dead_letter

max_attempts(n) with dead_letter(..) moves a row whose attempts are spent into another group, or into a table with the same columns.

The two steps come together at the mount site, and a mount that declares one without the other stops the service at startup. The cap reads the row's #[field(attempt)] column, which counts the deliveries, the first one included; a cap on a table without it stops startup too. A delivery that fails its n-th attempt moves the row instead of returning it: a table with a group column moves it to the dead_letter group, a table without one into the dead_letter table. A dead-letter table has the same columns as the queue's; on MySQL and MariaDB the move is two statements in one transaction. With max_attempts(1) every failure moves the row at once. A delayed retry obeys the cap too. The dead-letter group is an ordinary queue of the table: an InboxQueue of that name reads it, or an operator reviews it in place. A subscription by name refuses both steps at startup. A table may move its rows with its own SQL instead, through the DeadLetter event (Events).

Example

A payment the card network keeps declining is tried three times, then moves to the payments.failed group.

Macros
Manual

examples/sqlx_settlement_dead_letter.rs
use ruststream_sqlx::prelude::*;
use sqlx::Connection;
use sqlx::sqlite::SqlitePoolOptions;

/// One row of `payment_jobs`: `attempt` counts the deliveries the cap reads.
#[derive(Inbox, sqlx::FromRow)]
#[inbox(table = "payment_jobs")]
struct Charge {
    #[field(id, generated)]
    id: i64,
    #[field(group)]
    name: String,
    #[field(attempt, generated)]
    attempt: i16,
    #[field(locked_until)]
    locked_until: Option<DateTime<Utc>>,
    #[field(payload)]
    payload: Vec<u8>,
}

#[subscriber(InboxQueue::<Charge>::new("payments"))]
async fn charge(payment: &Payment) -> HandlerOutcome {
    if authorize(payment).await {
        return HandlerOutcome::ack();
    }
    HandlerOutcome::retry()
}

/// The service on the database `url` names.
fn service(url: &str) -> RustStream {
    let schema_url = url.to_owned();
    // Nothing connects here: the broker builds its pool when the service starts.
    let broker = SqlxBroker::from_url(url, SqlitePoolOptions::new());
    RustStream::new(AppInfo::new("payments", "0.1.0"))
        // The table, created before the subscription checks it.
        .on_startup(async move |()| {
            let mut conn = SqliteConnection::connect(&schema_url).await?;
            sqlx::raw_sql(SCHEMA).execute(&mut conn).await?;
            conn.close().await
        })
        .with_broker(broker.route::<Charge>("payments"), |b| {
            // Three attempts, the first included; the third failure moves the row into the
            // `payments.failed` group instead.
            b.include(charge)
                .max_attempts(nonzero!(3u32))
                .dead_letter("payments.failed");
        })
}

examples/manual/sqlx_settlement_dead_letter.rs
use ruststream_sqlx::prelude::*;
use sqlx::Connection;
use sqlx::sqlite::SqlitePoolOptions;

/// One row of `payment_jobs`: `attempt` counts the deliveries the cap reads.
#[derive(sqlx::FromRow)]
struct Charge {
    id: i64,
    attempt: i16,
    payload: Vec<u8>,
}

impl InboxTable for Charge {
    type Id = i64;
    type Table = InboxSpec<(Attempt, Lease<DateTime<Utc>>, Payload)>;
    const TABLE: Self::Table = InboxSpec::new("payment_jobs", Column::new("id").generated())
        .group(Column::new("name"))
        .attempt(Column::new("attempt").generated())
        .lease(Column::new("locked_until"))
        .payload(Column::new("payload"));

    fn id(&self) -> &i64 {
        &self.id
    }
}

impl AttemptRow for Charge {
    type Attempt = i16;

    fn attempt(&self) -> &i16 {
        &self.attempt
    }
}

impl PayloadRow for Charge {
    type Column = Vec<u8>;

    fn payload(&self) -> &[u8] {
        &self.payload
    }
}

struct ChargeCard;

impl Handle<Payment> for ChargeCard {
    async fn handle(
        &self,
        payment: &Payment,
        _outs: &(),
        _ctx: &mut Context<'_>,
    ) -> Result<(), HandlerOutcome> {
        if authorize(payment).await {
            return Ok(());
        }
        Err(HandlerOutcome::retry())
    }
}

/// The service on the database `url` names.
fn service(url: &str) -> RustStream {
    let schema_url = url.to_owned();
    // Nothing connects here: the broker builds its pool when the service starts.
    let broker = SqlxBroker::from_url(url, SqlitePoolOptions::new());
    RustStream::new(AppInfo::new("payments", "0.1.0"))
        // The table, created before the subscription checks it.
        .on_startup(async move |()| {
            let mut conn = SqliteConnection::connect(&schema_url).await?;
            sqlx::raw_sql(SCHEMA).execute(&mut conn).await?;
            conn.close().await
        })
        .with_broker(broker.route::<Charge>("payments"), |b| {
            let charge = subscriber("payments", ChargeCard)
                .map_source(|_| InboxQueue::<Charge>::new("payments"));
            // Three attempts, the first included; the third failure moves the row into the
            // `payments.failed` group instead.
            b.include(charge.build())
                .max_attempts(nonzero!(3u32))
                .dead_letter("payments.failed");
        })
}

Example​

Example