gistfile1.txt

    /**
     * <ul>
     *      <li>{@code keyCreator} creates the key which indexes the value to be saved</li>
     *      <li>{@code reader} reads the value stored with the key provided by {@code keyCreator}</li>
     *      <li>{@code creator} is a function that takes the value read by {@code reader} and returns a new value to be stored</li>
     * </ul>
     */
    public <T> void readWriteCompare(Provider<String> keyCreator, Provider<Optional<T>> reader, Function<Optional<T>, T> creator) {
        // This method protects against concurrent writes of the owner list using the read-write-compare technique
        // Starvation is hedged against with a random sleep, but is still possible
        // NOTE: database transactions would be nice :(
        while (true) {
            // read
            Optional<T> existing = reader.get();
            T toWrite = creator.apply(existing);

            // write
            save(keyCreator.get(), toWrite);

            // compare
            Optional<T> observed = reader.get();
            if (!observed.isPresent()) {
                throw new RuntimeException("Just wrote some stuff but can't find any thing");
            }
            boolean equal = toWrite.equals(observed.get());
            if (!equal) {
                try {
                    long delay = randomWriteDelay();
                    log.info("Delaying next write attempt for " + delay + " milliseconds");
                    Thread.sleep(delay);
                } catch (InterruptedException e) {
                    throw Throwables.propagate(e);
                }
            } else {
                break;
            }

        }
    }