docs/source/cpp/flight.rst
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.. default-domain:: cpp .. highlight:: cpp
Arrow Flight is an RPC framework for efficient transfer of Flight data over the network.
.. seealso::
:doc:Flight protocol documentation <../format/Flight>
Documentation of the Flight protocol, including how to use
Flight conceptually.
:doc:Flight API documentation <./api/flight>
C++ API documentation listing all of the various client and
server types.
C++ Cookbook <https://arrow.apache.org/cookbook/cpp/flight.html>_
Recipes for using Arrow Flight in C++.
Servers are subclasses of :class:arrow::flight::FlightServerBase. To
implement individual RPCs, override the RPC methods on this class.
.. code-block:: cpp
class MyFlightServer : public FlightServerBase { Status ListFlights(const ServerCallContext& context, const Criteria* criteria, std::unique_ptr<FlightListing>* listings) override { std::vector<FlightInfo> flights = ...; *listings = std::unique_ptr<FlightListing>(new SimpleFlightListing(flights)); return Status::OK(); } };
Each RPC method always takes a
:class:arrow::flight::ServerCallContext for common parameters and
returns a :class:arrow::Status to indicate success or
failure. Flight-specific error codes can be returned via
:func:arrow::flight::MakeFlightError.
RPC methods that return a value in addition to a status will use an
out parameter, as shown above. Often, there are helper classes
providing basic implementations of these out parameters. For instance,
above, :class:arrow::flight::SimpleFlightListing uses a vector of
:class:arrow::flight::FlightInfo objects as the result of a
ListFlights RPC.
To start a server, create a :class:arrow::flight::Location to
specify where to listen, and call
:func:arrow::flight::FlightServerBase::Init. This will start the
server, but won't block the rest of the program. Use
:func:arrow::flight::FlightServerBase::SetShutdownOnSignals to
enable stopping the server if an interrupt signal is received, then
call :func:arrow::flight::FlightServerBase::Serve to block until the
server stops.
.. code-block:: cpp
std::unique_ptrarrow::flight::FlightServerBase server; // Initialize server arrow::flight::Location location; // Listen to all interfaces on a free port ARROW_CHECK_OK(arrow::flight::Location::ForGrpcTcp("0.0.0.0", 0, &location)); arrow::flight::FlightServerOptions options(location);
// Start the server ARROW_CHECK_OK(server->Init(options)); // Exit with a clean error code (0) on SIGTERM ARROW_CHECK_OK(server->SetShutdownOnSignals({SIGTERM}));
std::cout << "Server listening on localhost:" << server->port() << std::endl; ARROW_CHECK_OK(server->Serve());
To connect to a Flight service, create an instance of
:class:arrow::flight::FlightClient by calling :func:Connect <arrow::flight::FlightClient::Connect>.
Each RPC method returns :class:arrow::Result to indicate the
success/failure of the request, and the result object if the request
succeeded. Some calls are streaming calls, so they will return a
reader and/or a writer object; the final call status isn't known until
the stream is completed.
When making a call, clients can optionally provide
:class:FlightCallOptions <arrow::flight::FlightCallOptions>. This
allows clients to set a timeout on calls or provide custom HTTP
headers, among other features. Also, some objects returned by client
RPC calls expose a Cancel method which allows terminating a call
early.
On the server side, no additional code is needed to implement
timeouts. For cancellation, the server needs to manually poll
:func:ServerCallContext::is_cancelled <arrow::flight::ServerCallContext::is_cancelled> to check if the
client has cancelled the call, and if so, break out of any processing
the server is currently doing.
TLS can be enabled when setting up a server by providing a certificate
and key pair to :func:FlightServerBase::Init <arrow::flight::FlightServerBase::Init>.
On the client side, use :func:Location::ForGrpcTls <arrow::flight::Location::ForGrpcTls> to construct the
:class:arrow::flight::Location to listen on.
.. warning:: Authentication is insecure without enabling TLS.
Handshake-based authentication can be enabled by implementing
:class:ServerAuthHandler <arrow::flight::ServerAuthHandler> and
providing this to the server during construction.
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. This is done by calling
:func:Authenticate <arrow::flight::FlightClient::Authenticate> with
the desired client authentication implementation.
On each RPC thereafter, the client handler's token is automatically
added to the call in the request headers. The server authentication
handler validates the token and provides the identity of the
client. On the server, this identity can be obtained from the
:class:arrow::flight::ServerCallContext.
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 subclassing :class:ServerMiddleware <arrow::flight::ServerMiddleware> and :class:ClientMiddleware <arrow::flight::ClientMiddleware>, then providing them when creating
the client or server.
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.
.. _flight-best-practices:
When using the default gRPC transport, options can be passed to it via
:member:arrow::flight::FlightClientOptions::generic_options. For example:
.. tab-set::
.. tab-item:: C++
.. code-block:: cpp
auto options = FlightClientOptions::Defaults();
// Set the period after which a keepalive ping is sent on transport.
options.generic_options.emplace_back(GRPC_ARG_KEEPALIVE_TIME_MS, 60000);
.. tab-item:: Python
.. code-block:: python
# Set the period after which a keepalive ping is sent on transport.
generic_options = [("GRPC_ARG_KEEPALIVE_TIME_MS", 60000)]
client = pyarrow.flight.FlightClient(server_uri, generic_options=generic_options)
Also see best gRPC practices_ and available gRPC keys_.
Creating and closing clients requires setup and teardown on the client and server side which can take away from actually handling RPCs. Reuse clients whenever possible to avoid this. Note that clients are thread-safe, so a single client can be shared across multiple threads.
