Using Docker Compose
Last updated on 2024-12-17 | Edit this page
Overview
Questions
- What is Docker Compose?
- Why and when would I use it?
- How can I translate my
docker runcommands into adocker-compose.ymlfile? - How can I make containers communicate with each other?
Objectives
- Learn how to run multiple containers together.
- Clean up our run command for once and for all.
We’ve manage to come a long way in making the SPUC container work better for us, but it still lacks a little something. If we want to open this service to our local community, we can hardly expect them to hit the API with a curl command!
Luckily, the SPUA released the SPUC Super Visualiser (SPUCSVi)! The SPUCSVi is a web-based tool that allows you to register unicorn sightings, and also see the record of unicorn sightings. Handily, the SPUA also made it available as a Docker container.
Now that you have seen several docker run commands, you
can well imagine how cumbersome running multiple containers can get.
Even more so if we want the different containers to play well with each
other.
Enter: Docker Compose!
Docker Compose is a tool for defining and running multi-container applications. With Compose, you use a YAML file to configure your application’s services. Then, with a single command, you create and start all the services from your configuration.
Let’s take a look at Docker Compose and see how it can help us run SPUC and SPUCSVi.
Running a container
As an initial step, we will learn how to run a container using Docker Compose.
The first thing we need to do is create a
docker-compose.yml file. All
docker-compose.yml files start with services:.
This is the root element under which we define the services we want to
run.
Next, let’s add the service for the SPUC container. We’ll call it
spuc and we will tell it what image to
use.
This is actually enough for us to run the container! But we won’t use
docker run any more.
Instead, we will use the base command docker compose. To
run the services, we add the command up, signalling that we
want to bring services up (i.e. start them).
OUTPUT
[+] Running 2/0
✔ Network docker_intro_default Created 0.1s
✔ Container docker_intro-spuc-1 Created 0.0s
Attaching to spuc-1
spuc-1 |
spuc-1 | \
spuc-1 | \
spuc-1 | \\
spuc-1 | \\\
spuc-1 | >\/7
spuc-1 | _.-(º \
spuc-1 | (=___._/` \ ____ ____ _ _ ____
spuc-1 | ) \ |\ / ___|| _ \| | | |/ ___|
spuc-1 | / / ||\ \___ \| |_) | | | | |
spuc-1 | / > /\\\ ___) | __/| |__| | |___
spuc-1 | j < _\ |____/|_| \____/ \____|
spuc-1 | _.-' : ``.
spuc-1 | \ r=._\ `. Space Purple Unicorn Counter
spuc-1 | <`\\_ \ .`-.
spuc-1 | \ r-7 `-. ._ ' . `\
spuc-1 | \`, `-.`7 7) )
spuc-1 | \/ \| \' / `-._
spuc-1 | || .'
spuc-1 | \\ (
spuc-1 | >\ >
spuc-1 | ,.-' >.'
spuc-1 | <.'_.''
spuc-1 | <'
spuc-1 |
spuc-1 |
spuc-1 | Welcome to the Space Purple Unicorn Counter!
spuc-1 |
spuc-1 | :::: Units set to Imperial Unicorn Hoove Candles [iuhc] ::::
spuc-1 |
spuc-1 | :: Try recording a unicorn sighting with:
spuc-1 | curl -X PUT localhost:8321/unicorn_spotted?location=moon\&brightness=100
spuc-1 |
spuc-1 | :: No plugins detected
spuc-1 |So we have our container running! With a couple of interesting bits of output to note:
- A container was created named
docker-intro-spuc-1. (The directory name is prepended to the container name) - A
networkwas created for the container - we will dig into what this means later! - The tool is running in the foreground, so we can see the output of the tool
We can stop the container by pressing [Ctrl+C] in the
terminal.
Configuring the container
We have managed to run our container, but we are still a way off from
reproducing our last run command. It’s ok, we need to add
more configuration to our Docker Compose file.
