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The use of a Correlation Identifier is key to working with the Camel Business Activity Monitoring Framework and can also be highly useful when testing with simulation or canned data such as with the Mock testing framework
Some EIP patterns will spin off a sub message, and in those cases, Camel will add a correlation id on the Exchange as a property with they key Exchange.CORRELATION_ID, which links back to the source Exchange. For example the Splitter, Multicast, Recipient List, and Wire Tap EIP does this.