GoLang

• 1: Agenda
• 2: Basics of Docker
• 3: Building and Running a Docker Container
• 4: Dockerize Your First Go Application
• 5: Working with Go Modules and Docker
• 6: Dockerize Multi-Container Go App Using Compose

1 - Agenda

2 - Basics of Docker

• Main Components of Docker System
• Test Your Knowledge

Basics of Docker

This section introduces the basic terminology of Docker.

Docker is a platform for developers and sysadmins to build, ship, and run applications. Docker lets you quickly assemble applications from components and eliminates the friction that can come when shipping code. Docker lets you test and deploy your code into production as fast as possible.

Docker simplifies software delivery by making it easy to build and share images that contain your application’s entire environment, or application operating system.

What does it mean by an application operating system ?

Your application typically requires a specific version of operating system, application server, runtime, and database server. It may also require configuration files, and multiple other dependencies. The application may need binding to specific ports and certain amount of memory. The components and configuration together required to run your application is what is referred to as application operating system.

You can certainly provide an installation script that will download and install these components. Docker simplifies this process by allowing to create an image that contains your application and infrastructure together, managed as one component. These images are then used to create Docker containers which run on the container virtualization platform, provided by Docker.

Main Components of Docker System

Docker has three main components:

• Images are the build component of Docker and are the read-only templates defining an application operating system.
• Containers are the run component of Docker and created from images. Containers can be run, started, stopped, moved, and deleted.
• Images are stored, shared, and managed in a registry and are the distribution component of Docker.
• DockerHub is a publicly available registry and is available at http://hub.docker.com.

In order for these three components to work together, the Docker Daemon (or Docker Engine) runs on a host machine and does the heavy lifting of building, running, and distributing Docker containers. In addition, the Client is a Docker binary which accepts commands from the user and communicates back and forth with the Engine.

The Client communicates with the Engine that is either co-located on the same host or on a different host. Client uses the pull command to request the Engine to pull an image from the registry. The Engine then downloads the image from Docker Store, or whichever registry is configured. Multiple images can be downloaded from the registry and installed on the Engine. Client uses the run run the container.

Docker Image

We’ve already seen that Docker images are read-only templates from which Docker containers are launched. Each image consists of a series of layers. Docker makes use of union file systems to combine these layers into a single image. Union file systems allow files and directories of separate file systems, known as branches, to be transparently overlaid, forming a single coherent file system.

One of the reasons Docker is so lightweight is because of these layers. When you change a Docker image—for example, update an application to a new version— a new layer gets built. Thus, rather than replacing the whole image or entirely rebuilding, as you may do with a virtual machine, only that layer is added or updated. Now you don’t need to distribute a whole new image, just the update, making distributing Docker images faster and simpler.

Every image starts from a base image, for example ubuntu, a base Ubuntu image, or fedora, a base Fedora image. You can also use images of your own as the basis for a new image, for example if you have a base Apache image you could use this as the base of all your web application images.

NOTE: By default, Docker obtains these base images from Docker Store.

Docker images are then built from these base images using a simple, descriptive set of steps we call instructions. Each instruction creates a new layer in our image. Instructions include actions like:

• Run a command
• Add a file or directory
• Create an environment variable
• Run a process when launching a container

These instructions are stored in a file called a Dockerfile. Docker reads this Dockerfile when you request a build of an image, executes the instructions, and returns a final image.

Docker Container

A container consists of an operating system, user-added files, and meta-data. As we’ve seen, each container is built from an image. That image tells Docker what the container holds, what process to run when the container is launched, and a variety of other configuration data. The Docker image is read-only. When Docker runs a container from an image, it adds a read-write layer on top of the image (using a union file system as we saw earlier) in which your application can then run.

Docker Engine

Docker Host is created as part of installing Docker on your machine. Once your Docker host has been created, it then allows you to manage images and containers. For example, the image can be downloaded and containers can be started, stopped and restarted.

Docker Client

The client communicates with the Docker Host and let’s you work with images and containers.

Check if your client is working using the following command:

docker -v

It shows the output:

Docker version 20.10.2, build 2291f61

NOTE: The exact version may differ based upon how recently the installation was performed.

