Load balancing rules in Azure - Step-by-Step Execution

Process Flow - Load balancing rules

Client sends request

↓

Load Balancer receives request

↓

Check load balancing rule

↓

Select backend pool based on rule

↓

Forward request to selected backend

↓

Backend processes request and responds

↓

Response sent back to client

The load balancer receives client requests, applies the load balancing rule to select a backend server, forwards the request, and returns the response.

Execution Sample

Azure

az network lb rule create \
  --resource-group MyResourceGroup \
  --lb-name MyLoadBalancer \
  --name MyLoadBalancingRule \
  --protocol Tcp \
  --frontend-port 80 \
  --backend-port 80 \
  --frontend-ip-name MyFrontEnd \
  --backend-pool-name MyBackEndPool

Creates a load balancing rule that forwards TCP traffic on port 80 from the frontend IP to backend pool servers on port 80.

Process Table

Step	Action	Input	Rule Evaluation	Backend Selected	Output
1	Client sends request	Request to LB IP:80	N/A	N/A	Request received by Load Balancer
2	Load Balancer checks rule	Request on port 80 TCP	Matches rule for port 80 TCP	Backend Pool MyBackEndPool	Rule matched
3	Select backend server	Backend Pool Members: VM1, VM2	Round-robin selection	VM1	Request forwarded to VM1
4	Backend processes request	Request at VM1 port 80	N/A	VM1	Response generated
5	Response sent back	Response from VM1	N/A	N/A	Response returned to client
6	Next request	Request to LB IP:80	Matches rule for port 80 TCP	VM2	Request forwarded to VM2
7	Backend processes request	Request at VM2 port 80	N/A	VM2	Response generated
8	Response sent back	Response from VM2	N/A	N/A	Response returned to client
9	Request on different port	Request to LB IP:443	No matching rule	N/A	Request dropped or rejected

💡 Execution stops when no matching load balancing rule is found or request is completed.

Status Tracker

Variable	Start	After Step 3	After Step 6	Final
Selected Backend	None	VM1	VM2	VM2
Request Port	80	80	80	443
Rule Matched	False	True	True	False

Key Moments - 3 Insights

Why does the load balancer forward the first request to VM1 and the second to VM2?

Because the load balancing rule uses round-robin distribution, it cycles through backend servers to balance load evenly, as shown in execution_table rows 3 and 6.

What happens if a request comes on a port not defined in any load balancing rule?

The load balancer does not forward the request to any backend and drops or rejects it, as seen in execution_table row 9.

Does the load balancer change the port number when forwarding the request to the backend?

No, the load balancer forwards the request to the backend on the backend port defined in the rule, which can be the same or different from the frontend port, as shown in the execution sample command.

Visual Quiz - 3 Questions

Look at the execution table, which backend server handles the second client request?

A VM3

B VM1

C VM2

D No backend selected

💡 Hint

Check the 'Backend Selected' column at Step 6 in the execution_table.

At which step does the load balancer determine that no matching rule exists for the request?

A Step 9

B Step 2

C Step 5

D Step 1

💡 Hint

Look for the step where 'Rule Evaluation' shows 'No matching rule' in the execution_table.

If the frontend port in the rule changed from 80 to 443, what would happen to requests on port 80?

A They would be forwarded to backend servers

B They would be dropped or rejected

C They would be forwarded to port 443 on backend

D They would be redirected to another load balancer

💡 Hint

Refer to the 'Rule Evaluation' and 'Output' columns in execution_table row 9.

Concept Snapshot

Load balancing rules define how incoming traffic is distributed to backend servers.
They specify frontend IP, protocol, frontend port, backend port, and backend pool.
The load balancer matches requests to rules and forwards them accordingly.
Common distribution methods include round-robin.
Requests not matching any rule are dropped or rejected.

Full Transcript

Load balancing rules in Azure define how the load balancer distributes incoming client requests to backend servers. When a client sends a request, the load balancer checks its rules to find a match based on protocol and port. If a rule matches, it selects a backend server from the backend pool, often using round-robin, and forwards the request. The backend processes the request and sends a response back through the load balancer to the client. Requests that do not match any rule are dropped or rejected. This process ensures even distribution of traffic and high availability.