This post explains some of the not so well-known features and configurations settings of the Azure App Service deployment slots. These can be used to modify the swap logic as well as to improve the application availability during and after the swap. Here is what you can do with them:
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If you use Azure Web App Deployment Slots then you may have noticed that sometimes the swap action may take a while to complete. This may happen when an application that is being swapped into production has a long warmup/initialization phase. It has to be warmed up on every VM in an App Service Plan and that sometimes takes minutes (more detailed description of what happens during the swap can be found in Most common deployment slot swap failures and how to fix them). This blog post explains how to see the progress and the result of the swap operation in the Azure Activity Log.
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Azure Web App Deployment Slots are used to deploy new versions of an application code into production with no interruption to the production traffic. In order to achieve this the swap process involves multiple steps that are performed to prepare the new version of the code to successfully handle the load once it is in production slot. Some of these steps may go wrong, especially when the new version of the code does not cooperate well. This in turn either causes the swap to fail or it results in swapping new code in production while it is still not ready to handle the production load. This post describes the most common reasons why this may happen and how to correct them.
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This blog post has been updated from its original version in order to use the correct names of the PowerShell cmdlets.
This blog post explains how to perform common management tasks for Azure Web App deployment slots by using Powershell cmdlets. To learn more about deployment slots refer to the Azure documentation and my previous blog posts: Azure Web App Deployment Slot Swap with Preview and How to warm up Azure Web App during deployment slots swap.
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Some time ago I had a blog post describing how to warm up an Azure Web App during deployment slots swap. In that post I explained the sequence of actions that happens during the swap. One important point in that explanation is that if a site has any app settings or connection strings that are marked as “Slot” then during slot swap those settings are read from target (e.g. Production) slot and applied to the site in the staging slot. That causes the restart of the site’s worker process so that those changes take effect and become visible as process environment variable.
The restart of the worker process is OK for the majority of the swaps. But sometimes it would be useful to pause right after the production settings were applied on the staging slot and before the actual swap of host names happens. Recently Azure Web Apps team has released a “Swap with Preview” feature that supports that use case. That feature will give you a chance to verify that the web site in the staging slot works fine with production settings. Also this will allow you to warm up/initialize the site in any way you want. For example you can generate some load on that site so that its cache is completely pre-populated. After you are satisfied with how the site works in staging slot you can complete the swap so that the site is moved to production slot and starts taking on production traffic. The site’s worker process will not be restarted during that step, which means that the worker process in production slot is exactly what you have tested in staging slot.
To demonstrate how Swap with Preview works I’ve created a site and added a “Slot” setting to it with name “SlotName” and value “Production“. Also I added a “Slot” connection string with value “ProductionConnectionString“.
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Azure Web App deployment slots are used to help roll out new versions of an app without downtime or cold start activation. New version is typically deployed to a staging slot, then after testing and final verification it gets swapped into a production slot. During the swap operation the Web App’s worker process may get restarted in order for some settings to take effect. Even though the swap does not proceed until the restarted worker process comes back online on every VM instance, it may still not be enough for application to be completely ready to take on production traffic. This post explains how you can use the recently enabled Application Initialization Module to completely warm up your application prior to swapping it into production.
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Windows Azure Web Sites supports Staged Publishing functionality, which allows you to create a staging site slot where you can publish a new version of the website and then test it before swapping to the production environment. By default the non-production deployment slot has its own hostname that you use to test the bits deployed there. Sometimes, however you may want to prevent users from browsing to your non-production deployment slot while you are testing it.
It is relatively easy to configure your site to block the http request if it is deployed to any non-production slot. For example let’s say I have a site ruslany.azurewebsites.net and I created a deployment slot for it and name the slot as ‘staging‘ (it is possible to give custom names to deployment slots now). The hostname for that deployment slot will be ‘ruslany-staging.azurewebsites.net‘. So to prevent access to the deployment slot from anybody except a few allowed IP addresses I can add the following rewrite rule to my site’s web.config file:
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In the beginning of the year Windows Azure Web Sites team has released a preview of the Staged Publishing functionality. The Staged Publishing allows you to create a staging site slot where you can publish a new version of the website and then test it before swapping to the production environment. This feature together with Continuous Deployment via GitHub, BitBucket or DropBox enables some very powerful deployment scenarios.
However the preview release did not provide the optimal experience for enabling Continuous Deployment (CD) for staging site. User had to configure a non-trivial workaround as described in blog post by Rick Rainey. Recently the Azure Web Sites team has released an update that fixes that problem and makes the setup of the CD with staged publishing very simple. This blog post describes how to enable CD from git repository located on BitBucket.
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