The exact version of Client and Server can be seen using docker version command. This shows the output as:

 docker version
Client: Docker Engine - Community
 Cloud integration: 1.0.7
 Version:           20.10.2
 API version:       1.41
 Go version:        go1.13.15
 Git commit:        2291f61
 Built:             Mon Dec 28 16:12:42 2020
 OS/Arch:           darwin/amd64
 Context:           default
 Experimental:      true

Server: Docker Engine - Community
 Engine:
  Version:          20.10.2
  API version:      1.41 (minimum version 1.12)
  Go version:       go1.13.15
  Git commit:       8891c58
  Built:            Mon Dec 28 16:15:28 2020
  OS/Arch:          linux/amd64
  Experimental:     true
 containerd:
  Version:          1.4.3
  GitCommit:        269548fa27e0089a8b8278fc4fc781d7f65a939b
 runc:
  Version:          1.0.0-rc92
  GitCommit:        ff819c7e9184c13b7c2607fe6c30ae19403a7aff
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0
ajeetraina@Ajeets-MacBook-Pro realtime-sensor-jetson % 

The complete set of commands can be seen using docker --help.

Test Your Knowledge

3 - Building and Running a Docker Container

• Build a Docker Image
• Running a Docker Container
• Example Go Application

Build a Docker Image

This section explains how to create a Docker image.

Dockerfile

Docker build images by reading instructions from a Dockerfile. A Dockerfile is a text document that contains all the commands a user could call on the command line to assemble an image. docker image build command uses this file and executes all the commands in succession to create an image.

build command is also passed a context that is used during image creation. This context can be a path on your local filesystem or a URL to a Git repository.

Dockerfile is usually called Dockerfile. The complete list of commands that can be specified in this file are explained at https://docs.docker.com/reference/builder/. The common commands are listed below:

Common commands for Dockerfile

Create your first image

Create a new directory hellodocker.

In that directory, create a new text file Dockerfile. Use the following contents:

FROM ubuntu:latest

CMD ["/bin/echo", "hello world"]

This image uses ubuntu as the base image. CMD command defines the command that needs to run. It provides a different entry point of /bin/echo and an argument “hello world”.

Build the image

  docker image build . -t helloworld

. in this command is the context for the command docker image build. -t adds a tag to the image.

The following output is shown:

Sending build context to Docker daemon  2.048kB
Step 1/2 : FROM ubuntu:latest
latest: Pulling from library/ubuntu
9fb6c798fa41: Pull complete 
3b61febd4aef: Pull complete 
9d99b9777eb0: Pull complete 
d010c8cf75d7: Pull complete 
7fac07fb303e: Pull complete 
Digest: sha256:31371c117d65387be2640b8254464102c36c4e23d2abe1f6f4667e47716483f1
Status: Downloaded newer image for ubuntu:latest
 ---> 2d696327ab2e
Step 2/2 : CMD /bin/echo hello world
 ---> Running in 9356a508590c
 ---> e61f88f3a0f7
Removing intermediate container 9356a508590c
Successfully built e61f88f3a0f7
Successfully tagged helloworld:latest

List the images

You can list the images available using docker image ls:

REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
helloworld          latest              e61f88f3a0f7        3 minutes ago       122MB
ubuntu              latest              2d696327ab2e        4 days ago          122MB

Other images may be shown as well but we are interested in these two images for now.

Run the container using the command:

docker container run helloworld

to see the output:

hello world

If you do not see the expected output, check your Dockerfile that the content exactly matches as shown above. Build the image again and now run it.

Change the base image from ubuntu to busybox in Dockerfile. Build the image again:

docker image build -t helloworld:2 .

and view the images using docker image ls command:

REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
helloworld          2                   7fbedda27c66        3 seconds ago       1.13MB
helloworld          latest              e61f88f3a0f7        5 minutes ago       122MB
ubuntu              latest              2d696327ab2e        4 days ago          122MB
busybox             latest              54511612f1c4        9 days ago          1.13MB

helloworld:2 is the format that allows to specify the image name and assign a tag/version to it separated by :.

Example Go Application

Create a main.go file with the following content:

Now that we have our server, let’s set about writing our Dockerfile and construct a container in which our Go application will live.

Create Dockerfile with following content:

Now that we have defined everything we need for our Go application to run in our Dockerfile we can now build an image using this file. In order to do that, we’ll need to run the following command:

$ docker build -t my-go-app .
Sending build context to Docker daemon   5.12kB
Step 1/6 : FROM golang:1.12.0-alpine3.9
 ---> d4953956cf1e
Step 2/6 : RUN mkdir /app
 ---> Using cache
 ---> be346f9ff24f
Step 3/6 : ADD . /app
 ---> eb420da7413c
Step 4/6 : WORKDIR /app
 ---> Running in d623a88e4a00
Removing intermediate container d623a88e4a00
 ---> ffc439c5bec5
Step 5/6 : RUN go build -o main .
 ---> Running in 15805f4f7685
Removing intermediate container 15805f4f7685
 ---> 31828faf8ae4
Step 6/6 : CMD ["/app/main"]
 ---> Running in 9d54463b7e84
Removing intermediate container 9d54463b7e84
 ---> 3f9244a1a240
Successfully built 3f9244a1a240
Successfully tagged my-go-app:latest

