Metrics

Metrics are an effective approach to monitoring applications at scale. They can be cheap to collect, making them suitable for performance sensitive operations. They can also be compact to report, making them suitable for high-volume scenarios. emit doesn't provide much infrastructure for collecting or sampling metrics. What it does provide is a standard way to report metric samples as events.

A standard kind of metric is a monotonic counter, which can be represented as an atomic integer. In this example, our counter is for the number of bytes written to a file, which we'll call bytes_written. We can report a sample of this counter as an event using some well-known properties:

#![allow(unused)]
fn main() {
extern crate emit;
fn sample_bytes_written() -> usize { 4643 }

let sample = sample_bytes_written();

emit::emit!(
    "{metric_agg} of {metric_name} is {metric_value}",
    evt_kind: "metric",
    metric_agg: "count",
    metric_name: "bytes_written",
    metric_value: sample,
);
}

Event {
    mdl: "my_app",
    tpl: "{metric_agg} of {metric_name} is {metric_value}",
    extent: Some(
        "2024-04-29T10:08:24.780230000Z",
    ),
    props: {
        "evt_kind": metric,
        "metric_name": "bytes_written",
        "metric_agg": "count",
        "metric_value": 4643,
    },
}

A metric produced by this example application in Prometheus.