Kubernetes is a popular container orchestration system that lets you automate application deployment, scaling and management tasks via simple command line calls. This guide describes how to build and deploy a simple dockerized web app to a Kubernetes cluster on CentOS 8.<\/p>\n\n\n\n\n\n
This guide is divided into three main parts:<\/p>\n\n\n\n
1. Testing out containers
2. Building a simple dockerized app using Dockerfile
3. Deploying the app to Kubernetes<\/p>\n\n\n\n
If you do not yet have a running Kubernetes cluster, have a look at our earlier tutorial on how to install Kubernetes on CentOS 8<\/a>.<\/p>\n\n\n\n Let\u2019s start by running a simple docker app to test the container platform.<\/p>\n\n\n\n 1. Run hello-world app with the following command:<\/p>\n\n\n\n You should see the app print \u201cHello from Docker!\u201d to your terminal towards the end of the output.<\/p>\n\n\n\n Running a known container pulls the required image from a public registry and saves it on your docker server.<\/p>\n\n\n\n 2. Check the list of docker images<\/p>\n\n\n\n After running the hello-world app once, it should show up on the images list.<\/p>\n\n\n\n Now let\u2019s try a more advanced docker app<\/p>\n\n\n\n 1. Run the Ubuntu docker app in interactive mode.<\/p>\n\n\n\n On success, you\u2019ll be brought directly to Ubuntu prompt.<\/p>\n\n\n\n 2. Test the internet connection from inside docker app by running a command that requires network access.<\/p>\n\n\n\n On Ubuntu\u2019s terminal prompt, run the following to check it has an internet connection.<\/p>\n\n\n\n On success, you should see something like the below.<\/p>\n\n\n\n Then stop the Ubuntu container with the following command.<\/p>\n\n\n\n In case you cannot access the internet, you might see something similar to the error underneath.<\/p>\n\n\n\n To fix this, make sure IP masquerade is enabled at the firewall and then try again.<\/p>\n\n\n\n OK, we\u2019ve completed the first part, now let us move on to the next step.<\/p>\n\n\n\n Dockerizing apps is a great way of creating consistent and reliable environments for many tasks regardless of the underlying operating system.<\/p>\n\n\n\n For this example, we are going to make a simple Golang web server that takes input via the server URL and prints out a hello message.<\/p>\n\n\n\n 1. Start by creating a new directory named \u201cgoserver\u201d<\/p>\n\n\n\n 2. Create a file named \u201cmain.go\u201d in the goserver directory<\/p>\n\n\n\n Enter the following code, then save the file.<\/p>\n\n\n\n 3. Create another new file named \u201cDockerfile\u201d with the following command.<\/p>\n\n\n\n Then add the following content to that file, save and exit the text editor.<\/p>\n\n\n\n 4. Next, build a docker image based on our goserver.<\/p>\n\n\n\n 5. Additionally, you may wish to clean unused images from the build process.<\/p>\n\n\n\n 6. Then check that your web app was added to the images list.<\/p>\n\n\n\n You should see a line such as in the example below.<\/p>\n\n\n\n 7. Afterwards, test run the web app.<\/p>\n\n\n\n 8. You can verify that the deployment was successful by checking the running images.<\/p>\n\n\n\n You should see the \u201cgoserver\u201d container running with the status \u201cUp\u201d.