Create a backend pool

3.2 Azure Application Gateway

📘Microsoft Azure Networking Solutions (AZ-700)

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A backend pool in Azure Application Gateway is basically the group of servers or services that will handle the traffic coming through the gateway. When users send requests to your application, the Application Gateway routes that traffic to one or more servers in the backend pool.

Think of it as the “team” that will respond to requests your gateway receives.

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1. Purpose of a Backend Pool

• It tells the Application Gateway where to send traffic.
• Supports load balancing between multiple backend resources.
• Can include different types of backend targets:
  • Azure Virtual Machines (VMs) – your own servers hosted in Azure.
  • Virtual Machine Scale Sets (VMSS) – automatically scalable sets of VMs.
  • App Services – Azure PaaS web apps.
  • IP Addresses or FQDNs – external or on-premises servers.

Example in IT context:
If you have three web servers hosting the same website, the backend pool ensures incoming traffic is distributed evenly to all three servers.

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2. Backend Pool Types

There are two main ways to define backend targets:

1. IP-based or FQDN-based
   • Use when backend is an IP address or DNS name.
   • Useful for non-Azure servers or on-premises servers.
   • Example: 10.0.0.5 or webserver.contoso.com.
2. Azure Resource-based
   • Use Azure VMs, VMSS, or App Services directly.
   • Azure automatically handles network details.
   • Example: adding a VM named WebVM1 in the backend pool.

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3. Steps to Create a Backend Pool in Azure Portal

1. Go to Azure Portal → Application Gateway → Backend pools → Add.
2. Give the backend pool a name (e.g., WebBackendPool).
3. Choose the target type:
   • IP addresses or FQDNs
   • Azure resource (VM, VMSS, App Service)
4. Add backend targets:
   • For VMs: select the VMs you want to include.
   • For IP/FQDN: type the IP addresses or DNS names of the servers.
5. Optionally configure HTTP settings:
   • These define how the gateway connects to the backend (port, protocol, etc.).
6. Click Add to create the backend pool.

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4. Important Configuration Points for Exams

• Backend pool name must be unique within the Application Gateway.
• Mixing target types: You can mix VMs and IPs in the same pool if needed.
• Health Probes: Application Gateway uses health probes to check if a backend is healthy. Only healthy backends receive traffic.
• HTTP Settings: Each backend pool is associated with HTTP settings defining:
  • Protocol (HTTP/HTTPS)
  • Port number (e.g., 80 for HTTP, 443 for HTTPS)
  • Cookie-based affinity (optional)

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5. Backend Pool in Action (IT Example)

• Scenario: You have a web app deployed on two Azure VMs (WebVM1 and WebVM2) and a third App Service (WebApp1).
• Backend Pool:
  • Includes WebVM1, WebVM2, and WebApp1.
  • Application Gateway receives incoming requests from users.
  • It checks which backend is healthy using health probes.
  • Routes traffic to healthy backends based on load-balancing rules.

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6. Exam Tips

• Know the types of backend targets (VM, VMSS, App Service, IP/FQDN).
• Understand the relationship between backend pools, HTTP settings, and listeners.
• Be familiar with health probes and why they are important.
• Remember: Backend pools are the destinations for your traffic, not the source.

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Quick Exam Summary Table

Feature	Explanation
Backend Pool	Group of servers or services that handle traffic from Application Gateway
Target Types	Azure VMs, VMSS, App Services, IP address, or FQDN
HTTP Settings	Defines protocol, port, and connection settings to backend
Health Probes	Monitors backend health; only healthy instances receive traffic
Load Balancing	Distributes traffic evenly among healthy backend instances

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This covers everything you need to create and understand backend pools for the AZ-700 exam, including all configurations, target types, and exam-focused points.