Prometheus client cleaning up counter prematurely? - go

Question

I am trying to write a program which exposes prometheus metrics.
It is a simple program, where I want to increment a counter for every time my "run" method is called on my struct.
import (
"log"
"net/http"
"time"
"github.com/prometheus/client_golang/prometheus"
"github.com/prometheus/client_golang/prometheus/promhttp"
)
type myStruct struct {
errorCount prometheus.Counter
}
func (s myStruct) initialize() {
s.errorCount = prometheus.NewCounter(prometheus.CounterOpts{
Name: "my_counter",
Help: "sample prometheus counter",
})
}
func (s myStruct) run() {
s.errorCount.Add(1)
}
func main() {
s := new(myStruct)
s.initialize()
http.Handle("/metrics", promhttp.Handler())
go func() {
for {
s.run()
time.Sleep(time.Second)
}
}()
log.Fatal(http.ListenAndServe(":8080", nil))
}
Above code fails with a "Failed to continue - bad access" error, every time I try to increment the counter. i.e. at this line
s.errorCount.Inc()
I am unable to determine why the counter suddenly disappears from memory (if I'm understanding the error message correctly).
I am determine if i am missing something fundamental w.r.t. Go, or am I using the prometheus client library incorrectly.

Answer 1

In initialise() s is being passed by value which means that in main() s.errorCount is nil.
Just change the declaration of initialise (and run) to take a pointer.
func (s *myStruct) initialize() {
...
A few more suggestions you might like to try:
func init() {
go func() {
http.Handle("/metrics", promhttp.Handler())
log.Fatal(http.ListenAndServe(":8080", nil))
}()
}
type myStruct struct {
errorCount prometheus.Counter
}
func NewMyStruct() *myStruct {
return &myStruct {
errorCount: prometheus.NewCounter(prometheus.CounterOpts {
Name: "my_counter",
Help: "sample prometheus counter",
}),
}
}
func (s *myStruct) run() {
s.errorCount.Add(1)
}
func main() {
s := NewMyStruct()
go func() {
for {
s.run()
time.Sleep(time.Second)
}
}()
// ... OR select{}
}