$bytes_sent

Description

The $bytes_sent variable in NGINX tracks the total number of bytes transmitted to the client during response. This variable is computed during the request processing phase and is available in all contexts where logging occurs. It reflects not only the raw response body but also any additional bytes sent, such as HTTP headers. The value of this variable is established when NGINX begins delivering the response to the client and continues to accumulate throughout the transmission process until the response is fully sent. Typically, $bytes_sent can vary significantly based on the type of content being delivered; for instance, static files will report the exact file size in bytes, while dynamic content generated by scripts can be less predictable. Additionally, and can affect the size reported due to differences in content length, headers, and compression. Overall, developers and system administrators utilize this variable primarily for analyzing bandwidth usage, calculating transfer statistics, and tuning server performance for efficient client response handling.

Config Example

log_format main '$remote_addr - $remote_user [$time_local] "$request" $status $bytes_sent';
access_log /var/log/nginx/access.log main;