Let’s recall our docker run command for the regular SPUC
container (rather than the one we made ourselves - we’ll get to that in
a bit):
BASH
docker run -d --rm --name spuc_container -p 8321:8321 -v ./print.config:/spuc/config/print.config -v spuc-volume:/spuc/output -v stats.py:/spuc/plugins/stats.py -e EXPORT=true spuacv/spuc:latest --units iuluThere are a lot of flags here! Each of these flags has a
corresponding key in Docker Compose. Lets order all the elements we want
in a table, so we can see what we need to add to our
docker-compose.yml file.
| Flag | Description |
|---|---|
-d |
Run the container in the background |
--rm |
Remove the container when it stops |
--name spuc_container |
Name the container spuc_container
|
-p 8321:8321 |
Map port 8321 on the host to port 8321 in the container |
-v ./print.config:/spuc/config/print.config |
Bind mount the ./print.config file into the
container |
-v spuc-volume:/spuc/output |
Persist the /spuc/output directory in a volume |
-v ./stats.py:/spuc/plugins/stats.py |
Bind mount the ./stats.py plugin into the
container |
-e EXPORT=true |
Set the environment variable EXPORT to
true
|
--units iulu |
Set the units to Imperial Unicorn Length Units |
We can now start translate this into a Docker Compose file bit by bit!
Running in the background
To run a docker compose stack in the background, we can use the
-d (for detach) flag when calling
docker compose up.
OUTPUT
[+] Running 1/1
✔ Container docker-intro-spuc-1 Started 0.2sOf course, this means we can no longer see the logs! But we can still
access them using the logs command.
OUTPUT
spuc-1 |
spuc-1 | \
spuc-1 | \
spuc-1 | \\
spuc-1 | \\\
spuc-1 | >\/7
spuc-1 | _.-(º \
spuc-1 | (=___._/` \ ____ ____ _ _ ____
spuc-1 | ) \ |\ / ___|| _ \| | | |/ ___|
spuc-1 | / / ||\ \___ \| |_) | | | | |
spuc-1 | / > /\\\ ___) | __/| |__| | |___
spuc-1 | j < _\ |____/|_| \____/ \____|
spuc-1 | _.-' : ``.
spuc-1 | \ r=._\ `. Space Purple Unicorn Counter
spuc-1 | <`\\_ \ .`-.
spuc-1 | \ r-7 `-. ._ ' . `\
spuc-1 | \`, `-.`7 7) )
spuc-1 | \/ \| \' / `-._
spuc-1 | || .'
spuc-1 | \\ (
spuc-1 | >\ >
spuc-1 | ,.-' >.'
spuc-1 | <.'_.''
spuc-1 | <'
spuc-1 |
spuc-1 |
spuc-1 | Welcome to the Space Purple Unicorn Counter!
spuc-1 |
spuc-1 | :::: Units set to Imperial Unicorn Hoove Candles [iuhc] ::::
spuc-1 |
spuc-1 | :: Try recording a unicorn sighting with:
spuc-1 | curl -X PUT localhost:8321/unicorn_spotted?location=moon\&brightness=100
spuc-1 |
spuc-1 | :: No plugins detected
spuc-1 |
spuc-1 |
spuc-1 | \
spuc-1 | \
spuc-1 | \\
spuc-1 | \\\
spuc-1 | >\/7
spuc-1 | _.-(º \
spuc-1 | (=___._/` \ ____ ____ _ _ ____
spuc-1 | ) \ |\ / ___|| _ \| | | |/ ___|
spuc-1 | / / ||\ \___ \| |_) | | | | |
spuc-1 | / > /\\\ ___) | __/| |__| | |___
spuc-1 | j < _\ |____/|_| \____/ \____|
spuc-1 | _.-' : ``.
spuc-1 | \ r=._\ `. Space Purple Unicorn Counter
spuc-1 | <`\\_ \ .`-.
spuc-1 | \ r-7 `-. ._ ' . `\
spuc-1 | \`, `-.`7 7) )
spuc-1 | \/ \| \' / `-._
spuc-1 | || .'
spuc-1 | \\ (
spuc-1 | >\ >
spuc-1 | ,.-' >.'
spuc-1 | <.'_.''
spuc-1 | <'
spuc-1 |
spuc-1 |
spuc-1 | Welcome to the Space Purple Unicorn Counter!
spuc-1 |
spuc-1 | :::: Units set to Imperial Unicorn Hoove Candles [iuhc] ::::
spuc-1 |
spuc-1 | :: Try recording a unicorn sighting with:
spuc-1 | curl -X PUT localhost:8321/unicorn_spotted?location=moon\&brightness=100
spuc-1 |
spuc-1 | :: No plugins detected
spuc-1 |Now… something a bit funny is happening here… why are we seeing the output twice?
