One of the premium features you get when using Azure Web Apps in a standard SKU is the deployment slots feature also known as staged deployment but it is actually more than that.
In this post I will go over the deployment slots concept and what you can do with it.
From a (standard) website you can create deployment slots which will actually be Azure Web App instances that are tied to that Website.
A deployment slot will carry the name of the Azure Web App + the name of the slot, for example:
If my Azure Web App is called mysite and I create a slot called staging then my slot will be an Azure Web App with the name mysite(staging) and its url will be http://mysite-staging.azurewebsites.net.
It's important to emphasize that the slot is in itself a regular Azure Web App, it will have its own app settings, connection string, any other configuration settings and even an scm site (https://mysite-staging.scm.azurewebsites.net).
In fact by default each Azure Web App has a single deployment slot called production which is the Azure Web App itself.
You can add more than one deployment slot.
The first feature of deployment slots is the Swap Slots and it's used for Staged Deployment
In short, the Swap operation will exchange the website's content between 2 deployment slots.
Later I'll explain what is swapped and what is not but note that swap is not about copying the content of the website but more about swapping DNS pointers.
So in our scenario we have the Production site with index.html that starts with Hello World and our staging slot has the same index.html but it starts with Yello World.
Before swap - http://mysite.azurewebsites.net/index.html will return Hello World...
After swap - http://mysite.azurewebsites.net/index.html will return Yello World...
Now to get this into a real life scenario.
Deploying your website in the traditional way, whether deploying via WebDeploy, FTP, git, CI or any other way, has weaknesses that may or may not concern you:
This is where staged deployment comes into play. Instead of deploying directly to our production website we create a deployment slot used for staging and we deploy our new bits there.
Then we "warm" our site (staging slot) by making requests to it and we can start testing our new bits verifying everything works as expected. Once we're ready we hit the Azure Portal's Swap button (or PowerShell/xplat cli command) and the slots will be swapped.
Our customers will not hit the "cold start" delay and we have more confidence in our new bits.
Since we want to test our website before going into production we have this manual step where we hit the Swap button to swap.
But if we only want to address the "cold start" delay we can configure the Auto Swap feature where the website automatically swaps a configured slot (in our case staging) with the Production slot after the deployment completes.
Currently auto-swap only works when deploying using WebDeploy (deploying through VS will usually use WebDeploy) and Continuous Integration (VSO, GitHub, Bitbucket). FTP and
git pushwill not cause an auto swap.Auto-swap can take a while to swap (1-2 minutes), until the swap completes any other attempts to deploy the website will fail.
To set this up you'll need to use the Azure PowerShell tool (download)
In PowerShell use the following command:
Set-AzureWebsite -Name mysite –Slot staging -AutoSwapSlotName production
This command will set Azure Web Apps to auto swap the staging slot into Production slot whenever staging is deployed.
You can use the operation logs in the (current) Azure portal to see the auto swap operation status.
One important concept to understand about deployment slots is how the configuration works.
A deployment slot is a full Azure Web App and as one it has all the same configurations as any Azure Web App. When you swap deployment slots there are some settings you actually need to keep with the slot and not swap them.
A setting that is not swapped is referred to as a setting that is sticky to the slot.
Some of the default settings that are sticky to the slot:
This also includes continuous integration settings - if you hooked your staging slot with a GitHub repository after a swap the hook will still exist between GitHub and the staging slot.
App settings and connection strings are not sticky to the slot and will remain with the website when swapped but we can configure selected app settings and connection strings to become sticky to the slot using a PowerShell command (not yet supported by the Azure portal).
Use this command in Azure PowerShell to set 2 app settings as sticky to the slot
Set-AzureWebsite -Name mysite -SlotStickyAppSettingNames @("myslot", "myslot2")
And this command to set 2 connection strings as sticky to the slot
Set-AzureWebsite -Name mysite -SlotStickyConnectionStringNames @("myconn", "myconn2")
Sticky to the slot configuration is website-wide configuration and affects all slots in that website.
Another great feature for deployment slots is the traffic routing also known as testing in production.
This feature will allow you to route traffic that is coming to your Azure Web App between your deployment slots based on percentage of the traffic.
This feature exists only in the new Azure preview portal.
In the portal under your website there is a tile called Testing in production, click on it to get to the "Testing in production" blade where you can direct traffic coming to your website between all of your deployment slots.
One usage scenario for this feature is A/B testing.
By default 100% of the traffic will go to the Production slot but you can create a new deployment slot with a slightly different version of your website (differs by what you want to A/B test) and add it there with a 50% value so 50% of your visitors will actually be served from the new slot.
Another scenario for this feature is having a dev slot that is a little less stable which gets 1% of the traffic so you can test feature currently being developed with real traffic.
For more information on this feature.
I hope that if the deployment slots were just a mysterious link/tile/concept before, you now know how to master them as they can bring lots of value to your production website.