Rewrite directive in Nginx - Time & Space Complexity

Time Complexity: Rewrite directive

O(n)

Understanding Time Complexity

We want to understand how the time it takes for nginx to process rewrite rules changes as we add more rules.

Specifically, how does the number of rewrite directives affect the work nginx does?

Scenario Under Consideration

Analyze the time complexity of the following nginx rewrite rules.

server {
    listen 80;
    server_name example.com;

    rewrite ^/oldpath/(.*)$ /newpath/$1 permanent;
    rewrite ^/blog/(.*)$ /news/$1 last;
    rewrite ^/user/(.*)$ /profile/$1 break;
}

This snippet shows three rewrite directives that change URLs based on patterns.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: nginx checks each rewrite rule one by one to see if the URL matches the pattern.
• How many times: It tries each rewrite directive in order until one matches or all are checked.

How Execution Grows With Input

As the number of rewrite rules grows, nginx has to check more patterns for each request.

Input Size (n)	Approx. Operations
3	3 pattern checks
10	10 pattern checks
100	100 pattern checks

Pattern observation: The number of checks grows directly with the number of rewrite rules.

Final Time Complexity

Time Complexity: O(n)

This means the time to process rewrites grows linearly with the number of rewrite directives.

Common Mistake

[X] Wrong: "Adding more rewrite rules does not affect performance because nginx is very fast."

[OK] Correct: Even though nginx is fast, each rewrite rule adds work because nginx checks them one by one, so more rules mean more checks and more time.

Interview Connect

Understanding how rewrite rules affect processing time helps you design efficient server configurations and shows you can think about performance in real setups.

Self-Check

What if we combined multiple rewrite rules into one using a more complex pattern? How would the time complexity change?

Practice

1. What is the main purpose of the rewrite directive in nginx?

easy

A. To limit the number of client connections

B. To change URLs based on patterns for redirection or rewriting

C. To configure SSL certificates for secure connections

D. To serve static files directly from the server

Solution

1. Step 1: Understand the role of rewrite directive

   The rewrite directive is used to modify URLs based on matching patterns.

2. Step 2: Identify its main use

   It helps redirect or rewrite URLs without changing the visible link to the user.

3. Final Answer:

   To change URLs based on patterns for redirection or rewriting -> Option B

4. Quick Check:

   Rewrite directive = URL modification [OK]

Hint: Rewrite changes URLs by pattern matching [OK]

Common Mistakes:

• Confusing rewrite with serving static files
• Thinking rewrite manages SSL or security
• Assuming rewrite limits connections

2. Which of the following is the correct syntax for a rewrite directive in nginx?

easy

A. rewrite ^/oldpath/(.*)$ /newpath/$1 permanent;

B. rewrite /oldpath/(.*) /newpath/$1 redirect

C. rewrite ^/oldpath/(.*)$ /newpath/$1 301

D. rewrite /oldpath/(.*)$ /newpath/$1 permanent

Solution

1. Step 1: Check the rewrite syntax format

   The correct syntax is: rewrite <pattern> <replacement> <flag>;

2. Step 2: Identify the correct flag and semicolon usage

   The flag must be a recognized keyword like 'permanent' and the line must end with a semicolon.

3. Final Answer:

   rewrite ^/oldpath/(.*)$ /newpath/$1 permanent; -> Option A

4. Quick Check:

   Correct syntax = rewrite + pattern + replacement + flag + semicolon [OK]

Hint: Rewrite ends with semicolon and uses flags like permanent [OK]

Common Mistakes:

• Omitting the semicolon at the end
• Using numeric flags instead of keywords
• Missing caret (^) in regex pattern

3. Given this nginx config snippet:

rewrite ^/blog/(.*)$ /newblog/$1 last;

What happens when a user visits /blog/article1?

medium

A. The user is redirected to /newblog/article1 with a 301 status

B. The URL changes to /newblog/article1 and nginx restarts processing

C. The URL stays /blog/article1 but content from /newblog/article1 is served

D. An error occurs because 'last' is an invalid flag

Solution

1. Step 1: Understand the 'last' flag in rewrite

   The 'last' flag performs an internal rewrite: nginx changes the URI internally and restarts location processing with the new URI. The browser URL remains unchanged.

2. Step 2: Apply to the example URL

   Visiting /blog/article1 matches the pattern, rewrites internally to /newblog/article1, and serves content from there without changing the visible URL.

3. Final Answer:

   The URL stays /blog/article1 but content from /newblog/article1 is served -> Option C

4. Quick Check:

   'last' flag = internal rewrite, URL unchanged [OK]

Hint: 'last' flag restarts processing with new URL [OK]

Common Mistakes:

• Confusing 'last' with external redirect
• Thinking URL visible to user changes
• Assuming 'last' flag causes error

4. Identify the error in this nginx rewrite directive:

rewrite ^/old/(.*)$ /new/$1 permanent

medium

A. Missing semicolon at the end of the directive

B. Incorrect regex pattern syntax

C. Invalid flag 'permanent' used

D. Rewrite directive cannot use variables like $1

Solution

1. Step 1: Check syntax completeness

   Every nginx directive must end with a semicolon (;). This line is missing it.

2. Step 2: Validate other parts

   The regex pattern and flag 'permanent' are correct, and $1 is valid for capturing groups.

3. Final Answer:

   Missing semicolon at the end of the directive -> Option A

4. Quick Check:

   Missing semicolon = syntax error [OK]

Hint: Always end rewrite lines with a semicolon [OK]

Common Mistakes:

• Forgetting semicolon at line end
• Misunderstanding regex syntax
• Thinking $1 is invalid in rewrite

5. You want to redirect all requests from /shop/ to /store/ permanently, but only if the URL ends exactly with /shop/ (no extra path after). Which rewrite directive achieves this?

hard

A. rewrite ^/shop/(.*)$ /store/$1 permanent;

B. rewrite ^/shop$ /store/ permanent;

C. rewrite /shop/ /store/ permanent;

D. rewrite ^/shop/$ /store/ permanent;

Solution

1. Step 1: Match only exact /shop/ URL

   The regex ^/shop/$ matches exactly '/shop/' with nothing after it.

2. Step 2: Redirect permanently to /store/

   The replacement is /store/ with the 'permanent' flag for 301 redirect.

3. Final Answer:

   rewrite ^/shop/$ /store/ permanent; -> Option D

4. Quick Check:

   Exact match with ^/shop/$ and permanent redirect [OK]

Hint: Use ^ and $ to match exact URL in rewrite [OK]

Common Mistakes:

• Using (.*) which matches extra paths
• Omitting anchors ^ and $
• Incorrect regex without end anchor