Docker - Orchestrating Containers using Swarm

Docker Swarm is a clustering and scheduling tool for Docker Containers. With Swarm, IT administrators and developers can establish and manage a Cluster of Docker nodes as a single virtual system. Swarm mode also exists natively for Docker Engine, the layer between the OS and container images. Swarm mode integrates the orchestration capabilities of Docker Swarm into Docker Engine 1.12. In this article, we will see how we can create swarm mode using docker containers but before that lets understand some of the terminology of docker swarm mode

Why do we want a Container Orchestration System?

imagine that you had to run hundreds of containers. You can easily see that if they are running in a distributed mode, there are multiple features that you will need from a management angle to make sure that the cluster is up and running, is healthy and more.

Some of these necessary features include:

• Health Checks on the Containers
• Launching a fixed set of Containers for a particular Docker image
• Scaling the number of Containers up and down depending on the load
• Performing rolling update of software across containers

Clustering - Clustering is an important feature for Container technology, because it creates a cooperative group of systems that can provide redundancy, enabling Docker Swarm failover if one or more nodes experience an outage

What does Swarm provides - A Docker Swarm cluster also provides administrators and developers with the ability to add or subtract container iterations as computing demands change.

Swarm manager - An IT administrator controls Swarm through a swarm manager, which orchestrates and schedules containers. The swarm manager allows a user to create a primary manager instance and multiple replica instances in case the primary instance fails. In Docker Engine's swarm mode, the user can deploy manager and worker nodes at runtime. This Enables multiple machines running Docker Engine to participate in a cluster, called swarm. The Docker engines contributing to a Swarm are said to be running in swarm mode.

Machines enter into the Swarm mode by either initializing a new swarm or by joining an existing swarm.

Manager node - The manager node performs cluster management and orchestration while the worker nodes perform tasks allocated by the manager.

Node - A Docker engine participating in a swarm is called a node. A node can either be a manager node or a worker node. A node is an instance of the Docker engine participating in the swarm. A manager node itself, unless configured otherwise, is also be a worker node.

Service - The central entity in the Docker Swarm infrastructure is called a service. A Docker swarm executes services. The user submits a service to the manager node to deploy and execute.

Task - A service is made up of many tasks. A task is the most basic work unit in a Swarm. A task is allocated to each worker node b the manager node.

Services can be scaled at runtime to handle extra load. The swarm manager natively supports internal load balancing to distribute tasks across the participating worker nodes. Also, the manager also supports ingress load balancing to control exposure of Docker services to the external world. The manager node also supports service discovery by automatically assigning a DNS entry to every service.

Lets create a Swarm mode and see how things work.

1.Create a Swarm Manager. Obtain your local host address to so that the swarm can be initialized

[root@ 10-149-66-36]# docker swarm init --advertise-addr 10.149.66.36

Swarm initialized: current node (4f2j4n02r0p8bs4mcu65h9dt7) is now a manager.

To add a worker to this swarm, run the following command: 

    docker swarm join \

    --token SWMTKN-1-0rxc91z9zbyg9pevtpr1s3f2jdpkhuwcgcbn1m7i4x15ku9y6f-3xn94z48fsjr0tbu1y6vzvv5v \

    10.149.66.36:2377

To add a manager to this swarm, run 'docker swarm join-token manager' and follow the instructions.

The docker swarm manager is now started. Once the docker swarm is initialized, check the "docker info" command to confirm that swarm has been started

Docker info

Swarm: active

 NodeID: 4f2j4n02r0p8bs4mcu65h9dt7

 Is Manager: true

 ClusterID: 7vdyrotpbxp4peonia1xp6hby

 Managers: 1

 Nodes: 1

 Orchestration:

  Task History Retention Limit: 5

 Raft:

  Snapshot Interval: 10000

  Heartbeat Tick: 1

  Election Tick: 3

 Dispatcher:

  Heartbeat Period: 5 seconds

 CA Configuration:

  Expiry Duration: 3 months

 Node Address: 10.149.66.36

So now the docker swarm is initialized, we need to now add worker nodes to the swarm so the services deployed to swarm will have the containers run on that worker node.

2. Go to remote machine and run the swarm join command ( make sure the docker engine is installed on the remote machine )

[root@ip-10-149-66-123]# hostname -I

10.149.66.123 172.17.0.1 172.18.0.1

[root@ip-10-149-66-123 centos]# docker swarm join --token SWMTKN-1-0rxc91z9zbyg9pevtpr1s3f2jdpkhuwcgcbn1m7i4x15ku9y6f-3xn94z48fsjr0tbu1y6vzvv5v 10.149.66.36:2377

This node joined a swarm as a worker.