Round robin load balancing_ means every client can have an open connection to
every server, causing an unexpected number of open connections and depleting
server resources.
When facing unexpected disconnects on long running connections use netstat to monitor the number of open connections. If number of connections is much greater than the number of clients it might cause issues.
For debugging, certain environment variables enable logging in gRPC. For
example, env GRPC_VERBOSITY=info GRPC_TRACE=http will print the initial
headers (on both sides) so you can see if gRPC established the connection or
not. It will also print when a message is sent, so you can tell if the
connection is open or not.
gRPC may not report connection errors until a call is actually made. Hence, to detect connection errors when creating a client, some sort of dummy RPC should be made.
Flight tries to reuse allocations made by gRPC to avoid redundant
data copies. However, experience shows that such data is frequently
misaligned. Some use cases might require data to have data type-specific
alignment (for example, for the data buffer of an Int32 array to be aligned
on a 4-byte boundary), which can be enforced
by setting :member:arrow::ipc::IpcReadOptions::ensure_alignment
to :member:arrow::ipc::Alignment::kDataTypeSpecificAlignment.
This uses the :member:arrow::ipc::IpcReadOptions::memory_pool
to allocate memory with aligned addresses, but only for mis-aligned data.
However, this creates data copies of your data received via Flight.
Unless gRPC data are copied as described above, allocations made by gRPC may not be tracked by the Arrow memory pool, and that memory usage behavior, such as whether free memory is returned to the system, is dependent on the allocator that gRPC uses (usually the system allocator).
A quick way of testing: attach to the process with a debugger and call
malloc_trim, or call :func:ReleaseUnused <arrow::MemoryPool::ReleaseUnused>
on the system pool. If memory usage drops, then likely, there is memory
allocated by gRPC or by the application that the system allocator was holding
on to. This can be adjusted in platform-specific ways; see an investigation
in ARROW-16697_ for an example of how this works on Linux/glibc. glibc malloc
can be explicitly told to dump caches.
gRPC will spawn up to max threads quota of threads for concurrent clients. Those threads are not necessarily cleaned up (a "cached thread pool" in Java parlance). glibc malloc clears some per thread state and the default tuning never clears caches in some workloads.
gRPC's default behaviour allows one server to accept many connections from many different clients, but if requests do a lot of work (as they may under Flight), the server may not be able to keep up. Configuring clients to retry with backoff (and potentially connect to a different node), would give more consistent quality of service.
.. tab-set::
.. tab-item:: C++
.. code-block:: cpp
auto options = FlightClientOptions::Defaults();
// Set the minimum time between subsequent connection attempts.
options.generic_options.emplace_back(GRPC_ARG_MIN_RECONNECT_BACKOFF_MS, 2000);
.. tab-item:: Python
.. code-block:: python
# Set the minimum time between subsequent connection attempts.
generic_options = [("GRPC_ARG_MIN_RECONNECT_BACKOFF_MS", 2000)]
client = pyarrow.flight.FlightClient(server_uri, generic_options=generic_options)
You may wish to limit the maximum batch size a client can submit to a server through
DoPut, to prevent a request from taking up too much memory on the server. On
the client-side, set :member:arrow::flight::FlightClientOptions::write_size_limit_bytes.
On the server-side, set the gRPC option GRPC_ARG_MAX_RECEIVE_MESSAGE_LENGTH.
The client-side option will return an error that can be retried with smaller batches,
while the server-side limit will close out the connection. Setting both can be wise, since
the former provides a better user experience but the latter may be necessary to defend
against impolite clients.
A stale call can be closed using
:member:arrow::flight::FlightCallOptions::stop_token. This requires recording the
stop token at call establishment time.
.. tab-set::
.. tab-item:: C++
.. code-block:: cpp
StopSource stop_source;
FlightCallOptions options;
options.stop_token = stop_source.token();
stop_source.RequestStop(Status::Cancelled("StopSource"));
flight_client->DoAction(options, {});
Use call timeouts. (This is a general gRPC best practice.)
.. tab-set::
.. tab-item:: C++
.. code-block:: cpp
FlightCallOptions options;
options.timeout = TimeoutDuration{0.2};
Status status = client->GetFlightInfo(options, FlightDescriptor{}).status();
.. tab-item:: Java
.. code-block:: java
Iterator<Result> results = client.doAction(new Action("hang"), CallOptions.timeout(0.2, TimeUnit.SECONDS));
.. tab-item:: Python
.. code-block:: python
options = pyarrow.flight.FlightCallOptions(timeout=0.2)
result = client.do_action(action, options=options)
Client timeouts are not great for long-running streaming calls, where it may be hard to choose a timeout for the entire operation. Instead, what is often desired is a per-read or per-write timeout so that the operation fails if it isn't making progress. This can be implemented with a background thread that calls Cancel() on a timer, with the main thread resetting the timer every time an operation completes successfully. For a fully-worked out example, see the Cookbook.
.. note:: There is a long standing ticket for a per-write/per-read timeout instead of a per call timeout (ARROW-6062_), but this is not (easily) possible to implement with the blocking gRPC API.
.. _best gRPC practices: https://grpc.io/docs/guides/performance/#general .. _gRPC keys: https://grpc.github.io/grpc/cpp/group__grpc__arg__keys.html .. _Round robin load balancing: https://github.com/grpc/grpc/blob/master/doc/load-balancing.md#round_robin .. _ARROW-15764: https://issues.apache.org/jira/browse/ARROW-15764 .. _ARROW-16697: https://issues.apache.org/jira/browse/ARROW-16697 .. _ARROW-6062: https://issues.apache.org/jira/browse/ARROW-6062 .. _gRPC: https://grpc.io/