The ExpVar middleware exposes runtime variables over HTTP in JSON format. Using it registers handlers on /debug/vars to provide insights into your application’s runtime state.
Installation
go get -u github.com/gofiber/fiber/v3
go get -u github.com/gofiber/fiber/v3/middleware/expvar
Signatures
func New(config ...Config) fiber.Handler
Usage
Basic Usage
package main
import (
"expvar"
"fmt"
"github.com/gofiber/fiber/v3"
expvarmw "github.com/gofiber/fiber/v3/middleware/expvar"
)
func main() {
app := fiber.New()
// Create custom variables
var count = expvar.NewInt("count")
// Register ExpVar middleware
app.Use(expvarmw.New())
app.Get("/", func(c fiber.Ctx) error {
count.Add(1)
return c.SendString(fmt.Sprintf("hello expvar count %d", count.Value()))
})
app.Listen(":3000")
}
Custom Configuration
app.Use(expvarmw.New(expvarmw.Config{
Next: func(c fiber.Ctx) bool {
// Skip middleware for certain paths
return c.Path() == "/health"
},
}))
Examples
Visit /debug/vars to see all exported variables:
curl http://127.0.0.1:3000
# hello expvar count 1
curl http://127.0.0.1:3000/debug/vars
# {
# "cmdline": ["./app"],
# "count": 1,
# "expvarHandlerCalls": 1,
# "expvarRegexpErrors": 0,
# "memstats": {...}
# }
?r= query parameter with a regular expression:
curl http://127.0.0.1:3000/debug/vars?r=c
# {
# "cmdline": ["./app"],
# "count": 1
# }
Configuration
Next
func(fiber.Ctx) bool
default:"nil"
Function to skip this middleware when it returns true.
Default Configuration
var ConfigDefault = Config{
Next: nil,
}
Best Practices
- Mount the middleware early in your middleware stack
- Use custom variables to track application-specific metrics
- Secure the
/debug/vars endpoint in production environments
- Consider using the
Next function to restrict access based on IP or authentication
- Combine with monitoring tools that can scrape JSON endpoints
Common Patterns
Track Request Counts by Route
var (
homeRequests = expvar.NewInt("home_requests")
apiRequests = expvar.NewInt("api_requests")
)
app.Get("/", func(c fiber.Ctx) error {
homeRequests.Add(1)
return c.SendString("Home")
})
app.Get("/api", func(c fiber.Ctx) error {
apiRequests.Add(1)
return c.JSON(fiber.Map{"status": "ok"})
})
Expose Custom Application State
var stats = expvar.NewMap("stats")
func init() {
stats.Set("start_time", expvar.Func(func() any {
return time.Now().Format(time.RFC3339)
}))
}
Restrict Access in Production
app.Use(expvarmw.New(expvarmw.Config{
Next: func(c fiber.Ctx) bool {
// Only allow localhost in production
if os.Getenv("ENV") == "production" {
return c.IP() != "127.0.0.1"
}
return false
},
}))