Why circuit breaker?

A flaky downstream service and a shared connection pool. Watch what happens when every caller waits out the timeout versus when a breaker fails fast.

Watch it explained

~27s animated walkthrough: a hung downstream call exhausting a connection pool vs. a breaker failing fast.

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Service A → Service B

Every request goes straight to the downstream service and holds a slot for the full duration, success or failure.

sent

succeeded

pool rejected

avg latency

—

Closed

Open

Half-Open

(breaker disabled — every call goes straight through)

Connection pool0 / 10 slots held

succeeding waiting on a timeout half-open trial free

pool utilization

timed-out waits

fast-failed

Last events

Run traffic to see breaker decisions.

Why this matters

Failures are expensive precisely because they're slow. A failing call still holds a pool slot for the full timeout — often longer than a successful call.
A full pool fails everyone, not just the doomed calls. Turn the breaker off and crank the failure rate — watch "pool rejected" climb even though some requests would have succeeded.
Failing fast is a feature. With the breaker on, a fast-failed request costs ~0ms instead of the full timeout, and the half-open probe checks recovery without risking the whole pool.

Downstream console

Make Service B flaky and watch the pool.

downstream failure ratePercent chance a call to Service B fails and hangs for the full timeout before erroring. 0% means Service B is fully healthy.

70% of calls fail

traffic (req/s)Incoming requests per simulated second from Service A.connection pool sizeTotal concurrent connections Service A can hold open to Service B. Once full, new requests — including ones that would have succeeded — are rejected outright.timeout duration (s)How long a doomed call waits before it errors out. This is what makes failures so expensive.simulation speedOnly changes animation/simulation playback speed.

Try this

Set failure rate to 70%+ and run traffic. The pool fills with red (timed-out) slots and stays full — watch “pool rejected” start climbing. Then flip the breaker on and rerun with the same settings.