The backend file

BackendFile in brane_cfg/backend.rs.

The backend file, or more commonly referenced as the backend.yml file, is a worker node configuration file that describes how to connect to the container execution backend. Its location is defined by the node.yml file.

The branectl tool can generate this file for you, using the branectl generate backend subcommand. See the chapter on installing a worker node for a realistic example.

Toplevel layout

The backend.yml file is written in YAML. It features only the following three toplevel fields:

• method: A map that defines the method of accessing the container execution backend. Can be one of the following options:
  • !Local: Connects to the Docker engine local to the node on which the worker node runs. This variant has the following two fields:
    • path [optional]: A string with the path to the Docker socket to connect to. If omitted, will default to /var/docker/run.sock.
    • version [optional]: A sequence of two numbers detailling the Docker client version to connect to. If omitted, will negotiate a client version on the fly.
• capabilities [optional]: A sequence of strings, each of which defines a capability of the computing backend. Currently supported capabilities are defined below. If omitted, no capabilities are enabled.
• hash_containers [optional]: A boolean that defines whether to hash containers, enabling container policies in the policies.yml file. It may give a massive performance boost when using many different larger containers (100MB+), although the hashes are cached as long as the containers are cached. If omitted, will default to 'true'.

A few example backend.yml files:

# Defines the simplest possible file, which is a local file with default options
method: !Local

# Defines a local file that has a different Docker socket path
method: !Local
  path: /home/amy/my-own-docker.sock

# Defines a default local backend that supports CUDA containers and explicitly hashes all containers
capabilities:
- cuda_gpu
method: !Local
hash_containers: true

In older versions of BRANE (<= 2.0.0), the tagged enum representation (e.g., !Local) was not yet supported. Instead, use the additional kind-field to distinguish. For example:

# This is the same as the first example
method:
  kind: local

# This is the same as the second example
method:
  kind: local
  path: /home/amy/my-own-docker.sock

...

Capabilities

The following capabilities can be used in the backend.yml file:

• cuda_gpu: States that the compute backend can provide a CUDA accelerator to containers who ask for that. See the requirements-field in the user's container.yml file.