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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 Continue reading 'How to warm up Azure Web App during deployment.

Choosing Azure App Services to host a web application means that you are using a managed platform and therefore have less responsibilities (such as OS updates and patching). Another specific feature that can be extremely useful is slots. Slots allow you to deploy your application to a separate live app service, warm it up and make sure it’s ready for use in production, and then swap the slots to provide seamless traffic redirection.

There are times when it is important for Azure Cloud Service application to know which deployment slot it is operating in. Worker Roles in particular need to know if certain functionality should be disabled when running in the Staging slot. Mar 17, 2017 The staging deployment slot of a Azure cloud service is not meant to serve as a staging environment. You should use separate cloud services and promote the same cloud package (cspkg) from the staging cloud service to the production cloud service, specifying the appropriate cloud configuration (cscfg). There are times when it is important for Azure Cloud Service application to know which deployment slot it is operating in. Worker Roles in particular need to know if certain functionality should be disabled when running in the Staging slot. This question is also to do with why one gets charged double for Cloud Service instances, and I believe that the answer in my case is due to the fact that I have a Cloud Service Staging Slot which is actually a seperate VM whereas a WebSite Staging slot is just another website instance on the same VM, so you pay the same.

You can slot swap manually (in the portal or command line) or you can automate the slot swap with Auto-swap or in a script.

I’ve been asked a few times recently about how to do this as part of a VSTS release pipeline and so I thought I’d outline my approach here.

Scenario

Let’s keep it pretty simple and say that the release pipeline for a web application is going to cover deploying to a Dev environment, then QA, then Staging and finally Production. Human approvals will be needed at points, but especially in approving the final release into Production. We will use slots so that the staging environment can be warmed-up, reviewed and then final approval triggers the slot swap.

VSTS Release Pipeline

We’ll create a release pipeline that has Dev, QA and Production:

Why no Staging? The Production environment will have both a staging and production slot and we’ll manage the transition between the two in that environment.

Why are Dev and QA separated? To reflect the app service separation. You could have Dev and QA as additional slots in production (you can have up to 15) but it makes sense to keep the production and non-production environments apart. This allows you to choose different hosting plans (size/scaling/cost) and also isolates impact, such as load testing in the QA environment.

Azure Web App Slot Setting

Could you have Dev and Test as separate slots in a single “non-production” environment? Yes, you could. I’ve chosen to split them so they have potentially different lifecycles but that’s more subjective.

Configuring slot swaps in Production

The Dev and QA environments orchestrate whatever flow you want in those environments (e.g. provision using ARM, deploy, run functional tests, run load tests etc.), and in this example don’t have any slot specific activity. Therefore let’s take a look at the production environment tasks:

(I’ve kept this to a minimum to keep it clearer, so no provisioning, running scripts or other activities.)

First deploy the web application to the staging slot in the production environment using the Azure App Service Deploy task:

Check the Deploy to slot option in the standard task, and then choose the Slot (I called the slot “staging” but you can use any name).

Next I’ve added an Agentless phase as it’s not going to delegate any actions to an agent, and added a Manual Intervention task. This causes the flow to pause until the manual intervention is completed by the specified approvers (groups and/or individuals). If it’s approved then it carries on, if it’s rejected (or times out and therefore is rejected) then the flow stops. In this case that works for me as I want the staging environment to be manually verified and then signed off as ok for final release to production. If it gets rejected then it never gets promoted from staging to production.

Finally, if the staging slot has been manually verified and is ready to be used, then we can initiate the actual slot swap (in an agent phase) using the Azure App Service Manage task:

The staging and production slots will then be swapped over, leaving the old production version in the staging app and the latest version in the production slot.

This flow supports the actual slot swaps being automated, but with manual approval. A nice side effect of the slot swap is that should there be a problem, the slot swap can be reversed to redeploy (rollback) the old version of the app now in the staging slot.

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Consider you have an Azure App Service Web App that handles a large amount of traffic and accesses a database, with an architecture similar to that shown in Figure 1. When you make a deployment you would not want to take the risk of deploying bugs or having significant downtime when you deploy a new release, this is what you would want to have a Deployment Slot. A deployment slot is an additional Azure App Service Web App instance (W3WP) which is ~bound to your production Azure App Service Web App of the same name and runs on the same App Service Plan (ASP) that I discuss here. This development slot lets you deploy you test or non-production ready code for testing prior to the release to the live Web App. The beauty of the deployment slot is that you can slick a button and the deployment slot which contains the new version is swapped with the production, and bang, the new version is live, with just a click.

Myncedcloud Rapid Identity

Notice in Figure 1 there is a SQL Server which has 2 SQL Azure databases on it, one called stickyslot-pro and the other stickyslot-tst. Additionally, there is an App Service Plan (ASP) called STICKYSLOT-ASP that runs an Azure App Service Web App called stickyslot with a deployment slot named testing. All of which are contained within a Resource Group called STICKSLOT-RG.

Figure 1, best case, App Service architecture diagramHere is a very good article that discusses sticky slots.

In this article I am concerned with the swapping of the Azure App Service Web App deployment slots from PRO (production) to TST (a testing instances). The thing is, by default, App Settings and database connection strings are NOT sticky to the slot and will follow the Web App when the test slot is swapped with the production slot. In this scenario, it means that when I swap my testing deployment slot, which is, by the way, getting its data from the stickslot-tst database, to production, the then swapped Web App will be pointing to the stickyslot-tst database instead of the stickyslot-pro database. Therefore, I need a way to swap the Web App development slots but keep the database connection string of the current production Web App pointing to production and the testing pointing to the testing database. This is achieved by making the setting ‘sticky to the slot’.

Assume that for my production Web App, I have 2 values I want to remain ‘production’ when I perform a swap. See Figure 2.

Figure 2, sticky slot settings that remain in production even when swapped with a testing slot

Notice there is a App setting with the name STICKSLOT with a Value = ProductionEnv

Azure Cloud Service Staging Slot