Using Docker in Cloud Foundry
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This topic provides information about how Docker works in Cloud Foundry and describes how to push an application with a Docker image. For information about Diego, see the Diego Architecture topic.
In addition to the default Linux environment provided by Cloud Foundry, operators can specify a Docker image that provides the file system used by the container. When pushing an app, you can specify the location of the docker image you want to use.
A Docker image consists of a collection of layers. Each layer consists of one or both of the following:
- Raw bits to download and mount. These bits form the file system.
- Metadata that describes commands, users, and environment for the layer.
This metadata includes the
CMDdirectives, and is specified in the Dockerfile.
Diego currently uses Garden-runC to construct Linux containers. Earlier versions of Diego used Garden-Linux. For more information, see the Garden topic.
Note: Elastic Runtime versions v1.8.8 and above use Garden-runC instead of Garden-Linux.
Both Docker and Garden-runC use libraries from the Open Container Initiative (OCI) to build Linux containers. After creation, these containers use name space isolation, or namespaces, and control groups, or cgroups, to isolate processes in containers and limit resource usage. These are common kernel resource isolation features used by all Linux containers.
Before Garden-runC creates a Linux container, it creates a file system that is mounted as the root file system of the container. Garden-runC supports mounting Docker images as the root file systems for the containers it creates.
When creating a container, both Docker and Garden-runC perform the following actions:
- Fetch and cache the individual layers associated with a Docker image
- Combine and mount the layers as the root file system
These actions produce a container whose contents exactly match the contents of the associated Docker image.
After Garden-runC creates a container, Diego runs and monitors the processes inside of it.
To determine which processes to run, the Cloud Controller fetches and stores the metadata associated with the Docker image. The Cloud Controller uses this metadata to perform the following actions:
- Runs the start command as the user specified in the Docker image
- Instructs Diego and the Gorouter to route traffic to the lowest-numbered port exposed in the Docker image
Note: When launching an application on Diego, the Cloud Controller honors any user-specified overrides such as a custom start command or custom environment variables.
The attack surface area for a Docker-based container running on Diego remains somewhat higher than that of a buildpack application because Docker allows users to fully specify the contents of their root file systems. A buildpack application runs on a trusted root filesystem.
Garden-runC provides features that allow the platform to run Docker images more securely in a multi-tenant context.
In particular, Cloud Foundry uses the
user-namespacing feature found on modern Linux kernels to ensure that users cannot gain escalated privileges on the host even if they escalate privileges within a container.
The Cloud Controller always runs Docker containers on Diego with user namespaces enabled. This security restriction prevents certain features, such as the ability to mount FuseFS devices, from working in Docker containers.
To mitigate security concerns, Cloud Foundry recommends that you run only trusted Docker containers on the platform. By default, the Cloud Controller does not allow Docker-based applications to run on the platform.
To allow Docker-based applications to run, a Cloud Controller administrator can enable the
diego_docker feature flag with the following command:
$ cf enable-feature-flag diego_docker
To disallow Docker-based applications, a Cloud Controller administrator can run the following command:
$ cf disable-feature-flag diego_docker
Note: Disabling the
diego_docker feature flag stops all Docker-based apps in your deployment within a few convergence cycles, on the order of a minute.
Follow these instructions to deploy updated or new Docker images using the Cloud Foundry Command Line Interface (cf CLI).
When pushing an app, you specify either a Docker image on Docker Hub or a URI to a trusted Docker registry. Diego does not support using private registries. Also, when using Diego, you must use a registry that supports the Docker Registry API V2.
cf push test-app -o cloudfoundry/MY-DOCKER-IMAGE to deploy a Docker image. Replace
MY-DOCKER-IMAGE with the name of an image from an accessible Docker Registry.
For example, the following command pushes the
test-app on Docker Hub to Cloud Foundry:
$ cf push test-app -o cloudfoundry/test-app
To deploy a Docker image using Docker trusted registries, run:
$ cf push my-app -o MY-PRIVATE-REGISTRY.DOMAIN:5000/image/name:v2
MY-PRIVATE-REGISTRY.DOMAIN with your domain name, which resolves to the private registry.
In this example, 5000 is the port on which your private registry serves traffic.
image/name is the name of the Docker image repository.
v2 is a tag in that repository that refers to a specific image.