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DockerSwarm.md

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Docker Swarm

Docker swarm is container orchestration tools build and manager by Docker.

It's a native clustering tool provided by Docker which provides high-availability and high-performance for your application by distributing it to all nodes inside the swarm cluster.

A Docker engine participating in a swarm is called a node.

Swarm uses the SwarmKit library for orchestration.

This toolkit uses Raft consensus algorithm to coordinate and decision making of a distributed system. SwarmKit is responsible for Orchestration, Scheduling and Cluster Management.

Features of Docker Swarm

Scaling

Services running on the cluster are easily scalable through a single parameter. You can declare the number of replicas a service should have and the swarm manager automatically adapts by adding or removing containers to maintain the desired state.

The swarm also provides resilience through reconciliation of the desired states. The manager continuously monitors the state of the nodes in the cluster. If a node goes offline, the manager will reconcile the difference in the desired state and the current state by redeploying the lost services on the available nodes.

Load balancing

Swarm manager uses ingress load balancing to expose the services you want to make available externally to the swarm..

Security

Docker Swarm comes with great security features out of the box. When a node joins the swarm, it uses a token that not only verifies itself but also verifies it is joining the swarm you think it is. From that moment on, all communication between nodes takes place using mutual TLS encryption. This encryption is all provisioned and managed automatically by the Swarm, so you never need to worry about renewing certificates, and other typical security hassles.

Important component of Docker swarm

  1. Service : Service define the task that need to be execute on manager and workder nodes.

  2. Task : Tasks are Docker containers that execute the commands you defined in the service.

  3. Manager Node: It delivers work (in the form of tasks) to worker nodes. and Monitor the worker node.

  4. Worker node: Worker nodes run tasks distributed by the manager node in the swarm.Each worker node runs an agent that reports back to the master node about the state of the tasks assigned to it, so the manager node can keep track of services and tasks running in the swarm.

Docker Swarm Cluster

Cluster

Docker Swarm contain multiple Docker Host that running on Docker Swarm mode. Each host may be work as Master or Worker. Master Host will work as Manager or Worker.

Formula for Creating the Master Host.

Master Or Manager = (n-1)/2

N= Number of Nodes

How to Initialize the Docker Swarm
 docker swarm init 
 
or

 docker swarm init --advertise-addr <MANAGER-IP>

Docker Service

Basic Comamnd of Docker Service

 create      Create a new service
  inspect     Display detailed information on one or more services
  logs        Fetch the logs of a service or task
  ls          List services
  ps          List the tasks of one or more services
  rm          Remove one or more services
  rollback    Revert changes to a service's configuration
  scale       Scale one or multiple replicated services
  update      Update a service

Create first Docker service example

docker service --name[nameofservice] [imagename] [action]   
docker service --name pingservice alpine ping www.google.com .  // New Service will create with the name of pingservice

List of service 

docker service ls // Give the list of available service 

Inspect the docker service  

docker service inspect pingservice 

Scaling the service 

docker service update pingservice --replicas=3 // Create the replicas for same service 

List of container runnong for that service 

docker ps -a  // all the running container will be visisble here 


Remove any container docker service will create new container automatically 

docker kill [container_name]
docker ps -a // after sometime you can see new container will created because we define that we need 4 replicas

Important Topic for Docker Swarm

  1. Docker Swarm node Promote and Demote
  2. Docker Swarm Visualizer
  3. Docker Service mode (Replicated and Global)
  4. Docker Swarm Availablity
Command to create Swarm Visualizer
docker service create \
  --name=viz \
  --publish=8080:8080/tcp \
  --constraint=node.role==manager \
  --mount=type=bind,src=/var/run/docker.sock,dst=/var/run/docker.sock \
  dockersamples/visualizer
  
demote      Demote one or more nodes from manager in the swarm
inspect     Display detailed information on one or more nodes
ls          List nodes in the swarm
promote     Promote one or more nodes to manager in the swarm
ps          List tasks running on one or more nodes, defaults to current node
rm          Remove one or more nodes from the swarm
update      Update a node

Docker Swarm Node Availability

There are 3 types of state present in the docker node.

  1. Active
  2. Pause
  3. Drain

Active - This is the default state of node. In this state node is ready to take new task from ther master or Manger node.

Pause - If Any node availability pause means that node is not ready to take new task from Master or Manager node.

Drain - Sometime if you have any planned maintanance acitity on that node so you can set the availability drain. So that node will not take any task from the master and Manger shift all the running container from that node to active node.

Command to change the availability -

Change availability of node from Active state to Pause .

docker node update --availability=pause

Change availability of node from Active state to Drain .

docker node update --availability=drain