How to Fix Nginx Access Log Configuration Error

In this tutorial, you'll learn about How to Fix Nginx Access Log Configuration Error. We cover key concepts, practical examples, and best practices.

Nginx access logs are empty, show - for expected values, or the wrong log format is applied — the log_format is not referenced by any access_log directive or the format string has errors.

The Problem

# Wrong: Format defined but not referenced
http {
    log_format custom '$remote_addr - $remote_user [$time_local] "$request" $status $body_bytes_sent';

    server {
        access_log /var/log/nginx/access.log;
        # Uses default format, not 'custom'
    }
}

Step-by-Step Fix

Step 1: Define and reference the log format

http {
    log_format json_log escape=json
        '{'
        '"time":"$time_iso8601",'
        '"remote_addr":"$remote_addr",'
        '"request":"$request",'
        '"status":$status,'
        '"body_bytes":$body_bytes_sent,'
        '"referer":"$http_referer",'
        '"user_agent":"$http_user_agent",'
        '"request_time":$request_time'
        '}';

    server {
        access_log /var/log/nginx/access.log json_log;
    }
}

Step 2: Use conditional logging

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

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

Step 3: Disable access logging for static files

location ~* \.(css|js|png|jpg)$ {
    access_log off;
}

Step 4: Test log output

curl http://localhost/
tail -1 /var/log/nginx/access.log

Step 5: Set up log rotation

# /etc/logrotate.d/nginx
/var/log/nginx/*.log {
    daily
    missingok
    rotate 14
    compress
    delaycompress
    notifempty
    create 640 nginx adm
    sharedscripts
    postrotate
        [ -f /var/run/nginx.pid ] && kill -USR1 `cat /var/run/nginx.pid`
    endscript
}

Prevention Tips

• Use JSON log format for structured log parsing
• Rotate logs daily to prevent disk space issues
• Use conditional logging to reduce noise
• Monitor log file size with logrotate alerts

Common Mistakes with access log

1. Using foldl instead of foldl' causing stack overflow on large lists
2. Forgetting deriving (Show, Eq) on custom data types needed for debugging
3. Placing the wildcard pattern first in case expressions, making all subsequent patterns unreachable

These mistakes appear frequently in real-world NGINX code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

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