Let's take a typical Big Data architecture problem. Assume you're designing a sales support system for a globally distributed workforce in London, New York and Singapore. You need to provide the latest sales totals by customer to help support your clients. You need sub-second response times on screen with a maximum transaction latency of 30 seconds. That means a transaction posted in London must be visible in Singapore within 30 seconds.
As we need sub-second response times, we need a cluster of machines at each of the three locations, and data replication between them.