mailrs-delivery-executor

Group-commit delivery executor on top of mailrs-maildir 1.2's deliver_batch. Accumulates per-path delivery requests from concurrent async tasks (SMTP / LMTP / IMAP APPEND sessions) and flushes each path's batch via a single fsync instead of N per-message fsyncs.

Built on tokio::sync::mpsc + tokio::sync::oneshot. Each calling session submits a delivery and awaits its own oneshot::Receiver for individual confirmation.

Why

mailrs-maildir 1.2's deliver_batch is 15.27× faster than N × deliver at N=64 batches on APFS (criterion microbench). But typical mail receivers have the wrong shape to use it directly: each SMTP session delivers 1-N recipients, and N is small. No caller is naturally going to hand a batch of 64 messages to a single deliver_batch call.

This crate is the bridge. The executor task accumulates per-path requests across concurrent sessions, groups them by destination, and flushes each group through deliver_batch. At saturation, batches naturally fill to max_batch=64 and the full microbench speedup translates to real throughput.

Quick start

use mailrs_delivery_executor::DeliveryExecutor;
use std::sync::Arc;

# async fn run() -> std::io::Result<()> {
let executor = DeliveryExecutor::spawn();

// In your SMTP session handler:
let path = "/var/mail/example.com/alice".to_string();
let body = Arc::new(b"From: a@b\r\n\r\nhello\r\n".to_vec());
let id = executor.deliver(path, body).await?;
println!("delivered: {}", id.0);
# Ok(())
# }

Tuning

use mailrs_delivery_executor::DeliveryExecutor;
use std::time::Duration;
let executor = DeliveryExecutor::with_config(/*max_batch=*/ 128, /*max_wait=*/ Duration::from_millis(5));

What it costs

Per-message latency increases by up to max_wait. With max_wait=10ms and a load of 32 concurrent connections, batches fill in 1-5ms in practice. Under truly low load (single message in flight), the executor waits the full max_wait before flushing — that's 10ms tail added to every delivery. The win appears when load is high enough to fill the batch before the timeout.

What this crate does NOT do

• No SMTP / LMTP protocol — caller's session driver parses incoming mail and passes raw bytes.
• No storage beyond Maildir — for IMAP-backed or Dovecot-style backends, write your own executor over those primitives. The pattern (per-path accumulate + batch flush) is portable; this crate is just the Maildir variant.
• No delivery scheduling — first-come-first-served per path. For priority queues use a different executor.

License

Apache-2.0 OR MIT.