1# Consumption Plan to Flex Consumption Plan Upgrade
2 
3> **Source**: Azure Functions Consumption Plan (Y1/Dynamic) on Linux
4> **Target**: Azure Functions Flex Consumption Plan (FC1/FlexConsumption)
5> **Platform**: Linux only (Windows support planned for future)
6> **Docs**: [Linux migration guide](https://learn.microsoft.com/en-us/azure/azure-functions/migration/migrate-plan-consumption-to-flex?pivots=platform-linux)
7 
8## Why Upgrade?
9 
10- **Faster cold starts** — always-ready instances mean functions respond more quickly
11- **Better scaling** — per-function scaling and concurrency controls
12- **Virtual network support** — connect to private networks and use private endpoints
13- **Active investment** — Flex Consumption is where new features land first
14 
15## What to Expect
16 
171. Your **code stays the same** — no rewriting required if on a supported language version
182. You'll **create a new app** alongside the existing one
193. New app runs in the **same resource group** with access to the same dependencies
204. You **control the timing** — test thoroughly before switching over
21 
22## Platform Notes
23 
24- Linux Consumption is being **retired September 30, 2028**
25- Azure CLI provides automated migration commands: `az functionapp flex-migration list` and `az functionapp flex-migration start`
26- The `flex-migration` commands handle assessment, app creation, and most configuration migration automatically
27- Deployment packages are in `squashfs` format stored in `scm-releases` blob container
28 
29## Prerequisites
30 
31- Azure CLI v2.77.0+
32- `resource-graph` extension: `az extension add --name resource-graph`
33- `jq` tool for JSON processing
34- Owner or Contributor role in the resource group
35- Permissions to create and manage function apps, storage accounts, App Insights resources, and managed identity role assignments
36 
37## Compatibility Requirements
38 
39### Supported Language Stacks
40 
41| Stack ID | Language | Supported? |
42|----------|----------|------------|
43| `dotnet-isolated` | .NET (isolated worker model) | ✅ Yes |
44| `node` | JavaScript/TypeScript | ✅ Yes |
45| `java` | Java | ✅ Yes |
46| `python` | Python | ✅ Yes |
47| `powershell` | PowerShell | ✅ Yes |
48| `dotnet` | .NET (in-process model) | ❌ No — must migrate to isolated first |
49| `custom` | Custom handlers | ✅ Yes |
50 
51### Known Limitations (Flex Consumption)
52 
53| Feature | Status | Impact |
54|---------|--------|--------|
55| Deployment slots | ❌ Not supported | Rearchitect to use separate apps |
56| TLS/SSL certificates | ❌ Not supported | Wait for support or find alternative |
57| Blob trigger (polling) | ❌ Only EventGrid source | Convert `LogsAndContainerScan` → `EventGrid` |
58| Azure Government | ❌ Not available | Cannot migrate yet |
59 
60## Upgrade Phases
61 
62### Phase 1: Assessment
63 
64Run all checks from [assessment.md](assessment.md). Use the automated eligibility check:
65 
66```bash
67# Automated eligibility check — scans all Linux Consumption apps
68az functionapp flex-migration list
69```
70 
71This returns `eligible_apps` and `ineligible_apps` arrays with specific reasons for any incompatibilities. For detailed manual checks, see [automation.md](automation.md).
72 
73### Phase 2: Pre-migration (Collect Everything)
74 
75Collect all settings and configurations from the existing app before creating the new one. See **Step-by-step scripts** in [automation.md](automation.md):
76 
771. Collect app settings
782. Collect application configurations (CORS, custom domains, HTTP version, TLS, client certs, etc.)
793. Identify managed identities and RBAC role assignments
804. Identify built-in authentication settings
815. Review inbound access restrictions
826. Get the code deployment package (if source code not in version control)
83 
84### Phase 3: Create Flex Consumption App
85 
86Run the automated migration command from [automation.md](automation.md) — Step 4.
