Consumption Plan to Flex Consumption Plan Upgrade

Source: Azure Functions Consumption Plan (Y1/Dynamic) on Linux Target: Azure Functions Flex Consumption Plan (FC1/FlexConsumption) Platform: Linux only (Windows support planned for future) Docs: Linux migration guide

Why Upgrade?

• Faster cold starts — always-ready instances mean functions respond more quickly
• Better scaling — per-function scaling and concurrency controls
• Virtual network support — connect to private networks and use private endpoints
• Active investment — Flex Consumption is where new features land first

What to Expect

1. Your code stays the same — no rewriting required if on a supported language version
2. You'll create a new app alongside the existing one
3. New app runs in the same resource group with access to the same dependencies
4. You control the timing — test thoroughly before switching over

Platform Notes

• Linux Consumption is being retired September 30, 2028
• Azure CLI provides automated migration commands: az functionapp flex-migration list and az functionapp flex-migration start
• The flex-migration commands handle assessment, app creation, and most configuration migration automatically
• Deployment packages are in squashfs format stored in scm-releases blob container

Prerequisites

• Azure CLI v2.77.0+
• resource-graph extension: az extension add --name resource-graph
• jq tool for JSON processing
• Owner or Contributor role in the resource group
• Permissions to create and manage function apps, storage accounts, App Insights resources, and managed identity role assignments

Compatibility Requirements

Supported Language Stacks

Known Limitations (Flex Consumption)

Upgrade Phases

Phase 1: Assessment

Run all checks from assessment.md. Use the automated eligibility check:

# Automated eligibility check — scans all Linux Consumption apps
az functionapp flex-migration list

This returns eligible_apps and ineligible_apps arrays with specific reasons for any incompatibilities. For detailed manual checks, see automation.md.

Phase 2: Pre-migration (Collect Everything)

Collect all settings and configurations from the existing app before creating the new one. See Step-by-step scripts in automation.md:

1. Collect app settings
2. Collect application configurations (CORS, custom domains, HTTP version, TLS, client certs, etc.)
3. Identify managed identities and RBAC role assignments
4. Identify built-in authentication settings
5. Review inbound access restrictions
6. Get the code deployment package (if source code not in version control)

Phase 3: Create Flex Consumption App

Run the automated migration command from automation.md — Step 4.

The flex-migration start command automatically:

• Assesses your source app for Flex Consumption compatibility
• Creates a new function app in the Flex Consumption plan
• Migrates app settings, identity assignments, storage mounts, CORS, custom domains, and access restrictions

💡 Use Microsoft Entra ID + managed identities instead of connection strings when creating the new app.

Identity-First Configuration (Functions)

Enterprise subscriptions commonly enforce policies blocking local auth. Configure identity-based access:

• Storage accounts: Use AzureWebJobsStorage__credential, __clientId, and service-specific URIs (__blobServiceUri, __queueServiceUri, __tableServiceUri)
• Application Insights: Use APPLICATIONINSIGHTS_AUTHENTICATION_STRING with Authorization=AAD
• When using User Assigned Managed Identity, pass managedIdentityClientId explicitly

Phase 4: Verify and Configure the New App

The flex-migration start command handles most configuration. Verify the results and manually configure anything it doesn't cover (see automation.md):

1. Verify migrated app settings, identities, custom domains
2. Configure built-in authentication (if used — not auto-migrated)
3. Configure scale and concurrency settings (if custom values needed)
4. Enable Application Insights monitoring

Phase 5: Deploy Code

⚠️ Code is NOT automatically migrated. The new app is created with config only — you must deploy code separately.

Use ask_user to present these options:

Your new Flex Consumption app <NEW_APP_NAME> has been created and configured. Now we need to deploy your function code. How would you like to proceed?

>

1. Update CI/CD pipeline — I'll help you update your Azure Pipelines or GitHub Actions workflow to target the new app
2. Deploy from local project — I'll run func azure functionapp publish <NEW_APP_NAME> from your project directory
3. Deploy existing package — I'll deploy the package we downloaded earlier from the original app

After user selects an option, execute the corresponding deployment method from automation.md — Step 5.

⚠️ After deployment, triggers immediately start processing. Review mitigation strategies for your trigger types.

After successful deployment, inform the user:

Code deployed! Next steps to consider:

>

• The original app is still running — keep it as rollback for a few days
• Update any clients/pipelines to point to the new URL
• Enable HTTPS-only and managed identity on the new app for better security
• When confident, you can delete the original app

Phase 6: Post-Upgrade Validation

1. Smoke test — Get the app’s default hostname via az functionapp show --query defaultHostName -o tsv, then hit that URL and confirm it returns a non-error response (HTTP 2xx/4xx, not connection refused or 503). This is the minimum bar before proceeding.
2. Verify the app is running on Flex Consumption (az functionapp show --query sku)
3. Test HTTP trigger endpoints (e.g. curl https://<DEFAULT_HOST>/api/<FUNCTION_NAME>)
4. Capture and compare performance benchmarks
5. Set up monitoring dashboards
6. Refine scale/concurrency settings
7. Update IaC (Bicep/Terraform) to reflect new plan

Phase 7: Cleanup (Optional)

• Keep the original app for a few days/weeks as rollback
• Consumption plan charges only for actual usage — low cost to keep idle
• When confident, delete using the command in automation.md — Step 7

Trigger Migration Risks

IaC Key Differences

Deprecated Settings (Do NOT Migrate)

These app settings are NOT supported in Flex Consumption and should be filtered out:

• WEBSITE_USE_PLACEHOLDER_DOTNETISOLATED
• AzureWebJobsStorage* (replaced by identity-based config)
• WEBSITE_MOUNT_ENABLED
• ENABLE_ORYX_BUILD
• FUNCTIONS_EXTENSION_VERSION (set via functionAppConfig)
• FUNCTIONS_WORKER_RUNTIME (set via functionAppConfig)
• FUNCTIONS_WORKER_RUNTIME_VERSION
• FUNCTIONS_MAX_HTTP_CONCURRENCY
• FUNCTIONS_WORKER_PROCESS_COUNT
• FUNCTIONS_WORKER_DYNAMIC_CONCURRENCY_ENABLED
• SCM_DO_BUILD_DURING_DEPLOYMENT
• WEBSITE_CONTENTAZUREFILECONNECTIONSTRING
• WEBSITE_CONTENTOVERVNET
• WEBSITE_CONTENTSHARE
• WEBSITE_DNS_SERVER
• WEBSITE_MAX_DYNAMIC_APPLICATION_SCALE_OUT
• WEBSITE_NODE_DEFAULT_VERSION
• WEBSITE_RUN_FROM_PACKAGE
• WEBSITE_SKIP_CONTENTSHARE_VALIDATION
• WEBSITE_VNET_ROUTE_ALL
• APPLICATIONINSIGHTS_CONNECTION_STRING (already created in new app)

Troubleshooting

Rollback

1. Restart the original app: az functionapp start --name <ORIGINAL_APP_NAME> --resource-group <RESOURCE_GROUP>
2. Redirect clients back to original resources (queues/topics/containers)
3. Revert DNS or custom domain changes
4. Delete the new Flex Consumption app if needed

References

• Flex Consumption plan overview
• How to use the Flex Consumption plan
• Azure CLI flex-migration commands (Linux only)
• Flex Consumption IaC samples
• Flex Consumption plan deprecations