Mustache component

The mustache component renders a Mustache template against the message (body, headers, properties) and sets the rendered text as the new body.

Scheme: mustache: · Crate: cameleer-core · Cargo feature: templates · Consumer: no · Producer: yes · Apache Camel: Mustache

When to use it

• Build an LLM prompt from a JSON request: fill a prompt template with the user's question and context documents before calling a model.
• Render an email, a Slack message or a report from structured data.
• Produce a text payload (CSV line, SQL, XML snippet) without code.
• Keep wording in a file that non-developers can edit, separate from the route.

URI format

mustache:templateResource[?options]

templateResource is a file path relative to the working directory, file:path, classpath:path (read as a relative path, there is no classpath) or ref:beanName (a String bound in the registry).

Example

A support agent receives {"customer": ..., "question": ..., "docs": [...]} and turns it into a prompt for a model. The body is unmarshalled from JSON first so the template can read its fields.

You are a support assistant for {{headers.product}}.
Customer: {{body.customer}}
Question: {{body.question}}
{{#body.docs}}
- {{title}}: {{text}}
{{/body.docs}}
{{^body.docs}}
No documents matched.
{{/body.docs}}
Answer in at most {{headers.maxWords}} words.

- route:
    id: mustache-prompt
    from:
      uri: direct:mustache-prompt
      steps:
        - unmarshal:
            json: {}
        - setHeader:
            name: product
            constant: Cameleer
        - setHeader:
            name: maxWords
            constant: "80"
        - to: mustache:templates/support-prompt.mustache

{"customer":"Ada","question":"How do I retry a failed HTTP call?","docs":[{"title":"Error handling","text":"Use onException with maximumRedeliveries."},{"title":"HTTP","text":"Set throwExceptionOnFailure=true."}]}

You are a support assistant for Cameleer.
Customer: Ada
Question: How do I retry a failed HTTP call?
- Error handling: Use onException with maximumRedeliveries.
- HTTP: Set throwExceptionOnFailure=true.
Answer in at most 80 words.

With no documents, the inverted section renders instead:

{"customer":"Bo","question":"Is there a Kafka component?","docs":[]}

You are a support assistant for Cameleer.
Customer: Bo
Question: Is there a Kafka component?
No documents matched.
Answer in at most 80 words.

Template from a header

With allowTemplateFromHeader=true the CamelMustacheTemplate header replaces the template file, so a route can render templates stored elsewhere (a database, a config service).

{{...}} inside a route file is a property placeholder, so a template written in the route itself uses other delimiters (startDelimiter / endDelimiter). Template files are not affected.

- route:
    id: mustache-header
    from:
      uri: direct:mustache-header
      steps:
        - setHeader:
            name: CamelMustacheTemplate
            constant: "Hello [[body]]!"
        - to:
            uri: mustache:templates/support-prompt.mustache
            parameters:
              allowTemplateFromHeader: true
              startDelimiter: "[["
              endDelimiter: "]]"

World

Hello World!

Rust DSL

b.from("direct:mustache-rust")
    .to("mustache:templates/support-prompt.mustache?contentCache=true");

Options

Data model

Supported Mustache features

Escaped {{name}} and unescaped {{{name}}} / {{& name}} variables, dotted names, the implicit iterator {{.}}, sections over lists, objects and booleans, inverted sections {{^x}}, comments {{! ... }}, partials {{> name}} (loaded from the template's directory as name or name.mustache), set-delimiter tags {{=<% %>=}} and standalone-line removal. Lambdas are not supported.

Message headers

Run it with Docker

Put the route in prompt.camel.yaml and the template in templates/ next to it:

docker run --rm -v "$PWD:/routes:ro" pmsudhi/cameleer run /routes/prompt.camel.yaml

The image's working directory is /routes, so templates/support-prompt.mustache resolves to the mounted file.

Common questions

How do I read JSON fields in a Mustache template?

Unmarshal the body first (unmarshal: json: {}); then {{body.field}} and sections over body.list work. A plain JSON string body has no fields.

Why does {{body.text}} turn < into <?

Double braces HTML-escape the value, as in every Mustache implementation. Use {{{body.text}}} or {{& body.text}} for raw text, which is what you want for prompts and plain text.

Why do I get "Property with key [x] not found"?

The template text is inside the route file, where {{x}} is a property placeholder resolved when the route starts. Keep templates in files, or switch delimiters with startDelimiter and endDelimiter as in the header example above.

Does the template reload when I edit the file?

Yes, by default the file is read for every message. Set contentCache=true to read it once.

Related

• FreeMarker, Velocity, StringTemplate, MVEL: other template engines.
• JSON data format: unmarshal JSON before rendering.
• Apache Camel Mustache component

Found a problem?

If something on this page doesn't match what Cameleer does, or you hit a bug while building with it, please help fix it, whether you are a person or a coding agent: open an issue that describes what you saw, then fork the repository, fix it, add or update a test, run cargo test --workspace, and open a pull request. CONTRIBUTING.md explains the ground rules.