28 May 2021

Golang setup docker with hot reloading

Today I’m going to show you how to easily add hot reloading functionality to your Golang project. I will use a simple Gin project as an example and docker-compose.

To make code as short as possible I will use Gin example with GET ping endpoint returning {"message": "pong"} as a response to request.

main.py

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package main

import "github.com/gin-gonic/gin"

func main() {
	r := gin.Default()
	r.GET("/ping", func(c *gin.Context) {
		c.JSON(200, gin.H{
			"message": "pong",
		})
	})
	r.Run() // listen and serve on 0.0.0.0:8080 (for windows "localhost:8080")
}

docker/Dockerfile

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FROM golang:1.16.4-stretch as build

ENV PROJECT_DIR=/project/app \
    GO111MODULE=auto \
    CGO_ENABLED=0

## Install required packages
## - git to fetch golang dependencies
RUN apt-get update \
    && apt-get install -y --no-install-recommends \
        git \
    && rm -rf /var/lib/apt/lists/*

## Set the current working directory inside the container
WORKDIR $PROJECT_DIR

## Install dependencies from go.mod file and verify it also
COPY go.mod .
COPY go.sum .
RUN go mod download
RUN go mod verify

## Install additional dependencies
RUN go get github.com/githubnemo/CompileDaemon

## Run watcher to automatically rebuild app after any change in code
CMD CompileDaemon --build="go build src/main.go" --command="./main" --color

At the end of the file, we are running CompileDaemon written in pure golang who will do all the job for us. In build we can optionally put out own command to build project by default is go build. command argument will run our binary created by build. Yes, is so easy :)

docker-compose.yml

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version: '3.8'

services:
  app:
    build:
      context: .
      dockerfile: ./docker/Dockerfile
    volumes:
      - .:/project/app
    restart: on-failure

Testing time!

Run docker-compose up --build and check if I not lied to you :)

...
app_1  | Running build command!
app_1  | Build ok.
app_1  | Restarting the given command.
app_1  | stdout: [GIN-debug] [WARNING] Creating an Engine instance with the Logger and Recovery middleware already attached.
app_1  | stdout: 
app_1  | stdout: [GIN-debug] [WARNING] Running in "debug" mode. Switch to "release" mode in production.
app_1  | stdout:  - using env:	export GIN_MODE=release
app_1  | stdout:  - using code:	gin.SetMode(gin.ReleaseMode)
app_1  | stdout: 
app_1  | stdout: [GIN-debug] GET    /ping                     --> main.main.func1 (3 handlers)
app_1  | stdout: [GIN-debug] Environment variable PORT is undefined. Using port :8080 by default
app_1  | stdout: [GIN-debug] Listening and serving HTTP on :8080

Change ping endpoint response to something else and save your file. You should see in your terminal new lines regarding rebuilding.

...
app_1  | Running build command!
app_1  | Build ok.
app_1  | Hard stopping the current process..
app_1  | Restarting the given command.
...

Full source from this post you can find here, have fun!