We’ve actually started the same container twice! We only
stopped the container when we pressed
[Ctrl+C], and didn’t remove it.
Removing the container when it stops
We can stop and remove the container with the down
command.
OUTPUT
[+] Running 2/2
✔ Container docker_intro-spuc-1 Removed 0.1s
✔ Network docker_intro_default Removed 0.2sIn practice, you only need to use down if you
need to remove the container. If you just want to stop it, you
can use [Ctrl+C] like we did before.
Naming the container
The next item in our list is the name of the container. We can name
the container using the container_name key.
YML
services:
spuc:
image: spuacv/spuc:latest
container_name: spuc_container # The name of the containerOUTPUT
[+] Running 2/0
✔ Network docker-intro_default Created 0.0s
✔ Container spuc_container Started 0.0sYou do not necessarily need to down your containers to
update the configuration, Docker Compose can be smart like
that.
You can update the docker-compose.yml file in your text
editor and then run docker compose up -d to see the changes
take effect.
Warning: This is not always foolproof! Some changes
will require a rebuild of the container. It is also worth noting that
Docker Compose does not save the status with which you started
your services. When you do a down, it will look at the
current file, and stop the services as described in that
file.
Exporting a port
Currently, if we attempt to record a sighting of a unicorn, we will get a connection refused error.
OUTPUT
curl: (7) Failed to connect to localhost port 8321 after 0 ms: Could not connect to serverThis is because we haven’t mapped the port from the container to the
host. We can do this using the ports key using the notation
host_port:container_port.
It’s worth noting the ports key is a list, so we can map
multiple ports if we need to, and that the host and container ports
don’t have to be the same!
YML
services:
spuc:
image: spuacv/spuc:latest
container_name: spuc_container
ports: # Starts the list of ports to map
- 8321:8321 # Maps port 8321 on the host to port 8321 in the containerOUTPUT
[+] Running 2/0
✔ Network docker-intro_default Created 0.0s
✔ Container spuc_container Created 0.0sNow we can record a unicorn sighting!
OUTPUT
{"message":"Unicorn sighting recorded!"}Bind mounts
As before, we want to make sure that our print configuration is being used by SPUC. We will use a bind mount for this - mapping a file from the host to the container.
As with the CLI, this is (confusingly) done using the
volumes key.
YML
services:
spuc:
image: spuacv/spuc:latest
container_name: spuc_container
ports:
- 8321:8321
volumes: # Starts the list of volumes/bind mounts
- ./print.config:/spuc/config/print.config # Bind mounts the print.config fileOUTPUT
[+] Running 2/2
✔ Network docker_intro_default Created 0.1s
✔ Container spuc_container Started 0.2sNow, if you record some sightings, you should see them formatted
according to the configuration in print.config.
As before, whether a bind mount or volume is performed is based on
whether the argument on the left of the colon is a name or a
path. If it is a path (i.e. starts with /
or ./), it generates a bind mount. Otherwise, it generates
a volume.
Volumes
Let’s add a volume to persist the unicorn sightings between runs of the container.
YML
services:
spuc:
image: spuacv/spuc:latest
container_name: spuc_container
ports:
- 8321:8321
volumes:
- ./print.config:/spuc/config/print.config
- spuc-volume:/spuc/output # Mounts the volume named spuc-volumeOUTPUT
service "spuc" refers to undefined volume spuc-volume: invalid compose projectOops! We forgot to declare the volume! Although we added the instruction to use the volume, we didn’t tell Docker Compose that we needed that volume.
We can do this by adding a volumes key to the file. The
volumes are separate from services, so they are declared at the
same level. To declare a named volume, we specify its name and end with
a :. We will do this at the end of the file.
YML
services:
spuc:
image: spuacv/spuc:latest
container_name: spuc_container
ports:
- 8321:8321
volumes:
- ./print.config:/spuc/config/print.config
- spuc-volume:/spuc/output # Mounts the volume named spuc-volume
volumes: # Starts section for declaring volumes
spuc-volume: # Declares a volume named spuc-volumeOUTPUT
[+] Running 2/2
✔ Volume "docker_intro_spuc-volume" Created 0.0s
✔ Container spuc_container Started 10.3sNow, if you record some sightings and then stop and start the container, you should see that the sightings are still there!
