Circuit breaker pattern

As explained in Python Concurrency with asyncio

A circuit breaker pattern is a fault-tolerance pattern that stops sending requests to a failing or slow service and resumes requests after a configured reset condition.

A circuit breaker prevents repeated calls to a failing or slow service from delaying users. After a configured number of errors within a time window, it opens, skips the service, and fails fast until the reset interval expires. section 10.3.5

Model the breaker with these states and transitions:

• Closed: send requests normally and record failures. section 10.3.5
• Open: stop calling the unhealthy service and return an error immediately. section 10.3.5
• After the reset interval, test the service; a successful test closes the breaker, while a failed test keeps it open. A simplified implementation may omit this half-open state. section 10.3.5

Protecting an inventory request

Wrap an asynchronous inventory callback with a breaker configured for a 0.5-second timeout, a five-second failure window, three allowed failures, and a 30-second reset interval. Repeated slow calls then fail fast instead of blocking the backend-for-frontend response. section 10.3.5 section 10.3.5

Figure 10.3 A circuit breaker that opens after two failures. Once opened, all requests will fail instantly.

Trace the breaker opening after two failures and observe subsequent requests failing instantly. section 10.3.5

How Python Concurrency with asyncio covers it

How deep it goes

Explains closed, open, and half-open states, then implements a simplified asynchronous breaker with configurable thresholds and timing.

Examples

Demonstrates slow cart calls, an inventory callback, repeated breaker requests, and a reset after five seconds.

What it assumes

Assumes asynchronous service calls and introduces the resilience problem through microservice aggregation.

What you can do after

Build it from scratch with asyncio, configure failure and reset thresholds, and apply it to asynchronous service requests.

How it connects

Connects microservices, backend-for-frontend aggregation, retries, concurrency, and downstream failure handling.

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Also explained in

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  The Art of CodeA circuit breaker is a fault-tolerance pattern that temporarily stops calls to an failing dependency after repeated failures and later permits limited recovery attempts.
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  gRPC Microservices in GoA circuit breaker is a fault-tolerance pattern that temporarily stops calls to a failing dependency to prevent cascading failures.
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  Google Anthos in ActionA circuit breaker is a distributed-systems fault-isolation mechanism that temporarily stops requests to an failing or unavailable service.
• 

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  Bootstrapping Microservices, Second Edition: With Docker, Kubernetes, GitHub Actions, and TerraformA circuit breaker is a resilience pattern that stops requests to a failing upstream service and resumes them after checking that the service has recovered.
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  Secure APIsA circuit breaker pattern prevents repeated calls to an unhealthy dependency and returns a controlled result when that dependency fails or responds unexpectedly.

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Related concepts

• open stateuses · An open state is a circuit-breaker state in which calls to a failing dependency are rejected or fail immediately without being attempted.
• half-open stateuses · A half-open state is a circuit-breaker state that permits a limited number of requests to test whether a previously failing dependency has recovered.
• cascading failureaddresses · A cascading failure is a chain reaction in which the failure or degradation of one component propagates through dependencies and causes further failures or instability.
• closed statehas part · The closed state is a circuit-breaker state in which requests pass normally to a dependency considered healthy.

This explainer was generated by Manning's knowledge graph from the text of Python Concurrency with asyncio. The section links point to the parts of the book it draws on.