Why virtual hosting serves multiple domains in Nginx - Performance Analysis

Time Complexity: Why virtual hosting serves multiple domains

O(n)

Understanding Time Complexity

We want to understand how nginx handles multiple websites using virtual hosting.

Specifically, how the work grows when more domains are added.

Scenario Under Consideration

Analyze the time complexity of this nginx virtual hosting setup.

server {
    listen 80;
    server_name example.com;
    root /var/www/example;
}

server {
    listen 80;
    server_name example.org;
    root /var/www/example_org;
}

server {
    listen 80;
    server_name example.net;
    root /var/www/example_net;
}
    

This config serves three different domains by matching the requested domain name.

Identify Repeating Operations

When a request comes, nginx checks each server block to find a matching domain.

• Primary operation: Comparing the requested domain to each server_name.
• How many times: Once per server block until a match is found or all checked.

How Execution Grows With Input

As the number of domains (server blocks) increases, nginx checks more names.

Input Size (n)	Approx. Operations
10	Up to 10 comparisons
100	Up to 100 comparisons
1000	Up to 1000 comparisons

Pattern observation: The work grows roughly in direct proportion to the number of domains.

Final Time Complexity

Time Complexity: O(n)

This means the time to find the right domain grows linearly as more domains are added.

Common Mistake

[X] Wrong: "nginx instantly knows the right domain no matter how many there are."

[OK] Correct: nginx checks domains one by one until it finds a match, so more domains mean more checks.

Interview Connect

Understanding how nginx matches domains helps you explain real server behavior clearly and confidently.

Self-Check

"What if nginx used a hash map to find domains instead of checking each one? How would the time complexity change?"