Azure Functions: Premium vs. Consumption

Introduction

Azure Functions, a cornerstone of Microsoft's serverless computing offering, empowers developers to build event-driven, scalable applications without managing infrastructure. At its core, Azure Functions provides two primary hosting plans for executing your code: the Consumption plan and the Premium plan. Understanding the nuances between these two plans is crucial for optimizing performance, managing costs, and ensuring the reliability of your serverless solutions.

This article delves into a detailed comparison of Azure Functions Premium and Consumption plans. We will explore their distinctive features, ideal use cases, and how to effectively choose the right plan for your specific workload. Developers, solution architects, and cloud administrators seeking to maximize their Azure Functions deployments will find this guide invaluable.

Why this matters

The choice between Azure Functions Consumption and Premium plans has significant implications for your application's performance, cost-efficiency, and operational reliability. Opting for the wrong plan can lead to issues ranging from unexpected cold starts impacting user experience to exorbitant cloud bills due to inefficient resource allocation. For businesses handling sensitive data or processing high-volume transactions, consistent performance and predictable costs are paramount, making the plan selection a critical architectural decision.

Furthermore, compliance requirements, especially those necessitating private network access for data processing, often dictate the need for features exclusive to the Premium plan. From a productivity standpoint, minimizing operational overhead associated with infrastructure management is a key benefit of serverless, and selecting the plan that best aligns with your team's expertise and monitoring capabilities is crucial. This decision directly impacts not only the technical solution but also the financial planning and risk management strategies of an organization.

Key concepts

• Serverless Computing: A cloud execution model where the cloud provider dynamically manages the allocation and provisioning of servers, allowing developers to focus solely on writing code.
• Cold Start: The delay experienced when a serverless function is invoked after a period of inactivity, as the underlying infrastructure needs to be provisioned and the function code loaded.
• Warm Instances: Pre-warmed instances of your function app that are kept ready to process requests, eliminating cold start delays.
• Virtual Network (VNet) Integration: The ability for an Azure Function app to connect securely to resources within a private Azure Virtual Network.
• Elastic Scale: The ability of an Azure Function app to automatically scale out (add more instances) or scale in (remove instances) based on incoming request load.
• Consumption Plan: A pay-per-execution and resource consumption model where you are only charged when your function runs. Offers automatic scaling and a free grant of executions.
• Premium Plan (Elastic Premium): A hosting plan that provides pre-warmed instances, VNet connectivity, longer run durations, and more powerful compute options, at a higher, more predictable cost.

Step-by-step implementation

Implementing an Azure Functions app with specific hosting plans typically begins in the Azure portal or via infrastructure-as-code. Here, we'll demonstrate deploying a Premium plan function app using Azure CLI, ensuring VNet integration.

1. Create a Resource Group: If you don't already have one, create a resource group to logically organize your resources.

``bash az group create --name "MyFunctionsRG" --location "eastus" ``

1. Create a Virtual Network and Subnet: For Premium functions requiring VNet integration, you need a dedicated subnet.

``bash az network vnet create --resource-group "MyFunctionsRG" --name "MyVNet" --address-prefix "10.0.0.0/16" az network vnet subnet create --resource-group "MyFunctionsRG" --vnet-name "MyVNet" --name "FunctionsSubnet" --address-prefixes "10.0.1.0/24" --service-endpoints Microsoft.Web ``

1. Create an Azure Storage Account: Functions require a general-purpose storage account.

``bash az storage account create --name "mypremiumfuncsa" --location "eastus" --resource-group "MyFunctionsRG" --sku Standard_LRS --kind StorageV2 ``

1. Create an Azure Functions Premium Plan: Specify the ElasticPremium SKU.

``bash az functionapp plan create --resource-group "MyFunctionsRG" --name "MyPremiumPlan" --location "eastus" --sku "EP1" --is-linux ``