However, we can now use a cool feature of Docker Compose - the ability to remove volumes when the container is removed.
We can do this using the -v flag with the
down command. Which tells Docker to remove any volumes
named in the volumes key.
You can confirm this by running docker volume ls before
and after running down.
OUTPUT
DRIVER VOLUME NAME
local docker_intro_spuc-volume
local spuc-volume
[+] Running 3/3
✔ Container spuc_container Removed 10.2s
✔ Volume docker_intro_spuc-volume Removed 0.0s
✔ Network docker_intro_default Removed 0.2s
DRIVER VOLUME NAME
local spuc-volumeSetting an environment variable
Next, we need to set the EXPORT environment variable to
true. This is done using the environment
key.
YML
services:
spuc:
image: spuacv/spuc:latest
container_name: spuc_container
ports:
- 8321:8321
volumes:
- ./print.config:/spuc/config/print.config
- spuc-volume:/spuc/output
environment: # Starts list of environment variables to set
- EXPORT=true # Sets the EXPORT environment variable to true
volumes:
spuc-volume:OUTPUT
[...]
spuc_container |
spuc_container | :::: Unicorn sightings export activated! ::::
spuc_container | :: Try downloading the unicorn sightings record with:
spuc_container | curl localhost:8321/export
spuc_container |We can see that the environment variable has been set by the output of the tool and the export functionality is now available.
Overriding the default command
Finally, lets set the units by overriding the command, as we did
before. For this, we use the command key.
YML
services:
spuc:
image: spuacv/spuc:latest
container_name: spuc_container
ports:
- 8321:8321
volumes:
- ./print.config:/spuc/config/print.config
- spuc-volume:/spuc/output
environment:
- EXPORT=true
command: ["--units", "iulu"] # Overrides the default command
volumes:
spuc-volume:OUTPUT
[...]
spuc_container |
spuc_container | :::: Units set to Intergalactic Unicorn Luminosity Units [iulu] ::::
spuc_container |
[...]Enabling the plugin
We’re nearly back to where we were with our docker run
command! The only thing we are missing is enabling the plugin.
We used a bind mount before to put the plugin file in the container, so lets try again:
YML
services:
spuc:
image: spuacv/spuc:latest
container_name: spuc_container
ports:
- 8321:8321
volumes:
- ./print.config:/spuc/config/print.config
- spuc-volume:/spuc/output
- ./stats.py:/spuc/plugins/stats.py # Mounts the stats.py plugin
environment:
- EXPORT=true
command: ["--units", "iulu"]
volumes:
spuc-volume:OUTPUT
[+] Running 1/1
✔ Container spuc_container Started 10.3sSeems to have worked, lets look at the logs to see if the plugin was loaded.
OUTPUT
spuc_container | Traceback (most recent call last):
spuc_container | File "/spuc/spuc.py", line 31, in <module>
spuc_container | __import__(f"{plugin_dir}.{plugin[:-3]}")
spuc_container | File "/spuc/plugins/stats.py", line 4, in <module>
spuc_container | import pandas as pd
spuc_container | ModuleNotFoundError: No module named 'pandas'Oh no! We’ve hit an error! The pandas library isn’t
installed in the container - which was the whole reason that we made our
own container in the first place!
Let’s go back to that.
Building containers in Docker Compose
We could use the tag we used when we built the container to use that image. However, this would mean that if we want to adjust our locally built container, we would have to rebuild it separately.
Instead, we can use the build key to tell Docker Compose
to build the container if needed.
To do that, we use the build key instead of the
image key:
YML
services:
spuc:
# image: spuacv/spuc:latest
build: # Instead of using the 'image' key, we use the 'build' key
context: . # Sets the build context (the directory in which the Dockerfile is located)
dockerfile: Dockerfile # Sets the name of the Dockerfile
container_name: spuc_container
ports:
- 8321:8321
volumes:
- ./print.config:/spuc/config/print.config
- spuc-volume:/spuc/output
- ./stats.py:/spuc/plugins/stats.py
environment:
- EXPORT=true
command: ["--units", "iulu"]This tells docker compose to look for the Dockerfile in the current directory. If needed, it will then build the container and tag it with the current directory and service names.
Now, we have to be a little careful with our up command! If we run
up, Docker Compose will default to checking if the image
exists and if it does, it will use that image. This is ok… unless we
have made changes to the Dockerfile!
