Define a container
Balena uses Docker containers to manage deployment and updates. You can use one or more containers to package your services with whichever environments and tools they need to run.
To ensure a service has everything it needs, you'll want to create a list of instructions for building a container image. Whether the build process is done on your device, on your workstation, or on the balena builders, the end result is a read-only image that ends up on your device. This image is used by the container engine (balena or Docker, depending on the balenaOS version) to kick off a running container.
Note: For additional information on working with Dockerfiles with balena see the services masterclass.
The instructions for building a container image are written in a Dockerfile - this is similar to a
Makefile in that it contains a recipe or set of instructions to build our container.
The syntax of Dockerfiles is fairly simple - at core there are 2 valid entries in a Dockerfile - comments, prepended with
# as in script files, and instructions of the format
FROM has to be the first instruction in any valid
Dockerfileand defines the base image to use as the basis for the container you're building.
RUN simply executes commands in the container - this can be of the format of a single line to execute, e.g.
RUN apt-get -y updatewhich will be run via
/bin/sh -c, or
[ "executable", "param1", "param2", ... ]which is executed directly.
ADD copies files from the current directory into the container, e.g.
ADD <src> <dest>. Note that if
<dest>doesn't exist, it will be created for you, e.g. if you specify a folder. If the
<src>is a local tar archive it will unpack it for you. It also allows the
<src>to be a url but will not unpack remote urls.
COPY is very similar to ADD, but without the compression and url functionality. According to the Dockerfile best practices, you should always use COPY unless the auto-extraction capability of ADD is needed.
CMD this command provides defaults for an executing container. This command will be run when the container starts up on your device, whereas RUN commands will be executed on our build servers. In a balena service, this is typically used to execute a start script or entrypoint for the user's service. CMD should always be the last command in your Dockerfile. The only processes that will run inside the container are the CMD command and all processes that it spawns.
For details on other instructions, consult the official Dockerfile documentation.
Using Dockerfiles with balena
To deploy a single-container release to balena, simply place a
Dockerfile at the root of your repository. A
docker-compose.yml file will be automatically generated, ensuring your container has host networking, is privileged, and has
/run/dbus bind mounted into the container. The default
docker-compose.yml will look something like this:
Note: If you have devices in your app that have a supervisor version lower than 7.22.0, then you should use the
io.resin.features. form of the labels to ensure that all devices obey the label. Earlier supervisor versions will not understand the
Releases with multiple services should include a
package.json in each service directory. A
docker-compose.yml file will need to be defined at the root of the repository, as discussed in our multicontainer documentation.
You can also include a
.dockerignore file with your project if you wish the builder to ignore certain files.
Note: You don't need to worry about ignoring
.git as the builders already do this by default.
One of the goals of balena is code portability and ease of use, so you can easily manage and deploy a whole fleet of different devices. This is why Docker containers were such a natural choice. However, there are cases where Dockerfiles fall short and can't easily target multiple different device architectures.
To allow our builders to build containers for multiple architectures from one code repository, we implemented simple Dockerfile templates.
It is now possible to define a
Dockerfile.template file that looks like this:
COPY package.json /package.json
RUN npm install
COPY src/ /usr/src/app
CMD ["node", "/usr/src/app/main.js"]
This template will build and deploy a Node.js project for any of the devices supported by balena, regardless of whether the device architecture is ARM or x86.
In this example, you can see the build variable
%%BALENA_MACHINE_NAME%%. This will be replaced by the machine name (i.e.:
raspberry-pi) at build time. See below for a list of machine names.
The machine name is inferred from the device type of the fleet you are deploying on. So if you have a NanoPi Neo Air fleet, the machine name will be
nanopi-neo-air and an
armv7hf architecture base image will be built.
Note: You need to ensure that your dependencies and Node.js modules are also multi-architecture, otherwise you will have a bad time.
Currently our builder supports the following build variables:
|The name of the fleet.
|The instruction set architecture for the base images associated with this device.
|The name of the yocto machine this board is base on. It is the name that you will see in most of the balena Docker base images. This name helps us identify a specific BSP.
|The hash corresponding to the release.
|The name of the service defined in the
Note: If your fleet contains devices of different types, the
%%BALENA_MACHINE_NAME%% build variable will not evaluate correctly for all devices. Your fleet services are built once for all devices, and the
%%BALENA_MACHINE_NAME%% variable will pull from the device type associated with the fleet, rather than the target device. In this scenario, you can use
%%BALENA_ARCH%% to pull a base image that matches the shared architecture of the devices in your fleet.
If you want to see an example of build variables in action, have a look at this basic openssh example.
Here are the supported machine names and architectures.
There are cases when you would need a higher granularity of control when specifying build instructions for different devices and architectures than a single Dockerfile template can provide. An example of this would be when different configuration or installation files are required for each architecture or device.
When creating a release, the balenaCloud build servers or the balena CLI tool (depending on the deployment method used) look at all available Dockerfiles and build the appropriate image using the following order of preference:
As an example, let's say you have two Dockerfiles available,
Dockerfile.template. Whenever you publish the application to balenaCloud, if the fleet
device-type is Raspberry Pi 3,
Dockerfile.raspberrypi3 will be selected as an exact match and for all other devices the builder will automatically select
Note that this feature works with the following commands:
balena build, and
When you push your code to your fleet, the build server generates a container for the environment your device operates in, deploys your code to it and runs
npm install to resolve npm dependencies, reporting progress to your terminal as it goes.
If the build executes successfully the release is deployed to your device where the supervisor runs it in place of any previously running containers, using
npm start to execute your code (note that if no start script is specified, it defaults to running
A good example of this is the text-to-speech application - here's its
"description": "Simple balena app that uses Google's TTS endpoint",
"preinstall": "bash deps.sh"
Note: We don't specify a
start script here which means node will default to running
server.js. We execute a bash script called
npm install tries to satisfy the code's dependencies. Let's have a look at that:-
apt-get install -y alsa-utils libasound2-dev
mv sound_start /usr/bin/sound_start
We use Raspbian as our contained operating system, so this script uses aptitude to install native packages before moving a script for our node code to use over to
/usr/bin (the install scripts runs with root privileges within the container.)
Note: With a plain Node.js project, our build server will detect compatible nodejs versions from the
package.json and build the container using a Docker image that satisfies the version requirement. If no version is specified then the default node version is
0.10.22 and it will be used if a node version is not specified. There will be an error if the specified node version is not in our registry. You can either try another node version or contact us to be supported. More details about Docker node images in our registry can be found here.
The balena Supervisor requires that the directory
/tmp/balena in containers be available for inter-container communication via update locks. Therefore, scripts should not attempt to remove this directory on startup.