<\/p>\n\n\n\n Your web app is now reachable both locally and over the internet.<\/p>\n\n\n\n 9. Call it on the command line, for example by using curl with the command below.<\/p>\n\n\n\n You should have curl available but if not, install curl using sudo dnf install curl<\/tt><\/p>\n\n\n\n You can also open the page in your web browser. Replace the public-ip-address<\/span> with the IP of your Master node.<\/p>\n\n\n\n http:\/\/public-ip-address:8090\/Joe<\/tt><\/p>\n\n\n\n You should see the following response.<\/p>\n\n\n\n Hello, Joe!<\/p>\n\n\n\n 12. Once done, stop the web server and remove the image.<\/p>\n\n\n\n Congratulations, you should now have an idea of what\u2019s included in making simple dockerized apps.<\/p>\n\n\n\n We\u2019ve now tested out the container platform and built our own dockerized web app, so the last thing to do is to deploy it on our Kubernetes cluster.<\/p>\n\n\n\nTesting out containers<\/h2>\n\n\n\n
docker run hello-world<\/pre>\n\n\n\n
docker images<\/pre>\n\n\n\n
REPOSITORY TAG IMAGE ID CREATED SIZE\nhello-world latest fce289e99eb9 15 months ago 1.84kB<\/pre>\n\n\n\n
docker run -it ubuntu bash<\/pre>\n\n\n\n
apt update<\/pre>\n\n\n\n
Get:1 http:\/\/security.ubuntu.com\/ubuntu bionic-security InRelease [88.7 kB]\nGet:2 http:\/\/archive.ubuntu.com\/ubuntu bionic InRelease [242 kB]\n...\nGet:17 http:\/\/archive.ubuntu.com\/ubuntu bionic-backports\/main amd64 Packages [2496 B]\nGet:18 http:\/\/archive.ubuntu.com\/ubuntu bionic-backports\/universe amd64 Packages [4247 B] Fetched 17.7 MB in 5s (3439 kB\/s)<\/pre>\n\n\n\n
exit<\/pre>\n\n\n\n
Err:1 http:\/\/security.ubuntu.com\/ubuntu bionic-security InRelease Temporary failure resolving 'security.ubuntu.com' Err:2 http:\/\/archive.ubuntu.com\/ubuntu<\/a> bionic InRelease Temporary failure resolving 'archive.ubuntu.com' ...<\/pre>\n\n\n\n
firewall-cmd --add-masquerade --permanent\nfirewall-cmd --reload<\/pre>\n\n\n\n
Building a simple dockerized app using Dockerfile<\/h2>\n\n\n\n
mkdir goserver<\/pre>\n\n\n\n
vi goserver\/main.go<\/pre>\n\n\n\n
package main\nimport ( \n \"fmt\"\n \"net\/http\"\n)\nfunc main() {\n http.HandleFunc(\"\/\", HelloServer)\n http.ListenAndServe(\":8080\", nil)\n}\nfunc HelloServer(w http.ResponseWriter, r *http.Request) {\n fmt.Fprintf(w, \"Hello, %s!n\", r.URL.Path[1:])\n}<\/pre>\n\n\n\nvi Dockerfile<\/pre>\n\n\n\n
FROM golang:alpine as builder\nWORKDIR \/build\nCOPY \/goserver .\nRUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -ldflags '-extldflags \"-static\"' -o main .\nFROM scratch\nCOPY --from=builder \/build\/main \/app\/\nWORKDIR \/app\nENV PORT=8080\nCMD [\".\/main\"]<\/pre>\n\n\n\n
docker build -f Dockerfile -t goserver .<\/pre>\n\n\n\n
docker system prune -f<\/pre>\n\n\n\n
docker images<\/pre>\n\n\n\n
REPOSITORY TAG IMAGE ID CREATED SIZE\ngoserver latest 6c0fb70f56fe 55 seconds ago 7.41MB<\/pre>\n\n\n\n
docker run -d -p 8090:8080 --name goserver goserver<\/pre>\n\n\n\n
docker ps -f name=goserver<\/pre>\n\n\n\n
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES\na16ebdf117b9 goserver \".\/main\" 54 seconds ago Up 53 seconds 0.0.0.0:8090->8080\/tcp goserver<\/pre>\n\n\n\n
curl http:\/\/localhost:8090\/Joe<\/pre>\n\n\n\n
docker stop goserver && docker rm goserver<\/pre>\n\n\n\n
Deploying a dockerized app to Kubernetes<\/h2>\n\n\n\n