By taking advantage of an asynchronous approach Starbucks also has to deal with the same challenges that asynchrony inherently brings. Take for example, correlation. Drink orders are not necessarily completed in the order they were placed. This can happen for two reasons. First, multiple baristas may be processing orders using different equipment. Blended drinks may take longer than a drip coffee. Second, baristas may 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 need to be matched up to the correct customer. Starbucks solves the problem with the same "pattern" we use in messaging architectures — they use a Correlation Identifier. 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 complete. In other countries, you have to correlate by the type of drink.
Are you stuck with a legacy PHP application? You might like my book because it gives you a step-by-step guide to improving your codebase, all while keeping it running the whole time.