During the last couple of weeks we've upgraded worker servers in the US and EU regions to support Windows Server 2012 R2, IIS 8.5 and .NET 4.5.2. Major upgrades like this can be a risky and lead to compatibility issues, and the upgrade was carefully planned and executed to maximize compatibility with running applications. Application performance and error rates have been closely monitored throughout the process and fortunately, chances are you haven't noticed a thing: We've detected migration-related issues with less than 0.1% of running applications.
Many of the new features and configuration improvements enabled by this upgrade will be gradually introduced over the coming months. This way we can ensure a continued painless migration and maintain compatibility with the previous Windows Server 2008 R2/IIS 7.5 setup, while we iron out any unexpected kinks if and when they crop up. A few changes have however already been deployed that we wanted to fill you in on.
WebSocket support and the beta region
Last year the beta region featuring experimental WS2012 and WebSockets support was introduced. The beta region allowed customers to test existing and new apps on the new setup while we prepared and optimized it for production use. This approach has been an important factor in learning about subtle differences between the server versions, and addressing pretty much all compatibility issues before upgrading the production regions. Thanks to all the customers who provided valuable feedback during the beta and helped ensure a smoother transition for everyone.
An important reason for the server upgrade was to support WebSocket connections. Now that the worker servers are running WS2012 and IIS 8.5 we've started doing just that. Applications in the old beta region have been merged into the production US region and the beta region is no longer available when you create a new application.
Most load balancers already support WebSockets and the upgrade is currently being rolled out to remaining load balancers. Apps created since August 14th fully support WebSockets and no configuration is necessary: AppHarbor will simply detect and proxy connections as expected when a client requests a
Some libraries, such as SignalR, will automatically detect and prefer WebSocket connections when supported by both the server and client. Until WebSocket connections are supported on all load balancers some apps may attempt and fail during the WebSocket handshake. This should not cause issues since these libraries will fall back to other supported transports, and affected apps will automatically be WebSocket-enabled when supported by the load balancers.
One of the major challenges that has held back this upgrade is a change in the way we throttle worker CPU usage. CPU limitations are the same as before, but the change can affect how certain CPU-intensive tasks are executed. Resources and documentation on this subject are limited, but testing shows that CPU time is more evenly scheduled across threads, leading to higher concurrency, consistency and stability within processes. While this is overall an improvement it can also affect peak performance on individual threads, and we're currently investigating various approaches to better support workloads affected by this.
For the curious, we previously used a CPU rate limit registry setting to limit CPU usage per user account, but this is no longer supported on Windows Server 2012. We now use a combination of IIS 8's built-in CPU throttling and a new CPU rate control for job objects to throttle background workers.
If you've experienced any issues with this upgrade or have feedback about the process, please don't hesitate to reach out.