How to implement a custom expvar.Var for tracking connection statistics in Go?

To implement a custom expvar.Var for tracking connection statistics in Go, you can follow these steps:

Create a new Go file for your custom expvar variable, for example connection_stats.go.
Start by importing the required packages:

package main

import (
	"expvar"
	"sync"
)

Define a struct to hold your connection statistics:

type ConnectionStats struct {
	sync.Mutex
	connections int
}

The sync.Mutex is used to provide thread-safe access to the connections variable.

Implement the String() method for your struct. This method will be called by expvar to retrieve the current value of your custom variable:

func (c *ConnectionStats) String() string {
	c.Mutex.Lock()
	defer c.Mutex.Unlock()
	return strconv.Itoa(c.connections)
}

In this example, strconv.Itoa is used to convert the connections variable to a string.

Create a NewConnectionStats() function to create and return a new ConnectionStats object:

func NewConnectionStats() *ConnectionStats {
	return &ConnectionStats{}
}

Finally, in your main code, initialize your custom expvar variable using the expvar.NewVar() function:

func main() {
	stats := NewConnectionStats()
	expvar.Publish("connections", stats)
	// ...
}

This will create a new expvar variable named "connections" and associate it with your ConnectionStats object.

Now, whenever you want to track connection statistics, you can increment or decrement the connections variable in your ConnectionStats object:

func handleConnection(stats *ConnectionStats) {
	// Increment the connection count
	stats.Lock()
	stats.connections++
	stats.Unlock()

	// ... (process connection)

	// Decrement the connection count
	stats.Lock()
	stats.connections--
	stats.Unlock()
}

Your custom expvar variable will automatically reflect the updated connection count. You can access the current value of the variable by visiting the /debug/vars endpoint in your application.