Ray Java API
============

Basic API
---------

``Ray.init()``
~~~~~~~~~~~~~~

Ray.init should be invoked before any other Ray functions to initialize
the runtime.

``@RayRemote``
~~~~~~~~~~~~~~

The ``@RayRemote`` annotation can be used to decorate static java
methods and classes. The former indicates that a target function is a remote
function, which is valid with the follow requirements.

- It must be a public static method.
- Its parameters and return value cannot be primitive types like int or
  double but can use wrapper classes like Integer or Double.
- The method must be deterministic for task reconstruction to behave correctly.

When the annotation is used for a class, the class becomes an actor class
(an encapsulation of state shared among many remote functions). The
member functions can be invoked using ``Ray.call``. The requirements for
an actor class are as follows.

- It must have a constructor without any parameters.
- Any inner class must be public static.
- It must not have a member field or method decorated using ``public static``,
  as the semantic is undefined with multiple instances of this same class on
  different machines
- An actor method must be decorated using ``public`` but not ``static``
- The other requirements are the same as for remote functions.

``Ray.call``
~~~~~~~~~~~~

Here is a simple example of invoking a remote function using ``Ray.call``.

.. code:: java

    RayObject<R> call(Func func, ...);

Here, ``func`` is the target method and the ``...``'s should be filled in with
the arguments to ``func``. In addition, the following must hold.

-  The return type of ``func`` must be ``R``.
-  Currently at most 6 parameters of ``func`` are allowed.
-  Each parameter must have the same type ``T`` as the corresponding parameter
   to ``func``, or it must have the type ``RayObject<T>`` to indicate a result
   from another ``Ray.call`` invocation.

The returned object is labeled as ``RayObject<R>`` and its value will be
put into the object store on the machine where the function call is executed.

``Ray.put``
~~~~~~~~~~~

You can also invoke ``Ray.put`` to explicitly place an object into the object
store.

.. code:: java

    public static <T> RayObject<T> put(T object);
    public static <T, TM> RayObject<T> put(T obj, TM metadata);

``RayObject<T>.get/getMeta``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. code:: java

    public class RayObject<T> {
        public T get() throws TaskExecutionException;
        public <M> M getMeta() throws TaskExecutionException;
    }

This method blocks the current thread until the requested data is ready and has
been fetched (if necessary) from a remote machine.

``Ray.wait``
~~~~~~~~~~~~

Calling ``Ray.wait`` will block the current thread and wait for specified
ray calls. It returns when at least ``numReturns`` calls are completed,
or the ``timeout`` expires. See multi-value support for ``RayList``.

.. code:: java

    public static WaitResult<T> wait(RayList<T> waitfor, int numReturns, int timeout);
    public static WaitResult<T> wait(RayList<T> waitfor, int numReturns);
    public static WaitResult<T> wait(RayList<T> waitfor);

Multi-value API
---------------

Multi-value Types
~~~~~~~~~~~~~~~~~

Multiple ``RayObject``'s can be placed in a single data
structure as a return value or as a ``Ray.call`` parameter through the
following container types.

``MultipleReturnsX<R0, R1, ...>``
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This consists of multiple heterogeneous values, with types ``R0``,
``R1``,... respectively. Currently this container type is only
supported as the return type of ``Ray.call``. Therefore you cannot use it
as the type of an input parameter.

``RayList<T>``
^^^^^^^^^^^^^^

This is a list of ``RayObject<T>``s, which inherits from ``List<T>`` in Java. It
can be used as the type for both a return value and a parameter value.

``RayMap<L, T>``
^^^^^^^^^^^^^^^^

A map of ``RayObject<T>``s with each indexed using a label with type
``L``, inherited from ``Map<L, T>``. It can be used as the type for both
a return value and a parameter value.

Enable multiple heterogeneous return values
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

At most four multiple heterogeneous return values are supported.
In order to let the runtime know the number of return values, we
supply the method of ``Ray.call_X`` as follows.

.. code:: java

    RayObjects2<R0, R1> call_2(Func func, ...);
    RayObjects3<R0, R1, R2> call_3(Func func, ...);
    RayObjects4<R0, R1, R2, R3> call_4(Func func, ...);

Note ``func`` must match the following requirements.

-  It must have a return value of type ``MultipleReturnsX``, and must be
   invoked using the corresponding ``Ray.call_X``.

Here is an example.

