1# App Service Deployment Slots
2 
3Zero-downtime deployments using staging slots.
4 
5## Basic Staging Slot
6 
7```bicep
8resource stagingSlot 'Microsoft.Web/sites/slots@2022-09-01' = {
9  parent: webApp
10  name: 'staging'
11  location: location
12  properties: {
13    serverFarmId: appServicePlan.id
14  }
15}
16```
17 
18## Slot Requirements — App Service
19 
20| SKU Tier | Slots Supported |
21|----------|-----------------|
22| Free/Shared | 0 |
23| Basic | 0 |
24| Standard | 5 |
25| Premium | 20 |
26 
27## Slot Requirements — Azure Functions
28 
29> ⚠️ Slot support for Azure Functions varies by OS and hosting plan.
30 
31| Hosting Plan | OS | Slots Supported |
32|---|---|---|
33| Flex Consumption (FC1) | Linux | ❌ 0 |
34| Consumption (Y1) | **Windows** | ✅ 1 staging slot |
35| Consumption (Y1) | Linux | ❌ 0 |
36| Elastic Premium (EP1-EP3) | Windows or Linux | ✅ 20 slots |
37| Dedicated (Standard+) | Windows or Linux | ✅ 5–20 slots |
38 
39> 💡 **For Azure Functions requiring deployment slots:**
40> - **Windows Consumption (Y1) supports 1 staging slot** — this is a supported platform capability.
41>   If you need it, use it. See the Bicep example below.
42> - Recommendation for new projects: prefer **Elastic Premium (EP1+)** (no cold starts, VNet integration)
43>   or a **Dedicated plan (Standard+)**. Y1 cold starts can affect slot swap warm-up reliability.
44> - **Linux Consumption and Flex Consumption do not support deployment slots.**
45 
46### Windows Consumption Function App with Staging Slot (Bicep)
47 
48```bicep
49resource functionAppPlan 'Microsoft.Web/serverfarms@2022-09-01' = {
50  name: '${resourcePrefix}-funcplan-${uniqueHash}'
51  location: location
52  sku: { name: 'Y1', tier: 'Dynamic' }
53  // No 'reserved: true' — Windows Consumption
54}
55 
56resource functionApp 'Microsoft.Web/sites@2022-09-01' = {
57  name: '${resourcePrefix}-${serviceName}-${uniqueHash}'
58  location: location
59  kind: 'functionapp'  // Windows (no 'linux' suffix)
60  identity: { type: 'SystemAssigned' }
61  properties: {
62    serverFarmId: functionAppPlan.id
63    httpsOnly: true
64    siteConfig: {
65      appSettings: [
66        { name: 'WEBSITE_NODE_DEFAULT_VERSION', value: '~20' }
67        { name: 'FUNCTIONS_EXTENSION_VERSION', value: '~4' }
68        { name: 'FUNCTIONS_WORKER_RUNTIME', value: 'node' }
69        { name: 'WEBSITE_CONTENTAZUREFILECONNECTIONSTRING', value: 'DefaultEndpointsProtocol=https;AccountName=${storageAccount.name};AccountKey=${storageAccount.listKeys().keys[0].value}' }
70        { name: 'WEBSITE_CONTENTSHARE', value: '${toLower(serviceName)}-prod' }
71        { name: 'AzureWebJobsStorage', value: 'DefaultEndpointsProtocol=https;AccountName=${storageAccount.name};AccountKey=${storageAccount.listKeys().keys[0].value}' }
72        { name: 'APPLICATIONINSIGHTS_CONNECTION_STRING', value: applicationInsights.properties.ConnectionString }
73      ]
74    }
75  }
76}
77 
78// Staging slot — only 1 staging slot supported on Consumption
79resource stagingSlot 'Microsoft.Web/sites/slots@2022-09-01' = {
80  parent: functionApp
81  name: 'staging'
82  location: location
83  kind: 'functionapp'
84  properties: {
85    serverFarmId: functionAppPlan.id
86    siteConfig: {
87      appSettings: [
88        { name: 'WEBSITE_NODE_DEFAULT_VERSION', value: '~20' }
89        { name: 'FUNCTIONS_EXTENSION_VERSION', value: '~4' }
90        { name: 'FUNCTIONS_WORKER_RUNTIME', value: 'node' }
91        { name: 'WEBSITE_CONTENTAZUREFILECONNECTIONSTRING', value: 'DefaultEndpointsProtocol=https;AccountName=${storageAccount.name};AccountKey=${storageAccount.listKeys().keys[0].value}' }
92        { name: 'WEBSITE_CONTENTSHARE', value: '${toLower(serviceName)}-staging' }  // MUST differ from production
93        { name: 'AzureWebJobsStorage', value: 'DefaultEndpointsProtocol=https;AccountName=${storageAccount.name};AccountKey=${storageAccount.listKeys().keys[0].value}' }
94        { name: 'APPLICATIONINSIGHTS_CONNECTION_STRING', value: applicationInsights.properties.ConnectionString }
95      ]
96    }
97  }
98}
99```
100 
101> ⚠️ `WEBSITE_CONTENTSHARE` **must be unique per slot** on Windows Consumption — each slot needs its own file share.
102> Use slot-sticky settings (via `slotConfigNames`) for `WEBSITE_CONTENTSHARE` and `WEBSITE_CONTENTAZUREFILECONNECTIONSTRING`
103> so these values do not swap with production.
104 
105## Deployment Flow
106 
1071. Deploy to staging slot
1082. Warm up and test staging
1093. Swap staging with production
1104. Rollback by swapping again if needed
111 
112## Slot Settings
113 
114Configure settings that should not swap:
115 
116```bicep
117resource slotConfigNames 'Microsoft.Web/sites/config@2022-09-01' = {
118  parent: webApp
119  name: 'slotConfigNames'
120  properties: {
121    appSettingNames: [
122      'APPLICATIONINSIGHTS_CONNECTION_STRING'
123    ]
124  }
125}
126```
127