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Ordering

On a subscription with message ordering, messages that share an ordering key reach a subscriber in the order they were published.

An ordering key is a string a publish carries, up to 1 KB. A subscription orders deliveries only when it was created with message ordering on, a setting that cannot change afterwards. A subscription create_with_topic creates has it on; one created elsewhere keeps its own setting. The order holds for messages of one key published to one region, so publishers of one key use the same locational endpoint. When a message of a key is redelivered, the messages published after it under that key come again too, acknowledged ones included. With a dead-letter policy on the subscription, the order may not hold.

A publish names its key in one of three places, and the first one present wins. The ordering_key step, or a transform, writes it into the message's settings. The partition-key header is the portable spelling. The policy's default key applies where neither named one. A delivery reports the key it arrived under in the partition-key header.

Example​

Three events of one order, published under its key, arrive in that order. The test runs the same body in process and against the emulator.

examples/pubsub_publishing_ordering.rs
/// The subscription `create_with_topic` creates has message ordering on, so the events of one
/// order arrive in the order they were published.
#[subscriber(
    GooglePubSub::new("ordering-order-events-workers").create_with_topic("ordering-order-events")
)]
async fn track(event: &OrderEvent, ctx: &mut Context<'_>) -> HandlerOutcome {
    let key = ctx
        .headers()
        .get_str(PARTITION_KEY_HEADER)
        .unwrap_or_default();
    tracing::info!(event.order_id, event.step, key, "order event");
    HandlerOutcome::ack()
}