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.. _flight-rpc:

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

Arrow Flight is an RPC framework for high-performance data services
based on Arrow data, and is built on top of gRPC_ and the :doc:`IPC
format <IPC>`.

Flight is organized around streams of Arrow record batches, being
either downloaded from or uploaded to another service. A set of
metadata methods offers discovery and introspection of streams, as
well as the ability to implement application-specific methods.

Methods and message wire formats are defined by Protobuf_, enabling
interoperability with clients that may support gRPC and Arrow
separately, but not Flight. However, Flight implementations include
further optimizations to avoid overhead in usage of Protobuf (mostly
around avoiding excessive memory copies).

.. _gRPC: https://grpc.io/
.. _Protobuf: https://developers.google.com/protocol-buffers/

RPC Methods and Request Patterns
================================

Flight defines a set of RPC methods for uploading/downloading data,
retrieving metadata about a data stream, listing available data
streams, and for implementing application-specific RPC methods. A
Flight service implements some subset of these methods, while a Flight
client can call any of these methods.

Data streams are identified by descriptors (the ``FlightDescriptor``
message), which are either a path or an arbitrary binary command. For
instance, the descriptor may encode a SQL query, a path to a file on a
distributed file system, or even a pickled Python object; the
application can use this message as it sees fit.

Thus, one Flight client can connect to any service and perform basic
operations. To facilitate this, Flight services are *expected* to
support some common request patterns, described next. Of course,
applications may ignore compatibility and simply treat the Flight RPC
methods as low-level building blocks for their own purposes.

See `Protocol Buffer Definitions`_ for full details on the methods and
messages involved.

Downloading Data
----------------

A client that wishes to download the data would:

.. mermaid:: ./Flight/DoGet.mmd
   :caption: Retrieving data via ``DoGet``.

#. Construct or acquire a ``FlightDescriptor`` for the data set they
   are interested in.

   A client may know what descriptor they want already, or they may
   use methods like ``ListFlights`` to discover them.
#. Call ``GetFlightInfo(FlightDescriptor)`` to get a ``FlightInfo``
   message.

   Flight does not require that data live on the same server as
   metadata. Hence, ``FlightInfo`` contains details on where the data
   is located, so the client can go fetch the data from an appropriate
   server. This is encoded as a series of ``FlightEndpoint`` messages
   inside ``FlightInfo``. Each endpoint represents some location that
   contains a subset of the response data.

   An endpoint contains a list of locations (server addresses) where
   this data can be retrieved from, and a ``Ticket``, an opaque binary
   token that the server will use to identify the data being
   requested.

   If ``FlightInfo.ordered`` is true, this signals there is some order
   between data from different endpoints. Clients should produce the
   same results as if the data returned from each of the endpoints was
   concatenated, in order, from front to back.

   If ``FlightInfo.ordered`` is false, the client may return data
   from any of the endpoints in arbitrary order. Data from any
   specific endpoint must be returned in order, but the data from
   different endpoints may be interleaved to allow parallel fetches.

   Note that since some clients may ignore ``FlightInfo.ordered``, if
   ordering is important and client support cannot be ensured,
   servers should return a single endpoint.

   The response also contains other metadata, like the schema, and
   optionally an estimate of the dataset size.
#. Consume each endpoint returned by the server.

   To consume an endpoint, the client should connect to one of the
   locations in the endpoint, then call ``DoGet(Ticket)`` with the
   ticket in the endpoint. This will give the client a stream of Arrow
   record batches.

   If the server wishes to indicate that the data is on the local
   server and not a different location, then it can return an empty
   list of locations. The client can then reuse the existing
   connection to the original server to fetch data. Otherwise, the
   client must connect to one of the indicated locations.

   The server may list "itself" as a location alongside other server
   locations.  Normally this requires the server to know its public
   address, but it may also use the special URI string
   ``arrow-flight-reuse-connection://?`` to tell clients that they may
   reuse an existing connection to the same server, without having to
   be able to name itself.  See `Connection Reuse`_ below.

   In this way, the locations inside an endpoint can also be thought
   of as performing look-aside load balancing or service discovery
   functions. And the endpoints can represent data that is partitioned
   or otherwise distributed.

