Fast functional circuit-breaker written in Clojure
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src updated to clojure 1.2.0 Aug 31, 2010
README updated readme for more details Aug 31, 2010
project.clj updated to clojure 1.2.0 Aug 31, 2010


Functional Implementation of Michael Nygaard's circuit breaker 
stability pattern using Clojure protocols and data types

A circuit breaker acts as an intemediary between a caller and a callee.
Typically the callee is an interface to an integration point, and
the caller is a client of that integration point.

If the circuit breaker detects a number of failures in the callee it 
'trips' (i.e., transitions to the open state) and prevents further 
calls to the callee for a fixed period (fail fast).

After this period it lets a single call go though and transitions
to the initial state or back to the open state depending on the outcome.

The circuit breaker is parameterized by a policy that decides
* how many failures are needed to trip
* how long before letting the single probe call pass
* what type of Exceptions from the integration point should
be considered errors to the circuit breaker
(e.g., one may want to exclude security related exceptions)


The implementation uses Clojure protocols and records (data type)
to implement a functional state machine.

The current state of a circuitbreaker is held in a clojure atom,
to updating is lock-free. States are immutable (record) objects
that satisfy the CircuitBreakerTransitions protocol. The protocol
functions are pure (transition) functions that map a state to a new state.

This implementation is easy to use from Java.
For an example see src/ and notes below.

Use leiningen 1.3.x.
krukow:~/Projects/clojure/circuitbreaker$ lein compile
Listening for transport dt_socket at address: 8030
Compiling net.higher-order.integration.circuit-breaker.states
Compiling net.higher-order.integration.circuit-breaker.atomic

To compile Java example.
krukow:~/Projects/clojure/circuitbreaker$ javac -cp lib/*:src:classes src/ -d classes/

To run:

krukow:~/Projects/clojure/circuitbreaker$ java -cp lib/*:src:classes C
Invoked with: KARL
Invoked with: 42
#:net.higher-order.integration.circuit-breaker.states.ClosedState{:policy #:net.higher-order.integration.circuit-breaker.states.TransitionPolicy{:max-fail-count 5, :timeout 5000, :is-error #<states$make_transition_policy$fn__91 net.higher_order.integration.circuit_breaker.states$make_transition_policy$fn__91@67c7980c>}, :fail-count 0}
null arg