Conditional logging in Nginx - Step-by-Step Execution

Process Flow - Conditional logging

Request Received

↓

Evaluate Condition

↓

Log Request

↓

Send Response

Nginx checks a condition for each request. If true, it logs the request; if false, it skips logging.

Execution Sample

Nginx

map $request_uri $loggable {
    default 1;
    ~^/health 0;
}

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

This config logs all requests except those starting with /health.

Process Table

Step	Request URI	Condition ($loggable)	Action	Log Output
1	/index.html	1 (true)	Log request	Logged /index.html
2	/health	0 (false)	Skip logging	No log
3	/api/data	1 (true)	Log request	Logged /api/data
4	/health/status	0 (false)	Skip logging	No log
5	/about	1 (true)	Log request	Logged /about
6	End of requests	-	-	-

💡 All requests processed; logging done only if condition is true.

Status Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
$request_uri	-	/index.html	/health	/api/data	/health/status	/about	-
$loggable	-	1	0	1	0	1	-

Key Moments - 2 Insights

Why does /health not get logged even though it is a valid request?

Because the condition $loggable is set to 0 for URIs starting with /health, as shown in rows 2 and 4 of the execution_table, so logging is skipped.

What does the 'if=$loggable' part do in the access_log directive?

It tells nginx to log the request only if $loggable is true (1). This is why requests with $loggable=0 are not logged, as seen in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of $loggable for the request '/api/data'?

A 0

B -

C 1

D Undefined

💡 Hint

Check row 3 under the 'Condition ($loggable)' column in the execution_table.

At which step does the logging condition evaluate to false?

A Step 2

B Step 3

C Step 1

D Step 5

💡 Hint

Look for rows where 'Condition ($loggable)' is 0 in the execution_table.

If we remove the line '~^/health 0;' from the map block, what happens to logging for '/health' requests?

A They will still not be logged

B They will be logged

C Logging will stop for all requests

D Nginx will error

💡 Hint

Refer to how the map sets default to 1 and the effect of the regex line in the variable_tracker.

Concept Snapshot

Conditional logging in nginx uses variables and the 'if' parameter in access_log.
Use 'map' to set a variable based on request properties.
Set 'if=$variable' in access_log to log only when variable is true.
Example: skip logging health checks by setting variable to 0 for /health URIs.
This reduces log noise and improves log relevance.

Full Transcript

In nginx, conditional logging lets you decide which requests to log based on a condition. We use the 'map' directive to create a variable, here called $loggable, which is 1 for most requests but 0 for those starting with /health. Then, in the access_log directive, we add 'if=$loggable' so nginx logs only when $loggable is true. The execution table shows requests with /health are skipped from logging because their $loggable is 0. This helps keep logs clean by ignoring routine health checks. Beginners often wonder why some requests are not logged; it's because the condition variable controls logging. Removing the regex line in the map would cause all requests to be logged, including /health. This method is simple and effective for controlling nginx logs.