To ensure that the image is built, we can run
docker compose build. This will build (all) the image(s)
specified inside our docker-compose.yml. After building, we
use the usual up command.
Alternatively, we can add the --build flag to the
up command, which results in Docker building the image
right before starting the container. This will rebuild the image every
time you run it, but use cached layers if they exist.
Let’s start our services using that flag, and verify that the plugin is loaded.
OUTPUT
[+] Building 9.2s (10/10) FINISHED docker:default
=> [spuc internal] load build definition from Dockerfile 0.0s
=> => transferring dockerfile: 250B 0.0s
=> [spuc internal] load metadata for docker.io/spuacv/spuc:latest 0.0s
=> [spuc internal] load .dockerignore 0.0s
=> => transferring context: 2B 0.0s
=> [spuc 1/4] FROM docker.io/spuacv/spuc:latest 0.1s
=> [spuc internal] load build context 0.0s
=> => transferring context: 546B 0.0s
=> [spuc 2/4] RUN pip install pandas 8.5s
=> [spuc 3/4] COPY stats.py /spuc/plugins/stats.py 0.0s
=> [spuc 4/4] COPY print.config /spuc/config/print.config 0.0s
=> [spuc] exporting to image 0.5s
=> => exporting layers 0.5s
=> => writing image sha256:b17d7f75ac398b083476cc3fda502875b1d1355b59ad2bdc9d0526f202be9c05 0.0s
=> => naming to docker.io/library/docker_intro-spuc 0.0s
=> [spuc] resolving provenance for metadata file 0.0s
[+] Running 1/1
✔ Container spuc_container Started 0.3s
[...]
spuc_container |
spuc_container | :::: Plugins loaded! ::::
spuc_container | :: Available plugins
spuc_container | stats.py
spuc_container |
[...]You should now have a container running with the stats plugin enabled!
You may have noticed that we ended up duplicating most of
the configuration from the Dockerfile within the
docker-compose.yml file.
In reality, if we are using Docker Compose we do not need to bake in
all of the configuration inside the Dockerfile. The only thing that was
different was the pip install pandas command. Therefore,
our dockerfile could be as simple as this:
Alternatively, we could simplify our docker-compose.yml
file by removing the duplicated configuration. However, this would make
the docker-compose.yml file less self-contained and more
dependent on the Dockerfile. It is usually a better idea to keep the
configuration in the docker-compose.yml file, as it makes
it easier to understand and maintain.
Connecting multiple services
We have now managed to replicate our docker run command
in a more readable and maintainable way.
There is an argument to be made that, even for running a single
service, Docker Compose is a useful tool. It brings an ephemeral run
command, that would need careful documentation to replicate, into a
single file that can be version controlled and shared. It is also much
easier on the eye than a long docker run command!
Where Docker Compose really shines, though, is when you have multiple services that need to be run together. And we happen to have another service that we need to run - SPUCSVi!
Adding SPUCSVi to our Docker Compose file
We can add SPUCSVi to our Docker Compose file in the same way that we
added SPUC, by adding another service to the services
key.
The SPUCSVi documentation helpfully provides a table of configuration options what we can use to configure the service, reproduced here:
| Item | Description | Default |
|---|---|---|
| Image Name | The name of the image to use | spuacv/spucsvi:latest |
| Port | The container port the service runs on | 8322 |
| SPUC_URL | An environment variable to set the URL of the SPUC service | http://spuc:8321 |
We can use this to add SPUCSVi to our Docker Compose file!
But how do we know the correct URL for the SPUC service? This touches on a couple of clever tricks that Docker Compose uses to make running multiple services easier.
First, Docker Compose creates a network for each stack that it starts. This means that, unless overridden, all services in the stack can communicate with each other.
Second, Docker Compose uses the service name as the hostname for the
service. This means that we can use the service name as the hostname in
the URL! For our service named spuc, the hostname would be
spuc with the protocol http prepended and
port appended i.e. http://spuc:8321.
Knowing this, we are able to add SPUCSVi to our Docker Compose file!
