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Docker : Docker Swarm - Defining Clustered Multi-Container Applications

This article describes how to use Docker Swarm to create clustered multi-container applications.

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Assumptions

If you plan on using the examples in this article, you are going to need a few things in place before you start.

Prerequisites

In order for this to work, without adjustment, you will need to perform the following prerequisites.

Clone the Git repository here. You will need to place the relevant installation media under the software directories of the "ol7_183" database build and the "ol7_ords" build.

Build the images.

# Build DB image
cd ~/dockerfiles/database/ol7_183
docker build -t ol7_183:latest .

# Build ORDS image
cd ~/dockerfiles/ords/ol7_ords
docker build -t ol7_ords:latest .

The example also uses a Portainer image, but that is downloaded automatically from Docker Hub when required.

Once you've completed these prerequisites without errors, we can move on.

Start Swarm

The swarm is started with the docker swarm init command. The node that starts the swarm automatically becomes the manager of the swarm.

$ docker swarm init
Swarm initialized: current node (elo30e3qmw0ot4gwcl5vpfhff) is now a manager.

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

    docker swarm join --token SWMTKN-1-3gdzi62dgi4wn8zv42w4uhh3axnjor4haqw054giyzfch76mlj-2g8bcl62d9rqfrf2q72hrkekg 10.0.2.15:2377

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

$

Notice the docker swarm join command in the output of the docker swarm init command. Not surprisingly, this is the command you use if you want any other machines to join the swarm. For this to work each machine will need to be accessible on the network, without any firewalls blocking them on the designated port.

Stack Definition

If you've used Docker Compose, the definition of a stack will look very familar, as it uses a compose file. Some examples still use a file name called "docker-compose.yml", while others use the name "docker-stack.yml" for the file name. I prefer the latter, as there are some options which are valid for Docker Compose, which are not valid for a stack definition.

For this article I'm using the "docker-stack.yml" file found here. This file defines a three services.

The "docker-stack.yml" contains the following entry for the ORDS service, which defines how is should be deployed. In this case we are expecting 2 containers, which the swarm will restart on failure, with each being limited to 1 CPU.

    deploy:
      replicas: 2
      restart_policy:
        condition: on-failure
      resources:
        limits:
          cpus: "1"

The rest of the definition matches what you would see in a regular Docker Compose file, minus the build instructions. If you reference any compose-specific parameters, they will be ignored and a warming displayed on screen.

Deploy Stack

We deploy a stack to the swarm using the docker stack deploy command, referencing the "docker-stack.yml" file using the "--compose-file" or "-c" flag.

$ cd ~/dockerfiles/swarm/ol7_183_ords
$ docker stack deploy --compose-file ./docker-stack.yml ords-stack
Creating network ords-stack_ordsnet
Creating network ords-stack_default
Creating service ords-stack_ords
Creating service ords-stack_db
Creating service ords-stack_portainer
$

The following commands give us some information about the stack. We can check what stacks are in the swarm using the docker stack ls command.

$ docker stack ls
NAME                SERVICES
ords-stack          3
$

We can see the services that are running, including the number of replicates of each, using the docker service ls command.

$ docker service ls
ID                  NAME                   MODE                REPLICAS            IMAGE                        PORTS
3yy4pncrars5        ords-stack_db          replicated          0/1                 ol7_183:latest               *:1521->1521/tcp
0eo2a8pi91tg        ords-stack_ords        replicated          2/2                 ol7_ords:latest              *:8080->8080/tcp, *:8443->8443/tcp
u7qnhf4l2tyd        ords-stack_portainer   replicated          1/1                 portainer/portainer:latest   *:9000->9000/tcp
$

We can display information about the processes running in a stack using the docker stack ps {stack name} command.

$ docker stack ps ords-stack
ID                  NAME                     IMAGE                        NODE                    DESIRED STATE       CURRENT STATE            ERROR               PORTS
j4rpczp0k6ch        ords-stack_portainer.1   portainer/portainer:latest   localhost.localdomain   Running             Running 5 minutes ago
jlrmo1g0yqta        ords-stack_db.1          ol7_183:latest               localhost.localdomain   Running             Running 17 seconds ago
rkvub2qez8ub        ords-stack_ords.1        ol7_ords:latest              localhost.localdomain   Running             Running 4 minutes ago
43usasqwjn6s        ords-stack_ords.2        ol7_ords:latest              localhost.localdomain   Running             Running 4 minutes ago
$

The docker ps command can still be used in the normal way of course.