   The client must consume all endpoints to retrieve the complete data
   set. The client can consume endpoints in any order, or even in
   parallel, or distribute the endpoints among multiple machines for
   consumption; this is up to the application to implement. The client
   can also use ``FlightInfo.ordered``. See the previous item for
   details of ``FlightInfo.ordered``.

   Each endpoint may have expiration time
   (``FlightEndpoint.expiration_time``). If an endpoint has expiration
   time, the client can get data multiple times by ``DoGet`` until the
   expiration time is reached. Otherwise, it is application-defined
   whether ``DoGet`` requests may be retried. The expiration time is
   represented as `google.protobuf.Timestamp`_.

   If the expiration time is short, the client may be able to extend
   the expiration time by ``RenewFlightEndpoint`` action. The client
   need to use ``DoAction`` with ``RenewFlightEndpoint`` action type
   to extend the expiration time. ``Action.body`` must be
   ``RenewFlightEndpointRequest`` that has ``FlightEndpoint`` to be
   renewed.

   The client may be able to cancel the returned ``FlightInfo`` by
   ``CancelFlightInfo`` action. The client need to use ``DoAction``
   with ``CancelFlightInfo`` action type to cancel the ``FlightInfo``.

.. _google.protobuf.Timestamp: https://protobuf.dev/reference/protobuf/google.protobuf/#timestamp

Downloading Data by Running a Heavy Query
-----------------------------------------

A client may need to request a heavy query to download
data. However, ``GetFlightInfo`` doesn't return until the query
completes, so the client is blocked. In this situation, the client
can use ``PollFlightInfo`` instead of ``GetFlightInfo``:

.. mermaid:: ./Flight/PollFlightInfo.mmd
   :caption: Polling a long-running query by ``PollFlightInfo``.

#. Construct or acquire a ``FlightDescriptor``, as before.
#. Call ``PollFlightInfo(FlightDescriptor)`` to get a ``PollInfo``
   message.

   A server should respond as quickly as possible on the first
   call. So the client shouldn't wait for the first ``PollInfo``
   response.

   If the query isn't finished, ``PollInfo.flight_descriptor`` has a
   ``FlightDescriptor``. The client should use the descriptor (not the
   original ``FlightDescriptor``) to call the next
   ``PollFlightInfo()``.  A server should recognize a
   ``PollInfo.flight_descriptor`` that is not necessarily the latest
   in case the client misses an update in between.

   If the query is finished, ``PollInfo.flight_descriptor`` is
   unset.

   ``PollInfo.info`` is the currently available results so far. It's
   a complete ``FlightInfo`` each time not just the delta between the
   previous and current ``FlightInfo``. A server should only append to
   the endpoints in ``PollInfo.info`` each time. So the client can
   run ``DoGet(Ticket)`` with the ``Ticket`` in the ``PollInfo.info``
   even when the query isn't finished yet. ``FlightInfo.ordered`` is
   also valid.

   A server should not respond until the result would be different
   from last time. That way, the client can "long poll" for updates
   without constantly making requests. Clients can set a short timeout
   to avoid blocking calls if desired.

   ``PollInfo.progress`` may be set. It represents progress of the
   query. If it's set, the value must be in ``[0.0, 1.0]``. The value
   is not necessarily monotonic or nondecreasing. A server may respond by
   only updating the ``PollInfo.progress`` value, though it shouldn't
   spam the client with updates.

   ``PollInfo.timestamp`` is the expiration time for this
   request. After this passes, a server might not accept the poll
   descriptor anymore and the query may be cancelled. This may be
   updated on a call to ``PollFlightInfo``. The expiration time is
   represented as `google.protobuf.Timestamp`_.

   A client may be able to cancel the query by the
   ``CancelFlightInfo`` action.

   A server should return an error status instead of a response if the
   query fails. The client should not poll the request except for
   ``TIMED_OUT`` and ``UNAVAILABLE``, which may not originate from the
   server.
#. Consume each endpoint returned by the server, as before.

Uploading Data
--------------

To upload data, a client would:

.. mermaid:: ./Flight/DoPut.mmd
   :caption: Uploading data via ``DoPut``.

#. Construct or acquire a ``FlightDescriptor``, as before.
#. Call ``DoPut(FlightData)`` and upload a stream of Arrow record
   batches.

   The ``FlightDescriptor`` is included with the first message so the
   server can identify the dataset.

