What is the Exchange in Camel — in-message, out-message, headers, and properties?

The Exchange is the message-passing container that travels through an entire route. When a consumer endpoint receives input, Camel wraps it in an Exchange and passes that object through every Processor. Everything a Processor needs — payload, metadata, and context — is accessed through the Exchange.

• In-message (exchange.getIn()): The current message with body, headers, and attachments. Processors read and write to this.
• Out-message (exchange.getOut()): The reply in InOut exchanges. In Camel 3.x writing to getOut() is discouraged because it creates a fresh Message and silently drops all headers set by prior processors.
• Headers: A Map<String, Object> on the current message. Components auto-populate keys like CamelFileName and JMSMessageID.
• Properties: Exchange-wide key-value pairs that persist across EIP steps like split() or aggregate().
• ExchangePattern: InOnly (fire-and-forget) or InOut (request-reply).
• Exception: Captured via exchange.getException() when a Processor throws.

from("direct:start")
    .process(exchange -> {
        String body = exchange.getIn().getBody(String.class);
        exchange.getIn().setHeader("processedBy", "OrderProcessor");
        exchange.setProperty("orderId", "ORD-1234"); // survives split
        exchange.getIn().setBody(body.toUpperCase());
    });