Circuit Breakers

The circuit breaker is what protects your application from a persistently failing dependency. Where retries handle the occasional blip, the breaker handles the outage: once a service fails too often, the breaker "opens" and further calls fail fast — immediately, without touching the struggling service — until it has had time to recover.

This is the mechanism described in Michael Nygard's Release It! and popularized by Martin Fowler. ExternalService implements it on top of the Erlang :fuse library, but you never call :fuse directly — the breaker is managed for you on every call.

Why fail fast?

When a dependency is down and you keep calling it, every caller blocks on timeouts, work piles up, and the failure spreads — a cascading failure. The breaker short-circuits that: after enough failures it stops you from even attempting the call, so callers get an immediate error they can handle (serve cached data, degrade gracefully, return 503) instead of hanging.

Unlike retries, which are per-call, the breaker is global to the service. If it trips, it trips for every caller in the system at once. That is precisely what makes it effective at preventing cascades.

Configuration

Configure the breaker with the :circuit_breaker option to use ExternalService or ExternalService.start/2:

use ExternalService,
  circuit_breaker: [
    tolerate: 5,                 # failures allowed within the window...
    within: :timer.seconds(1),   # ...this window, in milliseconds
    reset: :timer.seconds(5)     # stay open this long before resetting
  ]

So tolerate: 5, within: 1_000 means "open the breaker once there are more than 5 failures inside any 1-second window." After opening, the breaker stays open for :reset milliseconds, then closes again and calls resume under the same monitoring.

The :circuit_breaker option (and every key within it) is optional. Omit it to get the defaults above.

What counts as a failure?

The breaker is "melted" — pushed one step toward opening — on every call attempt that fails, where a failure is:

• the function returns :retry or {:retry, reason}, or
• the function returns a value matched by the :retry_on predicate, or
• the function raises an exception whose type is listed in the :retry_exceptions retry option.

Values your function simply returns — including its own {:error, reason} — are successes as far as the breaker is concerned and do not melt it.

When the breaker is open

A call made while the breaker is open does not invoke your function at all. Instead:

• call/3 returns {:error, %ExternalService.CircuitBreakerOpen{}},
• call!/3 raises ExternalService.CircuitBreakerOpen, and
• an [:external_service, :circuit_breaker, :blown] telemetry event is emitted.

See Error handling for how to deal with these.

Introspecting and resetting

You can ask about the breaker's state at any time. With the module front door:

MyApp.Stripe.available?()   #=> true when the breaker is closed
MyApp.Stripe.blown?()       #=> true when the breaker is open
MyApp.Stripe.reset()        #=> force the breaker closed

Or with the functional API:

ExternalService.available?(:payments)
ExternalService.blown?(:payments)
ExternalService.all_available?([:payments, :inventory])
ExternalService.reset(:payments)

A few semantics worth knowing:

• available?/1 is true only when the breaker is closed. A service that was never started reports false — it is not "ready to use."
• blown?/1 is the direct "is it open?" question. A service that was never started is not reported as blown (there is no breaker to be open); use available?/1 when you want "ready to use" semantics.
• all_available?/1 is true only if every listed service is available?/1 — handy for guarding work that depends on several services.
• Availability can change between the check and a subsequent call, so treat these as best-effort signals, not guarantees. They let you bail out early; they do not replace handling a CircuitBreakerOpen error from the call itself.

reset/1 forces the breaker closed immediately, discarding its recorded failures. It is mainly useful in tests and in operational tooling ("we fixed the upstream, stop failing fast now").

Fault injection (for testing)

The :fault_injection option makes the breaker fail a random fraction of calls, which is useful for exercising your own fallback and error-handling paths:

use ExternalService,
  circuit_breaker: [tolerate: 5, within: 1_000, fault_injection: 0.25]

This is a testing aid — leave it unset in production.

Choosing thresholds

There is no universally correct setting; it depends on the service's normal error rate and how costly a false trip is. Some rules of thumb:

• Set :tolerate/:within so the breaker tolerates normal transient noise but trips promptly on a real outage. Counting failures over a window (rather than consecutively) makes it robust to interleaved success and failure.
• Set :reset to roughly how long you expect a recovering service to need. Too short and you hammer a service that isn't ready; too long and you stay degraded after it has recovered.
• Remember the breaker is global to the service. Size it for aggregate traffic, not a single caller.