Note: If you have missed my previous articles on Docker, you can find them here.
Application deployment models evolution.
Getting started with Docker.
Docker file and images.
Publishing images to Docker Hub and re-using them
Docker- Find out what's going on
Docker Networking- Part 1
Docker Networking- Part 2Docker allows ease of deployment of "microservices" with containers. These containers run on different servers and need to be orchestrated. Docker swarm is Docker's built-in orchestration system.
So what is Docker Swarm? From Docker Documentation :
"A swarm consists of multiple Docker hosts which run in swarm mode and act as managers (to manage membership and delegation) and workers (which run swarm services). A given Docker host can be a manager, a worker, or perform both roles. When you create a service, you define its optimal state (number of replicas, network, and storage resources available to it, ports the service exposes to the outside world, and more). Docker works to maintain that desired state. For instance, if a worker node becomes unavailable, Docker schedules that node’s tasks on other nodes. A task is a running container that is part of a swarm service and managed by a swarm manager, as opposed to a standalone container."
A node in the Docker Swarm cluster can be a "manager" node or a "worker" node. Manager nodes "manage" tasks and perform cluster management functions. Worker node performs assigned tasks and report status back to the manager. If a host in swarm goes down, the manager node is responsible for allocating tasks and maintaining the "desired" state of swarm cluster.
Manager nodes maintain a database called the "Raft" database. The raft is synchronized between manager nodes in a swarm cluster and is used to manage workers, state of swarm among other things.
It is important to note that the manager node in a swarm cluster is a "worker' node with special permissions.
Let's check out how things work with Docker Swarm.
Note: If you intend to follow along, ensure you have created at least two VM's with docker installed in them. In my case, I am using ubuntu-20 servers inside Hyper-V.
By default, the Docker swarm is inactive.
root@ubuntu20-docker1:/home/sathish# docker info
Client:
Debug Mode: false
Server:
Containers: 2
Running: 0
Paused: 0
Stopped: 2
Images: 6
Server Version: 19.03.11
Storage Driver: overlay2
Backing Filesystem: extfs
Supports d_type: true
Native Overlay Diff: true
Logging Driver: json-file
Cgroup Driver: cgroupfs
Plugins:
Volume: local
Network: bridge host ipvlan macvlan null overlay
Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
Swarm: inactive
Runtimes: runc
Default Runtime: runc
Init Binary: docker-init
containerd version: 7ad184331fa3e55e52b890ea95e65ba581ae3429
runc version:
init version: fec3683
Security Options:
apparmor
seccomp
Profile: default
Kernel Version: 5.4.0-42-generic
Operating System: Ubuntu Core 16
OSType: linux
Architecture: x86_64
CPUs: 1
Total Memory: 3.817GiB
Name: ubuntu20-docker1
ID: AASS:4KTX:B6TN:WHC3:4FUF:KAN5:D7Z5:QJPQ:ETJW:7CO3:6LQ4:ATS6
Docker Root Dir: /var/snap/docker/common/var-lib-docker
Debug Mode: false
Username: wizardonwire
Registry: https://index.docker.io/v1/
Labels:
Experimental: false
Insecure Registries:
127.0.0.0/8
Live Restore Enabled: false
WARNING: No swap limit supportLet's enable swarm on first VM with "docker swarm init"
root@ubuntu20-docker1:/home/sathish# docker swarm init
Swarm initialized: current node (efhgyrygaplscg804h3wdgi82) is now a manager.
To add a worker to this swarm, run the following command:
docker swarm join --token SWMTKN-1-1pwpou1cx13ir6oias5mnqysbrmd2bgqgyl1izox2f6nrxjp3i-3mro3i1khnywex0k2cs1unf16 172.28.147.39:2377
To add a manager to this swarm, run 'docker swarm join-token manager' and follow the instructions.On the second VM, I am going to join this swarm cluster by copy-pasting the "docker swarm join" command.
root@sathish-ubuntu2:/home/sathish# docker swarm join --token SWMTKN-1-1pwpou1cx13ir6oias5mnqysbrmd2bgqgyl1izox2f6nrxjp3i-3mro3i1khnywex0k2cs1unf16 172.28.147.39:2377
This node joined a swarm as a worker.Nodes in a Swarm cluster can be seen with "docker node ls"
root@ubuntu20-docker1:/home/sathish# docker node ls
ID HOSTNAME STATUS AVAILABILITY MANAGER STATUS ENGINE VERSION
d9b5th39uktl17srsh7t8y2wx sathish-ubuntu2 Ready Active 19.03.11
efhgyrygaplscg804h3wdgi82 * ubuntu20-docker1 Ready Active Leader 19.03.11As we can see from the above, we have 1 manager and 1 worker node in the swarm cluster.
"docker service create" is used to create a "service" in the docker swarm cluster, this command is similar to "docker container run". With "docker service create" we can specify the number of instances of service required. Manager nodes in the cluster will create and maintain the number of replicas we specify.
