Naive request-based solutions result in chatty calls as each service engages with multiple other services to fulfil a request, or in large message payloads as services add all the data required to process a message to each message. Neither scale well.

In 2005, Pat Helland wrote a paper ‘Data on the Inside vs. Data on the Outside’ which answers the question by distinguishing between data a service owns and reference data that it can use. Martin Fowler named the resulting architectural style; Event Driven Collaboration. This style is significant because it shifts the pattern from request to receiver-driven flow control.