Learn the things to consider when using Azure App Service Deployment Slots

When using deployment slots in Azure App Service, there are several factors you need to consider to make the most out of this feature and ensure smooth deployments, testing, and rollbacks.

Here's a detailed breakdown of the key considerations.

App Service Plan and Pricing Tier

1. Plan Restrictions: The number of deployment slots available depends on the pricing tier of your App Service Plan:

   • Free/Shared plans: No slots available.

   • Basic: 1 slot (for staging or testing purposes).

   • Standard: Up to 5 slots.

   • Premium and Isolated: Up to 20 slots.

2. Cost Considerations: Deployment slots themselves don’t incur additional costs as long as they are part of the same App Service Plan. However, you may incur extra costs if you scale out (increase the number of instances) for these slots. Keep in mind that scaling a slot to multiple instances will increase your overall compute costs.

Slot-Specific Configuration

1. Environment Settings: Slots can have their own application settings (like appsettings.json in .NET apps), connection strings, and other environment variables. However, these settings can be swapped between slots or made slot-specific, depending on your configuration. Consider how environment variables (e.g., database connection strings, API keys) should differ between staging and production.

   • Sticky Settings: Settings that should not change during a swap (e.g., connection strings for production databases) should be configured as "sticky" so they don't swap along with other settings.

2. Slot-Specific App Settings: Use slot-specific settings for configurations that differ between environments (e.g., dev, test, prod). This ensures that sensitive information like database credentials or API keys don't accidentally get swapped into the wrong environment.

Traffic Routing and A/B Testing

Routing Traffic to Slots: Deployment slots allow you to route a portion of your traffic to staging or test slots. This is useful for:

1. A/B Testing: You can route a percentage of traffic to a different version of your app to compare performance or user experience.

2. Canary Releases: Gradually introduce new features by routing a small percentage of users to the new version of the app, ensuring it works as expected before full production rollout.

Consider traffic balancing if you want to send only a small fraction of users to a staging slot for testing purposes.

Zero-Downtime Deployment Considerations

Slot Swaps: Slot swapping is typically a zero-downtime operation, but certain factors may cause a brief delay or result in some requests experiencing increased latency:

1. Warm-Up: You can enable warm-up for slots before swapping, so the target slot (e.g., staging) is fully initialized before it goes live.

2. Cold Starts: If a slot hasn’t been receiving traffic for a while, it may experience "cold start" behavior when it is swapped into production.

   • This means the first few requests might have higher latency.

3. Swap Duration: The swap operation itself may take a few seconds, during which the app might experience a slight delay.

   • Ensure that your app is able to handle these brief interruptions gracefully.

Testing and Validation Before Swapping

1. Staging and Validation: Always test your app in the staging slot thoroughly before performing a swap.

This testing should include:

• Functional validation (ensure the app works as expected).

• Performance testing (ensure the new version performs well under load).

• User acceptance testing (UAT) or integration testing with other services.

2. Application Insights: Enable Application Insights for both staging and production slots to monitor how the app behaves in each environment. This allows you to catch issues early in staging before they reach production.

Rollback and Emergency Recovery

1. Rollbacks: If something goes wrong after a swap (e.g., bugs or performance degradation), you can immediately swap back to the previous slot. Ensure that the rollback process is well understood and that the staging slot contains a stable version of the app at all times.

2. Backup Strategy: Regularly backup your app’s configuration and content to ensure you can restore a previous version in case of critical failure.

Differences Between Slots

1. Shared Resources: All slots in the same App Service Plan share the same compute resources, so scaling the plan (e.g., increasing instances) will affect all slots, which could impact costs.

2. Slot-Specific Resources: While most resources (e.g., the app code and deployment artifacts) are shared, some resources can be isolated per slot, such as:

   • SSL Certificates: Certificates and custom domain bindings are specific to the slot they are associated with. A custom domain or SSL configuration must be explicitly configured for non-production slots if needed.

   • Networking: Some networking settings like VNET Integration or Private Endpoints need to be configured individually for each slot if you want them to behave differently in staging versus production.

Deployment Strategies

1. Blue/Green Deployment: This strategy involves deploying the new version of the app to a staging slot (the green environment) and then swapping it with the production slot (the blue environment). This minimizes downtime and risk by allowing you to validate the new version in staging first.

2. Canary Releases: Gradually increase the percentage of traffic routed to the new version deployed in a staging slot, helping you monitor the behavior of the new version under live conditions before full production rollout.

3. Rollout Schedule: You may want to schedule swaps and traffic routing adjustments based on your deployment pipeline, ensuring that they happen during off-peak hours for minimal disruption.

Scaling Considerations

1. App Service Plan Scaling: All slots share the same App Service Plan. If you scale your plan (increase instances, memory, CPU), it will apply to all slots. For different scaling needs between slots (e.g., more instances for production than for staging), you may need to use separate App Service Plans.

2. Scaling per Slot: If you need to scale slots individually, you will need to configure them in different plans (e.g., staging on a lower tier than production). However, keep in mind that scaling can increase costs.

Performance Monitoring and Alerts

1. Monitoring and Logging: Set up monitoring for both production and non-production slots using Azure Monitor, Application Insights, and Diagnostic Logs. Ensure that each slot is properly monitored, and that any performance degradation or failures in staging can be detected before promoting the app to production.

2. Health Checks: Enable health checks to ensure that your app is performing as expected in each slot, particularly the staging and production environments.

Security Considerations

1. Sensitive Information: Ensure that sensitive settings, like connection strings or API keys, are protected and properly isolated between environments. Use Azure Key Vault for storing secrets securely.

2. SSL/TLS Certificates: Make sure that SSL certificates are configured for production and staging slots if using custom domains, as certificates and domain bindings are slot-specific.

Deployment Tools and Automation

1. CI/CD Integration: Automate your deployments with Azure DevOps, GitHub Actions, or other CI/CD tools. This allows you to deploy to staging slots automatically as part of the deployment pipeline, test the new version, and swap to production when ready.

2. ARM Templates: Define your deployment slots and configurations as part of your infrastructure as code (IaC) strategy using ARM templates, which can help automate slot management during infrastructure provisioning and updates.

App Service Limits

1. Slot Limitations: Ensure that your app doesn’t exceed the limits of the App Service Plan, such as the number of slots, concurrent connections, or request limits. Monitor your app's performance and usage, and scale the plan accordingly.

2. Limits on Custom Domains: Custom domains are associated with slots. Be aware that you may have to configure a custom domain for each slot separately (e.g., staging.myapp.com).

Logging and Debugging in Slots

1. Log Streams: Use Azure App Service Diagnostics to view logs for different slots. This allows you to monitor logs independently for each slot, which is important for debugging issues in staging before promoting to production.

2. Kudu Console: You can access the Kudu console for each slot to manage your app, view file structures, and run diagnostic commands.

Summary

Using deployment slots effectively can help streamline your release process, minimize downtime, and reduce the risk of introducing bugs to production.

However, to fully take advantage of deployment slots, it’s important to carefully consider factors such as configuration management, scaling, monitoring, security, and traffic routing.

By balancing these considerations, you can ensure smooth, predictable app deployments and maintain high availability and performance for your users.