Round-robin (default) in Nginx - Step-by-Step Execution

Process Flow - Round-robin (default)

Client Request Arrives

↓

Select Next Server in List

↓

Forward Request to Selected Server

↓

Wait for Next Request

↩Back to Select Next Server

The load balancer receives each client request and forwards it to the next server in the list, cycling through servers evenly.

Execution Sample

Nginx

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

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

This config sets up two backend servers and uses round-robin to distribute requests between them.

Process Table

Request Number	Selected Server	Action	Result
1	backend1.example.com	Forward request	Request sent to backend1
2	backend2.example.com	Forward request	Request sent to backend2
3	backend1.example.com	Forward request	Request sent to backend1
4	backend2.example.com	Forward request	Request sent to backend2
5	backend1.example.com	Forward request	Request sent to backend1

💡 Cycle repeats after last server; round-robin continues indefinitely

Status Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5
Current Server Index	0	1	0	1	0	1

Key Moments - 2 Insights

Why does the server selection go back to backend1 after backend2?

Because round-robin cycles through the server list in order, after reaching the last server (backend2), it starts again from the first (backend1), as shown in execution_table rows 3 and 4.

Does round-robin consider server load or response time?

No, round-robin simply cycles through servers evenly without checking load or speed, so each request is sent to the next server in the list regardless of its state.

Visual Quiz - 3 Questions

Look at the execution table, which server handles the 4th request?

A backend1.example.com

B backend2.example.com

C backend3.example.com

D No server handles the 4th request

💡 Hint

Check the 'Selected Server' column for Request Number 4 in the execution_table.

At which request does the server selection index reset to 0?

A After Request 2

B After Request 3

C After Request 1

D After Request 5

💡 Hint

Look at the 'Current Server Index' in variable_tracker after each request.

If a third server is added, how will the selection change for the 6th request?

A It will go back to backend1.example.com

B It will skip the new server and select backend2.example.com

C It will select the new third server

D It will cause an error

💡 Hint

Round-robin cycles through all servers in order; adding a server adds it to the cycle.

Concept Snapshot

Round-robin load balancing in nginx cycles through backend servers in order.
Each new request goes to the next server in the list.
No server load or health checks affect selection by default.
Config example:
upstream backend { server backend1; server backend2; }
Requests rotate: backend1 -> backend2 -> backend1 -> ...

Full Transcript

Round-robin is the default method nginx uses to distribute client requests among backend servers. When a request arrives, nginx forwards it to the next server in the list, cycling through all servers evenly. This means the first request goes to the first server, the second to the second server, and so on. After the last server is used, nginx starts again from the first server. This cycle repeats indefinitely. The configuration defines the backend servers inside an upstream block. The proxy_pass directive sends requests to this upstream group. Round-robin does not consider server load or response time; it simply distributes requests evenly in order.