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Guides / Get started / Getting started guide / Part 7: 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 di�erently than
databases.

• Separate containers let you version and update versions in isolation.

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• 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, like the following diagram, it's best to run your app in
multiple containers.

Container networking
Remember that containers, by default, run in isolation and don't know anything about other

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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.

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 �rst and then attach the MySQL container at
startup.

1. Create the network.

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$ docker network create todo-app

2. Start a MySQL container and attach it to the network. You're also going to de�ne 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 .

Mac / Linux Windows (PowerShell) Windows (Command Prompt)

$ 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 previous command, you can see the --network-alias �ag. In a later section,
you'll learn more about this �ag.

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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.

3. To con�rm you have the database up and running, connect to the database and verify
that it connects.

$ 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.

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mysql> SHOW DATABASES;

You should see output that looks like this:

+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| sys |
| todos |
+--------------------+
5 rows in set (0.00 sec)

4. Exit the MySQL shell to return to the shell on your machine.

mysql> exit

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

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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 �nd 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.

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

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

2. Inside the container, you're going to use the dig command, which is a useful DNS tool.

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You're going to look up the IP address for the hostname mysql .

$ dig mysql

You should get output like the following.

; <<>> DiG 9.18.8 <<>> mysql

;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 32162
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;mysql. IN A

;; ANSWER SECTION:
mysql. 600 IN A 172.23.0.2

;; Query time: 0 msec

;; SERVER: 127.0.0.11#53(127.0.0.11)

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;; WHEN: Tue Oct 01 23:47:24 UTC 2019

;; MSG SIZE rcvd: 44

In the "ANSWER SECTION", you will see an A record for mysql that resolves to
172.23.0.2 (your IP address will most likely have a di�erent 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.

Run your app with MySQL

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

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• 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 �les in the
running container. You'll see many apps (including the MySQL image and the todo
app) also support env vars with a _FILE su�x to point to a �le containing the
variable.

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As an example, setting the MYSQL_PASSWORD_FILE var will cause the app to use the
contents of the referenced �le 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 �le contents.

You can now start your dev-ready container.

1. Specify each of the previous environment variables, as well as connect the container to
your app network. Make sure that you are in the getting-started-app directory when
you run this command.
Mac / Linux Windows (PowerShell) Windows (Command Prompt)

$ docker run -dp 127.0.0.1:3000:3000 \

-w /app -v "$(pwd):/app" \
--network todo-app \

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-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"

2. 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.

$ nodemon src/index.js
[nodemon] 2.0.20
[nodemon] to restart at any time, enter `rs`
[nodemon] watching dir(s): *.*
[nodemon] starting `node src/index.js`
Connected to mysql db at host mysql
Listening on port 3000

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3. Open the app in your browser and add a few items to your todo list.

4. Connect to the mysql database and prove that the items are being written to the
database. Remember, the password is secret .

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

And in the mysql shell, run the following:

mysql> select * from todo_items;

+--------------------------------------+--------------------+-----------+
| id | name | completed |
+--------------------------------------+--------------------+-----------+
| c906ff08-60e6-44e6-8f49-ed56a0853e85 | Do amazing things! | 0 |
| 2912a79e-8486-4bc3-a4c5-460793a575ab | Be awesome! | 0 |
+--------------------------------------+--------------------+-----------+

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Your table will look di�erent because it has your items. But, you should see them stored
there.

Summary
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.

Related information:

• docker CLI reference

• Networking overview

Next steps
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

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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.

Use Docker Compose

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