17 Multi container apps

Up to this point, you’ve been working with single container apps. But, now you will add MySQL to the application stack. The following question often arises - “Where will MySQL run? Install it in the same container or run it separately?” In general, each container should do one thing and do it well.

The following are a few reasons to run the container separately:

There’s a good chance you’d have to scale APIs and front-ends differently than databases.
Separate containers let you version and update versions in isolation.
While you may use a container for the database locally, you may want to use a managed service for the database in production. You don’t want to ship your database engine with your app then.
Running multiple processes will require a process manager (the container only starts one process), which adds complexity to container startup/shutdown.

And there are more reasons. So, it’s best to run your app in multiple containers.

17.1 Container networking

Remember that containers, by default, run in isolation and don’t know anything about other processes or containers on the same machine. So, how do you allow one container to talk to another? The answer is networking. If you place the two containers on the same network, they can talk to each other.

17.2 Start MySQL

There are two ways to put a container on a network:

Assign the network when starting the container.
Connect an already running container to a network.

In the following steps, you’ll create the network first and then attach the MySQL container at startup.

Create the network:

sudo docker network create todo-app

Start a MySQL container and attach it to the network. You’re also going to define a few environment variables that the database will use to initialize the database. To learn more about the MySQL environment variables, see the “Environment Variables” section in the MySQL Docker Hub listing.

sudo docker run -d \
     --network todo-app --network-alias mysql \
     -v todo-mysql-data:/var/lib/mysql \
     -e MYSQL_ROOT_PASSWORD=secret \
     -e MYSQL_DATABASE=todos \
     mysql:8.0

In the command above, you’ll see the --network-alias flag. In a later section, you’ll learn more about this flag.

You’ll notice a volume named todo-mysql-data in the above command that is mounted at /var/lib/mysql, which is where MySQL stores its data. However, you never ran a docker volume create command. Docker recognizes you want to use a named volume and creates one automatically for you.

To confirm you have the database up and running, connect to the database and verify that it connects.

sudo docker exec -it <mysql-container-id> mysql -u root -p

When the password prompt comes up, type in secret.

In the MySQL shell, list the databases and verify you see the todos database:

SHOW DATABASES;

You should see output that looks like this:

+--------------------+
| Database           |
+--------------------+
| information_schema |
| mysql              |
| performance_schema |
| sys                |
| todos              |
+--------------------+
5 rows in set (0.01 sec)

Exit the MySQL shell to return to the shell.

exit

You now have a todos database and it’s ready for you to use.

17.3 Connect to MySQL

Now that you know MySQL is up and running, you can use it. But, how do you use it? If you run another container on the same network, how do you find the container? Remember that each container has its own IP address.

To answer the questions above and better understand container networking, you’re going to make use of the nicolaka/netshoot container, which ships with a lot of tools that are useful for troubleshooting or debugging networking issues.

Start a new container using the nicolaka/netshoot image. Make sure to connect it to the same network.

sudo docker run -it --network todo-app nicolaka/netshoot

Inside the container, you’re going to use the dig command, which is a useful DNS tool. You’re going to look up the IP address for the hostname mysql.

dig mysql

You should get output like the following.

; <<>> DiG 9.18.13 <<>> mysql
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 35945
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;mysql.                         IN      A

;; ANSWER SECTION:
mysql.                  600     IN      A       172.18.0.2

;; Query time: 0 msec
;; SERVER: 127.0.0.11#53(127.0.0.11) (UDP)
;; WHEN: Sat May 13 11:00:51 UTC 2023
;; MSG SIZE  rcvd: 44

In the “ANSWER SECTION”, you will see an A record for mysql that resolves to 172.18.0.2 (your IP address will most likely have a different value). While mysql isn’t normally a valid hostname, Docker was able to resolve it to the IP address of the container that had that network alias. Remember, you used the --network-alias earlier.

What this means is that your app only simply needs to connect to a host named mysql and it’ll talk to the database.

Type exit to leave the Netshoot shell:

exit

17.4 Run your app with MySQL

The todo app supports the setting of a few environment variables to specify MySQL connection settings. They are:

MYSQL_HOST - the hostname for the running MySQL server
MYSQL_USER - the username to use for the connection
MYSQL_PASSWORD - the password to use for the connection
MYSQL_DB - the database to use once connected

While using env vars to set connection settings is generally accepted for development, it’s highly discouraged when running applications in production. Diogo Monica, a former lead of security at Docker, wrote a fantastic blog post explaining why.

A more secure mechanism is to use the secret support provided by your container orchestration framework. In most cases, these secrets are mounted as files in the running container. You’ll see many apps (including the MySQL image and the todo app) also support env vars with a _FILE suffix to point to a file containing the variable.

As an example, setting the MYSQL_PASSWORD_FILE var will cause the app to use the contents of the referenced file as the connection password. Docker doesn’t do anything to support these env vars. Your app will need to know to look for the variable and get the file contents.

You can now start your dev-ready container.

First, remove any old getting-started containers (keep the mysql container)

sudo docker remove -f <container-id>

Specify each of the environment variables above, as well as connect the container to your app network. As before, make sure that you are in the getting-started/app directory when you run this command:

sudo docker run -dp 3000:3000 \
   -w /app -v "$(pwd):/app" \
   --network todo-app \
   -e MYSQL_HOST=mysql \
   -e MYSQL_USER=root \
   -e MYSQL_PASSWORD=secret \
   -e MYSQL_DB=todos \
   node:18-alpine \
   sh -c "yarn install && yarn run dev"

If you look at the logs for the container (docker logs -f <container-id>), you should see a message similar to the following, which indicates it’s using the mysql database.

yarn install v1.22.19
[1/4] Resolving packages...
success Already up-to-date.
Done in 0.43s.
yarn run v1.22.19
$ nodemon src/index.js
[nodemon] 2.0.20
[nodemon] to restart at any time, enter `rs`
[nodemon] watching path(s): *.*
[nodemon] watching extensions: js,mjs,json
[nodemon] starting `node src/index.js`
Waiting for mysql:3306.
Connected!
Connected to mysql db at host mysql
Listening on port 3000

When you’re done watching the logs, exit out by hitting Ctrl+C.
Open the app in your browser and add a few items to your todo list.
Connect to the mysql database and prove that the items are being written to the database. Remember, the password is secret.

sudo  docker exec -it <mysql-container-id> mysql -p todos

And in the mysql shell, run the following:

SELECT * FROM todo_items;

This should output a table with your stored items.

To leave the MySQL-shell, simply type exit

17.5 Next steps

At this point, you have an application that now stores its data in an external database running in a separate container. You learned a little bit about container networking and service discovery using DNS.

But, there’s a good chance you are starting to feel a little overwhelmed with everything you need to do to start up this application. You have to create a network, start containers, specify all of the environment variables, expose ports, and more! That’s a lot to remember and it’s certainly making things harder to pass along to someone else.

In the next section, you’ll learn about Docker Compose. With Docker Compose, you can share your application stacks in a much easier way and let others spin them up with a single, simple command.