claro.circuit-breaker

This library provides a claro middleware to add circuit-breaking to resolution engines, using resilience4j's circuit-breaker implementation.

Usage

(require '[claro.engine :as engine]
         '[claro.middleware.circuit-breaker :as cb])

We can describe the different circuit-breakers by associating a name with a circuit-breaker configuration:

(def circuit-breakers
  {:db {:name                        "Database"
        :wait-duration-in-open-state 10000
        :record-failure-predicate    #(instance? IOException %)}})

Then we add the circuit-breaking middleware that returns a circuit-breaker name to use (or nil) for each batch-to-resolve:

(defonce engine
  (-> (engine/engine)
      (cb/wrap-circuit-breaker
        (fn [env batch]
          (if (uses-db? batch)
            :db))
        circuit-breakers)))

(uses-db? can be realised in a variety of ways, e.g. using a marker protocol that is implemented by all resolvables that need DB access.)

Now, every IOException thrown by the targeted resolvables will contribute to the circuit-breaker's failure rate. If it opens up, claro error values will be returned in place of the actual results:

@(engine {:result (->SomeResolvable)})
;; => {:result #<claro/error "circuit-breaker 'Database' is open."
;;                           {:failure-rate 60.0, :not-permitted-calls 2}>}

You can cause an exception to be thrown in these cases by passing :throw-when-open? in wrap-circuit-breaker's option map.

License

MIT License

Copyright (c) 2017 Yannick Scherer

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
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