Environment variable interpolation

Substitute environment variables into YAML route files with ${env:VAR} tokens. The tokens expand before YAML parsing, so they work in endpoint URIs, log messages, header values, and any other string field.

Syntax

# =============================================================================
# env-interpolation example routes
#
# Uses ${env:VAR_NAME} to inject runtime environment variables.
# Supports default values with ${env:VAR_NAME:-default} syntax.
# Substitution happens before YAML parsing — works in URIs, messages, values.
#
# Try setting these before running:
#   GREET_TARGET=log:my-app
#   POLL_PERIOD_MS=2000
#   LOG_PREFIX="[demo]"
#
# If unset, defaults kick in automatically (see route 2).
# =============================================================================

routes:
  # ---------------------------------------------------------------------------
  # Route 1: env var in log message and endpoint URI
  # ---------------------------------------------------------------------------
  - id: "env-greet"
    from: "timer:env-tick?period=${env:POLL_PERIOD_MS}&repeatCount=3"
    steps:
      - log: "${env:LOG_PREFIX} Firing env-interpolation route"
      - set_header:
          key: "app"
          value: "${env:APP_NAME}"
      - to: "${env:GREET_TARGET}"

  # ---------------------------------------------------------------------------
  # Route 2: env var with default fallback syntax
  # ---------------------------------------------------------------------------
  - id: "env-defaults"
    from: "timer:env-defaults?period=${env:DEFAULT_POLL_MS:-3000}&repeatCount=2"
    steps:
      - log: "${env:DEFAULT_LOG_PREFIX:-[defaults]} Using fallback defaults"
      - to: "${env:DEFAULT_TARGET:-log:info}"

${env:VAR} reads the variable VAR. ${env:VAR:-default} uses default when VAR is unset. An unset variable with no default fails route discovery. The error names the variable. Set a default or export the variable to avoid the failure.

How it works

The DSL loader (camel_dsl::interpolate_env) scans route source for ${env:...} patterns and replaces them before YAML parsing. Substituted values pass through sanitize_env_value, which strips control characters and newlines. This blocks newline injection from a hostile or malformed variable.

The PropertiesResolver type in camel-config exposes the same resolution as a public API for config-value placeholders.

Setup

    let config =
        CamelConfig::from_file("Camel.toml").map_err(|e| CamelError::Config(e.to_string()))?;

    let mut ctx = CamelContext::builder().build().await.unwrap(); // allow-unwrap
    ctx.register_component(TimerComponent::new());
    ctx.register_component(LogComponent::new());

    let routes = discover_routes(&config.routes).map_err(|e| CamelError::Config(e.to_string()))?;

The Camel.toml for this example is minimal. Route discovery and component registration follow the standard pattern.

[default]
routes = ["routes/**/*.yaml"]
log_level = "INFO"

When to use

• Twelve-factor apps: inject configuration that varies per deploy through the environment, not through files in source control.
• Secrets: pass credentials and tokens from the environment. The route file never stores the secret value.
• Per-environment endpoints: point routes at different brokers, HTTP hosts, or databases without editing route files.

Reference: PropertiesResolver in the Config crate