docs/source/flight.rst

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================
Arrow Flight RPC
================

Arrow Flight is an RPC framework for efficient transfer of Arrow data
over the network.

.. seealso::

   :external+arrow:doc:`Flight protocol documentation <format/Flight>`
        Documentation of the Flight protocol, including how to use
        Flight conceptually.

   :external+cookbook:doc:`Java Cookbook <flight>`
        Recipes for using Arrow Flight in Java.

Writing a Flight Service
========================

Flight servers implement the `FlightProducer`_ interface. For convenience,
they can subclass `NoOpFlightProducer`_ instead, which offers default
implementations of all the RPC methods.

.. code-block:: Java

    public class TutorialFlightProducer implements FlightProducer {
        @Override
        // Override methods or use NoOpFlightProducer for only methods needed
    }

Each RPC method always takes a ``CallContext`` for common parameters. To indicate
failure, pass an exception to the "listener" if present, or else raise an
exception.

.. code-block:: Java

    // Server
    @Override
    public void listFlights(CallContext context, Criteria criteria, StreamListener<FlightInfo> listener) {
        // ...
        listener.onError(
            CallStatus.UNAUTHENTICATED.withDescription(
                "Custom UNAUTHENTICATED description message.").toRuntimeException());
        // ...
    }

    // Client
    try{
        Iterable<FlightInfo> flightInfosBefore = flightClient.listFlights(Criteria.ALL);
        // ...
    } catch (FlightRuntimeException e){
        // Catch UNAUTHENTICATED exception
    }

To start a server, create a `Location`_ to specify where to listen, and then create
a `FlightServer`_ with an instance of a producer. This will start the server, but
won't block the rest of the program. Call ``FlightServer.awaitTermination``
to block until the server stops.

.. code-block:: Java

    class TutorialFlightProducer implements FlightProducer {
        @Override
        // Override methods or use NoOpFlightProducer for only methods needed
    }

    Location location = Location.forGrpcInsecure("0.0.0.0", 0);
    try(
        BufferAllocator allocator = new RootAllocator();
        FlightServer server = FlightServer.builder(
                allocator,
                location,
                new TutorialFlightProducer()
        ).build();
    ){
        server.start();
        System.out.println("Server listening on port " + server.getPort());
        server.awaitTermination();
    } catch (Exception e) {
        e.printStackTrace();
    }

.. code-block:: shell

    Server listening on port 58104

Using the Flight Client
=======================

To connect to a Flight service, create a `FlightClient`_ with a location.

.. code-block:: Java

    Location location = Location.forGrpcInsecure("0.0.0.0", 58104);

    try(BufferAllocator allocator = new RootAllocator();
        FlightClient client = FlightClient.builder(allocator, location).build()){
        // ... Consume operations exposed by Flight server
    } catch (Exception e) {
        e.printStackTrace();
    }

Cancellation and Timeouts
=========================

When making a call, clients can optionally provide ``CallOptions``. This allows
clients to set a timeout on calls. Also, some objects returned by client RPC calls
expose a cancel method which allows terminating a call early.

.. code-block:: Java

    Location location = Location.forGrpcInsecure("0.0.0.0", 58609);

    try(BufferAllocator allocator = new RootAllocator();
        FlightClient tutorialFlightClient = FlightClient.builder(allocator, location).build()){

        Iterator<Result> resultIterator = tutorialFlightClient.doAction(
                new Action("test-timeout"),
                CallOptions.timeout(2, TimeUnit.SECONDS)
        );
    } catch (Exception e) {
        e.printStackTrace();
    }

On the server side, timeouts are transparent. For cancellation, the server needs to manually poll
``setOnCancelHandler`` or ``isCancelled`` to check if the client has cancelled the call,
and if so, break out of any processing the server is currently doing.

.. code-block:: Java

    // Client
    Location location = Location.forGrpcInsecure("0.0.0.0", 58609);
    try(BufferAllocator allocator = new RootAllocator();
        FlightClient tutorialFlightClient = FlightClient.builder(allocator, location).build()){
        try(FlightStream flightStream = flightClient.getStream(new Ticket(new byte[]{}))) {
            // ...
            flightStream.cancel("tutorial-cancel", new Exception("Testing cancellation option!"));
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
    // Server
    @Override
    public void getStream(CallContext context, Ticket ticket, ServerStreamListener listener) {
        // ...
        listener.setOnCancelHandler(()->{
                    // Implement logic to handle cancellation option
                });
    }

Enabling TLS
============

TLS can be enabled when setting up a server by providing a
certificate and key pair to ``FlightServer.Builder.useTls``.

On the client side, use ``Location.forGrpcTls`` to create the Location for the client.

Enabling Authentication
=======================

.. warning:: Authentication is insecure without enabling TLS.

Handshake-based authentication can be enabled by implementing
``ServerAuthHandler``. Authentication consists of two parts: on
initial client connection, the server and client authentication
implementations can perform any negotiation needed. The client authentication
handler then provides a token that will be attached to future calls.

The client send data to be validated through ``ClientAuthHandler.authenticate``
The server validate data received through ``ServerAuthHandler.authenticate``.

Custom Middleware
=================

Servers and clients support custom middleware (or interceptors) that are called on every
request and can modify the request in a limited fashion. These can be implemented by implementing the
``FlightServerMiddleware`` and ``FlightClientMiddleware`` interfaces.

Middleware are fairly limited, but they can add headers to a
request/response. On the server, they can inspect incoming headers and
fail the request; hence, they can be used to implement custom
authentication methods.

Adding Services
===============

Servers can add other gRPC services. For example, to add the `Health Check service <https://github.com/grpc/grpc/blob/master/doc/health-checking.md>`_:

.. code-block:: Java

    final HealthStatusManager statusManager = new HealthStatusManager();
    final Consumer<NettyServerBuilder> consumer = (builder) -> {
      builder.addService(statusManager.getHealthService());
    };
    final Location location = forGrpcInsecure(LOCALHOST, 5555);
    try (
        BufferAllocator a = new RootAllocator(Long.MAX_VALUE);
        Producer producer = new Producer(a);
        FlightServer s = FlightServer.builder(a, location, producer)
            .transportHint("grpc.builderConsumer", consumer).build().start();
    ) {
      Channel channel = NettyChannelBuilder.forAddress(location.toSocketAddress()).usePlaintext().build();
      HealthCheckResponse response = HealthGrpc
              .newBlockingStub(channel)
              .check(HealthCheckRequest.getDefaultInstance());

      System.out.println(response.getStatus());
    }


:external+arrow:ref:`Flight best practices <flight-best-practices>`
===================================================================

See the :external+arrow:ref:`best practices for C++ <flight-best-practices>`.


.. _`FlightClient`: https://arrow.apache.org/docs/java/reference/org/apache/arrow/flight/FlightClient.html
.. _`FlightProducer`: https://arrow.apache.org/docs/java/reference/org/apache/arrow/flight/FlightProducer.html
.. _`FlightServer`: https://arrow.apache.org/docs/java/reference/org/apache/arrow/flight/FlightServer.html
.. _`NoOpFlightProducer`: https://arrow.apache.org/docs/java/reference/org/apache/arrow/flight/NoOpFlightProducer.html
.. _`Location`: https://arrow.apache.org/docs/java/reference/org/apache/arrow/flight/Location.html