X-Ray is a simple yet efficient way of debugging issues in production.
Normally the error log is expected to have low verbosity, with only critical errors being printed to disk on high volume servers.
With x-ray, we can query the server for more verbose error_log style instrumentation, and gain insight on what decisions are being made and what code path is executed.
X-Ray should be initialized in response to e.g. a special header, or based on client address, or both. When initialized, log lines are added to an in-memory buffer which is then pre-pended to the response.
See ngx_http_xray.h for function reference and available macros to get started. Log/Severity levels mirror nginx's error log levels (NGX_LOG_STDERR=0 - NGX_LOG_INFO=7).
The sample module demonstrates basic xray usage in a custom module.
$ curl http://server/foo/bar
<html>
<head><title>404 Not Found</title></head>
<body bgcolor="white">
<center><h1>404 Not Found</h1></center>
<hr><center>nginx/1.6.0</center>
</body>
</html>
$ curl -H "X-Ray-Level: 9" http://server/foo/bar
5 ngx_http_sample_rewrite_phase_handler: Hello, X-Ray!
7 ngx_http_get_input_header: get_header() key: X-Foo
7 ngx_http_get_input_header: i: 0, part: 000000000091A108 // 'User-Agent'
7 ngx_http_get_input_header: i: 1, part: 000000000091A108 // 'Host'
7 ngx_http_get_input_header: i: 2, part: 000000000091A108 // 'Accept'
7 ngx_http_get_input_header: i: 3, part: 000000000091A108 // 'X-Ray-Level'
<html>
<head><title>404 Not Found</title></head>
<body bgcolor="white">
<center><h1>404 Not Found</h1></center>
<hr><center>nginx/1.6.0</center>
</body>
</html>
$ curl -H "X-Foo: on" -H "X-Ray-Level: 5" http://server/foo/bar
5 ngx_http_sample_rewrite_phase_handler: Hello, X-Ray!
4 ngx_http_sample_rewrite_phase_handler: ,,__
4 ngx_http_sample_rewrite_phase_handler: .. .. / o._) .---.
4 ngx_http_sample_rewrite_phase_handler: /--'/--\ \-'|| .----. .' '.
4 ngx_http_sample_rewrite_phase_handler: / \_/ / | .' '..' '-.
4 ngx_http_sample_rewrite_phase_handler: .'\ \__\ __.'.' .' ì-._
4 ngx_http_sample_rewrite_phase_handler: )\ | )\ | _.'
4 ngx_http_sample_rewrite_phase_handler: // \\ // \\
4 ngx_http_sample_rewrite_phase_handler: ||_ \\|_ \\_
4 ngx_http_sample_rewrite_phase_handler: '--' '--'' '--'
<html>
<head><title>404 Not Found</title></head>
<body bgcolor="white">
<center><h1>404 Not Found</h1></center>
<hr><center>nginx/1.6.0</center>
</body>
</html>
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