An implementation of the circuit breaker pattern. Based on Netflix/Hystrix, implemented in pure Clojure.
The core concept of Short is the circuit. A circuit is a wrapper around a dependency (a single API, endpoint or query) that could potentially be flaky or unresponsive. The circuit tracks all calls to that API, and if enough calls fail (or timeout), it "breaks", no longer allowing subsequent calls through for a certain amount of time.
Technically, you can call any function through any circuit, though usually you'd create a circuit-per-dependency or similar. You DO NOT need to create a new circuit for each call; it's a gateway, not a command. In fact, doing this would lose any of the circuit state information from the previous call.
(def my-circuit (-> (circuit)
(with-strategy concurrency-limit 10)
(with-strategy consecutive-failures 5))
A circuit-> macro is provided to make this easier:
(def my-circuit (circuit-> (concurrency-limit 10)
(consecutive-failures 5)))
Strategies are order-dependent and wrap each other, like Ring middleware. The last added strategy will get the call/request first, then pass it up the chain; the return value (or exception) will flow back down the chain in the opposite order.
caching
Takes a clojure.core.cache/CacheProtocol implementation, and caches successful
responses. Can be configured to return a cached response on any request, or
only on failure.
concurrency-limit
Caps the maximum number of simultaneous calls that can be made to a dependency
to avoid overloading it if it hangs.
consecutive-failures
Breaks the circuit if N calls in a row fail.
fast-fail
If the circuit is open (broken), fail immediately with an exception rather than
making a call against the dependency.
reclose-ttl
Re-close the circuit after a certain amount of time has passed.
retry
Retry on failure up to a certain number of times, with a configurable retry
interval.
throttle
Limit the number of calls to the dependency that will be allowed through in a
certain amount of time. Any further calls will fail with an exception.
timeout
Add a configurable timeout (in milliseconds) to a circuit.