Backend pools and health probes in Azure - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a backend pool in Azure Load Balancer?

A backend pool is a group of virtual machines or instances that receive traffic from the load balancer. It acts like a team of servers ready to handle requests.

beginner

What is the purpose of a health probe in Azure Load Balancer?

A health probe checks if the backend servers are healthy and able to respond. It helps the load balancer send traffic only to healthy servers.

intermediate

How does a health probe determine if a backend server is healthy?

The health probe sends regular requests (like pings or HTTP checks) to the backend server. If the server responds correctly within a timeout, it is healthy.

intermediate

Why is it important to configure health probes correctly?

Correct health probes prevent sending traffic to servers that are down or slow, improving reliability and user experience.

intermediate

Can backend pools include virtual machines from different availability zones?

Yes, backend pools can include VMs from different availability zones to improve availability and fault tolerance.

What does a backend pool in Azure Load Balancer consist of?

A Network security groups

B Storage accounts for data backup

C Virtual machines or instances that receive traffic

D Azure Functions

Backend pools are groups of VMs or instances that handle incoming traffic from the load balancer.

What is the main role of a health probe?

A To check if backend servers are healthy

B To store user data

C To encrypt network traffic

D To create virtual machines

Health probes monitor backend servers to ensure they can respond to requests.

If a backend server does not respond to a health probe, what happens?

A Traffic is doubled to that server

B The server is automatically deleted

C The server is restarted

D The load balancer stops sending traffic to that server

The load balancer avoids sending traffic to unhealthy servers to maintain service quality.

Which protocol can health probes use to check backend servers?

A TCP, HTTP, or HTTPS

B FTP only

C SMTP only

D SSH only

Health probes can use TCP, HTTP, or HTTPS protocols to verify server health.

Why include VMs from different availability zones in a backend pool?

A To reduce storage costs

B To improve availability and fault tolerance

C To increase CPU speed

D To simplify network security

Using multiple availability zones helps keep services running even if one zone fails.

Explain what a backend pool is and why it is important in Azure Load Balancer.

Think of a backend pool as a team of servers ready to handle user requests.

Describe how health probes work and their impact on load balancer behavior.

Health probes are like health checkups for servers to keep traffic flowing smoothly.

Practice

1. What is the main purpose of a backend pool in Azure Load Balancer?

easy

A. To create virtual networks

B. To monitor the health of servers

C. To store user data securely

D. To group servers that will share incoming user requests

Solution

1. Step 1: Understand backend pool role

   A backend pool groups multiple servers to distribute incoming traffic among them.

2. Step 2: Differentiate from health probes

   Health probes check server status, but backend pools organize servers for load balancing.

3. Final Answer:

   To group servers that will share incoming user requests -> Option D

4. Quick Check:

   Backend pool = group servers for requests [OK]

Hint: Backend pools group servers; health probes check server status [OK]

Common Mistakes:

• Confusing backend pools with health probes
• Thinking backend pools store data
• Mixing backend pools with network creation

2. Which of the following is the correct way to define a health probe in Azure Load Balancer configuration?

easy

A. healthProbe: { protocol: 'TCP', port: 80, intervalInSeconds: 15, numberOfProbes: 2 }

B. healthProbe: { protocol: 'FTP', port: 21, intervalInSeconds: 10, numberOfProbes: 3 }

C. healthProbe: { protocol: 'HTTP', port: 8080, intervalInSeconds: 5, numberOfProbes: 1 }

D. healthProbe: { protocol: 'UDP', port: 53, intervalInSeconds: 20, numberOfProbes: 4 }

Solution

1. Step 1: Identify valid protocols for health probes

   Azure Load Balancer supports TCP and HTTP probes; FTP and UDP are invalid.

2. Step 2: Check probe parameters

   healthProbe: { protocol: 'TCP', port: 80, intervalInSeconds: 15, numberOfProbes: 2 } uses TCP on port 80 with reasonable intervals and probe counts, matching best practices.