YML
services:
spuc:
build:
context: .
dockerfile: Dockerfile
container_name: spuc_container
ports:
- 8321:8321
volumes:
- ./print.config:/spuc/config/print.config
- spuc-volume:/spuc/output
- ./stats.py:/spuc/plugins/stats.py
environment:
- EXPORT=true
command: ["--units", "iulu"]
spucsvi: # Declare a new service named spucsvi
image: spuacv/spucsvi:latest # Specify the image to use
container_name: spucsvi_container # Name the container spucsvi
ports: #
- "8322:8322" # Map port 8322 on the host to port 8322 in the container
environment: #
- SPUC_URL=http://spuc:8321 # Specify the SPUC_URL environment variable
volumes:
spuc-volume:Now both services will be started at the same time!
OUTPUT
[+] Running 3/3
✔ Network docker_intro_default Created 0.1s
✔ Container spuc_container Created 0.0s
✔ Container spucsvi_container Created 0.0s
Attaching to spuc_container, spucsvi_container
spuc_container |
spuc_container | \
spuc_container | \
spuc_container | \\
spuc_container | \\\
spuc_container | >\/7
spuc_container | _.-(º \
spuc_container | (=___._/` \ ____ ____ _ _ ____
spuc_container | ) \ |\ / ___|| _ \| | | |/ ___|
spuc_container | / / ||\ \___ \| |_) | | | | |
spuc_container | / > /\\\ ___) | __/| |__| | |___
spuc_container | j < _\ |____/|_| \____/ \____|
spuc_container | _.-' : ``.
spuc_container | \ r=._\ `. Space Purple Unicorn Counter
spuc_container | <`\\_ \ .`-.
spuc_container | \ r-7 `-. ._ ' . `\
spuc_container | \`, `-.`7 7) )
spuc_container | \/ \| \' / `-._
spuc_container | || .'
spuc_container | \\ (
spuc_container | >\ >
spuc_container | ,.-' >.'
spuc_container | <.'_.''
spuc_container | <'
spuc_container |
spuc_container |
spuc_container | Welcome to the Space Purple Unicorn Counter!
spuc_container |
spuc_container | :::: Units set to Imperial Unicorn Hoove Candles [iuhc] ::::
spuc_container |
spuc_container | :: Try recording a unicorn sighting with:
spuc_container | curl -X PUT localhost:8321/unicorn_spotted?location=moon\&brightness=100
spuc_container |
spuc_container | :::: Plugins loaded! ::::
spuc_container | :: Available plugins
spuc_container | stats.py
spuc_container |
spuc_container | :::: Unicorn sightings export activated! ::::
spuc_container | :: Try downloading the unicorn sightings record with:
spuc_container | curl localhost:8321/export
spuc_container |
spucsvi_container |
spucsvi_container | .-'''-. .-------. ___ _ _______ .-'''-. ,---. ,---..-./`)
spucsvi_container | / _ \\ _(`)_ \.' | | | / __ \ / _ \| / | |\ .-.')
spucsvi_container | (`' )/`--'| (_ o._)|| .' | | | ,_/ \__) (`' )/`--'| | | .'/ `-' \
spucsvi_container | (_ o _). | (_,_) /.' '_ | |,-./ ) (_ o _). | | _ | | `-'`"`
spucsvi_container | (_,_). '. | '-.-' ' ( \.-.|\ '_ '`) (_,_). '. | _( )_ | .---.
spucsvi_container | .---. \ :| | ' (`. _` /| > (_) ) __ .---. \ :\ (_ o._) / | |
spucsvi_container | \ `-' || | | (_ (_) _)( . .-'_/ )\ `-' | \ (_,_) / | |
spucsvi_container | \ / / ) \ / . \ / `-'`-' / \ / \ / | |
spucsvi_container | `-...-' `---' ``-'`-'' `._____.' `-...-' `---` '---'
spucsvi_container |
spucsvi_container | :::: SPUC Super Visualizer serving on localhost:8322 ::::
spucsvi_container |As the logs suggest, we can now view the SPUCSVi interface by
visiting localhost:8322 in our browser.
A visual treat awaits, and an easier way to record and view our unicorn sightings!
Networks
We briefly mentioned networks earlier, noting that, by default, Docker Compose creates a network for each stack.
However, by overriding the default network, we can perform some interesting tricks.
Now that we can record Unicorns using the SPUCSVi interface, we don’t need to be able to access the SPUC service directly.
This means we can isolate the SPUC service from the host network. This is a good security practice and helps keep things tidy.
