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Startup

A subscription opens once every shard it took has a reader and a starting cursor; a stream the service lacks fails startup unless the descriptor creates it.

Each subscription opens in three steps. First the descriptor is checked before any request: an empty stream name or a shard count below one fails startup with KinesisError::Invalid. Then one DescribeStreamSummary asks the service for the stream. A stream it does not have fails startup with KinesisError::Stream, which names the stream, so a mistyped name stops the service before it reads anything. Last, the subscription lists the stream's shards, takes a lease on each shard it can, and opens a reader with its starting cursor on each one.

The subscription is open when that first pass is done. From that moment every record published to the stream reaches it. A shard another instance holds is left to that instance, and the subscription takes it later if its lease lapses. A failure inside the first pass, such as a lease store that does not answer, reaches the subscription's stream after startup and is logged there.

One broker reads a stream through one subscription. A second subscription of the same broker to a stream it already reads is refused when it opens.

Example​

The stream payouts is the platform team's, and the service never creates it. On a stand without that stream, the service stops at startup.

examples/kinesis_broker_startup.rs
/// Pays out a settled order. The stream `payouts` is the platform team's: the service reads it
/// and never creates it.
#[subscriber(KinesisStream::new("payouts"))]
async fn pay(payout: &Payout) -> HandlerOutcome {
    tracing::info!(payout.order, payout.cents, "payout sent");
    HandlerOutcome::ack()
}

fn service(broker: KinesisBroker) -> RustStream {
    RustStream::new(AppInfo::new("payouts", "0.1.0")).with_broker(broker, |b| {
        b.include(pay);
    })
}