87 
88The `flex-migration start` command automatically:
89- Assesses your source app for Flex Consumption compatibility
90- Creates a new function app in the Flex Consumption plan
91- Migrates app settings, identity assignments, storage mounts, CORS, custom domains, and access restrictions
92 
93> 💡 Use Microsoft Entra ID + managed identities instead of connection strings when creating the new app.
94 
95### Identity-First Configuration (Functions)
96 
97Enterprise subscriptions commonly enforce policies blocking local auth. Configure identity-based access:
98 
99- **Storage accounts**: Use `AzureWebJobsStorage__credential`, `__clientId`, and service-specific URIs (`__blobServiceUri`, `__queueServiceUri`, `__tableServiceUri`)
100- **Application Insights**: Use `APPLICATIONINSIGHTS_AUTHENTICATION_STRING` with `Authorization=AAD`
101- When using User Assigned Managed Identity, pass `managedIdentityClientId` explicitly
102 
103### Phase 4: Verify and Configure the New App
104 
105The `flex-migration start` command handles most configuration. Verify the results and manually configure anything it doesn't cover (see [automation.md](automation.md)):
106 
1071. **Verify** migrated app settings, identities, custom domains
1082. **Configure** built-in authentication (if used — not auto-migrated)
1093. **Configure** scale and concurrency settings (if custom values needed)
1104. **Enable** Application Insights monitoring
111 
112### Phase 5: Deploy Code
113 
114> ⚠️ **Code is NOT automatically migrated.** The new app is created with config only — you must deploy code separately.
115 
116**Use `ask_user` to present these options:**
117 
118> 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?
119>
120> 1. **Update CI/CD pipeline** — I'll help you update your Azure Pipelines or GitHub Actions workflow to target the new app
121> 2. **Deploy from local project** — I'll run `func azure functionapp publish <NEW_APP_NAME>` from your project directory
122> 3. **Deploy existing package** — I'll deploy the package we downloaded earlier from the original app
123 
124After user selects an option, execute the corresponding deployment method from [automation.md](automation.md) — Step 5.
125 
126> ⚠️ After deployment, triggers immediately start processing. Review mitigation strategies for your trigger types.
127 
128**After successful deployment, inform the user:**
129 
130> Code deployed! Next steps to consider:
131>
132> - The original app is still running — keep it as rollback for a few days
133> - Update any clients/pipelines to point to the new URL
134> - Enable HTTPS-only and managed identity on the new app for better security
135> - When confident, you can delete the original app
136 
137### Phase 6: Post-Upgrade Validation
138 
1391. **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.
1402. Verify the app is running on Flex Consumption (`az functionapp show --query sku`)
1413. Test HTTP trigger endpoints (e.g. `curl https://<DEFAULT_HOST>/api/<FUNCTION_NAME>`)
1424. Capture and compare performance benchmarks
1435. Set up monitoring dashboards
1446. Refine scale/concurrency settings
1457. Update IaC (Bicep/Terraform) to reflect new plan
146 
147### Phase 7: Cleanup (Optional)
148 
149- Keep the original app for a few days/weeks as rollback
150- Consumption plan charges only for actual usage — low cost to keep idle
151- When confident, delete using the command in [automation.md](automation.md) — Step 7
152 
153## Trigger Migration Risks
154 
155| Trigger Type | Risk | Mitigation |
156|-------------|------|------------|
157| Azure Blob storage | High | Create separate container for event-based trigger in new app |
158| Azure Cosmos DB | High | Create dedicated lease container for new app; set `StartFromBeginning: false` |