We can now verify that our image exists on our machine by using docker images command:

$ docker images
REPOSITORY                                 TAG                 IMAGE ID            CREATED             SIZE
my-go-app                                  latest              3f9244a1a240        2 minutes ago       355MB$ docker images

In order to run this newly created image, we can use the docker run command and pass in the ports we want to map to and the image we wish to run.

$ docker run -p 8080:8081 -it my-go-app

• -p 8080:8081 - This exposes our application which is running on port 8081 within our container on http://localhost:8080 on our local machine.
• -it - This flag specifies that we want to run this image in interactive mode with a tty for this container process.
• my-go-app - This is the name of the image that we want to run in a container.

Awesome! Now if you go to http://localhost:8080 in your browser, you should see that the application is responds with Hello, "/".

Run Container in Background

You’ll notice that if we ctrl-c this within the terminal, it will kill the container. If we want to have it run permanently in the background, you can replace -it with -d to run this container in detached mode.

In order to view the list of containers running in the background you can use docker ps which should output something like this:

$ docker ps
CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS                    NAMES
70fcc9195865        my-go-app           "/app/main"              5 seconds ago       Up 3 seconds        0.0.0.0:8080->8081/tcp   silly_swirles

Go Modules

Let’s look at a more complex example which features imported modules. In this instance, we will need to add a step within our Dockerfile which does the job of downloading our dependencies prior to the go build command executing:

4 - Dockerize Your First Go Application

Create the main.go file with the following content:

Now that we have our server, let’s set about writing our Dockerfile and constructing the container in which our newly born Go application will live.

create dockerfile with following content:

Now that we have defined everything we need for our Go application to run in our Dockerfile we can now build an image using this file. In order to do that, we’ll need to run the following command:

$ docker build -t my-go-app .
Sending build context to Docker daemon   5.12kB
Step 1/6 : FROM golang:1.12.0-alpine3.9
 ---> d4953956cf1e
Step 2/6 : RUN mkdir /app
 ---> Using cache
 ---> be346f9ff24f
Step 3/6 : ADD . /app
 ---> eb420da7413c
Step 4/6 : WORKDIR /app
 ---> Running in d623a88e4a00
Removing intermediate container d623a88e4a00
 ---> ffc439c5bec5
Step 5/6 : RUN go build -o main .
 ---> Running in 15805f4f7685
Removing intermediate container 15805f4f7685
 ---> 31828faf8ae4
Step 6/6 : CMD ["/app/main"]
 ---> Running in 9d54463b7e84
Removing intermediate container 9d54463b7e84
 ---> 3f9244a1a240
Successfully built 3f9244a1a240
Successfully tagged my-go-app:latest

We can now verify that our image exists on our machine by typing docker images:

$ docker images
REPOSITORY                                 TAG                 IMAGE ID            CREATED             SIZE
my-go-app                                  latest              3f9244a1a240        2 minutes ago       355MB$ docker images

In order to run this newly created image, we can use the docker run command and pass in the ports we want to map to and the image we wish to run.

$ docker run -p 8080:8081 -it my-go-app

• -p 8080:8081 - This exposes our application which is running on port 8081 within our container on http://localhost:8080 on our local machine.
• -it - This flag specifies that we want to run this image in interactive mode with a tty for this container process.
• my-go-app - This is the name of the image that we want to run in a container.

Awesome, if we open up http://localhost:8080 within our browser, we should see that our application is successfully responding with Hello, "/".

Running our Container In the Background

You’ll notice that if we ctrl-c this within the terminal, it will kill the container. If we want to have it run permanently in the background, you can replace -it with -d to run this container in detached mode.

In order to view the list of containers running in the background you can use docker ps which should output something like this:

$ docker ps
CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS                    NAMES
70fcc9195865        my-go-app           "/app/main"              5 seconds ago       Up 3 seconds        0.0.0.0:8080->8081/tcp   silly_swirles

5 - Working with Go Modules and Docker

Let’s look at a more complex example which features imported modules. In this instance, we will need to add a step within our Dockerfile which does the job of downloading our dependencies prior to the go build command executing:

Goapp - Building Images Faster and Better With Multi-Stage Builds

Docker provides a set of standard practices to follow in order to keep your image size small - also covers multi-stage builds in brief.

