Arrow Flight RPC#

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

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).

Tabular Datasets

Debugging code using Arrow