---
title: Building the application
linkTitle: Understand the application
weight: 10 #
keywords: go, golang, prometheus, grafana, containerize, monitor
description: Learn how to create a Golang server to register metrics with Prometheus.
---

## Prerequisites

* You have a [Git client](https://git-scm.com/downloads). The examples in this section use a command-line based Git client, but you can use any client.

You will be creating a Golang server with some endpoints to simulate a real-world application. Then you will expose metrics from the server using Prometheus.

## Getting the sample application

Clone the sample application to use with this guide. Open a terminal, change
directory to a directory that you want to work in, and run the following
command to clone the repository:

```console
$ git clone https://github.com/dockersamples/go-prometheus-monitoring.git 
```

Once you cloned you will see the following content structure inside `go-prometheus-monitoring` directory,

```text
go-prometheus-monitoring
├── CONTRIBUTING.md
├── Docker
│   ├── grafana.yml
│   └── prometheus.yml
├── dashboard.json
├── Dockerfile
├── LICENSE
├── README.md
├── compose.yaml
├── go.mod
├── go.sum
└── main.go
```

- **main.go** - The entry point of the application.
- **go.mod and go.sum** - Go module files.
- **Dockerfile** - Dockerfile used to build the app.
- **Docker/** - Contains the Docker Compose configuration files for Grafana and Prometheus.
- **compose.yaml** - Compose file to launch everything (Golang app, Prometheus, and Grafana).
- **dashboard.json** - Grafana dashboard configuration file.
- **Dockerfile** - Dockerfile used to build the Golang app.
- **compose.yaml** - Docker Compose file to launch everything (Golang app, Prometheus, and Grafana).
- Other files are for licensing and documentation purposes.

## Understanding the application

The following is the complete logic of the application you will find in `main.go`.

```go
package main

import (
	"strconv"

	"github.com/gin-gonic/gin"
	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/client_golang/prometheus/promhttp"
)

// Define metrics
var (
	HttpRequestTotal = prometheus.NewCounterVec(prometheus.CounterOpts{
		Name: "api_http_request_total",
		Help: "Total number of requests processed by the API",
	}, []string{"path", "status"})

	HttpRequestErrorTotal = prometheus.NewCounterVec(prometheus.CounterOpts{
		Name: "api_http_request_error_total",
		Help: "Total number of errors returned by the API",
	}, []string{"path", "status"})
)

// Custom registry (without default Go metrics)
var customRegistry = prometheus.NewRegistry()

// Register metrics with custom registry
func init() {
	customRegistry.MustRegister(HttpRequestTotal, HttpRequestErrorTotal)
}

func main() {
	router := gin.Default()

	// Register /metrics before middleware
	router.GET("/metrics", PrometheusHandler())
	
	router.Use(RequestMetricsMiddleware())
	router.GET("/health", func(c *gin.Context) {
		c.JSON(200, gin.H{
			"message": "Up and running!",
		})
	})
	router.GET("/v1/users", func(c *gin.Context) {
		c.JSON(200, gin.H{
			"message": "Hello from /v1/users",
		})
	})

	router.Run(":8000")
}

// Custom metrics handler with custom registry
func PrometheusHandler() gin.HandlerFunc {
	h := promhttp.HandlerFor(customRegistry, promhttp.HandlerOpts{})
	return func(c *gin.Context) {
		h.ServeHTTP(c.Writer, c.Request)
	}
}

// Middleware to record incoming requests metrics
func RequestMetricsMiddleware() gin.HandlerFunc {
	return func(c *gin.Context) {
		path := c.Request.URL.Path
		c.Next()
		status := c.Writer.Status()
		if status < 400 {
			HttpRequestTotal.WithLabelValues(path, strconv.Itoa(status)).Inc()
		} else {
			HttpRequestErrorTotal.WithLabelValues(path, strconv.Itoa(status)).Inc()
		}
	}
}
```

In this part of the code, you have imported the required packages `gin`, `prometheus`, and `promhttp`. Then you have defined a couple of variables, `HttpRequestTotal` and `HttpRequestErrorTotal` are Prometheus counter metrics, and `customRegistry` is a custom registry that will be used to register these metrics. The name of the metric is a string that you can use to identify the metric. The help string is a string that will be shown when you query the `/metrics` endpoint to understand the metric. The reason you are using the custom registry is so avoid the default Go metrics that are registered by default by the Prometheus client. Then using the `init` function you are registering the metrics with the custom registry.