Getting Started with Docker Services

Let’s start with the smallest component of Docker swarm which is Docker services.

Initializing the swarm node#

If you are using the Docker Desktop for Windows or for Mac, the Docker swarm comes prebuilt. You can check that using docker swarm. To initialize the swarm mode, type docker swarm init.

＄ docker swarm initSwarm initialized: current node (ayxwwpio1sksk5hi2bdqxanf4) is now a manager.To add a worker to this swarm, run the following command:    docker swarm join --token SWMTKN-1-19vn56bmfcc2l20jmts9gg9b152vkn47ruu6vtfwgsl3646ha2-8lcseq2vaegpjqytnnw1xgksn 192.168.65.3:2377To add a manager to this swarm, run 'docker swarm join-token manager' and follow the instructions.

The node you started will be a manager node. We can add other machines or hosts to this swarm network using the command above with the provided token. But for now, we will see all the operations on a single worker-manager node only.

Creating services#

Like we have created containers using docker run command, a service does the same thing for a swarm cluster.

docker service create <service name> allows us to create a service on swarm with a defined number of containers in it.

Our aim is to create a load-balanced Flask app service and attach a database to it.

Swarm cluster only works with prebuilt images. Hence, you need to first have the image of the app in order to create the services.

For this lesson, we will use the prebuilt image of our app. So, pull the image using the docker pull venky8283/flask_app:3.0 command.

Now, let’s create the first service, which is our Flask login app.

Flask app#

Type docker service create --name web_app -p 5000:5000 venky8283/flask_app:3.0. This will create a service with one container in it, which is also called a task.

＄ docker service create --name web_app -p 5000:5000  venky8283/flask_app:3.0image flask_app:3.0 could not be accessed on a registry to recordits digest. Each node will access flask_app:3.0 independently,possibly leading to different nodes running differentversions of the image.rnlrh4e262aew2c15ldz71c5koverall progress: 1 out of 1 tasks1/1: running   [==================================================>]verify: Service converged

If the image is not pulled from the Docker Hub, it will warn us saying other swarm nodes may not have the same image, which is fine for now.

At this point, if you open localhost:5000 to see the app, you will get MySQLdb._exceptions.OperationalError MySQLdb._exceptions.OperationalError: (2005, "Unknown MySQL server host 'database' (-2)") error because we don’t have a database running yet.

So, the next task is to create a database service.

Database services#

We already have a mysql-server image locally so, let’s create a service by using it. Type docker service create --name database --env-file [.env file location] --mount type=bind,src=[location of init.sql],dst=/docker-entrypoint-initdb.d/init.sql mysql/mysql-server:5.7

＄ docker service create --name database --env-file .env --mount type=bind,src=/Users/venkateshachintalwar/Documents/Online_Projects/Docker/db/init.sql,dst=/docker-entrypoint-initdb.d/init.sql mysql/mysql-server:5.7is4ct57yaqu0mimuywhy05aspoverall progress: 1 out of 1 tasks1/1: running   [==================================================>]verify: Service converged

The command is a little bit longer. Let’s understand each argument of the command.

• --name: name of the service through which it can be reachable

• --mount: bind local file system to the container file system

• --env_file: to provide environment variables using the file

Type docker service ls to list all the services available on the swarm node.

Networking#

Both services are running now, but, you will get the same error as they are not connected. If you remember, we used the link argument to connect the database container. In swarm mode, the link option is not supported and we have to connect services using a network.

Services connected to the same network can talk to each other. So, to connect our database service to app service, we will create an “overlay network”. In swarm mode, we deal with distributed systems, hence, we use an overlay network to establish communication between services.

To create an overlay network, type docker network create --driver=overlay app. This will create a new network named ‘app’ on Docker host.

＄ docker network create --driver=overlay appriv3bf55f1xkemfjmm9w2tb5i＄ docker network lsNETWORK ID          NAME                DRIVER              SCOPEriv3bf55f1xk        app                 overlay             swarm

Updating services#

Now, we have our overlay network. Let’s attach it to both the services and let’s see if it solves the problem or not.

To update a service, type docker service update --network-add <network name> <service name/ID>.

＄ docker service update --network-add app web_appweb_appoverall progress: 1 out of 1 tasks1/1: running   [==================================================>]verify: Service converged＄ docker service update --network-add app databasedatabaseoverall progress: 1 out of 1 tasks1/1: running   [==================================================>]verify: Service converged

As soon as the network is updated, you will see that your app start working normally. Use the login credentials written in init.sql file to verify the working of the login app.

We have deployed our app using services on the swarm node. Check out all the containers using docker ps. Docker service offers a few more commands to help you work with services. You can explore using docker service --help and docker service <COMMAND> --help.