Circuit_Breaker.md

Circuit Breaker

• Motivation
• Solution
  • Concepts
  • Implementation
    • Basic mechanism
    • States
    • More Sophisticated Approaches
• Pros & Cons
  • Pros
  • Cons
• Consideration
• When To Use
• References

Motivation

Solution

Concepts

• Detects failures and prevents a failure from constantly recurring.

Implementation

Basic mechanism

• Adds circuit breaker in the middle of a client and service.
• Circuit breaker monitors failures by tracking the number of failures.
• Once the error rate exceeds some threshold, the circuit breaker will trip so that all further calls fail immediately.

States

• Circuit breaker retains a state over a series of calls, there are 3 common value:
  • Closed
    • This is the normal state. Circuit breaker doesn't intervene calls except counting the number of failures.
  • Open
    • Once the error rate exceeds a threshold, the circuit breaker trips and its state will be changed to "Open".
      • All further calls from client will be failed immediately.
      • Timeout timer will be started for changing the state from "Open" to "Half open" when timeout.
  • Half open
    • Circuit breaker accepts a limited number of calls:
      • If all those calls are successful, the circuit breaker will change the state to "Closed" and will all the calls as normal.
      • If any call fails, the circuit breaker will reverts back the state to "Open" and block all the calls.

More Sophisticated Approaches

• General
  • Log all failed calls to enable an administrator to monitor the health of the operation.
  • Provide a manual option for an administrator to close or open a circuit breaker.
  • Changes in a circuit breaker’s state should always be logged.
  • A circuit breaker's current state should be exposed for querying and monitoring.
• In the Closed state
  • Handle different types of errors more specifically.
  • Check frequency of errors rather than number of errors.
  • Have different thresholds for different types of errors.
• In the Open state
  • Rather than using a timer to determine when to switch to the Half-Open state, a circuit breaker can instead periodically ping the service to determine whether it's become available again.
  • Rather than simply failing quickly, a circuit breaker could also record the details of each call to a journal and arrange for these calls to be replayed when the service becomes available.

Pros & Cons

Pros

• Prevents a failure from constantly recurring.
  • Reduce resources tied up in operations which are likely to fail.
  • Avoid waiting on timeouts for the client.
  • Avoid putting load on a struggling server.

Cons

• Negatively affects on performance.

Consideration

Topic	Consideration	Possible Solution Options

When To Use

References

• Web Article: Circuit breaker design pattern | https://en.wikipedia.org/wiki/Circuit_breaker_design_pattern
• Web Article: 断路器设计模式 | https://zh.wikipedia.org/wiki/%E6%96%B7%E8%B7%AF%E5%99%A8%E8%A8%AD%E8%A8%88%E6%A8%A1%E5%BC%8F
• Web Article: CircuitBreaker | https://martinfowler.com/bliki/CircuitBreaker.html
• Web Article: Circuit Breaker pattern | https://docs.microsoft.com/en-us/azure/architecture/patterns/circuit-breaker
• Book: Michael N.(2018). Chapter 5 Stability Patterns, Release It! (pp. 91-128). Pragmatic Bookshelf