Build Low-Latency, VNET-Secure Serverless APIs with Azure Functions Flex Consumption

Are you struggling to build secure, low-latency APIs on Azure without spinning up expensive always-on infrastructure?

Traditional serverless models like the Azure Functions Consumption Plan are great for scaling, but they fall short when it comes to VNET integration and consistent low latency. Enterprises often need to connect serverless APIs to internal databases or secure networks — and until recently, that meant upgrading to Premium Plans or sacrificing the cost benefits of serverless.

That’s where the Azure Functions Flex Consumption Plan changes the game. It brings together the elasticity of serverless, the security of VNETs, and latency performance that matches dedicated infrastructure — all while keeping your costs optimized.

What is Azure Functions Flex Consumption?

Azure Functions Flex Consumption is the newest hosting plan designed to power enterprise-grade serverless applications. It offers more control and flexibility without giving up the pay-per-use efficiency of the traditional Consumption Plan.

Key capabilities include:

• – Low Latency: Near-instant cold start response with pre-warmed instances.
• – VNET Integration: Securely connect to private endpoints, databases, and internal services.
• – Elastic Scaling: Automatically scales up during traffic spikes and scales down to zero when idle.
• – Optimized Cost: Pay only for active usage — no idle VM costs.

Why This Matters

APIs are the backbone of every digital product. In industries like finance, retail, and healthcare, response times and data security are mission critical. Flex Consumption ensures your serverless APIs are always ready, fast, and safely contained within your private network — ideal for internal or hybrid architectures.

VNET Integration: Security Without Complexity

Security has always been the biggest limitation of traditional serverless plans. With Flex Consumption, Azure Functions can now run inside your Virtual Network (VNET).

This allows your Functions to:

• a. Access private resources such as databases or microservices through Private Endpoints.
• b. Stay isolated from the public internet, reducing attack surfaces.
• c. Use Managed Identity for secure authentication with other Azure resources.
• d. Support hybrid connectivity scenarios where on-premises systems are integrated securely with cloud APIs.

In short: You can now build fully private, VNET-secure APIs without maintaining dedicated infrastructure.

Building a VNET-Secure Serverless API: Step-by-Step

Step 1: Create a Function App in Flex Consumption Plan

az functionapp create \
  --resource-group MyResourceGroup \
  --name MyFlexFuncApp \
  --plan MyFlexPlan \
  --runtime dotnet-isolated \
  --vnet MyVnet

Step 2: Configure VNET Integration

• -Go to your Function App → Networking → VNET Integration.
• -Select the subnet you want to integrate with (ensure it supports Private Endpoints).

Step 3: Deploy Your API Code
Use Azure DevOps, GitHub Actions, or VS Code to deploy your function app just like any other Azure Function.

Step 4: Secure Your API

• -Enable Private Endpoint access.
• -Restrict inbound traffic using Application Gateway or Front Door.
• -Use Azure Managed Identity to connect securely with databases and storage accounts.

How It Compares to Other Hosting Plans

Flex Consumption truly combines the best of both worlds – the agility of serverless and the power of enterprise networking.

Real-World Use Case Example

A large retail enterprise needed to modernize its internal inventory API system.
They were running on Premium Functions Plan for VNET access but were overpaying due to idle resource costs.

After migrating to Flex Consumption, they achieved:

• –65% cost reduction in monthly compute charges.
• –Sub-second cold starts on API requests.
• –Secure, private network isolation within the existing VNET.

This allowed them to maintain compliance, improve responsiveness, and simplify their architecture — all with minimal migration effort.

To conclude, in today’s API-driven world, you shouldn’t have to choose between speed, cost, and security. With Azure Functions Flex Consumption, you can finally deploy VNET-secure, low-latency serverless APIs that scale seamlessly and stay protected inside your private network.

Next Step:
Start by migrating one of your internal APIs to the Flex Consumption Plan. Test the latency, monitor costs, and see the difference in performance.

We hope you found this blog useful, and if you would like to discuss anything, you can reach out to us at transform@cloudFronts.com