Struct Chained 

pub struct Chained<S, K>
where
    S: Source,
    K: Sink,
{
    pub source: S,
    pub sink: K,
}

A sink CHAINED into a source’s task: the source’s batches go straight to the sink on the same thread — no edge, no hop, no second thread to wake (the scoping note’s chaining, for the stage that needs no reshuffle). At a barrier the sink makes its output durable FIRST, then the source records its position; a sink that cannot (a failed flush) leaves the epoch without a position, so the commit that follows commits nothing newer. The task reports one Snapshot per epoch and no Ack: the ack is implied by the order.

Fields

source: S

sink: K

Trait Implementations

impl<S, K> Source for Chained<S, K>

where S: Source, K: Sink,

fn poll(&mut self, out: &mut Out) -> Poll

fn on_barrier(&mut self, epoch: u64) -> Vec<u8>

A barrier for epoch: record the position the epoch covers; the bytes are the source’s own record of it, opaque to the runtime (a single-partition source writes it with position_record, the shape the checkpoint reads back).

fn position(&mut self) -> Vec<u8>

The position now, in the record on_barrier writes — read once when the source runs dry, so every checkpoint taken after it still names where it ended (a restore then resumes it there, past everything it emitted, instead of replaying it from the start). An unbounded source never runs dry: the default records nothing.

fn on_commit(&mut self, epoch: u64)

Every task reported epoch: the position recorded at its barrier may be committed.

fn on_stop(&mut self)

The task is ending (the source ran dry, the coordinator stopped it, or it failed): the last chance to flush anything chained into this task.