How does the Dead Letter Channel work and how do you configure error handling in Camel?

The Dead Letter Channel (DLC) is a default error handler that retries failed exchanges a configurable number of times and, on exhaustion, routes the Exchange to a designated dead-letter endpoint. It prevents message loss when downstream systems are temporarily unavailable.

// Configure Dead Letter Channel on the RouteBuilder:
public class MyRoutes extends RouteBuilder {
    @Override
    public void configure() {
        errorHandler(deadLetterChannel("jms:queue:DLQ")
            .maximumRedeliveries(3)
            .redeliveryDelay(1000)         // 1s initial delay
            .backOffMultiplier(2.0)         // doubles: 1s, 2s, 4s
            .retryAttemptedLogLevel(LoggingLevel.WARN)
            .logExhausted(true)
            .useOriginalMessage());         // send original msg, not transformed

        from("jms:queue:orders")
            .to("http://payment-service/pay");
    }
}

When all retries are exhausted, Camel adds an exception header (CamelExceptionCaught) to the Exchange before routing to the DLQ. useOriginalMessage() ensures the pristine inbound message reaches the DLQ, not a partially transformed version. The DLC can be scoped at route level (inside configure()) or globally (in a parent class). For per-exception handling, combine with onException().