$ docker ps -a
CONTAINER ID        IMAGE                        COMMAND                  CREATED             STATUS                   PORTS                NAMES
66dc6c991f51        portainer/portainer:latest   "/portainer"             7 minutes ago       Up 6 minutes             9000/tcp             ords-stack_portainer.1.j4rpczp0k6chpj5b6acgqxl44
fe92be4d84f8        ol7_183:latest               "/bin/sh -c 'exec ${…"   7 minutes ago       Up 6 minutes (healthy)   1521/tcp, 5500/tcp   ords-stack_db.1.jlrmo1g0yqtagbjbz3rn7gyff
6991979026f1        ol7_ords:latest              "/bin/sh -c 'exec ${…"   7 minutes ago       Up 7 minutes (healthy)   8080/tcp, 8443/tcp   ords-stack_ords.2.43usasqwjn6s9wl03y3uyzdvx
ca3b6a58ebe2        ol7_ords:latest              "/bin/sh -c 'exec ${…"   7 minutes ago       Up 7 minutes (healthy)   8080/tcp, 8443/tcp   ords-stack_ords.1.rkvub2qez8ubtxewibuiab0ep
$

Scale a Service

We can scale up or down the replicas for a specific service using the docker service scale command. The following command increases the number of replicates from 2 to 5 for the ORDS service, using the service name listed with the docker service ls command. The output from the command shows the additional containers starting.

$ docker service scale ords-stack_ords=5
ords-stack_ords scaled to 5
overall progress: 5 out of 5 tasks
1/5: running   [==================================================>]
2/5: running   [==================================================>]
3/5: running   [==================================================>]
4/5: running   [==================================================>]
5/5: running   [==================================================>]
verify: Service converged
$

$ docker stack ps ords-stack
ID                  NAME                     IMAGE                        NODE                    DESIRED STATE       CURRENT STATE                ERROR               PORTS
j4rpczp0k6ch        ords-stack_portainer.1   portainer/portainer:latest   localhost.localdomain   Running             Running 14 minutes ago
jlrmo1g0yqta        ords-stack_db.1          ol7_183:latest               localhost.localdomain   Running             Running 9 minutes ago
rkvub2qez8ub        ords-stack_ords.1        ol7_ords:latest              localhost.localdomain   Running             Running 13 minutes ago
43usasqwjn6s        ords-stack_ords.2        ol7_ords:latest              localhost.localdomain   Running             Running 13 minutes ago
rnw0i26gudqn        ords-stack_ords.3        ol7_ords:latest              localhost.localdomain   Running             Running about a minute ago
a64bj66nv9rh        ords-stack_ords.4        ol7_ords:latest              localhost.localdomain   Running             Running about a minute ago
kdh39395zdc8        ords-stack_ords.5        ol7_ords:latest              localhost.localdomain   Running             Running about a minute ago
$

We can scale down the service also.

$ docker service scale ords-stack_ords=2
ords-stack_ords scaled to 2
overall progress: 2 out of 2 tasks
1/2: running   [==================================================>]
2/2: running   [==================================================>]
verify: Service converged
$

$ docker stack ps ords-stack
ID                  NAME                     IMAGE                        NODE                    DESIRED STATE       CURRENT STATE            ERROR               PORTS
j4rpczp0k6ch        ords-stack_portainer.1   portainer/portainer:latest   localhost.localdomain   Running             Running 16 minutes ago
jlrmo1g0yqta        ords-stack_db.1          ol7_183:latest               localhost.localdomain   Running             Running 11 minutes ago
rkvub2qez8ub        ords-stack_ords.1        ol7_ords:latest              localhost.localdomain   Running             Running 15 minutes ago
43usasqwjn6s        ords-stack_ords.2        ol7_ords:latest              localhost.localdomain   Running             Running 15 minutes ago
$

Remove Stack

The docker stack rm command will remove the specified stack.

$ docker stack rm ords-stack
Removing service ords-stack_db
Removing service ords-stack_ords
Removing service ords-stack_portainer
Removing network ords-stack_default
Removing network ords-stack_ordsnet
$

Depending on the container shutdowns, it can take some time to complete, so you might want to check the status using the docker stack ps command, until all the processes are gone.

$ docker stack rm ords-stack
Removing service ords-stack_db
Removing service ords-stack_ords
Removing service ords-stack_portainer
Removing network ords-stack_default
Removing network ords-stack_ordsnet
$

$ docker stack ps ords-stack
ID                  NAME                          IMAGE               NODE                    DESIRED STATE       CURRENT STATE           ERROR               PORTS
jlrmo1g0yqta        3yy4pncrars5wk9bvucs1nf2l.1   ol7_183:latest      localhost.localdomain   Remove              Running 5 seconds ago
$

$ docker stack ps ords-stack
nothing found in stack: ords-stack
$

Leave Swarm

A machine can leave a swarm using the docker swarm leave command. The "-f" flag allows the swarm manager to leave the swarm.

$ docker swarm leave -f
Node left the swarm.
$

Considerations

Just some things to consider when using Docker Swarm.

For more information see:

Hope this helps. Regards Tim...

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