How to Use Go’s expvar Package for Real‑Time Metrics and Performance Monitoring

When profiling Go applications, pprof is commonly used to collect CPU and memory data, but it can miss real‑time context in complex, concurrent programs. To capture performance bottlenecks more efficiently, you can expose probing data through the expvar package, which publishes variables as JSON at /debug/vars.

Package expvar provides a standardized interface to public variables, such as operation counters in servers. It exposes these variables via HTTP at /debug/vars in JSON format.

Below is a minimal example that starts an HTTP server and registers the default expvar handler:

package main

import (
    _ "expvar"
    "fmt"
    "net/http"
)

func main() {
    http.HandleFunc("/hi", func(w http.ResponseWriter, r *http.Request) {
        w.Write([]byte("hi
"))
    })
    fmt.Println(http.ListenAndServe("localhost:8080", nil))
}

Visiting http://localhost:8080/debug/vars returns a JSON object that includes the default cmdline and memstats fields:

{
    "cmdline": ["/path/to/your/app"],
    "memstats": { /* runtime.Memstats fields */ }
}

To expose custom metrics, define an expvar.Map, set values, and publish it:

package main

import (
    "expvar"
    "fmt"
    "net/http"
)

var customVar = expvar.NewMap("custom")

func init() {
    customVar.Set("hi_count", new(expvar.Int))
    expvar.Publish("custom", customVar)
}

func main() {
    http.HandleFunc("/hi", func(w http.ResponseWriter, r *http.Request) {
        customVar.Add("hi_count", 1)
        w.Write([]byte("hi
"))
    })
    fmt.Println(http.ListenAndServe("localhost:8080", nil))
}

After calling /hi twice, the JSON endpoint shows the incremented counter:

{
    "cmdline": ["/path/to/app"],
    "custom": {"hi_count": 2},
    "memstats": "..."
}

For visualizing the exported data, you can use expvarmon. Install it with:

go get github.com/divan/expvarmon

Then run:

expvarmon -ports="8080" -vars="custom.hi_count,mem:memstats.Alloc,mem:memstats.Sys,mem:memstats.HeapAlloc,mem:memstats.HeapInuse,duration:memstats.PauseNs,duration:memstats.PauseTotalNs"

The tool renders the selected metrics in a terminal UI, making the output more readable.

Compared with pprof, which focuses on CPU and memory profiling, expvar provides a lightweight way to expose live application state and custom counters, suitable for long‑running services that need continuous health monitoring.

In summary, expvar is a powerful, easy‑to‑use package for exporting runtime statistics and custom metrics, and it integrates well with external monitoring systems such as Prometheus, InfluxDB, or Grafana.