Least connections in Nginx - Step-by-Step Execution

Process Flow - Least connections

Client Request Arrives

↓

Check Backend Servers

↓

Find Server with Least Active Connections

↓

Forward Request to That Server

↓

Increase Active Connection Count for That Server

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Server Handles Request

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Decrease Active Connection Count After Response

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Ready for Next Request

When a client request comes, nginx checks all backend servers and sends the request to the one with the fewest active connections, balancing load dynamically.

Execution Sample

Nginx

upstream backend {
    least_conn;
    server backend1.example.com;
    server backend2.example.com;
}

server {
    location / { proxy_pass http://backend; }
}

This config sets up nginx to forward requests to backend servers using the least connections method.

Process Table

Step	Incoming Request	Backend Servers Active Connections	Selected Server	Action
1	Request 1	backend1=0, backend2=0	backend1	Forward to backend1, active connections backend1=1
2	Request 2	backend1=1, backend2=0	backend2	Forward to backend2, active connections backend2=1
3	Request 3	backend1=1, backend2=1	backend1	Forward to backend1, active connections backend1=2
4	Request 4	backend1=2, backend2=1	backend2	Forward to backend2, active connections backend2=2
5	Request 5	backend1=2, backend2=2	backend1	Forward to backend1, active connections backend1=3
6	Request 1 completes	backend1=3, backend2=2	N/A	Decrease backend1 active connections to 2
7	Request 2 completes	backend1=2, backend2=2	N/A	Decrease backend2 active connections to 1
8	Request 3 completes	backend1=2, backend2=1	N/A	Decrease backend1 active connections to 1
9	Request 4 completes	backend1=1, backend2=1	N/A	Decrease backend2 active connections to 0
10	Request 5 completes	backend1=1, backend2=0	N/A	Decrease backend1 active connections to 0

💡 All requests completed, active connections back to zero

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7	After Step 8	After Step 9	After Step 10
backend1 active connections	0	1	1	2	2	3	2	2	1	1	0
backend2 active connections	0	0	1	1	2	2	2	1	1	0	0

Key Moments - 3 Insights

Why does nginx choose backend1 for the first request when both servers have zero connections?

When both servers have equal active connections (0), nginx picks the first server listed, as shown in step 1 of the execution_table.

What happens to active connections when a request finishes?

Active connections for the server handling that request decrease by one, as seen in steps 6 to 10 where connections drop after requests complete.

If backend2 has fewer active connections, will nginx always pick it?

Yes, nginx always forwards to the server with the least active connections, demonstrated in steps 2 and 4 where backend2 is chosen due to fewer connections.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, how many active connections does backend1 have after forwarding the request?

A 1

B 2

C 0

D 3

💡 Hint

Check the 'Backend Servers Active Connections' and 'Action' columns at step 3.

At which step does backend2 first have 2 active connections?

A Step 3

B Step 5

C Step 4

D Step 2

💡 Hint

Look at the 'Backend Servers Active Connections' column for backend2 in the execution_table.

If a new request arrives when backend1=1 and backend2=0, which server will nginx select?

A backend2

B backend1

C Either one randomly

D None, request is queued

💡 Hint

Refer to the selection logic shown in steps 2 and 4 where the server with fewer connections is chosen.

Concept Snapshot

nginx least connections load balancing:
- Use 'least_conn;' in upstream block
- nginx forwards requests to server with fewest active connections
- Active connections increase on request start, decrease on completion
- Balances load dynamically based on current server usage
- Helps avoid overloading busy servers

Full Transcript

This visual execution shows how nginx uses the least connections method to balance incoming requests. When a request arrives, nginx checks all backend servers and forwards the request to the one with the fewest active connections. Active connections increase when a request is forwarded and decrease when the request completes. The execution table traces five requests arriving and completing, showing how nginx picks servers and updates connection counts. Key moments clarify why nginx picks the first server when connections are equal, how connections decrease after requests finish, and how nginx always picks the least busy server. The quiz tests understanding of connection counts and server selection at different steps.