Prefix match in Nginx - Step-by-Step Execution

Process Flow - Prefix match

Request URL arrives

↓

Check location blocks

↓

Match prefix location?

↓

Serve content or proxy

Nginx checks the request URL against location blocks using prefix matching to find the best matching path prefix.

Execution Sample

Nginx

location /images/ {
    root /data;
}

location / {
    root /var/www/html;
}

This config serves URLs starting with /images/ from /data, others from /var/www/html.

Process Table

Step	Request URL	Location Tested	Prefix Match?	Action Taken
1	/images/cat.png	/images/	Yes	Select this location
2	/images/cat.png	/	Yes	Keep searching for longer prefix
3	/images/cat.png	No more locations	N/A	Serve from /images/ location root /data
4	/about.html	/images/	No	Skip this location
5	/about.html	/	Yes	Select this location
6	/about.html	No more locations	N/A	Serve from / location root /var/www/html

💡 Nginx picks the longest matching prefix location or default location if no prefix matches.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Selected Location	None	/images/	/images/	/images/	/images/
Request URL	/images/cat.png	/images/cat.png	/images/cat.png	/images/cat.png	/images/cat.png

Key Moments - 2 Insights

Why does nginx continue checking other locations after finding a prefix match?

Nginx looks for the longest prefix match, so even if a prefix matches early (like '/'), it keeps checking for a longer prefix (like '/images/') as shown in steps 1 and 2.

What happens if no prefix matches the request URL?

Nginx uses the default location (usually '/') to serve the request, as shown in steps 4 to 6 for '/about.html'.

Visual Quiz - 3 Questions

Look at the execution table, which location is selected for the URL '/images/cat.png'?

A No location

B /images/

C /

D /about/

💡 Hint

Check steps 1 to 3 in the execution table where '/images/' is selected as the longest prefix match.

At which step does nginx decide to serve content from the '/' location for '/about.html'?

A Step 6

B Step 4

C Step 5

D Step 3

💡 Hint

Look at steps 4 to 6 where '/images/' is skipped and '/' is selected finally.

If a new location 'location /images/cat/' is added, how would the execution table change for '/images/cat.png'?

A It would select '/'

B It would still select '/images/'

C It would select '/images/cat/' as the longest prefix match

D It would cause an error

💡 Hint

Longest prefix match means the most specific prefix wins, so '/images/cat/' would be chosen.

Concept Snapshot

Nginx prefix match:
- Checks request URL against location prefixes
- Chooses longest matching prefix
- If none match, uses default location
- Example: /images/ matches URLs starting with /images/
- Order matters only for regex, prefix matches longest prefix wins

Full Transcript

When nginx receives a request URL, it checks all location blocks to find the best prefix match. It starts by testing each location's prefix against the URL. If a prefix matches, nginx remembers it but continues checking for a longer prefix match. If no longer prefix is found, nginx uses the best match to serve content. If no prefix matches, it uses the default location, usually '/'. For example, a request to '/images/cat.png' matches both '/' and '/images/', but nginx selects '/images/' because it is longer and more specific. This process ensures requests are routed to the correct content folder or proxy target.