Struct pravega_client::event::EventReader

pub struct EventReader {
    pub id: Reader,
    /* private fields */
}

Read events from Stream.

An event reader fetches data from its assigned segments as a SegmentSlice, where a SegmentSlice represents data from a Pravega Segment. It provides the following APIs.

1. A method to initialize the event reader EventReader#init
2. A method to obtain a SegmentSlice to read events from a Pravega segment. The user can use the SegmentSlice’s iterator API to fetch individual events from a given Segment Slice. EventReader#acquire_segment.
3. A method to release the Segment back at the given offset. EventReader#release_segment_at. This method needs to be invoked only the user does not consume all the events in a SegmentSlice.
4. A method to mark the reader as offline.EventReader#reader_offline. This method ensures the segments owned by this readers are transferred to other readers in the reader group.

Examples

use pravega_client_config::{ClientConfigBuilder, MOCK_CONTROLLER_URI};
use pravega_client::client_factory::ClientFactory;
use pravega_client_shared::{ScopedStream, Scope, Stream};

#[tokio::main]
async fn main() {
   let config = ClientConfigBuilder::default()
        .controller_uri(MOCK_CONTROLLER_URI)
        .build()
        .expect("creating config");
    let client_factory = ClientFactory::new(config);
    let stream = ScopedStream {
        scope: Scope::from("scope".to_string()),
        stream: Stream::from("stream".to_string()),
    };
    // Create a reader group to read data from the Pravega stream.
    let rg = client_factory.create_reader_group("rg".to_string(), stream).await;
    // Create a reader under the reader group. The segments of the stream are assigned among the
    // readers which are part of the reader group.
    let mut reader1 = rg.create_reader("r1".to_string()).await;
    // read all events from a given segment slice.
    if let Some(mut segment_slice) =  reader1.acquire_segment().await.expect("Failed to acquire segment since the reader is offline") {
        while let Some(event) = segment_slice.next() {
            println!("Event read is {:?}", event);
        }
    }
    // read one event from the a given  segment slice and return it back.
    if let Some(mut segment_slice) = reader1.acquire_segment().await.expect("Failed to acquire segment since the reader is offline") {
        if let Some(event) = segment_slice.next() {
            println!("Event read is {:?}", event);
            // release the segment slice back to the reader.
            reader1.release_segment(segment_slice).await;
        }
    }
}

Fields

id: Reader

Implementations

impl EventReader

pub async fn release_segment( &mut self, slice: SegmentSlice ) -> Result<(), EventReaderError>

Release a partially read segment slice back to event reader.

Note: it may return an error indicating that the reader has already been removed. This means that another thread removes this reader from the ReaderGroup probably due to the host of this reader is assumed dead.

pub async fn release_segment_at( &mut self, slice: SegmentSlice, offset: i64 ) -> Result<(), EventReaderError>

Release a segment back to the reader and also indicate the offset up to which the segment slice is consumed.

Note: it may return an error indicating that the reader has already been removed. This means that another thread removes this reader from the ReaderGroup probably due to the host of this reader is assumed dead.

pub async fn reader_offline(&mut self) -> Result<(), EventReaderError>

Mark the reader as offline after calling the reader_offline_internal. Note: it may return an error indicating that the reader has already been removed. This means that another thread removes this reader from the ReaderGroup probably due to the host of this reader is assumed dead.

pub async fn acquire_segment( &mut self ) -> Result<Option<SegmentSlice>, EventReaderError>

This function returns a SegmentSlice from the data received from the SegmentStore(s). Individual events can be read from the data received using SegmentSlice.next().

Invoking this function multiple times ensure multiple SegmentSlices corresponding to different Segments of the stream are received. In-case we receive data for an already acquired SegmentSlice this method waits until SegmentSlice is completely consumed before returning the data.

Note: it may return an error indicating that the reader is not online. This means that another thread removes this reader from the ReaderGroup probably because the host of this reader is assumed dead.

Trait Implementations

impl Drop for EventReader

fn drop(&mut self)

Destructor for reader invoked. This will automatically invoke reader_offline().