Interpolation

Blip interpolates environment variables and monitor variables in the config file and plan files.

Environment variable

${FOO}

Environment variable with default value

${FOO:-default}

Monitor variable

%{monitor.VAR}

${} and %{} are always required.

Environment variable interpolation is a simple implementation of the shell standard. In Blip, only the two cases shown above are supported, and default must be a literal value (it cannot be another ${}).

Monitor variables are scoped to (only work within) a single monitor. For example:

monitors:
  - hostname: db.local
    username: metrics
    tags:
      hostname: %{monitor.hostname}

The result is monitors.tags.hostname = "db.local" because %{monitor.hostname} refers to the local monitors.hostname variable. Blip is remarkably flexible, so this works the other way, too:

monitors:
  - hostname: %{monitor.tags.hostname}
    username: metrics
    tags:
      hostname: db.local

The result is monitors.hostname = "db.local" because %{monitor.tags.hostname} refers to the local monitors.tags.hostname variable.

Singular “monitor” in %{monitor.VAR}, not plural, to emphasize that the reference is only to the single monitor in which it appears

You can use both in a single value, like:

tls:
  ca: "${SECRETS_DIR}/%{monitor.hostname}"

monitors:
  - hostname: db1
  - hostname: db2

Top-level tls.ca specifies a monitor default that applies to all monitors that don’t explicitly set the variable. If SECRETS_DIR = /secrets, the result is:

monitors:
  - hostname: db1
    tls:
      ca: /secrets/db1
  - hostname: db2
    tls:
      ca: /secrets/db2