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Both terms, robustness and resiliency, express the ability of an application to handle unexpected scenarios. However, these terms differ in their expected behavior under these circumstances as compared to their normal behavior. A system is robust if it can withstand unexpected situations and continue to function to some degree. This will likely be suboptimal behavior, as compared to normal behavior. On the other hand, a system is resilient if it continues exhibiting its normal behavior, potentially taking a finite amount of time before being able to do so. I put forward the following examples from the book to illustrate the difference.

In Chapter 2, you will learn to enforce time-outs for command-line application functionality that is executing a user-specified program. By enforcing time-outs, we avoid the scenario where the application continues to hang indefinitely because of bad user output. Since we configure an upper bound on how long we want to allow the user-specified command to be executed, we will exit with an error when this duration expires before the command could be completed. This is not the normal behavior of the application—that we should wait for the command to complete—but this suboptimal behavior is necessary to allow the application to recover from an unexpected situation, such as the user-specified command taking longer than expected. You will find similar examples throughout, notably when sending or receiving network requests in Chapters 4, 7, 10, and 11. We will refer to these techniques as introducing robustness in our applications.

In Chapter 10, you will learn to handle transient failures in your gRPC client applications. You will write your applications in a manner in which they can tolerate temporary failures that are likely to be resolved soon. We refer to this as introducing resilient behavior in our applications. However, we also introduce an upper time limit, which we allow to resolve the potentially temporary failure. If this time limit is exceeded, we consider that the operation cannot be completed. Thus, we introduce robustness as well.

To summarize, resiliency and robustness both aim to handle unexpected situations in our applications, and this book uses these terms to refer to such techniques.