``DoPut`` allows the server to send response messages back to the
client with custom metadata. This can be used to implement things like
resumable writes (e.g. the server can periodically send a message
indicating how many rows have been committed so far).

Exchanging Data
---------------

Some use cases may require uploading and downloading data within a
single call. While this can be emulated with multiple calls, this may
be difficult if the application is stateful. For instance, the
application may wish to implement a call where the client uploads data
and the server responds with a transformation of that data; this would
require being stateful if implemented using ``DoGet`` and
``DoPut``. Instead, ``DoExchange`` allows this to be implemented as a
single call. A client would:

.. mermaid:: ./Flight/DoExchange.mmd
   :caption: Complex data flow with ``DoExchange``.

#. Construct or acquire a ``FlightDescriptor``, as before.
#. Call ``DoExchange(FlightData)``.

   The ``FlightDescriptor`` is included with the first message, as
   with ``DoPut``. At this point, both the client and the server may
   simultaneously stream data to the other side.

Authentication
==============

Flight supports a variety of authentication methods that applications
can customize for their needs.

"Handshake" authentication
  This is implemented in two parts. At connection time, the client
  calls the ``Handshake`` RPC method, and the application-defined
  authentication handler can exchange any number of messages with its
  counterpart on the server. The handler then provides a binary
  token. The Flight client will then include this token in the headers
  of all future calls, which is validated by the server authentication
  handler.

  Applications may use any part of this; for instance, they may ignore
  the initial handshake and send an externally acquired token (e.g. a
  bearer token) on each call, or they may establish trust during the
  handshake and not validate a token for each call, treating the
  connection as stateful (a "login" pattern).

  .. warning:: Unless a token is validated on every call, this pattern
               is not secure, especially in the presence of a layer
               7 load balancer, as is common with gRPC, or if gRPC
               transparently reconnects the client.

Header-based/middleware-based authentication
  Clients may include custom headers with calls. Custom middleware can
  then be implemented to validate and accept/reject calls on the
  server side.

`Mutual TLS (mTLS)`_
  The client provides a certificate during connection establishment
  which is verified by the server. The application does not need to
  implement any authentication code, but must provision and distribute
  certificates.

  This may only be available in certain implementations, and is only
  available when TLS is also enabled.

Some Flight implementations may expose the underlying gRPC API as
well, in which case any `authentication method supported by gRPC
<https://grpc.io/docs/guides/auth/>`_ is available.

.. _Mutual TLS (mTLS): https://grpc.io/docs/guides/auth/#supported-auth-mechanisms

.. _flight-location-uris:

Location URIs
=============

Flight is primarily defined in terms of its Protobuf and gRPC
specification below, but Arrow implementations may also support
alternative transports (see :ref:`status-flight-rpc`).  Clients and
servers need to know which transport to use for a given URI in a
Location, so Flight implementations should use the following URI
schemes for the given transports:

+----------------------------+--------------------------------+
| Transport                  | URI Scheme                     |
+============================+================================+
| gRPC (plaintext)           | grpc: or grpc+tcp:             |
+----------------------------+--------------------------------+
| gRPC (TLS)                 | grpc+tls:                      |
+----------------------------+--------------------------------+
| gRPC (Unix domain socket)  | grpc+unix:                     |
+----------------------------+--------------------------------+
| (reuse connection)         | arrow-flight-reuse-connection: |
+----------------------------+--------------------------------+
| HTTP (1)                   | http: or https:                |
+----------------------------+--------------------------------+

Notes:

* \(1) See :ref:`flight-extended-uris` for semantics when using
   http/https as the transport. It should be accessible via a GET request.

Connection Reuse
----------------

"Reuse connection" above is not a particular transport.  Instead, it
means that the client may try to execute DoGet against the same server
(and through the same connection) that it originally obtained the
FlightInfo from (i.e., that it called GetFlightInfo against).  This is
interpreted the same way as when no specific ``Location`` are
returned.

This allows the server to return "itself" as one possible location to
fetch data without having to know its own public address, which can be
useful in deployments where knowing this would be difficult or
impossible.  For example, a developer may forward a remote service in
a cloud environment to their local machine; in this case, the remote
service would have no way to know the local hostname and port that it
is being accessed over.

