Things to know about Azure App Service Deployment Slots

Azure App Service Deployment Slots provide a powerful way to manage different versions of your web application for testing, staging, or production.

A deployment slot is a live app that is deployed to a specific environment but not yet exposed to the public.

Once you're ready, you can swap the slot with your production environment to ensure zero downtime.

Here’s a detailed look at what you should know about deployment slots.

What are Deployment Slots?

Definition: Deployment slots are separate environments within the same Azure App Service Plan where you can deploy different versions of your app (e.g., dev, test, staging, production).

Each slot is a fully functional app, but it is isolated from the production app.

Slot Types:

Common slots include:

1. Production: The main environment that users interact with.

2. Staging: A pre-production environment where new versions of the app are tested.

3. Development/QA: Additional slots for testing or development purposes.

Benefits of Using Deployment Slots

1. Zero-Downtime Deployments: Deployment slots enable zero-downtime deployment by swapping the staging and production slots. You can test the new version of the app in the staging slot and, when ready, swap it to production with minimal disruption.

2. Easy Rollback: If there are issues with the new deployment, you can easily swap back to the previous version.

3. Testing in Production-Like Environment: You can test the app in a production-like environment with real traffic in the staging slot before making it live.

4. A/B Testing: With multiple slots, you can direct a percentage of traffic to different versions of the app, enabling A/B testing scenarios.

5. Blue/Green Deployments: By using separate slots for staging and production, you can simulate blue/green deployments where two different versions of the application can be tested in parallel.

How Deployment Slots Work

1. Separate URLs: Each deployment slot has a unique URL that’s different from the production URL (e.g., myapp-staging.azurewebsites.net).

2. Shared Resources: All slots share the same App Service Plan (e.g., compute resources), but they can have different configurations, settings, and code.

3. Configuration Isolation: Slots have their own configuration settings, app settings, and connection strings, so you can configure each slot differently. However, by default, these settings can be swapped between slots if needed.

Slot Configuration and Settings

Application Settings and Connection Strings:

Slots can have specific application settings and connection strings that differ from the production slot, allowing you to test different configurations.

1. Slot-Specific Settings: Some settings (like database connections or API keys) may differ between staging and production. For example, a staging slot might point to a staging database while production points to the live database.

2. Sticky Settings: Certain settings (like connection strings or app settings) can be slot-specific. These are not swapped when performing a slot swap.

3. Configuration Swap: You can choose whether to swap application settings and connection strings when swapping slots. For example, you may choose to not swap a production database connection string when promoting a staging app to production.

Swapping Slots

1. Swap Process: The swap operation moves the content and settings from one slot to another. For instance, when you swap from staging to production, your staging app becomes the new production app, and the previous production app becomes the new staging app.

2. Warm-up Process: When you perform a slot swap, you can optionally enable the warm-up feature. This allows the new app in the target slot to pre-load any necessary resources before the swap is completed, minimizing downtime.

3. Swap with Preview: You can preview the swap before it happens, giving you the chance to ensure that everything is correct before making the swap live.

Routing Traffic to Slots

Slot Traffic Routing:

You can route a percentage of traffic to a staging slot for testing or A/B testing purposes without fully swapping.

This allows you to test how the app performs under real-world traffic conditions before fully promoting it to production.

This can be configured in the Traffic Routing section of the Azure portal.

Example:

You can route 90% of traffic to production and 10% to staging to observe the new version’s performance.

Slot-Specific Debugging

1. Slot Logs: Logs for each slot are isolated. You can view logs specific to a slot to debug issues without interfering with production logs.

2. Custom Domain for Slots: If necessary, you can set a custom domain to point to a specific deployment slot (such as staging.myapp.com), enabling easier access and testing.

Use Cases for Deployment Slots

1. Testing New Versions of the App: Use a staging slot to deploy new versions of your app, test them, and verify they work before promoting them to production.

2. Hotfixes and Rollbacks: Deploy hotfixes to a staging slot and then swap it to production. If there’s an issue, you can swap back to the previous version.

3. Blue/Green Deployment: One of the most common use cases for deployment slots. You can deploy a new version to the staging slot, test it, and then swap it to production, making the deployment process seamless and without downtime.

4. Canary Releases: With traffic routing, you can gradually release new features to a subset of users for monitoring before fully rolling them out to all users.

5. A/B Testing: Route a percentage of users to different deployment slots running slightly different versions of your app to gather feedback or test different user experiences.

Scaling Slots

Scaling Slots:

The scaling settings (e.g., the number of instances) are shared across all slots in the same App Service Plan.

If you need different scaling behaviors for different slots (e.g., more instances for production and fewer for staging), you’ll need to use separate App Service Plans for those slots.

Pricing and Limits

1. Number of Slots: The number of slots available depends on your App Service Plan tier:

2. Free and Shared plans don’t support deployment slots.

3. Basic allows 1 slot.

4. Standard allows up to 5 slots.

5. Premium and Isolated plans allow up to 20 slots.

6. Cost: Deployment slots do not incur additional charges as long as they are within the same App Service Plan. However, you may incur additional costs for scaling out the app (e.g., more instances).

Managing Slots Programmatically

1. Azure CLI: You can manage deployment slots via the Azure CLI. Commands such as az webapp deployment slot create and az webapp deployment slot swap let you manage slots from the command line.

2. ARM Templates: Deployment slots can also be defined in Azure Resource Manager (ARM) templates to automate their creation and management as part of your infrastructure as code strategy.

Limitations and Considerations

1. Slot Swap Time: While Azure guarantees zero downtime for swaps, there can still be brief periods where some requests might experience latency due to the swap process. You should test your app thoroughly in staging to ensure no issues occur.

2. Slot-Specific Resources: Not all resources can be isolated to slots (e.g., some persistent services like external databases or file storage are outside the app's slots).

3. Cold Starts: When a slot (especially staging) is not receiving traffic, it may experience a "cold start" (a slight delay on first requests), as Azure may deallocate the instance when no traffic is hitting the slot.

4. Custom Domains and SSL: Custom domains and SSL certificates apply only to the production slot unless you configure them for other slots as well.

Summary

Azure App Service deployment slots are a valuable feature for safely deploying and testing new versions of your app with minimal risk.

They help ensure zero-downtime deployments, streamline rollback procedures, and provide flexibility for staging, A/B testing, and blue/green deployments.

By using slots effectively, you can improve the reliability and stability of your app, while also optimizing the deployment process for both new features and bug fixes.