159| Azure Event Grid | Medium | Recreate event subscriptions; ensure idempotent functions |
160| Azure Event Hubs | Medium | Create new consumer group for new app |
161| Azure Service Bus | High | Create new topic/queue; update senders; drain original before shutdown |
162| Azure Storage Queue | High | Create new queue; update senders; drain original before shutdown |
163| HTTP | Low | Update clients to target new app URL |
164| Timer | Low | Offset schedules during cutover to avoid simultaneous execution |
165 
166## IaC Key Differences
167 
168| Property | Consumption | Flex Consumption |
169|----------|-------------|------------------|
170| SKU | `Y1` (Dynamic) | `FC1` (FlexConsumption) |
171| Plan required | Optional (auto-created) | Required (must be explicit) |
172| OS | Linux | Linux only |
173| Configuration | App settings | `functionAppConfig` section |
174| Storage | `WEBSITE_CONTENTSHARE` setting | `deployment.storage` in `functionAppConfig` |
175 
176## Deprecated Settings (Do NOT Migrate)
177 
178These app settings are NOT supported in Flex Consumption and should be filtered out:
179 
180- `WEBSITE_USE_PLACEHOLDER_DOTNETISOLATED`
181- `AzureWebJobsStorage*` (replaced by identity-based config)
182- `WEBSITE_MOUNT_ENABLED`
183- `ENABLE_ORYX_BUILD`
184- `FUNCTIONS_EXTENSION_VERSION` (set via `functionAppConfig`)
185- `FUNCTIONS_WORKER_RUNTIME` (set via `functionAppConfig`)
186- `FUNCTIONS_WORKER_RUNTIME_VERSION`
187- `FUNCTIONS_MAX_HTTP_CONCURRENCY`
188- `FUNCTIONS_WORKER_PROCESS_COUNT`
189- `FUNCTIONS_WORKER_DYNAMIC_CONCURRENCY_ENABLED`
190- `SCM_DO_BUILD_DURING_DEPLOYMENT`
191- `WEBSITE_CONTENTAZUREFILECONNECTIONSTRING`
192- `WEBSITE_CONTENTOVERVNET`
193- `WEBSITE_CONTENTSHARE`
194- `WEBSITE_DNS_SERVER`
195- `WEBSITE_MAX_DYNAMIC_APPLICATION_SCALE_OUT`
196- `WEBSITE_NODE_DEFAULT_VERSION`
197- `WEBSITE_RUN_FROM_PACKAGE`
198- `WEBSITE_SKIP_CONTENTSHARE_VALIDATION`
199- `WEBSITE_VNET_ROUTE_ALL`
200- `APPLICATIONINSIGHTS_CONNECTION_STRING` (already created in new app)
201 
202## Troubleshooting
203 
204| Issue | Remediation |
205|-------|-------------|
206| Cold start performance issues | Review concurrency settings; check for missing dependencies |
207| Missing bindings | Verify extension bundles; update binding configurations |
208| Permission errors | Check identity assignments and role permissions |
209| Network connectivity | Validate access restrictions and networking settings |
210| Missing App Insights | Recreate the Application Insights connection |
211| App fails to start | Check portal Diagnose & Solve; review App Insights Failures blade |
212| Triggers not processing | Verify binding configs, connection strings, consumer groups |
213 
214## Rollback
215 
2161. Restart the original app: `az functionapp start --name <ORIGINAL_APP_NAME> --resource-group <RESOURCE_GROUP>`
2172. Redirect clients back to original resources (queues/topics/containers)
2183. Revert DNS or custom domain changes
2194. Delete the new Flex Consumption app if needed
220 
221## References
222 
223- [Flex Consumption plan overview](https://learn.microsoft.com/en-us/azure/azure-functions/flex-consumption-plan)
224- [How to use the Flex Consumption plan](https://learn.microsoft.com/en-us/azure/azure-functions/flex-consumption-how-to)
225- [Azure CLI flex-migration commands](https://learn.microsoft.com/en-us/cli/azure/functionapp/flex-migration) (Linux only)
226- [Flex Consumption IaC samples](https://github.com/Azure-Samples/azure-functions-flex-consumption-samples/tree/main/IaC)
227- [Flex Consumption plan deprecations](https://learn.microsoft.com/en-us/azure/azure-functions/functions-app-settings#flex-consumption-plan-deprecations)
228