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Blue-green deployments in IIS

With some custom scripting you can achieve reduced downtime deployments in IIS on a single server, without the need for an external load-balancer. This might help if you only deploy a single instance of your application, or you cannot control the load-balancer itself but you can control IIS and your deployments.

In this case the blue/green are not separate environments, they are different web site and application pool instances, but the basic premise is exactly the same. The general idea is:

  1. Deploy a new instance of your web application and warm it up.
  2. Use an on-server reverse-proxy to seamlessly switch new incoming requests to the new instance.
  3. Delete the old instance once it has finished processing outstanding requests.

A reverse-proxy or some kind of router is required Changing the configuration of a web site in IIS (like physical path or bindings) always results in the application pool being recycled. The default IIS websites and application pools step in Octopus will try to reuse an existing web site in IIS (or create one for you), and as the last step it will update the physical path in IIS. This causes a minimum of downtime, especially if you have allowed overlapping rotation on your application pool. However, to achieve truly zero-downtime deployments of IIS Web Applications, you must use a reverse-proxy or some kind of routing technology.

General steps for zero-downtime deployments in IIS

Every scenario is slightly different which is why this page is written more as a general guide than a step-by-step walk-through. This rough example should provide a strong starting point to reduce downtime for your deployments to IIS.

The general steps for this kind of deployment would be:

  1. Use a custom script step to calculate a new port number so we can configure a binding you can use to warm up the new instance of your application. See this blog post for more details.
  • The new port number should end up in a variable like #{Octopus.Action[Calculate port number].Output.Port}.
  1. Use the IIS Websites and Application Pools step to deploy a new instance of your web application into a new Web Site and Application Pool.
  • Use an expression like MyApp-#{Octopus.Release.CurrentForEnvironment.Number} for the Web Site Name and Application Pool Name.
  • Configure a binding to http://localhost:#{Octopus.Action[Calculate port number].Output.Port}.
  1. Make sure your new instance is warmed up and completely ready to process requests.
  • This may involve making some requests to the localhost binding you configured earlier.
  1. Start routing new requests to the new instance.
  • You might decide to perform on-the-fly reconfiguration of your on-server reverse-proxy (ARR, IIS Web Farm, NGINX, etc).
  • Alternatively you might configure the on-server reverse-proxy to use health checks to determine which instances are able handle requests.
  1. Use a custom script step to delete the old Web Site and Application Pool.
  • The name for the previous instance can be calculated by an expression like this: MyApp-#{Octopus.Release.PreviousForEnvironment.Number}.
  • You may want to wait for outstanding web requests to finish processing using something like this: Get-Item IIS:\AppPools\MyApp-#{Octopus.Release.PreviousForEnvironment.Number} | Get-WebRequest.

Using application request routing (ARR)

You can achieve this kind of result by using ARR as a reverse proxy to your Web Site. You will need to configure a Web Farm in IIS and use ARR to route requests to the Web Farm. You can then choose how you want to switch between active instances of your application. Kevin Reed has written a nice blog post on how he achieves Blue/Green deployments using ARR.

Using NGINX

You can achieve this kind of result using an NGINX server as a reverse proxy to your Web Site. The latest versions of NGINX provide easier support for on-the-fly reconfiguration.

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Page updated on Sunday, January 1, 2023

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