When you need to break up a composite message into smaller parts, but provide access to any of the parts on demand, use a Claim Check.
Consider the Claim Check a unique identifier used to store and access a checked item. You pass as part of a processing message the identity of the specific checked item; its Claim Check. Each step in the process can use the Claim Check to retrieve all or part of the composite message content as needed.
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The full text is now available as part of my book Reactive Enterprise with Actor Model on Safari Books Online.