Services are rarely exists in isolation; they have dependencies, both upstream and downstream. Every server will receive requests from clients (either UI or other services ) that are counting on the service to honor the advertised contract and to live up to its SLAs. Each service also has a bunch of downstream dependencies (other services) that it depends on to get work done.
To understand the impact of dependencies on the overall system's, reliability, let's consider the Hotel Search service in the travel website we are building. Let's say we have built it to very high reliability levels (as described in the previous section) and we have an uptime requirement of four-nines (99.99% availability) to the clients. Now, the Hotel Search service depends on several other microservices, such as the Pricing Engine, Catalog Service, and Wallet, to display the...