Substitution (dynamic templates)

Substitution provides a powerful mechanism to dynamically transform application configuration that is tailored for each device.

Substitution

Synpse provides the ability to expand, or substitute, application and device metadata to facilitate dynamic application configurations.

Available Environment Variables

• Device environment variables (defined by user)

• Synpse environment variables (automatically set, reference here)

Dynamic Application Spec

Most of the values in the application spec can be changed, for example:

• Container name

• Image name (you can adapt image name based on architecture)

• Environment variables

• Secrets

• Volumes

Application name, scheduling configuration or the yaml structure itself cannot be changed as it will prevent yaml from being correctly marshalled.

Example Image Based On CPU Arch

If you have an application that is deployed on multiple devices of various CPU architectures (amd64, arm, arm64), you can specify image substitution (see available environment variables):

name: example-app
scheduling:
  type: AllDevices
spec:
  containers:
  - name: my-app
    image: docker.io/my-org/my-app-${SYNPSE_DEVICE_ARCH}:latest

On a regular amd64 (x86) machine it will be:

name: example-app
scheduling:
  type: AllDevices
spec:
  containers:
  - name: my-app
    image: docker.io/my-org/my-app-amd64:latest

While on a 32-bit arm machine the spec will become:

name: example-app
scheduling:
  type: AllDevices
spec:
  containers:
  - name: my-app
    image: docker.io/my-org/my-app-arm:latest 

Example Device Env Substitution

Let's say you have set an environment variable on the device called:

REGION=sillicon-valley

Now, you can use that environment variable to create something else, for example a URL on which the device will be accessible by users (assuming you have configured domain, etc.):

name: example-app
scheduling:
  type: AllDevices
spec:
  containers:
  - name: my-app
    image: docker.io/my-org/my-app:latest
    env:
    - name: PUBLIC_ADDRESS
      value: https://${REGION}.example.com

And your application will then see an environment variable which equals

PUBLIC_ADDRESS=https://sillicon-valley.example.com

String Operations

Synpse provides partial emulation for bash string operations. This can be used to manipulate string values prior to substitution.

• Example variable substitution with substring:

name: example-app
scheduling:
  type: AllDevices
spec:
  containers:
  - name: my-app
    image: docker.io/my-org/my-app:latest
    env:
    - name: CUSTOMER
      value: ${CUSTOMER_ID:0:4} # Getting first 4 symbols

Synpse emulates the below string operations:

${parameter^}
${parameter^^}
${parameter,}
${parameter,,}
${parameter:position}
${parameter:position:length}
${parameter#substring}
${parameter##substring}
${parameter%substring}
${parameter%%substring}
${parameter/substring/replacement}
${parameter//substring/replacement}
${parameter/#substring/replacement}
${parameter/%substring/replacement}
${#parameter}
${parameter=default}
${parameter:=default}
${parameter:-default}

Synpse makes a best-effort to emulate these operations however we do not promise perfect emulation.

Escaping

If you do not want the system to evaluate an expression it must be escaped:

name: example-app
scheduling:
  type: AllDevices
spec:
  containers:
  - name: my-app
    image: docker.io/my-org/my-app:latest
    env:
    - name: DEVICE_NAME
      value: $${MY_DEVICE_NAME}

Caveats

If your environment variable contains *, it will be automatically quoted to avoid parsing issues. For example:

    env:
    - name: MY_DOMAIN
      value: ${MY_DOMAIN}

Where MY_DOMAIN=*.example.com will be rendered as:

    env:
    - name: MY_DOMAIN
      value: *.example.com

However, the following example

    env:
    - name: MY_DOMAIN
      value: https://${MY_DOMAIN}

Will be rendered as:

    env:
    - name: MY_DOMAIN
      value: https://"*.example.com"

Which can lead to issues in your applications. In these scenarios, try to avoid using environment variable substitution.