For compatibility reasons, the URI should always be
``arrow-flight-reuse-connection://?``, with the trailing empty query
string.  Java's URI implementation does not accept ``scheme:`` or
``scheme://``, and C++'s implementation does not accept an empty
string, so the obvious candidates are not compatible.  The chosen
representation can be parsed by both implementations, as well as Go's
``net/url`` and Python's ``urllib.parse``.

.. _flight-extended-uris:

Extended Location URIs
----------------------

In addition to alternative transports, a server may also return
URIs that reference an external service or object storage location.
This can be useful in cases where intermediate data is cached as
Apache Parquet files on cloud storage or is otherwise accessible
via an HTTP service. In these scenarios, it is more efficient to be
able to provide a URI where the client may simply download the data
directly, rather than requiring a Flight service to read it back into
memory and serve it from a ``DoGet`` request.

To avoid the complexities of Flight clients having to implement support
for multiple different cloud storage vendors (e.g. AWS S3, Google Cloud),
we extend the URIs to only allow an HTTP/HTTPS URI where the client can
perform a simple GET request to download the data. Authentication can be
handled either by negotiating externally to the Flight protocol or by the
server sending a presigned URL that the client can make a GET request to.
This should be supported by all current major cloud storage vendors, meaning
only the server needs to know the semantics of the underlying object store APIs.

When using an extended location URI, the client should ignore any
value in the ``Ticket`` field of the ``FlightEndpoint``. The
``Ticket`` is only used for identifying data in the context of a
Flight service, and is not needed when the client is directly
downloading data from an external service.

Clients should assume that, unless otherwise specified, the data is
being returned using the :ref:`format-ipc` just as it would
via a ``DoGet`` call. If the returned ``Content-Type`` header is a generic
media type such as ``application/octet-stream``, the client should still assume
it is an Arrow IPC stream. For other media types, such as Apache Parquet,
the server should use the appropriate IANA Media Type that a client
would recognize.

Finally, the server may also allow the client to choose what format the
data is returned in by respecting the ``Accept`` header in the request.
If multiple formats are requested and supported, the choice of which to
use is server-specific. If none of the requested content-types are
supported, the server may respond with either 406 (Not Acceptable),
415 (Unsupported Media Type), or successfully respond with a different
format that it does support, along with the correct ``Content-Type``
header.

Error Handling
==============

Arrow Flight defines its own set of error codes. The implementation
differs between languages (e.g. in C++, Unimplemented is a general
Arrow error status while it's a Flight-specific exception in Java),
but the following set is exposed:

+----------------+-------------------------------------------+
|Error Code      |Description                                |
+================+===========================================+
|UNKNOWN         |An unknown error. The default if no other  |
|                |error applies.                             |
+----------------+-------------------------------------------+
|INTERNAL        |An error internal to the service           |
|                |implementation occurred.                   |
+----------------+-------------------------------------------+
|INVALID_ARGUMENT|The client passed an invalid argument to   |
|                |the RPC.                                   |
+----------------+-------------------------------------------+
|TIMED_OUT       |The operation exceeded a timeout or        |
|                |deadline.                                  |
+----------------+-------------------------------------------+
|NOT_FOUND       |The requested resource (action, data       |
|                |stream) was not found.                     |
+----------------+-------------------------------------------+
|ALREADY_EXISTS  |The resource already exists.               |
+----------------+-------------------------------------------+
|CANCELLED       |The operation was cancelled (either by the |
|                |client or the server).                     |
+----------------+-------------------------------------------+
|UNAUTHENTICATED |The client is not authenticated.           |
+----------------+-------------------------------------------+
|UNAUTHORIZED    |The client is authenticated, but does not  |
|                |have permissions for the requested         |
|                |operation.                                 |
+----------------+-------------------------------------------+
|UNIMPLEMENTED   |The RPC is not implemented.                |
+----------------+-------------------------------------------+
|UNAVAILABLE     |The server is not available. May be emitted|
|                |by the client for connectivity reasons.    |
+----------------+-------------------------------------------+

External Resources
==================

- https://arrow.apache.org/blog/2019/10/13/introducing-arrow-flight/
- https://arrow.apache.org/blog/2018/10/09/0.11.0-release/
- https://www.slideshare.net/JacquesNadeau5/apache-arrow-flight-overview

Protocol Buffer Definitions
===========================

.. literalinclude:: ../../../format/Flight.proto
   :language: protobuf
   :linenos: