Conditional logging in Nginx - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the effect of this nginx config snippet?

Given this nginx configuration snippet, what will be the effect on logging?

map $status $loggable {
    ~^[23] 0;
    default 1;
}

access_log /var/log/nginx/access.log combined if=$loggable;

A Logging will be disabled completely.

B All requests will be logged regardless of status code.

C Only requests with status codes starting with 2 or 3 will be logged; others will be ignored.

D Requests with status codes starting with 2 or 3 will NOT be logged; all others will be logged.

💡 Hint

Look at the map directive and the if condition in access_log.

The map directive sets $loggable to 0 for status codes starting with 2 or 3, meaning logging is disabled for those. The access_log directive logs only if $loggable is 1, so only other status codes are logged.

❓ Configuration

intermediate

Choose the correct nginx config to log only client errors (4xx)

Which nginx configuration snippet will log only requests with client error status codes (4xx)?

A

map $status $log_client_error {
    default 1;
}
access_log /var/log/nginx/client_error.log combined if=$log_client_error;

B

map $status $log_client_error {
    ~^4 1;
    default 0;
}
access_log /var/log/nginx/client_error.log combined if=$log_client_error;

C

map $status $log_client_error {
    ~^[23] 1;
    default 0;
}
access_log /var/log/nginx/client_error.log combined if=$log_client_error;

D

map $status $log_client_error {
    ~^5 1;
    default 0;
}
access_log /var/log/nginx/client_error.log combined if=$log_client_error;

💡 Hint

Client errors have status codes starting with 4.

The map sets $log_client_error to 1 only when status code starts with 4, enabling logging only for client errors.

❓ Troubleshoot

advanced

Why does conditional logging not work as expected?

An nginx config uses:

map $status $loggable {
    ~^2 0;
    default 1;
}
access_log /var/log/nginx/access.log combined if=$loggable;

But logs still include 2xx responses. What is the likely cause?

A The variable $status is not available at the time of logging evaluation.

B The regex ~^2 matches only status code '2', not '200' or '201'.

C The access_log directive does not support the 'if' parameter.

D The map directive syntax is incorrect and causes a fallback to logging all.

💡 Hint

Consider when $status variable is set during request processing.

The $status variable is set after the log phase, so using it in 'if' for access_log does not work as expected. This causes logging to ignore the condition and log all requests.

🔀 Workflow

advanced

Order the steps to enable conditional logging for error responses only

Put these steps in the correct order to configure nginx to log only error responses (status 4xx and 5xx):

A 1,4,2,3

B 4,1,2,3

C 1,2,4,3

D 4,2,1,3

💡 Hint

Start by defining the variable, then specify patterns, then use it, then reload.

First define the variable with map, then specify regex patterns inside it, then use the variable in access_log, finally reload nginx to apply.

✅ Best Practice

expert

Which approach is best for conditional logging of slow requests in nginx?

You want to log only requests that take longer than 2 seconds to process. Which nginx configuration approach is best?

A Use the 'if' parameter in access_log with a variable set by a map on $request_time with a numeric comparison (e.g., $request_time > 2).

B Use the 'if' parameter in access_log with a variable set by a map on $request_time > 2.

C Use the 'if' parameter in access_log with a variable set by a map on $request_time matching a regex for values > 2.

D Use the 'if' parameter in access_log with a variable set by a map on $request_time with a threshold using the 'geo' module.

💡 Hint

Nginx map works with string matching, not numeric comparisons.

Nginx map cannot do numeric comparisons directly. Using regex to match $request_time strings representing values greater than 2 seconds is the practical approach.