Docker Swarm| Step by Step | What is Docker Swarm | How to create Docker Swarm

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Hi, I am Raghav & Today we will learn :
1. What is Docker Swarm
2. Why to use it
3. How to create and manage Docker Swarm
4. Create service on docker swarm
5. Scaling services up and down
6. Features/Helpful Tips
A swarm is a group of machines that are running Docker and joined into a cluster 
Docker Swarm is a tool for Container Orchestration

Let’s take an example
You have 100 containers
You need to do
- Health check on every container
- Ensure all containers are up on every system
- Scaling the containers up or down depending on the load
- Adding updates/changes to all the containers

Orchestration - managing and controlling multiple docker containers as a single service
Tools available - Docker Swarm, Kubernetes, Apache Mesos

Pre-requisites
1. Docker 1.13 or higher
2. Docker Machine (pre installed for Docker for Windows and Docker for Mac)https://docs.docker.com/machine/install-machine/#installing-machine-directly
https://docs.docker.com/get-started/part4/

Step 1 : Create Docker machines (to act as nodes for Docker Swarm) Create one machine as manager and others as workers
docker-machine create --driver hyperv manager1 docker-machine create --driver virtualbox manager1

docker-machine:Error with pre-create check: “exit status 126”
https://stackoverflow.com/questions/38696164/docker-machineerror-with-pre-create-check-exit-status-126
brew cask install virtualbox;

Create one manager machine
and other worker machines

Step 2 : Check machine created successfully
docker-machine ls
docker-machine ip manager1

Step 3 : SSH (connect) to docker machine
docker-machine ssh manager1

Step 4 : Initialize Docker Swarm docker swarm init --advertise-addr MANAGER_IP
docker node ls
(this command will work only in swarm manager and not in worker)

Step 5 : Join workers in the swarm
Get command for joining as worker
In manager node run command
docker swarm join-token worker
This will give command to join swarm as worker

docker swarm join-token manager
This will give command to join swarm as manager

SSH into worker node (machine) and run command to join swarm as worker

In Manager Run command - docker node ls to verify worker is registered and is ready

Do this for all worker machines

Step 6 : On manager run standard docker commands
docker info
check the swarm section
no of manager, nodes etc

Now check docker swarm command options
docker swarm

Step 7 : Run containers on Docker Swarm
docker service create --replicas 3 -p 80:80 --name serviceName nginx
Check the status:
docker service ls
docker service ps serviceName
Check the service running on all nodes
Check on the browser by giving ip for all nodes

Step 8 : Scale service up and down
On manager node
docker service scale serviceName=2

Inspecting Nodes (this command can run only on manager node)
docker node inspect nodename
docker node inspect self
docker node inspect worker1

Step 9 : Shutdown node
docker node update --availability drain worker1

Step 10 : Update service
docker service update --image imagename:version web
docker service update --image nginx:1.14.0 serviceName

Step 11 : Remove service
docker service rm serviceName
docker swarm leave : to leave the swarm
docker-machine stop machineName : to stop the machine
docker-machine rm machineName : to remove the machine

REFERENCES:
https://docs.docker.com/get-started/part4/#create-a-cluster
https://rominirani.com/docker-swarm-tutorial-b67470cf8872

FAQs & Helpful Tips:
A swarm is a group of machines that are running Docker and joined into a cluster
A cluster is managed by swarm manager
The machines in a swarm can be physical or virtual. After joining a swarm, they are referred to as nodes
Swarm managers are the only machines in a swarm that can execute your commands, or authorise other machines to join the swarm as workers

Workers are just there to provide capacity and do not have the authority to tell any other machine what it can and cannot do

you can have a node join as a worker or as a manager. At any point in time, there is only one LEADER and the other manager nodes will be as backup in case the current LEADER opts out

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Raghav