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Creating Kubernetes Replication Controller in 5 Minutes

Let’s look at the following diagram to create Kubernetes Replication Controller:

What to remember when creating ReplicationController

  • apiVersion: v1
  • You can define number of replicas, selector and template under spec
  • spec->template has its own spec section along side metadata
  • You specify containers’ related attributes under template‘s spec

Create ReplicationController YAML file example

apiVersion: v1
kind: ReplicationController
metadata:
  name: nginx
spec:
  replicas: 3
  selector:
    app: nginx
  template:
    metadata:
      name: nginx
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx
        ports:
        - containerPort: 80
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Quering PostgreSQL with Docker

Recently I had to write a bash script to truncate all tables on a test database server. Using pgadmin is a real pain since I have at lot of tables and databases. In addition, I need to do this quite frequently.

And I also didn’t want to install postgres on my machine. The requirement is so small and a full installation is not necessary.

Using PostgreSQL client with Docker

Then I found a docker image that contains only the psql client (here)! You can run almost any commands in psql using this image. Here is an example:

docker run -it --rm jbergknoff/postgresql-client postgresql://user:pass@host:5432/db

For example, if you want to run a select, simply type:

docker run -it --rm jbergknoff/postgresql-client postgresql://user:pass@host:5432/db -c "select * from table"

One drawback of this image is it has paging and I couldn’t reuse the image. I need to use docker run every time I want to query.

For my need, I would like something that I can start a container once and run SQL commands many times.

PostgreSQL client docker image improved

So I created a new docker image based on the above image that you can download here:

https://hub.docker.com/r/codingpuss/postgres-client

Similarly, run

docker run -dit --network=local_net --name=pgclient codingpuss/postgres-client

To create a container. Now, you can reuse it as many time as you like to query from any PostgreSQL database like this:

 docker exec -it pgclient psql postgresql://postgres:1@pgdb:5432/postgres -c 'select * from pg_catalog.pg_tables'

And here is the result:

querying postgres database using client in docker

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Install Kubernetes Cluster With Kubeadm On Ubuntu Step By Step

I’ve been trying installing a Kubernetes cluster for while following the official documentation without any success. It turned out the official documentation was missing some important steps (or they put the missing steps else where I couldn’t find). Anyways, if you are struggling to get a Kubernetes up and running, this step by step tutorial is for you.

My environment

I’m going to setup a k8s cluster with 1 master node and 1 worker node. Once you have a master node up and running, adding one or more worker nodes does not require extra expertise.

I also use VirtualBox running two identical Ubuntu 18.04 VM. I guess that the newer Ubuntu versions should work fine (haven’t tested).

Step by step setting up Kubernetes cluster

There are some steps you need to run on all machine and some you need to run on specific node only (master/worker). We are covering the common steps first:

Steps that need to run on all nodes

Step 1: Disable swap

To disable swap, simply remove the line with swap in /etc/fstab

sudo vim /etc/fstab

As you can see, previously, I had swap enabled in line 12. I commented out that line with the # symbol.

Save the file and swap is off.

Step 2: Install docker run time

sudo apt-get install \
    apt-transport-https \
    ca-certificates \
    curl \
    gnupg \
    lsb-release
    
curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo gpg --dearmor -o /usr/share/keyrings/docker-archive-keyring.gpg

echo \
  "deb [arch=amd64 signed-by=/usr/share/keyrings/docker-archive-keyring.gpg] https://download.docker.com/linux/ubuntu \
  $(lsb_release -cs) stable" | sudo tee /etc/apt/sources.list.d/docker.list > /dev/null
  
 
sudo apt-get update
sudo apt-get install -y docker-ce docker-ce-cli containerd.io

Step 3: Configure cgroup

I couldn’t find any clear instruction on the official documentation to set systemd cgroup (I don’t really know what that means). However, it can be done in very simple command. Make sure you switch to root first:

cat > /etc/docker/daemon.json <<EOF
{
  "exec-opts": ["native.cgroupdriver=systemd"],
  "log-driver": "json-file",
  "log-opts": {
    "max-size": "100m"
  },
  "storage-driver": "overlay2",
  "storage-opts": [
    "overlay2.override_kernel_check=true"
  ]
}
EOF

Make sure you restart docker after this:

systemctl restart docker

Step 4: Install kubeadm, kubelet, kubectl

You can easily install these k8s tools using the following commands:

sudo curl -fsSLo /usr/share/keyrings/kubernetes-archive-keyring.gpg https://packages.cloud.google.com/apt/doc/apt-key.gpg

echo "deb [signed-by=/usr/share/keyrings/kubernetes-archive-keyring.gpg] https://apt.kubernetes.io/ kubernetes-xenial main" | sudo tee /etc/apt/sources.list.d/kubernetes.list

sudo apt-get update
sudo apt-get install -y kubelet kubeadm kubectl
sudo apt-mark hold kubelet kubeadm kubectl

Now, we have reached the end of the common steps. Next, let’s set up the master node.

Step 5: Commands to run on the master node

On the master node, run:

kubeadm init

This step would take quite a while since kubeadm is going to download from the internet. It took me around 5 minutes to finish but this time can vary. If everything goes right, you should see a similar message as below:

running kubeadm init on master node

Make sure to copy the line starts with kubeadm join...

You will need to run that on worker nodes later.

Still on the master node, switch to a regular user (if you haven’t) and run the following command:

mkdir -p $HOME/.kube
sudo cp -i /etc/kubernetes/admin.conf $HOME/.kube/config
sudo chown $(id -u):$(id -g) $HOME/.kube/config

Finally, you need to install network plugin for the master node (super important!)

 sudo kubectl apply -f "https://cloud.weave.works/k8s/net?k8s-version=$(sudo kubectl version | base64 | tr -d '\n')"

After the network plugin applied if you run kubectl cluster-info now, you should see the following:

Step 6: Commands on worker nodes

Now you have the master node ready. Simply run the command kubeadm join... you copied previously and run it on the worker node. In my case the command is:

kubeadm join 192.168.1.98:6443 --token 0mfz2s.4xt0waiyfnpxiyt9 \
        --discovery-token-ca-cert-hash sha256:12e48d3bbfb435536618fc293a77950c13ac975fbea934c49c39abe4b7335ce1

You should see the message saying the node has successfully joined the cluster:

joining cluser on worker node

If you see something like that, that means the node has joined successfully.

Back to the master node, run:

kubectl get nodes

You should see the worker is not ready:

worker node joins successfully

Don’t worry, after a few minute max, you should see this:

set up kubernetes cluster successfully

Congratulations! You have successfully install a Kubernetes cluster!

Running a test deployment

Let’s create a simple deployment with nginx and see if it works.

As you can see, the deploy has successfully created. Let’s check the pods:

Our nginx pod is at the top!