Let's create 6 replicas of httpd container and expose it on port 80
root@ubuntu20-docker1:/home/sathish# docker service create --replicas 6 --name web -p 80:80 httpd
2z19wj64sy37pb4zdg964ql7e
overall progress: 6 out of 6 tasks
1/6: running [==================================================>]
2/6: running [==================================================>]
3/6: running [==================================================>]
4/6: running [==================================================>]
5/6: running [==================================================>]
6/6: running [==================================================>]
verify: Service converged
root@ubuntu20-docker1:/home/sathish# docker service ls
ID NAME MODE REPLICAS IMAGE PORTS
2z19wj64sy37 web replicated 6/6 httpd:latest *:80->80/tcpThe 6 replicas are distributed between 2 nodes in the cluster (3 on manager node-VM1 and 3 on worker node- VM2)
VM1
root@ubuntu20-docker1:/home/sathish# docker container ls
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
778d7a58c47f httpd:latest "httpd-foreground" 40 seconds ago Up 36 seconds 80/tcp web.1.qvgpda49crnpitvm2rlv4iro8
d4ee4ba4f116 httpd:latest "httpd-foreground" 40 seconds ago Up 37 seconds 80/tcp web.3.zo9olsog2aaf2tncekwfvt3si
71f5b072d0eb httpd:latest "httpd-foreground" 40 seconds ago Up 36 seconds 80/tcp web.5.riqbg58kuhcwyxkwenq8q0v57
VM2
root@sathish-ubuntu2:/home/sathish# docker container ls
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
a12cfd234da1 httpd:latest "httpd-foreground" 59 seconds ago Up 56 seconds 80/tcp web.2.zjqsd9c0b888ycggrltu3y2dk
438c62087888 httpd:latest "httpd-foreground" 59 seconds ago Up 55 seconds 80/tcp web.6.d3lkzimfmm1hpjhgrm8z6x2iw
4bc31cfb251d httpd:latest "httpd-foreground" 59 seconds ago Up 56 seconds 80/tcp web.4.k6bl1ui3si9kgk66h896at28nAs I mentioned before, manger nodes in a swarm are responsible for maintaining the desired state- in our case 6 replicas. I am going to reboot VM2 and check if 3 replicas are recreated on VM1
root@ubuntu20-docker1:/home/sathish# docker node ls
ID HOSTNAME STATUS AVAILABILITY MANAGER STATUS ENGINE VERSION
d9b5th39uktl17srsh7t8y2wx sathish-ubuntu2 Down Active 19.03.11
efhgyrygaplscg804h3wdgi82 * ubuntu20-docker1 Ready Active Leader 19.03.11
root@ubuntu20-docker1:/home/sathish# docker service ls
ID NAME MODE REPLICAS IMAGE PORTS
2z19wj64sy37 web replicated 3/6 httpd:latest *:80->80/tcp
root@ubuntu20-docker1:/home/sathish# docker service ls
ID NAME MODE REPLICAS IMAGE PORTS
2z19wj64sy37 web replicated 3/6 httpd:latest *:80->80/tcp
root@ubuntu20-docker1:/home/sathish# docker service ls
ID NAME MODE REPLICAS IMAGE PORTS
2z19wj64sy37 web replicated 4/6 httpd:latest *:80->80/tcp
root@ubuntu20-docker1:/home/sathish# docker service ls
ID NAME MODE REPLICAS IMAGE PORTS
2z19wj64sy37 web replicated 6/6 httpd:latest *:80->80/tcp
root@ubuntu20-docker1:/home/sathish# docker container ls
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
d9571617924e httpd:latest "httpd-foreground" About a minute ago Up About a minute 80/tcp web.2.k9ymuict2lrohpunmn8urm7mb
97488d952a90 httpd:latest "httpd-foreground" About a minute ago Up About a minute 80/tcp web.6.82ka87f7snlu01es2p2ohk5v2
06b415ae6c1a httpd:latest "httpd-foreground" About a minute ago Up About a minute 80/tcp web.4.125w9c50pqkf9xq9vgegdc5kt
778d7a58c47f httpd:latest "httpd-foreground" 6 minutes ago Up 6 minutes 80/tcp web.1.qvgpda49crnpitvm2rlv4iro8
d4ee4ba4f116 httpd:latest "httpd-foreground" 6 minutes ago Up 6 minutes 80/tcp web.3.zo9olsog2aaf2tncekwfvt3si
71f5b072d0eb httpd:latest "httpd-foreground" 6 minutes ago Up 6 minutes 80/tcp web.5.riqbg58kuhcwyxkwenq8q0v57Once I brought down the worker node the manger node detected the event and brought up 3 more replicas.
That's all for today folks!! In the next article, I will talk about multi-host container networking and overlay driver. Thanks for your time
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