Least connections in Nginx - Commands & Configuration

Introduction

When you have multiple servers handling requests, you want to send new requests to the server with the fewest active connections. This helps balance the load evenly and keeps your app fast and reliable.

When you run a website on several servers and want to avoid overloading any single server.

When some servers are slower and you want to send fewer requests to busy servers.

When you want to improve user experience by reducing wait times during high traffic.

When you want to automatically distribute traffic without manual intervention.

When you want a simple way to balance load without complex health checks.

Config File - nginx.conf

nginx.conf

http {
    upstream myapp {
        least_conn;
        server 192.168.1.101:80;
        server 192.168.1.102:80;
        server 192.168.1.103:80;
    }

    server {
        listen 80;

        location / {
            proxy_pass http://myapp;
        }
    }
}

This configuration sets up a group of servers named myapp. The least_conn directive tells Nginx to send new requests to the server with the fewest active connections. The server blocks list the backend servers by IP and port. The proxy_pass forwards incoming requests to this group.

Commands

Check the Nginx configuration file for syntax errors before applying changes.

Terminal

nginx -t

Expected Output

nginx: the configuration file /etc/nginx/nginx.conf syntax is ok

nginx: configuration file /etc/nginx/nginx.conf test is successful

Reload Nginx to apply the new configuration without stopping the service.

Terminal

systemctl reload nginx

Expected Output

No output (command runs silently)

Send a simple request to the Nginx server to verify it is forwarding requests correctly.

Terminal

curl -I http://localhost

Expected Output

HTTP/1.1 200 OK

Server: nginx

Date: Thu, 01 Jun 2024 12:00:00 GMT

Content-Type: text/html

Content-Length: 612

Connection: keep-alive

Key Concept

If you remember nothing else from this pattern, remember: least connections sends new requests to the backend server with the fewest active connections to balance load evenly.

Common Mistakes

Forgetting to include the least_conn directive inside the upstream block.

Without least_conn, Nginx uses the default round-robin method, ignoring connection counts.

Always add least_conn inside the upstream block to enable this load balancing method.

Not testing the configuration with nginx -t before reloading.

Syntax errors can cause Nginx to fail to reload, leading to downtime.

Run nginx -t to verify configuration syntax before reloading.

Reloading Nginx without proper permissions or using the wrong command.

Reload may fail silently or cause service interruption.

Use sudo systemctl reload nginx or run as root to reload safely.

Summary

Configure the upstream block with least_conn to balance requests by active connections.

Test the configuration syntax with nginx -t before applying changes.

Reload Nginx to apply the new load balancing method without downtime.

Verify the setup by sending requests and checking responses.

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