Script

The Script step is a Message Translator from Hohpe and Woolf. It runs an inline Rhai script against the exchange so the route can mutate headers, properties, and body in one step. rust-camel registers Rhai as the scripting language.

        // Step 4: .script() — mutating Rhai expression tags the order and
        //         appends a status suffix. Changes propagate back to the Exchange.
        .script(
            "rhai",
            r#"
            headers["processed"] = true;
            let status = if headers["priority"] == "high" { "PRIORITY" } else { "STANDARD" };
            body = body + " [" + status + "]";
            "#,
        )
        // Step 5: log every message (after enrichment)
        .to("log:all-orders?showBody=true&showHeaders=true")

- id: script-demo
  from: timer:tick?period=1000
  steps:
    - script:
        language: rhai
        source: |
          headers["tenant"] = "acme"
          body = body + "_processed"
    - to: log:scripted?showBody=true&showHeaders=true

The .script("rhai", source) call hands the script text to the Rhai engine. The engine exposes three variables on the exchange: headers and properties as mutable maps, and body as the current body string. The script reads and writes all three. Assignments to headers["key"] and body propagate to the next step in the pipeline.

Script handles logic that one expression cannot express. A Rhai block runs conditionals, loops, and several mutations in a single step. Transform evaluates one expression and writes one result. A route that needs a single body replacement uses Transform. A route that needs branching logic or several field changes uses Script.

Script interprets Rhai at runtime, so it runs slower than a native .process(|ex| ...) closure. The closure compiles to Rust and reads the full Exchange type. A route that needs maximum throughput uses process. A route that needs logic it can change without a rebuild uses Script.

Per ADR-0001, the step compiles into a Service<Exchange> in the Tower middleware pipeline. The Rhai integration is documented in camel-language-api/CONTEXT.md.

The example source is at examples/language-rhai.