Connection limiting (limit_conn) in Nginx - Step-by-Step Execution

Process Flow - Connection limiting (limit_conn)

Client sends request

↓

Check current connections count

↓

Is count < limit?

No→Reject connection with error

Yes↓

Allow connection

↓

Process request

↓

Connection closes, decrement count

This flow shows how nginx checks the number of active connections per key and either allows or rejects new connections based on the limit.

Execution Sample

Nginx

limit_conn_zone $binary_remote_addr zone=addr:10m;
limit_conn addr 2;

server {
  listen 80;
  location / {
    proxy_pass http://backend;
  }
}

This config limits each client IP to 2 simultaneous connections to the server.

Process Table

Step	Client IP	Current Connections	Condition (connections < 2?)	Action	Result
1	192.168.1.10	0	0 < 2 = Yes	Allow connection	Connection accepted, count=1
2	192.168.1.10	1	1 < 2 = Yes	Allow connection	Connection accepted, count=2
3	192.168.1.10	2	2 < 2 = No	Reject connection	Connection rejected with 503 error
4	192.168.1.11	0	0 < 2 = Yes	Allow connection	Connection accepted, count=1
5	192.168.1.10	1 (after one closes)	1 < 2 = Yes	Allow connection	Connection accepted, count=2
6	192.168.1.10	2	2 < 2 = No	Reject connection	Connection rejected with 503 error

💡 Connections rejected when client IP reaches limit of 2 active connections.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6
connections_192.168.1.10	0	1	2	2	1	2	2
connections_192.168.1.11	0	0	0	0	1	1	1

Key Moments - 2 Insights

Why does the connection get rejected at step 3 even though the client is the same?

At step 3, the current connections for 192.168.1.10 is already 2, which equals the limit. The condition 'connections < 2' is false, so nginx rejects the new connection as shown in the execution_table row 3.

What happens when one connection closes for a client IP?

When a connection closes, nginx decrements the count for that client IP. For example, after step 4, one connection from 192.168.1.10 closed, reducing its count from 2 to 1, allowing new connections again as seen in step 5.

Visual Quiz - 3 Questions

Look at the execution table, what is the connection count for 192.168.1.10 at step 2?

A 1

B 0

C 2

D 3

💡 Hint

Check the 'Current Connections' column for step 2 in the execution_table.

At which step does nginx reject a connection from 192.168.1.10 due to limit reached?

A Step 1

B Step 3

C Step 4

D Step 5

💡 Hint

Look for 'Reject connection' action for 192.168.1.10 in the execution_table.

If the limit was increased to 3, what would happen at step 3?

A Connection would be rejected

B No change, connection count resets

C Connection would be allowed

D Server would crash

💡 Hint

Refer to the condition 'connections < limit' in the execution_table and imagine limit=3.

Concept Snapshot

limit_conn_zone defines a shared memory zone keyed by client IP.
limit_conn sets max simultaneous connections per key.
Nginx checks current connections before accepting new ones.
If limit reached, new connections are rejected with error.
Counts decrement when connections close.
Useful to protect server from overload by single clients.

Full Transcript

This visual execution shows how nginx limits simultaneous connections per client IP using limit_conn. When a client connects, nginx checks how many active connections that IP has. If below the limit, nginx allows the connection and increments the count. If the limit is reached, nginx rejects the new connection with an error. When a connection closes, nginx decrements the count. The example limits each IP to 2 connections. Steps show connections accepted or rejected based on this limit, helping prevent overload from too many connections by one client.