22-to-30.md

* Update 22-to-30.md

Co-authored-by: Rick Anderson <3605364+Rick-Anderson@users.noreply.github.com>
---
 aspnetcore/migration/22-to-30.md | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/aspnetcore/migration/22-to-30.md b/aspnetcore/migration/22-to-30.md
index 9abb2dcf92..f3fb5a8d4a 100644
--- a/aspnetcore/migration/22-to-30.md
+++ b/aspnetcore/migration/22-to-30.md
@@ -428,6 +428,20 @@ If you have trouble with <xref:System.IO.TextWriter> implementations or other st
 
 For more information, see [[Announcement] AllowSynchronousIO disabled in all servers (dotnet/AspNetCore #7644)](https://github.com/dotnet/AspNetCore/issues/7644).
 
+### Output formatter buffering
+
+[Newtonsoft.Json](https://www.newtonsoft.com/json), <xref:System.Xml.Serialization.XmlSerializer>, and <xref:System.Runtime.Serialization.DataContractSerializer> based output formatters only support synchronous serialization. To allow these formatters to work with the [AllowSynchronousIO](https://github.com/dotnet/aspnetcore/issues/7644) restrictions of the server, MVC buffers the output of these formatters before writing to disk. As a result of buffering, MVC will include the Content-Length header when responding using these formatters.
+
+<xref:System.Text.Json> supports asynchronous serialization and consequently the `System.Text.Json` based formatter does not buffer. Consider using this formatter for improved performance.
+
+To disable buffering, applications can configure <xref:Microsoft.AspNetCore.Mvc.MvcOptions.SuppressOutputFormatterBuffering> in their startup:
+
+```csharp
+services.AddControllers(options => options.SuppressOutputFormatterBuffering = true)
+```
+
+Note that this may result in the application throwing a runtime exception if `AllowSynchronousIO` isn't also configured.
+
 ### Microsoft.AspNetCore.Server.Kestrel.Https assembly removed
 
 In ASP.NET Core 2.1, the contents of *Microsoft.AspNetCore.Server.Kestrel.Https.dll* were moved to *Microsoft.AspNetCore.Server.Kestrel.Core.dll*. This was a non-breaking update using `TypeForwardedTo` attributes. For 3.0, the empty *Microsoft.AspNetCore.Server.Kestrel.Https.dll* assembly and the NuGet package have been removed.