This chapter activates T7 (State Machines) and T11 (Feedback Loops).
T7 began in Book 1, Chapter 1, where logical statements established the binary values (True/False) from which all computational state is built. In Book 1, Chapter 29, state machines formalised how systems transition between discrete states based on inputs. A circuit breaker is exactly a state machine: three states (CLOSED, OPEN, HALF-OPEN), transition rules based on failure counts and timeouts, and actions associated with each state (pass requests through, reject requests, probe the dependency).
T11 (Feedback Loops) continues from Chapter 5 (rate limiting as a feedback loop) and Chapter 13 (queues as flow control). The circuit breaker is another instance of T11: it measures a signal (failure rate), applies a threshold, and changes behaviour when the threshold is exceeded — a feedback loop that adapts the system’s behaviour to prevent further damage. The pattern recurs throughout the reliability landscape because feedback loops are the fundamental mechanism for maintaining stable behaviour in the presence of disturbances.