network-pattern-circuit-breaker

in-memory calls vs remote calls

• remote calls can fail, hang without response until timeout.
• if there are many callers on unresponsive supplier. critical resource failure across system may result.

circuit-breaker pattern

• wrap function call in circuit breaker object that monitors failures
• if failure reaches threshold. -> circuit breaker returns all further calls as error.
• usage - Hystrix, Resilience4j

ruby code example

class CircuitBreaker...

  attr_accessor :invocation_timeout, :failure_threshold, :monitor
  def initialize &block
    @circuit = block
    @invocation_timeout = 0.01
    @failure_threshold = 5
    @monitor = acquire_monitor
    reset
  end

if circuit-breaker open return error
if closed calls block.

class CircuitBreaker...

  def call args
    case state
    when :closed
      begin
        do_call args
      rescue Timeout::Error
        record_failure
        raise $!
      end
    when :open then raise CircuitBreaker::Open
    else raise "Unreachable Code"
    end
  end
  def do_call args
    result = Timeout::timeout(@invocation_timeout) do
      @circuit.call args
    end
    reset
    return result
  end

for timeout. failure counter, resets if success

class CircuitBreaker...

  def record_failure
    @failure_count += 1
    @monitor.alert(:open_circuit) if :open == state
  end
  def reset
    @failure_count = 0
    @monitor.alert :reset_circuit
  end

Source

https://martinfowler.com/bliki/CircuitBreaker.html

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network-call-pattern-OOO-threshold failure-return