max_outstanding

max_outstanding caps how many deliveries may be unsettled at once; Pub/Sub stops sending at the cap and resumes below it.

A delivery is outstanding from the moment the service receives it until a handler settles it. The limit travels with the streaming pull, and the service holds the stream to it. It applies to one stream, not to the subscription as a whole: two instances of the service each hold their own share. The default is the client library's 1000 messages; a value of zero or less sets no limit. A delivery held by retry_after stays outstanding until it comes back. The client also caps outstanding bytes at 100 MiB, a limit the descriptor leaves at its default.

Example

At most 50 orders are unsettled at once.

Macros

examples/pubsub_subscriptions_flow.rs

/// At most 50 orders are unsettled at once; the service holds the rest back.
#[subscriber(
    GooglePubSub::new("flow-orders-workers")
        .create_with_topic("flow-orders")
        .max_outstanding(50)
)]
async fn index(order: &Order) -> HandlerOutcome {
    tracing::info!(order.id, "order indexed");
    HandlerOutcome::ack()
}

Manual

examples/manual/pubsub_subscriptions_flow.rs

/// At most 50 orders are unsettled at once; the service holds the rest back.
fn orders() -> GooglePubSub {
    GooglePubSub::new("flow-orders-workers")
        .create_with_topic("flow-orders")
        .max_outstanding(50)
}