Arrow Flight RPC¶
Arrow Flight is an RPC framework for efficient transfer of Flight data over the network.
See also
- Flight protocol documentation
Documentation of the Flight protocol, including how to use Flight conceptually.
- Flight API documentation
C++ API documentation listing all of the various client and server types.
- C++ Cookbook
Recipes for using Arrow Flight in C++.
Writing a Flight Service¶
Servers are subclasses of arrow::flight::FlightServerBase
. To
implement individual RPCs, override the RPC methods on this class.
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
arrow::flight::ServerCallContext
for common parameters and
returns a arrow::Status
to indicate success or
failure. Flight-specific error codes can be returned via
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, arrow::flight::SimpleFlightListing
uses a vector of
arrow::flight::FlightInfo
objects as the result of a
ListFlights
RPC.
To start a server, create a arrow::flight::Location
to
specify where to listen, and call
arrow::flight::FlightServerBase::Init()
. This will start the
server, but won’t block the rest of the program. Use
arrow::flight::FlightServerBase::SetShutdownOnSignals()
to
enable stopping the server if an interrupt signal is received, then
call arrow::flight::FlightServerBase::Serve()
to block until the
server stops.
std::unique_ptr<arrow::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());
Using the Flight Client¶
To connect to a Flight service, create an instance of
arrow::flight::FlightClient
by calling Connect
.
Each RPC method returns 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.
Cancellation and Timeouts¶
When making a call, clients can optionally provide
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
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.
Enabling TLS¶
TLS can be enabled when setting up a server by providing a certificate
and key pair to FlightServerBase::Init
.
On the client side, use Location::ForGrpcTls
to construct the
arrow::flight::Location
to listen on.
Enabling Authentication¶
Warning
Authentication is insecure without enabling TLS.
Handshake-based authentication can be enabled by implementing
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
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
arrow::flight::ServerCallContext
.
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 subclassing ServerMiddleware
and 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.
Alternative Transports¶
The standard transport for Arrow Flight is gRPC. The C++
implementation also experimentally supports a transport based on
UCX. To use it, use the protocol scheme ucx:
when starting a
server or creating a client.
UCX Transport¶
Not all features of the gRPC transport are supported. See Flight RPC for details. Also note these specific caveats:
The server creates an independent UCP worker for each client. This consumes more resources but provides better throughput.
The client creates an independent UCP worker for each RPC call. Again, this trades off resource consumption for performance. This also means that unlike with gRPC, it is essentially equivalent to make all calls with a single client or with multiple clients.
The UCX transport attempts to avoid copies where possible. In some cases, it can directly reuse UCX-allocated buffers to back
arrow::Buffer
objects, however, this will also extend the lifetime of associated UCX resources beyond the lifetime of the Flight client or server object.Depending on the transport that UCX itself selects, you may find that increasing
UCX_MM_SEG_SIZE
from the default (around 8KB) to around 60KB improves performance (UCX will copy more data in a single call).