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/C.java and notes below.

Compiling.
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
Compiling net.higher-order.integration.circuit-breaker.java
Compiling net.higher-order.integration.circuit-breaker.java.AtomicCircuitBreaker


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

To run:

krukow:~/Projects/clojure/circuitbreaker$ java -cp lib/*:src:classes C
Invoked with: KARL
KARL
Invoked with: 42
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
...

krukow:~/Projects/clojure/circuitbreaker$