Your Coffee Shop Doesn't Use Two-Phase Commit

Y ou know you're a geek when going to the coffee shop gets you thinking about interaction patterns between loosely coupled systems. This happened to me on a recent trip to Japan. One of the more familiar sights in Tokyo is the numerous Starbucks coffee shops, especially around Shinjuku and Roppongi. While waiting for my " Hotto Cocoa, " I started thinking about how a coffee shop processes customer orders. As a business, the coffee shop is naturally interested in maximizing order throughput, because more fulfilled orders mean more revenue. Interestingly, the optimization for throughput results in a concurrent and asynchronous processing model: when you place your order, the cashier marks a coffee cup with your order and places it into a queue. This queue is literally a line of coffee cups on top of the espresso machine. The queue decouples the cashier and barista, letting the cashier continue to take orders even when the barista is backed up. It also allows multiple baristas to start servicing the queue if the store gets busy, without impacting the cashier. Asynchronous processing models can be highly efficient but are not without challenges. If the real world writes the best stories, then maybe we can learn something from Starbucks about designing successful asynchronous mes-saging solutions. Correlation For example, the asynchronous processing model means that drink orders aren't necessarily completed in the same sequence in which they were placed. This can happen for two different reasons. First, multiple baristas might be processing orders using different equipment. Blended drinks usually take longer to make than drip coffee , so a drip coffee ordered last might be delivered first. Second, baristas can make multiple drinks in one batch to optimize processing time. As a result, Starbucks has a correlation problem. Drinks are delivered out of sequence and must be matched up with the correct customer. Starbucks solves the problem with the same " pattern " we use in messaging architectures— they use a correlation identifier. 1 In the US, most Starbucks use an explicit correlation identifier by writing your name on the cup and calling it out when the drink is ready. In other countries, they often correlate by drink type. The correlation issue became very apparent in Japan, where I had difficulties understanding the baristas calling out the drinks. My approach was to order extra large " venti " drinks because they're uncommon and therefore …