Struct arrow_flight::sql::server::PeekableFlightDataStream

pub struct PeekableFlightDataStream { /* private fields */ }

A wrapper around Streaming<FlightData> that allows “peeking” at the message at the front of the stream without consuming it. This is needed because sometimes the first message in the stream will contain a FlightDescriptor in addition to potentially any data, and the dispatch logic must inspect this information.

Example

PeekableFlightDataStream::peek can be used to peek at the first message without discarding it; otherwise, PeekableFlightDataStream can be used as a regular stream. See the following example:

use arrow_array::RecordBatch;
use arrow_flight::decode::FlightRecordBatchStream;
use arrow_flight::FlightDescriptor;
use arrow_flight::error::FlightError;
use arrow_flight::sql::server::PeekableFlightDataStream;
use tonic::{Request, Status};
use futures::TryStreamExt;

#[tokio::main]
async fn main() -> Result<(), Status> {
    let request: Request<PeekableFlightDataStream> = todo!();
    let stream: PeekableFlightDataStream = request.into_inner();

    // The first message contains the flight descriptor and the schema.
    // Read the flight descriptor without discarding the schema:
    let flight_descriptor: FlightDescriptor = stream
        .peek()
        .await
        .cloned()
        .transpose()?
        .and_then(|data| data.flight_descriptor)
        .expect("first message should contain flight descriptor");

    // Pass the stream through a decoder
    let batches: Vec<RecordBatch> = FlightRecordBatchStream::new_from_flight_data(
        request.into_inner().map_err(|e| e.into()),
    )
    .try_collect()
    .await?;
}

Implementations

impl PeekableFlightDataStream

pub fn into_inner(self) -> Streaming<FlightData>

Convert this stream into a Streaming<FlightData>. Any messages observed through Self::peek will be lost after the conversion.

pub fn into_peekable(self) -> Peekable<Streaming<FlightData>>

Convert this stream into a Peekable<Streaming<FlightData>>. Preserves the state of the stream, so that calls to Self::peek and Self::poll_next are the same.

pub async fn peek(&mut self) -> Option<&Result<FlightData, Status>>

Peek at the head of this stream without advancing it.

Trait Implementations

impl Stream for PeekableFlightDataStream

type Item = Result<FlightData, Status>

Values yielded by the stream.

fn poll_next( self: Pin<&mut Self>, cx: &mut Context<'_> ) -> Poll<Option<Self::Item>>

Attempt to pull out the next value of this stream, registering the current task for wakeup if the value is not yet available, and returning None if the stream is exhausted.

fn size_hint(&self) -> (usize, Option<usize>)

Returns the bounds on the remaining length of the stream.