Rewrite flags (last, break, redirect, permanent) in Nginx - Time & Space Complexity

Time Complexity: Rewrite flags (last, break, redirect, permanent)

O(n)

Understanding Time Complexity

When nginx processes rewrite rules, it uses flags to decide what to do next. Understanding how these flags affect processing helps us see how the work grows as more rules or requests come in.

We want to know how the number of rewrite rules and their flags impact the total steps nginx takes to handle a request.

Scenario Under Consideration

Analyze the time complexity of this nginx rewrite snippet with different flags.

location /example/ {
    rewrite ^/example/(.*)$ /newpath/$1 last;
    rewrite ^/old/(.*)$ /oldpath/$1 break;
    rewrite ^/temp/(.*)$ /tempredirect/$1 redirect;
    rewrite ^/perm/(.*)$ /permanent/$1 permanent;
}
    

This snippet shows four rewrite rules with different flags controlling how nginx continues processing after each rewrite.

Identify Repeating Operations

Look at what repeats when nginx processes these rules.

• Primary operation: nginx checks each rewrite rule in order for a match.
• How many times: It can check multiple rules per request, but flags like last and break can stop or restart the checks early.

How Execution Grows With Input

As the number of rewrite rules grows, nginx may check more rules per request.

Input Size (rules)	Approx. Operations (rule checks)
10	Up to 10 checks, but often fewer due to flags
100	Up to 100 checks, but flags can reduce this
1000	Up to 1000 checks, flags still help limit checks

Pattern observation: More rules mean more checks, but flags like last and break help stop or restart processing early, preventing all rules from always running.

Final Time Complexity

Time Complexity: O(n)

This means nginx may check each rewrite rule once per request, but flags can reduce how many it actually checks.

Common Mistake

[X] Wrong: "All rewrite rules always run for every request regardless of flags."

[OK] Correct: Flags like last and break stop or restart processing early, so nginx often skips checking all rules.

Interview Connect

Understanding how nginx processes rewrite rules and flags shows you can reason about request handling efficiency. This skill helps you explain how servers manage work as configurations grow.

Self-Check

What if we replaced all last flags with break? How would the time complexity change?

Practice

1. What does the last flag do in an nginx rewrite directive?

easy

A. Stops processing the current location and restarts the rewrite phase with the new URI

B. Sends a temporary redirect (HTTP 302) to the client

C. Stops processing the rewrite directives and continues with the current location

D. Sends a permanent redirect (HTTP 301) to the client

Solution

1. Step 1: Understand the last flag behavior

   The last flag tells nginx to stop processing current rewrite directives and restart the rewrite phase with the new URI.

2. Step 2: Compare with other flags

   Unlike break which stops rewriting internally, last restarts processing, and unlike redirect or permanent which send redirects, it does not send a client redirect.

3. Final Answer:

   Stops processing the current location and restarts the rewrite phase with the new URI -> Option A

4. Quick Check:

   last = restart rewrite phase [OK]

Hint: Remember: last restarts rewrite, break stops internally [OK]

Common Mistakes:

• Confusing last with break flag
• Thinking last sends a redirect
• Mixing last with permanent redirect

2. Which of the following is the correct syntax to perform a permanent redirect in nginx using the rewrite directive?

easy

A. rewrite ^/old$ /new break;

B. rewrite ^/old$ /new last;

C. rewrite ^/old$ /new permanent;

D. rewrite ^/old$ /new redirect;

Solution

1. Step 1: Identify the flag for permanent redirect

   The permanent flag sends an HTTP 301 status code indicating a permanent redirect.

2. Step 2: Check syntax correctness

   The syntax rewrite ^/old$ /new permanent; correctly applies the permanent redirect flag.

3. Final Answer:

   rewrite ^/old$ /new permanent; -> Option C

4. Quick Check:

   Permanent redirect uses permanent flag [OK]

Hint: Permanent redirect uses 'permanent' flag at end [OK]

Common Mistakes:

• Using 'last' or 'break' instead of 'permanent' for redirects
• Omitting the semicolon at the end
• Confusing 'redirect' with 'permanent'

3. Given the nginx configuration snippet:

location /test {
  rewrite ^/test/(.*)$ /new/$1 redirect;
}

What will happen when a client requests /test/page?

medium

A. The client will receive a 301 permanent redirect to /new/page

B. The rewrite will restart processing with the new URI /new/page

C. The request will be internally rewritten to /new/page without redirect

D. The client will receive a 302 temporary redirect to /new/page

Solution

1. Step 1: Understand the redirect flag

   The redirect flag sends an HTTP 302 temporary redirect to the client.

2. Step 2: Analyze the rewrite rule

   The rewrite matches /test/page and rewrites it to /new/page with a temporary redirect.

3. Final Answer:

   The client will receive a 302 temporary redirect to /new/page -> Option D

4. Quick Check:

   redirect = 302 temporary redirect [OK]

Hint: redirect flag means 302 temporary redirect [OK]

Common Mistakes:

• Confusing redirect with permanent
• Thinking redirect rewrites internally
• Assuming last or break flags here

4. You have this nginx rewrite rule:

rewrite ^/oldpath/(.*)$ /newpath/$1 last

But it causes a 500 Internal Server Error. What is the likely problem?

medium

A. Missing semicolon at the end of the rewrite directive

B. Using 'last' flag instead of 'break' causes infinite loop

C. The regex pattern is invalid

D. Rewrite directive must be inside a location block

Solution

1. Step 1: Check syntax of rewrite directive

   Every nginx directive must end with a semicolon. The given rewrite lacks a semicolon.

2. Step 2: Understand error cause

   Missing semicolon causes nginx to fail parsing config, leading to 500 error.

3. Final Answer:

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

4. Quick Check:

   Missing semicolon = syntax error = 500 error [OK]

Hint: Always end rewrite directives with semicolon [OK]

Common Mistakes:

• Assuming last flag causes infinite loop
• Ignoring missing semicolon syntax errors
• Thinking regex pattern is wrong without checking

5. You want to rewrite URLs starting with /app to /app/index.html internally without redirecting the client, and stop further rewrite processing. Which rewrite directive and flag should you use?

hard

A. rewrite ^/app$ /app/index.html last;

B. rewrite ^/app$ /app/index.html break;

C. rewrite ^/app$ /app/index.html redirect;

D. rewrite ^/app$ /app/index.html permanent;

Solution

1. Step 1: Identify internal rewrite without client redirect

   The break flag stops rewrite processing and serves the rewritten URI internally without redirecting the client.

2. Step 2: Match requirement to flag

   Using break with the rewrite stops further rewrites and serves /app/index.html internally.

3. Final Answer:

   rewrite ^/app$ /app/index.html break; -> Option B

4. Quick Check:

   break = internal rewrite stop [OK]

Hint: Use break to rewrite internally and stop further rewrites [OK]

Common Mistakes:

• Using last which restarts rewrite phase
• Using redirect or permanent which send client redirects
• Not stopping rewrite processing causing unexpected behavior