Azure Load Balancer (Layer 4) - Step-by-Step Execution

Process Flow - Azure Load Balancer (Layer 4)

Client sends request

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Azure Load Balancer receives request

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Check load balancing rules

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Select backend VM based on health and distribution

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Forward request to selected VM

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VM processes request and sends response

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Load Balancer forwards response back to client

The Azure Load Balancer receives client requests, selects a healthy backend VM using Layer 4 rules, forwards the request, and returns the response.

Execution Sample

Azure

1. Client sends TCP request to Load Balancer IP
2. Load Balancer checks rules and health probes
3. Load Balancer forwards request to healthy VM
4. VM processes and responds
5. Load Balancer sends response back to client

This sequence shows how Azure Load Balancer routes a TCP request to a backend VM and returns the response.

Process Table

Step	Action	Load Balancer Decision	Backend VM Selected	Response Sent
1	Client sends TCP request to Load Balancer IP	Received request	None yet	No
2	Load Balancer checks load balancing rules	Rule matched for TCP port 80	None yet	No
3	Load Balancer checks health probes	Backend VM1 healthy, VM2 unhealthy	VM1 selected	No
4	Load Balancer forwards request to VM1	Request forwarded	VM1	No
5	VM1 processes request	Processing	VM1	No
6	VM1 sends response to Load Balancer	Response received	VM1	No
7	Load Balancer forwards response to client	Response forwarded	VM1	Yes
8	End of request cycle	Request complete	VM1	Yes

💡 Request cycle ends after response is forwarded back to client.

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 7	Final
Request State	Not received	Received	Received	Forwarded to VM1	Response forwarded	Complete
Selected Backend VM	None	None	VM1	VM1	VM1	VM1
Response State	None	None	None	None	Received from VM1	Sent to client

Key Moments - 3 Insights

Why does the Load Balancer select VM1 and not VM2?

Because the health probe check at step 3 shows VM1 is healthy and VM2 is unhealthy, so only VM1 is selected as backend.

Does the Load Balancer inspect the content of the request?

No, Azure Load Balancer works at Layer 4, so it only looks at IP and port, not the request content, as shown in step 2.

When is the client response sent back?

At step 7, after the backend VM processes the request and sends the response to the Load Balancer, which then forwards it to the client.

Visual Quiz - 3 Questions

Look at the execution table, which backend VM is selected at step 3?

A VM1

B VM2

C None

D Both VM1 and VM2

💡 Hint

Check the 'Backend VM Selected' column at step 3 in the execution table.

At which step does the Load Balancer forward the response back to the client?

A Step 4

B Step 6

C Step 7

D Step 8

💡 Hint

Look at the 'Response Sent' column in the execution table to find when it changes to 'Yes'.

If VM1 became unhealthy, what would change in the execution table at step 3?

A Load Balancer would select VM2 if healthy

B Load Balancer would select VM1 anyway

C Load Balancer would not forward the request

D Load Balancer would forward to both VMs

💡 Hint

Refer to the 'Load Balancer Decision' and 'Backend VM Selected' columns at step 3.

Concept Snapshot

Azure Load Balancer (Layer 4):
- Operates at transport layer (TCP/UDP)
- Routes traffic based on IP and port
- Uses health probes to select healthy backend VMs
- Forwards client requests to backend VMs
- Returns VM responses back to clients
- Does not inspect request content

Full Transcript

Azure Load Balancer at Layer 4 receives client TCP requests and uses load balancing rules and health probes to select a healthy backend VM. It forwards the request to the selected VM, which processes it and sends a response back. The Load Balancer then forwards this response to the client, completing the request cycle. It works by routing based on IP and port without inspecting the request content.