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The Synchrony Budget

For building a system of distributed services, one concept I think is very valuable to keep in mind is what I call the synchrony budget: as much as possible, a service should minimize the number of synchronous requests which it makes to other services.

I remember a birthday party a few years back where one of my friends was on call and had to patch the inventory table of an e-commerce application after a rack of flowers had been tossed over in the warehouse. In all likelihood, we’ll be better off sending a customer a $10 voucher as an apology in the rare case of accepting an order for an item without inventory, instead of spending our synchrony budget and establishing a synchronous call flow for this process. That being said, for an e-commerce application it may be actually feasible to make synchronous calls to the payment service by default, but fall back to asynchronous processing in case of failures.

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