To do this we need to stop exposing the ports for SPUC, by removing
the ports key from the SPUC service:
YML
services:
spuc:
build:
context: .
dockerfile: Dockerfile
container_name: spuc_container
# ports: # We can remove these two lines
# - 8321:8321
volumes:
- ./print.config:/spuc/config/print.config
- spuc-volume:/spuc/output
- ./stats.py:/spuc/plugins/stats.py
environment:
- EXPORT=true
command: ["--units", "iulu"]
spucsvi:
image: spuacv/spucsvi:latest
container_name: spucsvi
ports:
- "8322:8322"
environment:
- SPUC_URL=http://spuc:8321
volumes:
spuc-volume:Now, the SPUC service is only accessible from within the Docker network! Try doing a curl to register a sighting and you wont be able to. However, you can still register sightings through the SPUCSVi interface.
This can be taken further to create networks with very limited purposes. For example in a typical web app you may make it so that the frontend can connect only to backend, but not to the database.
You may have noticed that the network that Docker Compose created for
our stack is named docker_intro_default. This is because
Docker Compose uses the name of the directory that the
docker-compose.yml file is in as the name of the
network.
If you want to specify the name of the network, you can use the
networks key in the docker-compose.yml file.
You also need to specify the network name for each service that you want
to connect to the network.
For example, to specify the network name as
spuc_network, you would add the following to the file:
YML
services:
spuc:
build:
context: .
dockerfile: Dockerfile
container_name: spuc_container
volumes:
- ./print.config:/spuc/config/print.config
- spuc-volume:/spuc/output
- ./stats.py:/spuc/plugins/stats.py
environment:
- EXPORT=true
command: ["--units", "iulu"]
networks: # Starts list of networks to connect this service to
- spuc_network # Connects to the spuc_network network
spucsvi:
image: spuacv/spucsvi:latest
container_name: spucsvi
ports:
- "8322:8322"
environment:
- SPUC_URL=http://spuc:8321
networks: # Starts list of networks to connect this service to
- spuc_network # Connects to the spuc_network network
volumes:
spuc-volume:
networks: # Starts section for declaring networks
spuc_network: # Declares a network for spuc
name: spuc_network # Specifies the name of the networkDepends on
There is an important problem that we haven’t addressed yet - what happens if the SPUCSVi service starts before the SPUC service?
This is a common problem when running multiple services together - services that depend on each other need to start in a specific order.
Docker Compose has a solution to this - the depends_on
key.
We can use this key to tell Docker Compose that the SPUCSVi service depends on the SPUC service.
YML
services:
spuc:
build:
context: .
dockerfile: Dockerfile
container_name: spuc_container
volumes:
- ./print.config:/spuc/config/print.config
- spuc-volume:/spuc/output
- ./stats.py:/spuc/plugins/stats.py
environment:
- EXPORT=true
command: ["--units", "iulu"]
spucsvi:
image: spuacv/spucsvi:latest
container_name: spucsvi
ports:
- "8322:8322"
environment:
- SPUC_URL=http://spuc:8321
depends_on: # Starts section for declaring dependencies
- spuc # Declares that the spucsvi service depends on the spuc service
volumes:
spuc-volume:Now, when we run docker compose up, the SPUCSVi service
will wait until SPUC has started before it starts.
But there is a catch! The depends_on key only ensures
that the service is started in the correct order. It doesn’t check if
the service is ready!
This can be a problem if a service is fast to start but slow to be ready. For example, a database service may start quickly, but take a while to be ready to accept connections.
To address this, Docker Compose allows you to define a
healthcheck for a service. This is a command that is run
periodically (from inside the container) to check if the service is
ready. The command failing (returning a non-zero exit code)
means that the service is not ready.
We can try this out by adding a healthcheck to the SPUC
service. Since we don’t want SPUCSVi to start until the record of
unicorn sightings is ready, we can use the curl command to
check if the /export endpoint is available. We need to add
the --fail flag to curl to ensure that it
returns a non-zero exit code if the endpoint is not available.
The other change we need to make is to add a condition
to the depends_on key in the SPUCSVi service. This tells
Docker Compose to only start the service if the service it depends on is
healthy, rather than just started.