1. Create the Azure Function App and integrate with VNet: Link the function app to the Premium plan, storage account, and the VNet subnet.

```bash SUBNET_ID=$(az network vnet subnet show --resource-group "MyFunctionsRG" --vnet-name "MyVNet" --name "FunctionsSubnet" --query id -o tsv)

az functionapp create --resource-group "MyFunctionsRG" --consumption-plan-location "eastus" \ --name "MyPremiumFunctionApp" \ --storage-account "mypremiumfuncsa" \ --functions-version 4 \ --os-type Linux \ --runtime node \ --plan "MyPremiumPlan" \ --https-only true \ --vnet-route-all-enabled true \ --subnet "$SUBNET_ID" ``` This sequence provisions a Function App within a Premium plan, configured for Linux Node.js runtime, and integrated into a specific subnet of your Virtual Network, which is essential for private network access scenarios.

Example configuration

The following JSON snippet represents an Azure Resource Manager (ARM) template defining an Azure Functions Premium Plan. This provides a declarative way to provision the resources with desired settings.

{
  "$schema": "https://schema.management.azure.com/schemas/2019-08-01/deploymentTemplate.json",
  "contentVersion": "1.0.0.0",
  "parameters": {
    "appName": {
      "type": "string",
      "metadata": {
        "description": "The name of the function app."
      }
    },
    "location": {
      "type": "string",
      "defaultValue": "[resourceGroup().location]",
      "metadata": {
        "description": "Location for all resources."
      }
    },
    "storageAccountName": {
      "type": "string",
      "metadata": {
        "description": "The name of the storage account."
      }
    },
    "functionWorkerRuntime": {
      "type": "string",
      "defaultValue": "dotnet",
      "allowedValues": [
        "dotnet",
        "node",
        "python",
        "java",
        "powershell",
        "custom"
      ],
      "metadata": {
        "description": "The worker runtime for the function app."
      }
    },
    "vnetSubnetId": {
      "type": "string",
      "metadata": {
        "description": "Resource ID of the subnet for VNet integration."
      }
    }
  },
  "resources": [
    {
      "type": "Microsoft.Web/serverfarms",
      "apiVersion": "2020-12-01",
      "name": "[parameters('appName')]",
      "location": "[parameters('location')]",
      "sku": {
        "name": "EP1",
        "tier": "ElasticPremium",
        "size": "1",
        "family": "EP",
        "capacity": 1
      },
      "kind": "functionapp",
      "properties": {
        "isSpot": false,
        "isXenon": false,
        "reserved": true,
        "perSiteScaling": false,
        "elasticSku": {
          "name": "EP1",
          "tier": "ElasticPremium"
        }
      }
    },
    {
      "type": "Microsoft.Web/sites",
      "apiVersion": "2020-12-01",
      "name": "[parameters('appName')]",
      "location": "[parameters('location')]",
      "kind": "functionapp",
      "dependsOn": [
        "[resourceId('Microsoft.Web/serverfarms', parameters('appName'))]"
      ],
      "properties": {
        "serverFarmId": "[resourceId('Microsoft.Web/serverfarms', parameters('appName'))]",
        "siteConfig": {
          "appSettings": [
            {
              "name": "FUNCTIONS_WORKER_RUNTIME",
              "value": "[parameters('functionWorkerRuntime')]"
            },
            {
              "name": "AzureWebJobsStorage",
              "value": "[concat('DefaultEndpointsProtocol=https;AccountName=', parameters('storageAccountName'), ';AccountKey=', listKeys(resourceId('Microsoft.Storage/storageAccounts', parameters('storageAccountName')), '2021-09-01').keys[0].value, ';EndpointSuffix=core.windows.net')]"
            }
          ],
          "cors": {
            "allowedOrigins": [
              "https://ms.portal.azure.com",
              "https://learn.microsoft.com"
            ],
            "supportCredentials": false
          }
        },
        "clientAffinityEnabled": false,
        "httpsOnly": true,
        "virtualNetworkSubnetId": "[parameters('vnetSubnetId')]",
        "vnetRouteAllEnabled": true
      }
    }
  ]
}

This ARM template deploys both an ElasticPremium App Service Plan (Microsoft.Web/serverfarms) and an Azure Function App (Microsoft.Web/sites) configured to use that plan. It also includes properties for VNet integration (virtualNetworkSubnetId and vnetRouteAllEnabled), highlighting a key benefit of the Premium plan.

Common pitfalls

• Underestimating Cold Starts on Consumption: Assuming all workloads are suitable for Consumption without considering varying latency tolerances. Bursty, interactive APIs are often poor candidates due to potential cold start delays.
• Over-provisioning Premium Instances: Deploying a Premium plan with too many pre-warmed instances (e.g., EP2 or EP3 when EP1 suffices) for a low-traffic application, leading to unnecessary costs.
• Ignoring VNet Integration requirements: Attempting to access private network resources from a Consumption plan function, which inherently lacks VNet integration capabilities, leading to connectivity failures.
• Neglecting App Service Plan Scaling: Not configuring appropriate auto-scaling rules for the Premium plan, resulting in either under-scaling (performance bottlenecks) or over-scaling (cost inefficiency).
• Misunderstanding Billing Models: Failing to grasp that Consumption is truly pay-per-execution, while Premium has a base cost per instance regardless of execution count, plus per-execution charges. This can lead to unexpected bills.
• Choosing Premium for Trivial Workloads: Deploying simple, infrequent background tasks on a Premium plan when a Consumption plan would be significantly more cost-effective.

Best practices

• Assess Workload Characteristics: For new projects, thoroughly evaluate your function's invocation frequency, expected latency, and dependency on private network resources. Applications with low-to-medium, inconsistent traffic and high latency tolerance are good candidates for Consumption. High-throughput, low-latency, or VNet-dependent applications should default to Premium, aligning with the performance pillars of the Azure Well-Architected Framework.
• Prioritize Performance-Critical Functions for Premium: Identify functions that are critical to user experience or sensitive business processes where cold starts are unacceptable. Migrate or deploy these on a Premium plan to leverage pre-warmed instances.
• Implement VNet Integration for Secure Access: When functions need to securely access databases, APIs, or other resources residing within a private Azure VNet, utilize the VNet integration feature of the Premium plan. This adheres to Zero Trust principles by restricting network flow.
• Right-Size Premium Plans: Start with the smallest Premium SKU (e.g., EP1) and monitor performance. Adjust the instance size and maximum scale-out limits based on actual load metrics, adhering to the Cost Optimization pillar of the Well-Architected Framework.
• Leverage Azure Advisor for Cost Optimization: Regularly review Azure Advisor recommendations for your Function Apps. It can often suggest opportunities to optimize costs by right-sizing or switching plans based on usage patterns.
• Monitor and Alarm on key metrics: Utilize Azure Monitor to track cold starts, latency, and execution counts for Consumption plans. For Premium plans, monitor CPU, memory, and instances to ensure efficient resource utilization and prevent overspending. This aligns with the Operational Excellence pillar.

Further reading

• Azure Functions hosting options
• Azure Functions Premium plan
• Performance considerations for Azure Functions
• Understand Azure Functions billing
• Integrate Azure Functions with an Azure Virtual Network