.. code:: java

    public class MultiRExample {
        public static void main(String[] args) {
            Ray.init();
            RayObjects2<Integer, String> refs = Ray.call_2(MultiRExample::sayMultiRet);
            Integer obj1 = refs.r0().get();
            String obj2 = refs.r1().get();
            Assert.assertTrue(obj1.equals(123));
            Assert.assertTrue(obj2.equals("123"));
        }

        @RayRemote
        public static MultipleReturns2<Integer, String> sayMultiRet() {
            return new MultipleReturns2<Integer, String>(123, "123");
        }
    }

Return with ``RayList``
~~~~~~~~~~~~~~~~~~~~~~~

We use ``Ray.call_n`` to do so, which is similar to ``Ray.call`` except it has
an additional parameter ``returnCount`` which specifies the number of return
``RayObject<R>``s in ``RayList<R>``. This is because Ray needs to know the
number of return values before the method is actually executed.

.. code:: java

    RayList<R> call_n(Func func, Integer returnCount, ...);

Here is an example.

.. code:: java

    public class ListRExample {
        public static void main(String[] args) {
            Ray.init();
            RayList<Integer> ns = Ray.call_n(ListRExample::sayList, 10, 10);
            for (int i = 0; i < 10; i++) {
                RayObject<Integer> obj = ns.Get(i);
                Assert.assertTrue(i == obj.get());
            }
        }

        @RayRemote
        public static List<Integer> sayList(Integer count) {
            ArrayList<Integer> rets = new ArrayList<>();
            for (int i = 0; i < count; i++)
                rets.add(i);
            return rets;
        }
    }

Return with ``RayMap``
~~~~~~~~~~~~~~~~~~~~~~

This is similar to ``RayList`` case, except that now each return
``RayObject<R>`` in ``RayMap<L,R>`` has a given label when
``Ray.call_n`` is made.

.. code:: java

    RayMap<L, R> call_n(Func func, Collection<L> returnLabels, ...);

Here is an example.

.. code:: java

    public class MapRExample {
        public static void main(String[] args) {
            Ray.init();
            RayMap<Integer, String> ns = Ray.call_n(MapRExample::sayMap,
                    Arrays.asList(1, 2, 4, 3), "n_futures_");
            for (Entry<Integer, RayObject<String>> ne : ns.EntrySet()) {
                Integer key = ne.getKey();
                RayObject<String> obj = ne.getValue();
                Assert.assertTrue(obj.get().equals("n_futures_" + key));
            }
        }

        @RayRemote(externalIO = true)
        public static Map<Integer, String> sayMap(Collection<Integer> ids,
                                                String prefix) {
            Map<Integer, String> ret = new HashMap<>();
            for (int id : ids) {
                ret.put(id, prefix + id);
            }
            return ret;
        }
    }

Enable ``RayList`` and ``RayMap`` as parameters
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. code:: java

    public class ListTExample {
        public static void main(String[] args) {
            Ray.init();
            RayList<Integer> ints = new RayList<>();
            ints.add(Ray.put(new Integer(1)));
            ints.add(Ray.put(new Integer(1)));
            ints.add(Ray.put(new Integer(1)));
            RayObject<Integer> obj = Ray.call(ListTExample::sayReadRayList，
                                            (List<Integer>)ints);
            Assert.assertTrue(obj.get().equals(3));
        }

        @RayRemote
        public static int sayReadRayList(List<Integer> ints) {
            int sum = 0;
            for (Integer i : ints) {
                sum += i;
            }
            return sum;
        }
    }

Actor Support
-------------

Create Actors
~~~~~~~~~~~~~

A regular class annotated with ``@RayRemote`` is an actor class.

.. code:: java

    @RayRemote
    public class Adder {
      public Adder() {
        sum = 0;
      }

      public Integer add(Integer n) {
        return sum += n;
      }

      private Integer sum;
    }

Whenever you call ``Ray.create()`` method, an actor will be created, and
you get a local ``RayActor`` of that actor as the return value.

.. code:: java

    RayActor<Adder> adder = Ray.create(Adder.class);

Call Actor Methods
~~~~~~~~~~~~~~~~~~

The same ``Ray.call`` or its extended versions (e.g., ``Ray.call_n``) is
applied, except that the first argument becomes ``RayActor``.

.. code:: java

    RayObject<R> Ray.call(Func func, RayActor<Adder> actor, ...);
    RayObject<Integer> result1 = Ray.call(Adder::add, adder, 1);
    RayObject<Integer> result2 = Ray.call(Adder::add, adder, 10);
    result2.get(); // 11