Decomposing external commitments into internal targets
An external SLA is a contractual commitment to customers: we will respond to support tickets within 4 hours, or our uptime will be 99.9 percent. To keep that promise, the organization must chain a series of internal SLAs that back it up. Response time of 4 hours might require the support team to triage in 1 hour, engineering to confirm a bug in 2 hours, product to approve a fix in 30 minutes, and deployment to ship in 30 minutes. Each internal SLA must hold, or the external SLA breaks.
The cascade is a tree, not a simple chain. Uptime depends on multiple systems staying up: database availability, load balancer availability, application server availability. Each system's uptime SLA is a component of the total. If the database has 99.95 percent uptime and the load balancer has 99.98 percent uptime, the combined system is lower than either: roughly 99.93 percent. The math gets worse with more components.
Where cascades break and accountability gets murky
In practice, the cascade breaks when internal SLAs are either too aggressive or too loose. Too aggressive and they are always missed, creating a culture where SLAs are fiction. Too loose and the external SLA is unachievable even with perfect execution of the internals. The second failure mode is common: the organization promises 99.99 percent uptime but only has 99 percent SLAs on the components that feed it, a mathematical impossibility.
The other failure point is accountability. When the database team misses their SLA and the external SLA is missed, who is responsible? If accountability is not clear, blame spirals. A proper SLA cascade assigns accountability at each level: the database team owns their 99.95 percent, the application team owns 99.96 percent, the support team owns response time, and leadership reconciles misses against the contract. This clarity is what makes SLAs a tool for improvement instead of theater.