This module adds logging encoder named transform. The module accepts a template with the placeholders are surrounded by braces {} and filled by values extracted from the stucture of the JSON log encoder. The JSON configuration looks like this:

{
	"encoder": "transform",
	"template": "{...}"
}

The nesting is traversed using >. For example, to print the uri field, the traversal is templated as {request>uri}.

{
	"request": {
		"method": "GET",
		"uri": "/",
		"proto": "HTTP/2.0",
		...
}

The Caddyfile configuration accepts the template immediately following the encoder name, and can be ommitted to assume Apache Common Log Format.

log {
	format transform [<template>] {
		placeholder <string>
		message_key <key>
		level_key   <key>
		time_key    <key>
		name_key    <key>
		caller_key  <key>
		stacktrace_key <key>
		line_ending  <char>
		time_format  <format>
		level_format <format>
	}
}

The syntax of template is defined by the package github.com/buger/jsonparser. Objects are traversed using the key name. Arrays can be traversed by using the format [index], as in [0]. For example, to get the first element in the User-Agent array, the template is {request>headers>User-Agent>[0]}.

The module comes with one special value of {common_log} for the Apache Common Log format to simplify configuration

 format transform "{common_log}"

The more spelled out way of doing it is:

format transform `{request>remote_addr} - {request>user_id} [{ts}] "{request>method} {request>uri} {request>proto}" {status} {size}` {
	time_format "02/Jan/2006:15:04:05 -0700"
}

The more spelled out way of doing it is:

format transform `{request>remote_addr} - {request>user_id} [{ts}] "{request>method} {request>uri} {request>proto}" {status} {size} "{request>headers>Referer>[0]}" "{request>headers>User-Agent>[0]}"` {
        time_format "02/Jan/2006:15:04:05 -0700"
}

First, the xcaddy command:

$ go install github.com/caddyserver/xcaddy/cmd/xcaddy@latest

Then build Caddy with this Go module plugged in. For example:

$ xcaddy build --with github.com/caddyserver/transform-encoder