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

↓

Server Handles Request

↓

Decrease Active Connection Count After Response

↓

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.

Practice

1. What does the least_conn directive do in an nginx upstream block?

easy

A. Sends new requests to the server with the fewest active connections

B. Sends new requests to the server with the fastest response time

C. Sends new requests to servers in a round-robin order

D. Sends new requests to the server with the most active connections

Solution

1. Step 1: Understand the purpose of least_conn

   The least_conn directive is used to balance load by sending requests to the server with the fewest active connections.

2. Step 2: Compare with other load balancing methods

   Other methods like round-robin or fastest response time differ; least_conn focuses on active connections count.

3. Final Answer:

   Sends new requests to the server with the fewest active connections -> Option A

4. Quick Check:

   Least connections = fewest active connections [OK]

Hint: Least connections means pick server with fewest active users [OK]

Common Mistakes:

• Confusing least connections with round-robin
• Thinking it picks fastest server instead
• Assuming it sends to server with most connections

2. Which of the following is the correct syntax to enable least connections load balancing in nginx?

easy

A. upstream backend { least_conn; server 192.168.1.1; }

B. upstream backend { least_connections; server 192.168.1.1; }

C. upstream backend { least_conn_load; server 192.168.1.1; }

D. upstream backend { leastconn; server 192.168.1.1; }

Solution

1. Step 1: Recall the exact directive name

   The correct directive to enable least connections is least_conn inside the upstream block.

2. Step 2: Verify syntax correctness

   Options A, B, and D use incorrect directive names or misspellings.

3. Final Answer:

   upstream backend { least_conn; server 192.168.1.1; } -> Option A

4. Quick Check:

   Correct directive = least_conn [OK]

Hint: Use exact directive name 'least_conn' inside upstream block [OK]

Common Mistakes:

• Using 'least_connections' instead of 'least_conn'
• Misspelling the directive as 'leastconn' or 'least_conn_load'
• Placing directive outside the upstream block

3. Given this nginx upstream configuration:
<pre>upstream backend { least_conn; server 10.0.0.1; server 10.0.0.2; } If 10.0.0.1 has 5 active connections and 10.0.0.2 has 3 active connections, where will the next request be sent?

medium

A. To 10.0.0.1 because it is listed first

B. To both servers equally

C. To 10.0.0.2 because it has fewer active connections

D. To 10.0.0.1 because it has more active connections

Solution

1. Step 1: Identify the load balancing method

   The least_conn method sends requests to the server with the fewest active connections.

2. Step 2: Compare active connections of servers

   10.0.0.1 has 5 connections, 10.0.0.2 has 3 connections, so the next request goes to 10.0.0.2.

3. Final Answer:

   To 10.0.0.2 because it has fewer active connections -> Option C

4. Quick Check:

   Least connections picks server with 3 connections [OK]

Hint: Pick server with smallest active connections count [OK]

Common Mistakes:

• Choosing server listed first instead of fewest connections
• Assuming equal distribution regardless of connections
• Picking server with more connections by mistake

4. You configured nginx with least_conn but all requests keep going to the same server. What is the most likely cause?

medium

A. The servers have equal active connections, so nginx picks the first server always

B. The least_conn directive is misspelled in the config

C. The upstream block is missing the server directives

D. The nginx service was not reloaded after configuration change

Solution

1. Step 1: Check if configuration changes are active

   If nginx is not reloaded after config changes, old settings remain active causing unexpected behavior.

2. Step 2: Consider other causes

   Misspelling or missing servers cause errors or no load balancing, but requests going to one server usually means config not applied.

3. Final Answer:

   The nginx service was not reloaded after configuration change -> Option D

4. Quick Check:

   Reload nginx after config changes [OK]

Hint: Always reload nginx after config edits to apply changes [OK]

Common Mistakes:

• Ignoring the need to reload nginx after config update
• Assuming equal connections cause all traffic to one server
• Not verifying server directives exist in upstream block

5. You have three backend servers with varying response times and active connections:

• Server A: 2 active connections, slow response
• Server B: 5 active connections, fast response
• Server C: 2 active connections, medium response

Using least_conn load balancing, which server will receive the next request and why?

hard

A. Requests will be evenly distributed among all servers

B. Server A, because it has the fewest active connections and slow response is ignored

C. Server C, because it has the fewest active connections and better response than A

D. Server B, because it has the fastest response time despite more connections

Solution

1. Step 1: Understand least_conn prioritizes active connections

   It sends requests to servers with the fewest active connections, ignoring response time.

2. Step 2: Compare servers with fewest connections

   Server A and C both have 2 connections, but least_conn does not consider response speed, so it picks the first with fewest connections, Server A.

3. Final Answer:

   Server A, because it has the fewest active connections and slow response is ignored -> Option B

4. Quick Check:

   Least connections ignores speed, picks fewest active connections [OK]

Hint: Least connections ignores speed; picks server with fewest active connections [OK]

Common Mistakes:

• Assuming fastest response server is chosen with least_conn
• Thinking least_conn balances by response time
• Expecting even distribution regardless of connections