3. Final Answer:

   healthProbe: { protocol: 'TCP', port: 80, intervalInSeconds: 15, numberOfProbes: 2 } -> Option A

4. Quick Check:

   Valid protocol and settings = healthProbe: { protocol: 'TCP', port: 80, intervalInSeconds: 15, numberOfProbes: 2 } [OK]

Hint: Use TCP or HTTP protocols for health probes in Azure [OK]

Common Mistakes:

• Using unsupported protocols like FTP or UDP
• Setting too few probes causing false negatives
• Choosing invalid port numbers

3. Given this health probe configuration:

protocol: 'HTTP', port: 8080, intervalInSeconds: 10, numberOfProbes: 3

If the backend server responds successfully on the first two probes but fails on the third, what will Azure Load Balancer do?

medium

A. Mark the server as unhealthy immediately after the first failure

B. Mark the server as unhealthy after three consecutive failures

C. Ignore the probe results and keep the server in the pool

D. Keep the server as healthy because two successful probes passed

Solution

1. Step 1: Understand numberOfProbes meaning

   numberOfProbes defines how many consecutive failed probes mark a server unhealthy.

2. Step 2: Analyze probe results

   Only one failure occurred, so the server is still healthy until 3 consecutive failures happen.

3. Final Answer:

   Mark the server as unhealthy after three consecutive failures -> Option B

4. Quick Check:

   3 failures needed to mark unhealthy = Mark the server as unhealthy after three consecutive failures [OK]

Hint: Server unhealthy after consecutive failed probes count reached [OK]

Common Mistakes:

• Marking unhealthy after a single failure
• Ignoring the numberOfProbes setting
• Assuming any failure removes server immediately

4. You configured a backend pool with three servers and a health probe. One server is always marked unhealthy even though it is running fine. What is the most likely cause?

medium

A. Backend pool has too many servers

B. Load balancer is not assigned a public IP

C. Health probe is configured with the wrong port or protocol

D. The backend server is overloaded with traffic

Solution

1. Step 1: Check health probe configuration

   If the probe uses the wrong port or protocol, it will fail to get a healthy response from the server.

2. Step 2: Rule out other causes

   Too many servers or missing public IP do not cause probe failures; overload affects performance but not probe success directly.

3. Final Answer:

   Health probe is configured with the wrong port or protocol -> Option C

4. Quick Check:

   Wrong probe config causes false unhealthy status [OK]

Hint: Check probe port and protocol if server shows unhealthy wrongly [OK]

Common Mistakes:

• Blaming backend pool size for probe failures
• Ignoring probe configuration details
• Assuming public IP affects probe health

5. You want to ensure high availability for your web app using Azure Load Balancer. You have three backend servers and want to configure health probes and backend pools. Which combination best improves reliability and performance?

hard

A. Create a backend pool with all three servers and a TCP health probe on port 80 with 5-second intervals and 2 probes

B. Create separate backend pools for each server and no health probes

C. Create a backend pool with two servers only and an HTTP health probe on port 8080 with 30-second intervals and 5 probes

D. Create a backend pool with all servers and a health probe using unsupported protocol FTP

Solution

1. Step 1: Use a single backend pool for load distribution

   Grouping all servers in one backend pool balances traffic and improves availability.

2. Step 2: Configure a valid health probe with appropriate settings

   TCP probe on port 80 with short intervals and few probes quickly detects unhealthy servers.

3. Step 3: Evaluate other options

   Separate pools reduce load balancing benefits; no probes risk sending traffic to bad servers; unsupported protocols cause failures.

4. Final Answer:

   Create a backend pool with all three servers and a TCP health probe on port 80 with 5-second intervals and 2 probes -> Option A

5. Quick Check:

   All servers + valid probe = best reliability [OK]

Hint: Use all servers in one pool with valid health probe for best uptime [OK]

Common Mistakes:

• Splitting servers into multiple pools unnecessarily
• Skipping health probes
• Using unsupported protocols for probes