Once the above command ran successfully, the worker node is joined to the manager node.

3. Once the docker swarm is initialized, manager node is added. we need to confirm the details. On the master node run the "docker ps" command as

[root@ip-10-149-66-36 yum.repos.d]# docker node ls

ID              HOSTNAME                            STATUS  AVAILABILITY  MANAGER STATUS

4f2j4n02r0p8b   *ip-10-149-66-36.ca.com Ready    Active             Leader

6p440yuk3g44k   ip-10-149-66-123          Ready    Active

From the above command we can see that the IP address "10.149.66.36" is set to manager or leader. the other IP address "10.149.66.123" is added as node or worker node.

4. Deploy a Service

Now lets deploy a ping service.

[root@ip-10-149-66-36]# docker service create --replicas 1 --name helloworld alpine ping docker.com

e00irbaijlk9n4h6yz1219mz2

A Service with the name "helloworld" is deployed. In order to check if the service is running, use the "docker service ls" command as below

[root@ip-10-149-66-36 yum.repos.d]# docker service ls

ID               NAME        REPLICAS  IMAGE   COMMAND

e00irbaijlk9  helloworld  0/1          alpine     ping docker.com

In order to get more details about the Service, use the "docker inspect" command with service ID as

[root@ip-10-149-66-36 yum.repos.d]# docker service inspect --pretty e00irbaijlk9

ID:             e00irbaijlk9n4h6yz1219mz2

Name:           helloworld

Mode:           Replicated

 Replicas:      1

Placement:

UpdateConfig:

 Parallelism:   1

 On failure:    pause

ContainerSpec:

 Image:         alpine

 Args:          ping docker.com

Resources:

Now lets check the status of service using the "docker service ps" command as

[root@ip-10-149-66-36 yum.repos.d]# docker service ps e00irbaijlk9

ID       NAME           IMAGE   NODE            DESIRED STATE  CURRENT STATE           ERROR

9s7*   helloworld.1  alpine    ip-10-149-66-123 Running        Running 43 seconds ago

In this case, the one instance of the helloworld service is running on the worker2 node. You may see the service running on your manager node. By default, manager nodes in a swarm can execute tasks just like worker nodes.

Check the same details on the worker node as

[root@ip-10-149-66-123 centos]# docker ps

CONTAINER ID     IMAGE         COMMAND             CREATED            STATUS      PORTS       NAMES 

a827c2d976d5     alpine:latest "ping docker.com"   2 minutes ago       Up 2 minutes                            helloworld.1.9s7cyf913h6dsneh41cgsfy7i

Now we have the swarm manager is up and running. the Worker node is added to swarm manager. A service with the name "helloworld" is deployed and we can see that it is running on the worker node.

5. Scale the Service

Once you have deployed a service to a swarm, you are ready to use the Docker CLI to scale the number of containers in the service. Containers running in a service are called “tasks.”

[root@ip-10-149-66-36]# docker service scale e00irbaijlk9=3

e00irbaijlk9 scaled to 3

Now the service is scaled from 1 container to 3 containers. We can see that 3 containers are running either on the manager node or worker node. Though the swarm manager is manager node ,it can still hold certain containers up and running. The details of the containers running can be checked out using,

[root@ip-10-149-66-36 yum.repos.d]# docker service ps helloworld

ID                         NAME              IMAGE   NODE                    DESIRED STATE  CURRENT STATE            ERROR

9s7cyf913h6dsneh41cgsfy7i  helloworld.1      alpine  ip-10-149-66-123        Running        Running 3 minutes ago

48bj29pr4dzvtm6odt72brp4c  helloworld.2      alpine  ip-10-149-66-36.ca.com  Running        Starting 46 seconds ago

bapvp1km9bwrof66ujup79eld  helloworld.3      alpine  ip-10-149-66-36.ca.com  Running        Starting 46 seconds ago

From the above output we can see that the service is up and running on 3 containers. Two containers are running on manager node and 1 is running on swarm node.

6. Delete the Service

A service can be deleted using "docker service rm" command. This is done on the manager node as

[root@ip-10-149-66-36 yum.repos.d]# docker service rm helloworld

helloworld

[root@ip-10-149-66-36 yum.repos.d]# docker service inspect helloworld

[]

Error: no such service: helloworld

At the same time on the worker node, when ran the "docker ps" will not show any services running.

This is a small introduction about the docker swarm and how to implement those. More to come. Happy learning :-)