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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.

Environment variables

Volumes

Last modified 1yr ago