Master Nginx Log Formatting: Customize, Test, and Optimize Your Access Logs

Introduction

In Nginx, controlling log output is straightforward thanks to the HttpLogModule, which allows you to set the log format and related directives.

2. HttpLogModule Module

2.1 Example

2.2 Directives

access_log Syntax: access_log [format [buffer=size]] | off Default: access_log logs/access.log combined; Context: http, server, location, if in location, limit_except

log_format Syntax: log_format name [escape=default|json|none] string ...; Default: log_format combined "..."; Context: http

open_log_file_cache Syntax:

open_log_file_cache max=N [inactive=time] [min_uses=N] [valid=time]; | open_log_file_cache off;

Default: open_log_file_cache off; Context: http, server, location max – maximum number of file descriptors stored in the cache (LRU eviction). inactive – time after which unused descriptors are removed (default 10 s). min_uses – minimum accesses before a descriptor is cached (default 1). valid – time to check for the existence of the same‑named file (default 60 s). off – disables the cache.

2.3 Test

View the default log format:

After sending a request, the log shows entries such as two "- -" fields – the first comes from the log_format definition, the second is a placeholder for a null $remote_user.

log_format  main  '$remote_addr - $remote_user [$time_local] "$request" $status $body_bytes_sent "$http_referer" "$http_user_agent" "$http_x_forwarded_for" "$request_time"';

log_format  remote_main  '$remote_addr - $remote_user [$time_local] "$request" $status $body_bytes_sent "$http_referer" "$http_user_agent" "$http_x_forwarded_for" "$request_time" "$upstream_response_time" "$upstream_addr" "$upstream_status"';

For faster anomaly detection, define logs per location instead of a global http block.

access_log  /var/log/nginx/app.log  buffer=32k  flush=5s;

Note: with buffering, logs are written only when the buffer reaches 32 KB or after 5 seconds, whichever comes first.

2.4 Test

Execute a request and inspect the log:

curl http://10.0.10.158/

The log clearly shows:

Requested URI: GET /apache/index.html Total request time: 0.002 s

Backend processing time: 0.002 s

Backend socket: 10.0.10.159:8080 Backend status:

200