Circuit Breaker

The advent of electrical wiring

Not too long ago, when electrical wiring was first being built into houses, many people fell victim to physics. The unfortunates would plug too many appliances into their circuit. Each appliance drew a certain amount of current. When current is resisted, it produces heat proportional to the square of the current times the resistance (shown mathematically as $I2R$). Because houses lacked superconducting home wiring, this hidden coupling between electronic gizmos made the wires in the walls get hot, sometimes hot enough to catch fire.

The electrical fuses

The fledgling energy industry found a partial solution to the problem of resistive heating in the form of fuses. The entire purpose of an electrical fuse is to burn up before the house does. It’s a component designed to fail first, thereby controlling the overall failure mode. This brilliant device worked well, except for two flaws. First, a fuse is a disposable, one-time use item; therefore, it’s possible to run out of them. Second, residential fuses (in the United States) were about the same diameter as copper pennies. Together, these two flaws led many people to conduct experiments with homemade, high-current, low-resistance fuses (that is, a 3/4-inch disk of copper).

Residential fuses have gone the way of the rotary dial telephone. Now, circuit breakers protect overeager gadget hounds from burning their houses down. The principle is the same: detect excess usage, fail first, and open the circuit. More abstractly, the circuit breaker exists to allow one subsystem (an electrical circuit) to fail (excessive current draw, possibly from a short circuit) without destroying the entire system (the house). Furthermore, once the danger has passed, the circuit breaker can be reset to restore full function to the system.

We can apply the same technique to software by wrapping dangerous operations with a component that can circumvent calls when the system is not healthy. This differs from retries, in that circuit breakers exist to prevent operations rather than re-execute them.

In the normal “closed” state, the circuit breaker executes operations as usual. These can be calls out to another system, or they can be internal operations that are subject to timeout or other execution failure. If the call succeeds, nothing extraordinary happens. If it fails, however, the circuit breaker makes a note of the failure. Once the number of failures (or the frequency of failures, in more sophisticated cases) exceeds a threshold, the circuit breaker trips and “opens” the circuit, as shown in the following figure.