What Is the Circuit Breaker Pattern? Closed, Open and Half-Open Explained
Your payment gateway stops answering. Your Rails app keeps calling it anyway, hundreds of times, and every one of those calls sits there holding a thread. The gateway is the thing that broke, but it is your application that goes down.
The circuit breaker pattern exists for exactly this. This video follows one example all the way through: a Rails checkout calling an external payment gateway, what the three breaker states do, and what a breaker still cannot do for you.
What you will learn:
- Why a slow dependency is worse than a fast failure
- How waiting calls exhaust a thread pool without a single error being raised
- The electrical analogy, and where it actually holds
- CLOSED, OPEN and HALF-OPEN, and what moves between them
- Why an open breaker can reject at once or fall back
- What the half-open probe is for, and why it is a single request
- Which numbers you configure: the failure threshold and the recovery wait
- Why the timeout on the call matters as much as the breaker around it
- Why one breaker per dependency beats one breaker per app
The honest part: a circuit breaker does not repair the service behind it, and it does not guarantee your app stays up. It contains a failure. Set the thresholds badly and it will reject perfectly good traffic. And it does not replace timeouts, limited retries or real error handling - those are separate layers, and retries in particular have to be coordinated with the breaker, or uncoordinated retrying will deepen the outage you were trying to survive.
A note on the code: it is pseudocode. Rails has no built-in circuit breaker.
Chapters:
00:00 - One service down, the whole app slow
00:10 - Why waiting is worse than failing
00:25 - The breaker in your wall, and in your code
00:40 - Closed, open, half-open
01:00 - A Rails payment service example
01:25 - Benefits and trade-offs
01:45 - The key takeaway
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