Let’s start without using multi-stage builds.

Dockerfile
FROM golang
ADD . /app
WORKDIR /app
RUN go build # This will create a binary file named app
ENTRYPOINT /app/app

Build and run the image

docker build -t goapp .
~/g/helloworld ❯❯❯ docker run -it --rm goapp
Hello World!!

Now let us check the image size

 ❯❯❯ docker images | grep goapp
goapp                                          latest              b4221e45dfa0        18 seconds ago      805MB

New Dockerfile

Re-build and run the image

❯❯❯ docker run -it --rm goapp
Hello World!!

Let us check the image again

❯❯❯ docker images | grep goapp
goapp                                          latest              100f92d756da        8 seconds ago       8.15MB

What’s happening here?

We are building the image in two stages. First, we are using a Golang base image, copying our code inside it and building our executable file App. Now in the next stage, we are using a new Alpine base image and copying the binary which we built earlier to our new stage. Important point to note here is that the image built at each stage is entirely independent.

• Stage 0

# Build executable stage
FROM golang
ADD . /app
WORKDIR /app
RUN go build
ENTRYPOINT /app/app

• Stage 1

# Build final image
FROM alpine:latest
RUN apk --no-cache add ca-certificates
WORKDIR /root/
COPY --from=0 /app/app .
CMD ["./app”]

Note the line COPY —from=0 /app/app -> this lets you access data from inside the image built in previous image.

6 - Dockerize Multi-Container Go App Using Compose

The app fetches the quote of the day from a public API hosted at http://quotes.rest then it caches the result in Redis. For subsequent API calls, the app will return the result from Redis cache instead of fetching it from the public API

create following file structure :

go-docker-compose
   ↳ model
       ↳ quote.go
   ↳ app.go

Run Go application

git clone https://github.com/docker-community-leaders/dockercommunity/
cd /content/en/examples/Golang/go-docker-compose
go mod init https://github.com/docker-community-leaders/dockercommunity/content/en/examples/Golang/go-docker-compose
go build

Run binaries

$ ./go-docker-compose
2019/02/02 17:19:35 Starting Server
$ curl http://localhost:8080/qod
The free soul is rare, but you know it when you see it - basically because you feel good, very good, when you are near or with them.

Dockerize above go application

application;s services via Docker Compose

Our application consists of two services -

• App service that contains the API to display the “quote of the day”.
• Redis which is used by the app to cache the “quote of the day”.

Let’s define both the services in a docker-compose.yml file

Running the application with docker compose

$ docker-compose up
Starting go-docker-compose_redis_1 ... done
Starting go-docker-compose_app_1   ... done
Attaching to go-docker-compose_redis_1, go-docker-compose_app_1
redis_1  | 1:C 02 Feb 2019 12:32:45.791 # oO0OoO0OoO0Oo Redis is starting oO0OoO0OoO0Oo
redis_1  | 1:C 02 Feb 2019 12:32:45.791 # Redis version=5.0.3, bits=64, commit=00000000, modified=0, pid=1, just started
redis_1  | 1:C 02 Feb 2019 12:32:45.791 # Warning: no config file specified, using the default config. In order to specify a config file use redis-server /path/to/redis.conf
redis_1  | 1:M 02 Feb 2019 12:32:45.792 * Running mode=standalone, port=6379.
redis_1  | 1:M 02 Feb 2019 12:32:45.792 # WARNING: The TCP backlog setting of 511 cannot be enforced because /proc/sys/net/core/somaxconn is set to the lower value of 128.
redis_1  | 1:M 02 Feb 2019 12:32:45.792 # Server initialized
redis_1  | 1:M 02 Feb 2019 12:32:45.792 # WARNING you have Transparent Huge Pages (THP) support enabled in your kernel. This will create latency and memory usage issues with Redis. To fix this issue run the command 'echo never > /sys/kernel/mm/transparent_hugepage/enabled' as root, and add it to your /etc/rc.local in order to retain the setting after a reboot. Redis must be restarted after THP is disabled.
redis_1  | 1:M 02 Feb 2019 12:32:45.793 * DB loaded from disk: 0.000 seconds
redis_1  | 1:M 02 Feb 2019 12:32:45.793 * Ready to accept connections
app_1    | 2019/02/02 12:32:46 Starting Server

The docker-compose up command starts all the services defined in the docker-compose.yml file. You can interact with the Go service using curl -

$ curl http://localhost:8080/qod
A show of confidence raises the bar

Stopping all the services with docker compose

$ docker-compose down