Interruttore is a circuit-breaker implementation for Clojure/ClojureScript.
Circuit breakers help you write simpler, more robust code in cases where failures might happen (e.g. network or db is down).
If you want to know more about this design pattern have a look at this awesome blog post by Martin Fowler.
[interruttore "0.1.3"]
Still alpha, interfaces, namespaces, functionality may change every time; not tested, use at your own risk.
If you need to use a battle tested circuit breaker implementation (and more), please have a look at diehard.
This library provides two ways to wrap a function with a circuit breaker:
- Exception based
- Based on the return value of the wrapped function (richer API)
(require '[interruttore.core :as cb])
(defn to-be-wrapped [a b]
{:result :ok
:value (/ a b)})
(def wrapped (cb/make-circuit-breaker to-be-wrapped {:max-retries 1
:exceptions [ArithmeticException]
:retry-after-ms 10}))
;; happy case
(wrapped 2 2) ;; => {:status :closed :value 1}
;; failing case, will return a failure, internally it will keep track of the
;; count of the failures, but at this time the circuit is still closed
(wrapped 2 0) ;; => {:status :closed :result :soft-failure}
;; after this failure the circuit is in the open state
(wrapped 2 0) ;; => {:status :open
;; :reason :max-retries
;; :result :soft-failure
;; :retry-after (now-plus-10-ms)}
;; wrapped start to work again, we are in the semi-open state now
(wrapped 4 2) ;; => {:status :semi-open :result :ok :value 2}
;; fail again, but this time the circuit will be opened right after this call
;; instead of waiting for another failure
(wrapped 2 0) ;; => {:status :open
;; :reason :max-retries
;; :retry-after (now-plus-10-ms)}
(defn to-be-wrapped [result]
result)
(def wrapped (cb/make-circuit-breaker to-be-wrapped {:max-retries 1
:retry-after-ms 10}))
;; happy case
(wrapped {:result :ok :value 1}) ;; => {:status :closed :result :ok :value 1}
;; failing case, retry max-retry times and wait retry-after-ms before retrying
(wrapped {:result :soft-failure
:retry-after (in-a-minute)}) ;; => {:status :closed
;; :result :soft-failure}
(wrapped {:result :soft-failure
:retry-after (in-a-minute)}) ;; => {:status :open
;; :result :soft-failure
;; :reason :max-retries
;; :retry-after (in-a-minute)}
;; wrapped start to work again, we are in the semi-open state now
(wrapped {:result :ok :value 2}) ;; => {:status :semi-open :result :ok :value 2}
;; fail again, but this time it opens the circuit right after the call
(wrapped {:result :soft-failure
:retry-after (in-a-minute)}) ;; => {:status :open
;; :result :ok
;; :reason :max-retries
;; :retry-after (in-a-minute)}
Return based approach, so far, seems very close to the exception based one, if we exclude the possibility, for the wrapped function, to tell exactly until when the circuit must stay open; this method provides an additional result key :hard-failure which will open the circuit without retrying to call the wrapped function. This can be convenient, for example, in case an external API has a quota; it does not make any sense to call the external API again today if we have exceed our daily quota.
(cb/reset wrapped) ;; reset the circuit to its initial state
;; fail hard this time, so it does not retry
(wrapped {:result :hard-failure
:retry-after (tomorrow)}) ;; => {:status :open
;; :result :hard-failure
;; :reason :hard-failure
;; :retry-after :some-date-time}
The map returned by the wrapped can provide another key, :reason, that can be
used to replace the builtin :reason returned when the circuit is open, here is
an example:
(cb/reset wrapped) ;; reset the circuit to its initial state
;; fail hard, this time providing a :reason
(wrapped {:result :hard-failure
:reason :daily-quota-exceeded
:retry-after (tomorrow)}) ;; => {:status :open
;; :result :hard-failure
;; :reason :daily-quota-exceeded
;; :retry-after :some-date-time}
- get feedback: is it useful? missing something? received some very useful feedback; open for the next round.
- put everything on clojars
- better documentation
- def-circuit-breaker macro?
- support to ClojureScript (should require few changes exp datetime handling)
Copyright © 2020 Francesco Pischedda
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