The key to engineering resilience is to understand that there is no fault prevention—we should design to handle failures. Protecting the system's SLAs means building insulation to the faults that can happen to dependencies. In our design, we need to do the following:

• Be nice to the broken service: If the dependent service is down, we should not bombard it with more requests, thus allowing it time to recover.
• Gracefully degrade: Our clients should get a clear and timely error message
• Provision monitoring/alerts: We should be able to monitor the health of our dependents in the same way as we do monitoring of our own systems.

Though this sounds daunting, the good folks at Netflix have architected a comprehensive solution to enable applications to build such resilience. It's called Hystrix, and we will discuss it now.