Error nginx: [emerg] unknown "upstream_first_byte_time" variable

[opiumfor@…]

Hello!

I've added the $upstream_first_byte_time to my log format in nginx.conf like this:

log_format json escape=json
...here goes some other fields...
'"upstream_first_byte_time": "$upstream_first_byte_time", '
...here goes some other fields...
;

But unfortunately the output of "nginx -V && nginx -T" is:

nginx version: nginx/1.19.0
built by gcc 9.2.0 (Alpine 9.2.0)
built with OpenSSL 1.1.1d 10 Sep 2019 (running with OpenSSL 1.1.1g 21 Apr 2020)
TLS SNI support enabled
configure arguments: --prefix=/etc/nginx --sbin-path=/usr/sbin/nginx --modules-path=/usr/lib/nginx/modules --conf-path=/etc/nginx/nginx.conf --error-log-path=/var/log/nginx/error.log --http-log-path=/var/log/nginx/access.log --pid-path=/var/run/nginx.pid --lock-path=/var/run/nginx.lock --http-client-body-temp-path=/var/cache/nginx/client_temp --http-proxy-temp-path=/var/cache/nginx/proxy_temp --http-fastcgi-temp-path=/var/cache/nginx/fastcgi_temp --http-uwsgi-temp-path=/var/cache/nginx/uwsgi_temp --http-scgi-temp-path=/var/cache/nginx/scgi_temp --with-perl_modules_path=/usr/lib/perl5/vendor_perl --user=nginx --group=nginx --with-compat --with-file-aio --with-threads --with-http_addition_module --with-http_auth_request_module --with-http_dav_module --with-http_flv_module --with-http_gunzip_module --with-http_gzip_static_module --with-http_mp4_module --with-http_random_index_module --with-http_realip_module --with-http_secure_link_module --with-http_slice_module --with-http_ssl_module --with-http_stub_status_module --with-http_sub_module --with-http_v2_module --with-mail --with-mail_ssl_module --with-stream --with-stream_realip_module --with-stream_ssl_module --with-stream_ssl_preread_module --with-cc-opt='-Os -fomit-frame-pointer' --with-ld-opt=-Wl,--as-needed

2020/06/05 13:49:21 [emerg] 1#1: unknown "upstream_first_byte_time" variable
nginx: [emerg] unknown "upstream_first_byte_time" variable
nginx: configuration file /etc/nginx/nginx.conf test failed

What I am doing wrong?

[Maxim Dounin]

The ​$upstream_first_byte_time variable in only available in the stream module. For http, consider using the ​$upstream_header_time variable instead.

For further questions on how to configure nginx, please consider ​support options available.

[opiumfor@…]

Thanks, Maxim, it's clear now. But this should probably be covered little bit more in documentation because it confused me why the other variables are worked as expected, for example the upstream_connect_time and so on.