YML
services:
spuc:
build:
context: .
dockerfile: Dockerfile
container_name: spuc_container
volumes:
- ./print.config:/spuc/config/print.config
- spuc-volume:/spuc/output
- ./stats.py:/spuc/plugins/stats.py
environment:
- EXPORT=true
command: ["--units", "iulu"]
healthcheck: # Starts section for declaring healthchecks
test: ["CMD", "curl", "--fail", "http://spuc:8321/export"] # Specifies the healthcheck command (ran from inside the container)
interval: 3s # Specifies the interval between healthchecks
timeout: 2s # Specifies the timeout for the healthcheck
retries: 5 # Specifies the number of retries before failing completely
spucsvi:
image: spuacv/spucsvi:latest
container_name: spucsvi
ports:
- "8322:8322"
environment:
- SPUC_URL=http://spuc:8321
depends_on:
spuc: # This changed from a list (- spuc) to a mapping (spuc:)
condition: service_healthy # Specifies further conditions for starting the service
volumes:
spuc-volume:Now, when we run docker compose up, the SPUCSVi service
will only start when the SPUC service is ready.
This is a little hard to see in action as the SPUC service starts so
quickly. To be able to see it, let’s add a sleep command to
the entrypoint of the SPUC service to simulate a slow
start.
YML
services:
spuc:
build:
context: .
dockerfile: Dockerfile
container_name: spuc_container
volumes:
- ./print.config:/spuc/config/print.config
- spuc-volume:/spuc/output
- ./stats.py:/spuc/plugins/stats.py
environment:
- EXPORT=true
command: ["--units", "iulu"]
entrypoint: ["sh", "-c", "sleep 5 && python /spuc/spuc.py"] # Adds a sleep command to the entrypoint to simulate a slow start
healthcheck:
test: ["CMD", "curl", "--fail", "http://spuc:8321/export"]
interval: 3s
timeout: 2s
retries: 5
spucsvi:
image: spuacv/spucsvi:latest
container_name: spucsvi
ports:
- "8322:8322"
environment:
- SPUC_URL=http://spuc:8321
depends_on:
spuc:
condition: service_healthy
volumes:
spuc-volume:OUTPUT
[+] Running 3/3
✔ Network docker_intro_default Created 0.1s
✔ Container spuc_container Healthy 6.7s
✔ Container spucsvi_container Started 6.8sAs yoy can see, the SPUCSVi service only started after the SPUC service was healthy.
To simulate a service that does not pass the healthcheck, we can set
the EXPORT environment variable to false in
the SPUC service. This will mean that the export endpoint is not
available, so the healthcheck will fail.
YML
services:
spuc:
build:
context: .
dockerfile: Dockerfile
container_name: spuc_container
volumes:
- ./print.config:/spuc/config/print.config
- spuc-volume:/spuc/output
- ./stats.py:/spuc/plugins/stats.py
environment:
- EXPORT=false # Sets the EXPORT environment variable to false
command: ["--units", "iulu"]
healthcheck:
test: ["CMD", "curl", "--fail", "http://spuc:8321/export"]
interval: 3s
timeout: 2s
retries: 5
spucsvi:
image: spuacv/spucsvi:latest
container_name: spucsvi
ports:
- "8322:8322"
environment:
- SPUC_URL=http://spuc:8321
depends_on:
spuc:
condition: service_healthy
volumes:
spuc-volume:OUTPUT
[+] Running 3/3
✔ Network docker_intro_default Created 0.1s
✘ Container spuc_container Error 15.7s
✔ Container spucsvi_container Created 0.0s
dependency failed to start: container spuc_container is unhealthyThe SPUC service shows an error, because it failed all 5 retries, and the SPUCSVi service was not started.
Warning: Even though unhealthy, the
spuc_container is running. You can check this by running
docker ps.
Key Points
- Docker Compose is a tool for defining and running multi-container
stacks in a YAML file. They can also serve as a way of structuring and
documenting
docker runcommands for single containers. - Instructions are saved in a
docker-compose.ymlfile, where services, networks, and volumes are defined. - Each
serviceis a separate container, and it can be fully configured from within the file. - Bind mounts and
volumescan be declared for each service, and they can be shared between containers too. - You can define
networks, which can be used to connect or isolate containers from each other. - All the services, volumes and networks are started together using
the
docker compose upcommand. - They can be stopped using the
docker compose downcommand. - Container images can be built as the services are spun up by using
the
--buildflag. - The order in which services start can be controlled using the
depends_onkey. - A
healthcheckcan be defined to verify the status of a service. These are commands run from within the container to make sure it is ready.