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https://github.com/TPei/circuit_breaker

Implementation of the circuit breaker pattern in crystal
https://github.com/TPei/circuit_breaker

circuit-breaker circuit-breaker-pattern crystal design-pattern

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Implementation of the circuit breaker pattern in crystal

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# circuit_breaker



###### This project is being built weekly with the latest crystal version (works with v1.7.2 🎉)



Simple Implementation of the [circuit breaker pattern](http://martinfowler.com/bliki/CircuitBreaker.html) in Crystal.



## What??!?



> The basic idea behind the circuit breaker is very simple. You wrap a protected function call in a circuit breaker object, which monitors for failures. Once the failures reach a certain threshold, the circuit breaker trips, and all further calls to the circuit breaker return with an error, without the protected call being made at all. Usually you'll also want some kind of monitor alert if the circuit breaker trips. - Martin Fowler



Given a certain error threshold, timeframe and timeout window, a breaker can be used to monitor criticial command executions. Circuit breakers are usually used to prevent unnecessary requests if a server ressource et al becomes unavailable. This protects the server from additional load and allows it to recover and relieves the client from requests that are doomed to fail.



Wrap API calls inside a breaker, if the error rate in a given time frame surpasses a certain threshold, all subsequent calls will fail for a given duration.



## Installation



Add to your shard.yml



```yaml

dependencies:

  circuit_breaker:

    github: tpei/circuit_breaker

    branch: master

```



and then install the library into your project with



```bash

$ crystal deps

```



## Usage



Create a new breaker:

```crystal

require "circuit_breaker"



breaker = CircuitBreaker.new(

  threshold: 5, # % of errors before you want to trip the circuit

  timewindow: 60, # in s: anything older will be ignored in error_rate

  reenable_after: 300 # after x seconds, the breaker will allow executions again

)

```



Then wrap whatever you like:

```crystal

breaker.run do

  my_rest_call()

end

```



### Handling CircuitBreaker trips



The Breaker will open and throw an CircuitOpenException for all subsequent calls, once the threshold is reached. You can of course catch these exceptions and do whatever you want :D

```crystal

begin

  breaker.run do

    my_rest_call()

  end

rescue exc : CircuitOpenException

  log "happens to the best of us..."

  42

end

```



After the given reenable time, the circuit will transition to "half open". This will completely reset the circuit if the next execution succeeds, but reopen the circuit and reset the timer if the next execution fails.



### Handling only certain error types



If you are feeling really funky, you can also limit the exception classes to monitor. You might want to catch `RandomRestError`, but not `ArgumentError`, so do this:

```crystal

breaker = CircuitBreaker.new(

  threshold: 5,

  timewindow: 60,

  reenable_after: 300,

  handled_errors: [RandomRestError.new]

)



breaker.run

  raise ArgumentError.new("won't count towards the error rate")

end

```



### Ignoring certain error types



Conversely, you can also add custom errors to ignore and count all others:

```crystal

breaker = CircuitBreaker.new(

  threshold: 5,

  timewindow: 60,

  reenable_after: 300,

  ignored_errors: [ArgumentError.new]

)



breaker.run

  raise ArgumentError.new("won't count towards the error rate")

end

```



Unfortunately this both won't match against exception subclasses just yet, so at the moment you have to specify the exact class to monitor and can't just use `RestException` to match every subclass like `RestTimeoutException < RestException`...



## Thanks

Special thanks goes to Pedro Belo on whose ruby circuit breaker implementation ([CB2](https://github.com/pedro/cb2)) this is loosely based.