https://www.cloudflare.com/learning/performance/glossary/what-is-latency/
Latency is the time it takes for data to pass from one point on a network to another.
(Yes this is the right definition of networking latency)
NOTE: According to this definition, the time output from ping command is not latency.
In the following output, time=35.2 ms, which is RTT (round-trip time). It's pretty double of latency
(aka, the latency in this case is about 35.2/2 ~ 17.6ms. However, RTT = 2 * latency is not always true.
$ ping 1.1.1.1 -c 1
PING 1.1.1.1 (1.1.1.1) 56(84) bytes of data.
64 bytes from 1.1.1.1: icmp_seq=1 ttl=49 time=35.2 ms
https://developer.mozilla.org/en-US/docs/Web/Performance/Understanding_latency (or https://web.archive.org/web/20240922180416/https://developer.mozilla.org/en-US/docs/Web/Performance/Understanding_latency)
Network latency is the time it takes for a data request to get from the computer making the request, to the computer responding. Including the time it takes for a byte of data to make it from the responding computer back to the requesting computer. It is generally measured as a round trip delay.
https://jmeter.apache.org/usermanual/glossary.html
Latency. JMeter measures the latency from just before sending the request to just after the first response has been received. Thus the time includes all the processing needed to assemble the request as well as assembling the first part of the response, which in general will be longer than one byte. Protocol analysers (such as Wireshark) measure the time when bytes are actually sent/received over the interface. The JMeter time should be closer to that which is experienced by a browser or other application client.
In many log configurations, latency is defined as below
"latency": "$upstream_response_time s"
See more with, for example https://www.google.com/search?q="latency"%3A+"%24upstream_response_time+s".
Elasticsearch has an intergration documentation with this format
https://web.archive.org/web/20240922181130/https://www.elastic.co/docs/current/integrations/nginx_ingress_controller
So what is $upstream_response_time? According to nginx's documentation (https://nginx.org/en/docs/http/ngx_http_upstream_module.html#var_upstream_response_time)
$upstream_response_timekeeps time spent on receiving the response from the upstream server; the time is kept in seconds with millisecond resolution. Times of several responses are separated by commas and colons like addresses in the $upstream_addr variable.
Their blogs gives more details (https://blog.nginx.org/blog/using-nginx-logging-for-application-performance-monitoring)
$request_time– Full request time, starting when NGINX reads the first byte from the client and ending when NGINX sends the last byte of the response body
$upstream_connect_time– Time spent establishing a connection with an upstream server
$upstream_header_time– Time between establishing a connection to an upstream server and receiving the first byte of the response header
$upstream_response_time– Time between establishing a connection to an upstream server and receiving the last byte of the response body