Customize Compose Bridge

Table of contents

Requires: Docker Desktop 4.43.0 and later

You can customize how Compose Bridge converts your Docker Compose files into platform-specific formats.

This page explains how Compose Bridge uses templating to generate Kubernetes manifests and how you can customize these templates for your specific requirements and needs, or how you can build your own transformation.

How it works

Compose bridge uses transformations to let you convert a Compose model into another form.

A transformation is packaged as a Docker image that receives the fully-resolved Compose model as /in/compose.yaml and can produce any target format file under /out.

Compose Bridge includes a default Kubernetes transformation using Go templates, which you can customize by replacing or extending templates.

Template syntax

Compose Bridge makes use of templates to transform a Compose configuration file into Kubernetes manifests. Templates are plain text files that use the Go templating syntax. This enables the insertion of logic and data, making the templates dynamic and adaptable according to the Compose model.

When a template is executed, it must produce a YAML file which is the standard format for Kubernetes manifests. Multiple files can be generated as long as they are separated by ---

Each YAML output file begins with custom header notation, for example:

#! manifest.yaml

In the following example, a template iterates over services defined in a compose.yaml file. For each service, a dedicated Kubernetes manifest file is generated, named according to the service and containing specified configurations.

{{ range $name, $service := .services }}
---
#! {{ $name }}-manifest.yaml
# Generated code, do not edit
key: value
## ...
{{ end }}

Input model

You can generate the input model by running docker compose config.

This canonical YAML output serves as the input for Compose Bridge transformations. Within the templates, data from the compose.yaml is accessed using dot notation, allowing you to navigate through nested data structures. For example, to access the deployment mode of a service, you would use service.deploy.mode:

# iterate over a yaml sequence
{{ range $name, $service := .services }}
 # access a nested attribute using dot notation
 {{ if eq $service.deploy.mode "global" }}
kind: DaemonSet
 {{ end }}
{{ end }}

You can check the Compose Specification JSON schema for a full overview of the Compose model. This schema outlines all possible configurations and their data types in the Compose model.

Helper functions

As part of the Go templating syntax, Compose Bridge offers a set of YAML helper functions designed to manipulate data within the templates efficiently:

Function	Description
`seconds`	Converts a duration into an integer (seconds).
`uppercase`	Converts a string to uppercase.
`title`	Capitalizes the first letter of each word.
`safe`	Converts a string into a safe identifier (replaces non-lowercase characters with `-`).
`truncate`	Removes the first N elements from a list.
`join`	Joins list elements into a single string with a separator.
`base64`	Encodes a string as base64 (used for Kubernetes secrets).
`map`	Maps values using `“value -> newValue”` syntax.
`indent`	Indents string content by N spaces.
`helmValue`	Outputs a Helm-style template value.

In the following example, the template checks if a healthcheck interval is specified for a service, applies the seconds function to convert this interval into seconds and assigns the value to the periodSeconds attribute.

{{ if $service.healthcheck.interval }}
            periodSeconds: {{ $service.healthcheck.interval | seconds }}{{ end }}
{{ end }}

Customize the default templates

As Kubernetes is a versatile platform, there are many ways to map Compose concepts into Kubernetes resource definitions. Compose Bridge lets you customize the transformation to match your own infrastructure decisions and preferences, with varying level of flexibility and effort.

Modify the default templates

You can extract templates used by the default transformation docker/compose-bridge-kubernetes:

$ docker compose bridge transformations create --from docker/compose-bridge-kubernetes my-template

The templates are extracted into a directory named after your template name, in this case my-template. It includes:

A Dockerfile that lets you create your own image to distribute your template
A directory containing the templating files

Edit, add, or remove templates as needed.

You can then use the generated Dockerfile to package your changes into a new transformation image, which you can then use with Compose Bridge:

$ docker build --tag mycompany/transform --push .

Use your transformation as a replacement:

$ docker compose bridge convert --transformations mycompany/transform

Model Runner templates

The default transformation also includes templates for applications that use LLMs:

model-runner-deployment.tmpl
model-runner-service.tmpl
model-runner-pvc.tmpl
/overlays/model-runner/kustomization.yaml
/overlays/desktop/deployment.tmpl

These templates can be extended or replaced to change how Docker Model Runner is deployed or configured.

For more details, see Use Model Runner.

Add your own templates

For resources that are not managed by Compose Bridge's default transformation, you can build your own templates.

The compose.yaml model may not offer all the configuration attributes required to populate the target manifest. If this is the case, you can then rely on Compose custom extensions to better describe the application, and offer an agnostic transformation.

For example, if you add x-virtual-host metadata to service definitions in the compose.yaml file, you can use the following custom attribute to produce Ingress rules:

{{ $project := .name }}
#! {{ $name }}-ingress.yaml
# Generated code, do not edit
apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: virtual-host-ingress
  namespace: {{ $project }}
spec:
  rules:  
{{ range $name, $service := .services }}
{{ range index $service "x-virtual-host" }}
  - host: ${{ . }}
    http:
      paths:
      - path: "/"
        backend:
          service:
            name: ${{ name }}
            port:
              number: 80  
{{ end }}
{{ end }}

Once packaged into a Docker image, you can use this custom template when transforming Compose models into Kubernetes in addition to other transformations:

$ docker compose bridge convert \
    --transformation docker/compose-bridge-kubernetes \
    --transformation mycompany/transform

Build your own transformation

While Compose Bridge templates make it easy to customize with minimal changes, you may want to make significant changes, or rely on an existing conversion tool.

A Compose Bridge transformation is a Docker image that is designed to get a Compose model from /in/compose.yaml and produce platform manifests under /out. This simple contract makes it easy to bundle an alternate transformation using Kompose:

FROM alpine

# Get kompose from github release page
RUN apk add --no-cache curl
ARG VERSION=1.32.0
RUN ARCH=$(uname -m | sed 's/armv7l/arm/g' | sed 's/aarch64/arm64/g' | sed 's/x86_64/amd64/g') && \
    curl -fsL \
    "https://github.com/kubernetes/kompose/releases/download/v${VERSION}/kompose-linux-${ARCH}" \
    -o /usr/bin/kompose
RUN chmod +x /usr/bin/kompose

CMD ["/usr/bin/kompose", "convert", "-f", "/in/compose.yaml", "--out", "/out"]

This Dockerfile bundles Kompose and defines the command to run this tool according to the Compose Bridge transformation contract.