Learn the things to know about creating Deployment slots in Azure App Service

Creating deployment slots in Azure App Service is a useful way to manage and streamline the deployment process for web applications.

Deployment slots allow you to deploy different versions of your app without affecting the production environment, giving you more control over testing, staging, and rollbacks.

Here are key things to know when creating and managing deployment slots.

What are Deployment Slots?

Deployment slots are separate environments within an Azure App Service (Web App) where you can deploy your app.

By default, you have a single "production" slot, but you can create additional slots for staging, testing, or other purposes.

Each slot has its own hostname (e.g., myapp-staging.azurewebsites.net), so you can deploy and test without affecting the live version.

Common Use Cases for Deployment Slots

1. Staging: Create a staging slot to deploy the new version of the app and test it before going live.

2. Blue/Green Deployment: One slot (e.g., blue) is active for production, while the other (e.g., green) is used to test new features. After testing, you swap the slots to make the green version live.

3. Canary Releases: Deploy a new version to a slot, then gradually route a portion of traffic to it to test its behavior under real-world conditions.

4. Rollback: If something goes wrong with a deployment, you can quickly swap back to a previous version by swapping slots.

How to Create Deployment Slots

1. Azure Portal: Go to your App Service in the Azure portal, select "Deployment Slots" from the left-hand menu, and click “Add Slot.” You can choose to clone the configuration of the production slot or start from scratch.

2. Azure CLI: You can also create deployment slots via the Azure CLI using the az webapp deployment slot create command.

3. ARM Templates: Deployment slots can also be created and managed using Azure Resource Manager (ARM) templates, which is useful for infrastructure as code (IaC) scenarios.

Slot Settings and Configuration

1. Configuration and Environment Variables: Each slot has its own app settings, connection strings, and environment variables. However, you can choose to share settings between slots or configure them separately for each one.

2. Application Insights: You can set up separate Application Insights configurations for each slot or use a shared one to monitor both production and non-production traffic.

3. Scaling and Pricing: Each deployment slot inherits the scale settings and pricing of the main app service plan, so it's important to factor this into your cost and scaling strategy.

Slot Swapping

1. Swap Operation: The primary feature of deployment slots is the ability to swap between slots (e.g., swap a staging slot with the production slot). This is an atomic operation that minimizes downtime, and the app will continue to run while the swap occurs.

2. Warm-Up Time: When swapping slots, Azure keeps the app "warm" by keeping it running in the background before the actual swap happens. This reduces downtime, but you can adjust the warm-up behavior to fine-tune the swap process.

3. Sticky Settings: Some app settings (like connection strings or custom domains) can be marked as "sticky," meaning they stay with a particular slot when swapping. This is useful for things like database connections or external APIs that shouldn't change across slots.

Traffic Routing

1. Partial Slot Swapping: You can route a percentage of traffic to a specific slot for canary releases or testing. This is done using the slot traffic splitting feature.

2. Custom Domains: You can configure custom domains for your slots, allowing them to be accessed independently of the main production slot.

Managing Deployment Slots

1. Continuous Deployment: Azure supports CI/CD pipelines with deployment slots. You can configure a pipeline to deploy to a specific slot (e.g., staging) first, then swap it with production after tests.

2. Manual Deployments: In addition to CI/CD, you can manually deploy to a slot using Azure Portal, Azure CLI, or even FTP.

3. Slot Debugging: You can also attach debuggers to individual slots, so you can debug issues in staging or test environments without affecting production.

Slot-Specific Features

1. Diagnostics and Monitoring: Each slot has its own diagnostic logs and metrics. You can monitor performance, errors, and diagnostics separately for each slot.

2. Scaling: You can scale each slot independently if needed, but typically, you will scale the entire App Service Plan, which affects all slots.

3. Slot Settings and Configurations: App settings, connection strings, and app configurations can be set per slot, which gives you flexibility in managing different environments.

Limitations

1. Slot Limitations: A single Azure App Service plan allows a maximum of 5 slots per app, though this limit may vary depending on your pricing tier. If you need more slots, you might need to upgrade to a higher service plan.

2. Startup Time: New slots take a few seconds to start up when first created or swapped, which can cause brief downtime if not managed properly.

3. Custom Domain Limitations: You can't map a custom domain directly to a deployment slot (other than the production slot), though you can use CNAMEs to route traffic to a specific slot.

Best Practices

1. Test Thoroughly: Use slots like staging to thoroughly test your app before moving it to production. Automate tests as part of your CI/CD pipeline to ensure quality.

2. Monitor Performance: Always monitor performance and error logs for each slot separately so you can spot any issues early.

3. Keep Slot Settings in Sync: Make sure to keep the configuration in sync across slots where appropriate (e.g., environment variables, secrets). Use slot settings for specific configurations that differ between environments.

Summary

Deployment slots in Azure App Service provide a powerful mechanism for managing app deployments with minimal downtime and risk.

They enable sophisticated deployment strategies like blue/green deployments, canary releases, and continuous delivery.

However, you should manage configuration differences between slots carefully, monitor app performance, and ensure you’re following best practices for slot usage and scaling.