#Snippets

August 23, 2020

#Create, test and run a Java project with Gradle using Docker and no IDE

In order to create and manage Java projects, its common to employ the help of an IDE like IntelliJ IDEA or Eclipse.

However, if you would rather use only the command line, Docker can be a great help.

For convenience and reproducibility, create a Dockerfile in your local filesystem with the following contents.

FROM openjdk:14

ENV GRADLE_HOME=/opt/gradle/gradle-6.4.1
ENV PATH=${GRADLE_HOME}/bin:${PATH}

RUN yum install -y wget unzip

RUN wget https://services.gradle.org/distributions/gradle-6.4.1-bin.zip -P /tmp &&\
    unzip -d /opt/gradle /tmp/gradle-*.zip &&\
    rm -rf /tmp/*

Build the Docker image and run a container (takes a while). Again, for convenience, -v and -w are used so that file creations and modifications made inside the container are reflected outside.

docker run --rm -it -v ${PWD}:/app -w /app $(docker build -q .) bash

Now, we can use gradle to create and manage a project from the command line.

# Create a new project
# The "application" template most likely will suit your needs
gradle init

# Run unit tests (if you enabled JUnit during project creation)
gradle test

# Run your code
gradle run

Write source code in src/main/java/<your_package> and test code in src/test/java/<your_package>.

Further reading

Official Gradle guide for Java applications

Docker command line

Gradle command line

List of JUnit Assertions

Section break
August 02, 2020

#Read Kubernetes API data using Golang

Get the API client.

import (
  "fmt"
  "context"
  "k8s.io/client-go/kubernetes"
  "k8s.io/client-go/tools/clientcmd"
  metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
)

// use the current context in kubeconfig
config, err := clientcmd.BuildConfigFromFlags("", "<path_to_KUBECONFIG>")
if err != nil {
  panic(err.Error())
}

// create the clientset
clientset, err := kubernetes.NewForConfig(config)
if err != nil {
  panic(err.Error())
}

Usage examples:

  • read data about servers
nodeList, err := clientset.CoreV1().Nodes().List(context.TODO(), metav1.ListOptions{})
if err != nil {
  panic(err.Error())
}
  • read data about pods
namespace := "example"  // leave empty to get data from all namespaces
podList, err := clientset.CoreV1().Pods(namespace).List(context.TODO(), metav1.ListOptions{})
if err != nil {
  panic(err.Error())
}

In the above examples, ListOptions can take two important strings, field selectors and label selectors. This can be used for filtering results.

metav1.ListOptions{
  LabelSelector: "labelName=labelKey",
  FieldSelector: "spec.nodeName=<node_name>",  // Example for filtering by node name
}

Field selectors and label selectors on this case operate in the same way that the CLI options for kubectl do. So, the same rules apply here.

Further reading

Official docs on Field Selectors and Labels.

Section break
August 02, 2020

#Read Kubernetes metrics-server data using Golang

First, get the metrics API client.

import (
  "fmt"
  "context"
  "k8s.io/client-go/kubernetes"
  "k8s.io/client-go/tools/clientcmd"
  metricsv "k8s.io/metrics/pkg/client/clientset/versioned"
)

// Get the config
config, err := clientcmd.BuildConfigFromFlags("", "<path_to_KUBECONFIG>")
if err != nil {
  panic(err.Error())
}

// Get the metrics client
metricsClientset, err := metricsv.NewForConfig(config)
if err != nil {
  panic(err.Error())
}

Get the data and store it in podMetricsList array.

namespace := "example"  // leave empty to get data from all namespaces
podMetricsList, err := metricsClientset.MetricsV1beta1().PodMetricses(namespace).List(context.TODO(), metav1.ListOptions{})
if err != nil {
  panic(err.Error())
}

Iterate over the results found.

for _, v := range podMetricsList.Items {
  fmt.Printf("%s\n", v.GetName())
  fmt.Printf("%s\n", v.GetNamespace())
  fmt.Printf("%vm\n", v.Containers[0].Usage.Cpu().MilliValue())
  fmt.Printf("%vMi\n", v.Containers[0].Usage.Memory().Value()/(1024*1024))
}

Note: if you’re inspecting pods that may have more than one container, you’ll need to iterate over v.Containers as well.

Section break
July 16, 2020

#Bootstrap a Go Cobra application with Go Modules

To bootstrap a Go application using Cobra Generator and Go Modules, start by installing the Cobra command line:

# Install Cobra
go get github.com/spf13/cobra/cobra

Initialize your Cobra project, but remember to use your package’s fully qualified name, as in: github.com/laurybueno/kubectl-hoggers. Don’t worry if your package is not publicly available yet. And there is no need to put your source code inside GOPATH anymore (since Go 1.11 at least):

cobra init --pkg-name <fully_qualified_package_name>

Initialize your new module:

go mod init <fully_qualified_package_name>

Get recursively all its dependencies and install it:

go get -d -v ./...
go install -v ./...

Your app’s command line is now ready to be used:

kubectl-hoggers

Further reading

Docker and Go: https://www.docker.com/blog/docker-golang/

Go Modules: https://github.com/golang/go/wiki/Modules

Section break