Function drain_to 

pub async fn drain_to<S, T>(
    store: &mut S,
    transport: &mut T,
    topic: &str,
) -> Result<usize>
where
    S: Store,
    T: Transport,

Drains store onto transport, publishing each record to topic, oldest first.

Each record is sent before it is removed from the store, so a send failure leaves that record and every record after it buffered in order to retry later: nothing is lost and nothing is reordered. Delivery is at-least-once, since a crash between a successful send and the record’s removal redelivers it on the next run.

This is the “forward” half of store-and-forward: buffer with a Store while offline, then call this when a link appears.

Arguments

• store - the queue to drain.
• transport - a connected transport to publish on.
• topic - the topic every record is published to.

Returns

The number of records forwarded once the store is drained empty.

Errors

Returns the transport’s error if a send fails, leaving the unsent records buffered in order, or Error::Io if the store cannot be read.

Examples

use pamoja_core::{Store, Transport};
use pamoja_sync::{drain_to, MemoryStore};

let mut outbox = MemoryStore::new();
outbox.append(b"21.5").await?;
let forwarded = drain_to(&mut outbox, transport, "sensors/1/temperature").await?;
assert_eq!(forwarded, 1);