Prefix match in Nginx - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a prefix match in nginx configuration?

A prefix match in nginx is a way to match the beginning part of a URL path to decide how to handle the request. It checks if the URL starts with a specific string.

beginner

How do you define a prefix match location block in nginx?

You define it by using location /prefix/ { ... } without any special symbols. This matches any URL starting with '/prefix/'.

intermediate

What is the difference between prefix match and exact match in nginx?

Prefix match checks if the URL starts with a string, while exact match uses = to match the URL exactly with no extra characters.

intermediate

What happens if multiple prefix matches apply to a request in nginx?

Nginx chooses the longest matching prefix. For example, if both /app/ and /app/api/ match, it picks /app/api/.

beginner

Can prefix matches in nginx use regular expressions?

No, prefix matches are simple string matches. Regular expressions require ~ or ~* modifiers in location blocks.

Which nginx location block uses prefix match?

A location /images/ { }

B location = /images/ { }

C location ~ /images/ { }

D location ^~ /images/ { }

The block without any special symbols uses prefix match. '=' is exact match, '~' is regex, '^~' is prefix match with priority.

If you have location /app/ and location /app/api/, which one handles /app/api/users?

A location /app/api/

B Both equally

C location /app/

D Neither

Nginx picks the longest prefix match, so /app/api/ handles /app/api/users.

What symbol marks an exact match in nginx location?

A ~

B =

C ^~

D *

The '=' symbol marks an exact match in nginx location blocks.

Can prefix match location blocks use regular expressions?

A Yes, always

B Only with =

C No, never

D Only with ^~

Prefix matches are simple string matches and do not support regex.

What does the '^~' modifier do in nginx location?

A Prefix match with higher priority

B Regex match

C Exact match

D Case-insensitive match

The '^~' modifier tells nginx to use prefix match and stop searching further if matched.

Explain how nginx chooses a location block when multiple prefix matches exist.

Think about which prefix is the longest that fits the URL.

Describe the difference between prefix match and exact match in nginx location blocks.

Consider how nginx treats the URL string in each case.

Practice

1. What does prefix match in nginx do when routing requests?

easy

A. It matches requests based on the start of the URL path.

B. It matches requests only if the full URL matches exactly.

C. It matches requests based on the file extension.

D. It matches requests randomly to any location block.

Solution

1. Step 1: Understand prefix match concept

   Prefix match means nginx checks if the URL path starts with a specific string.

2. Step 2: Compare with other matching types

   Exact match requires full URL match, file extension match is unrelated, random match is invalid.

3. Final Answer:

   It matches requests based on the start of the URL path. -> Option A

4. Quick Check:

   Prefix match = start of URL path [OK]

Hint: Prefix match checks URL start, not full or random [OK]

Common Mistakes:

• Confusing prefix match with exact match
• Thinking prefix match checks file extensions
• Assuming prefix match is random

2. Which of the following is the correct syntax for a prefix match location in nginx?

easy

A. location = /images/ { }

B. location ^~ /images/ { }

C. location /images/ { }

D. location ~ /images/ { }

Solution

1. Step 1: Identify prefix match syntax

   Prefix match uses plain location /prefix/ { } without modifiers.

2. Step 2: Understand other modifiers

   = is exact match, ^~ is prefix but with higher priority, ~ is regex match.

3. Final Answer:

   location /images/ { } -> Option C

4. Quick Check:

   Plain location = prefix match [OK]

Hint: Plain location block means prefix match [OK]

Common Mistakes:

• Using = or ^~ modifiers for simple prefix match
• Confusing regex (~) with prefix match
• Missing trailing slash in prefix

3. Given this nginx config snippet:

location /app/ {
  proxy_pass http://backend1;
}
location /app/api/ {
  proxy_pass http://backend2;
}

Which backend will handle a request to /app/api/users?

medium

A. Both backends in round-robin

B. http://backend2

C. No backend, 404 error

D. http://backend1

Solution

1. Step 1: Understand prefix match selection

   nginx selects the location with the longest matching prefix for plain prefix locations (no modifiers).

2. Step 2: Compare prefixes

   Both /app/ (length 5) and /app/api/ (length 9) match /app/api/users, but /app/api/ is longer, so it wins.

3. Final Answer:

   http://backend2 -> Option B

4. Quick Check:

   Longest prefix wins [OK]

Hint: Longest matching prefix wins [OK]

Common Mistakes:

• Thinking config order (first match) wins instead of longest prefix
• Thinking regex or exact match applies here
• Believing nginx load balances both backends

4. You wrote this nginx config:

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

Requests to /static/css/style.css return 404. What is the likely error?

medium

A. root directive path is incorrect

B. location block should use = modifier

C. Missing trailing slash in location prefix

D. proxy_pass is missing

Solution

1. Step 1: Understand root directive behavior

   root appends the full request URI (/static/css/style.css) to /var/www/html, looking for /var/www/html/static/css/style.css.

2. Step 2: Identify likely cause

   If static files are at /var/www/html/css/style.css (without static/), root path doesn't strip prefix, causing 404. (Use alias /var/www/html; to strip.)

3. Final Answer:

   root directive path is incorrect -> Option A

4. Quick Check:

   root + full URI (no prefix strip) [OK]

Hint: root appends full URI; alias strips prefix [OK]

Common Mistakes:

• Confusing root (full URI) with alias (strips prefix)
• Thinking trailing slash strips prefix for root
• Adding unnecessary modifiers like =

5. You want nginx to route all requests starting with /api/ to http://backend_api and all others to http://backend_web, ensuring the /api/ prefix has priority even if regex locations follow. Which config correctly uses prefix match for this?

hard

A. location ~ /api/ {
proxy_pass http://backend_api;
}
location / {
proxy_pass http://backend_web;
}

B. location /api/ {
proxy_pass http://backend_api;
}
location / {
proxy_pass http://backend_web;
}

C. location = /api/ {
proxy_pass http://backend_api;
}
location / {
proxy_pass http://backend_web;
}

D. location ^~ /api/ {
proxy_pass http://backend_api;
}
location / {
proxy_pass http://backend_web;
}

Solution

1. Step 1: Use ^~ for prefix priority

   The ^~ modifier tells nginx to stop searching if prefix matches, ensuring /api/ routes to backend_api even over later regex.

2. Step 2: Default location for others

   The plain location / { } catches all other requests to backend_web.

3. Final Answer:

   location ^~ /api/ { proxy_pass http://backend_api; } location / { proxy_pass http://backend_web; } -> Option D

4. Quick Check:

   ^~ prefix match for priority routing [OK]

Hint: Use ^~ to prioritize prefix match over regex [OK]

Common Mistakes:

• Not using ^~ causes regex or exact match to override
• Using = modifier limits to exact /api/ only
• Using ~ makes it regex, not prefix match