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//! The coordinator: the node that received the proposal watches the cluster during the
//! settle period and then writes the decision.
//!
//! It aborts if, at any point before deciding:
//! * an operator requested an abort,
//! * a competing proposal for the same epoch appeared (both coordinators abort),
//! * a peer it considered went dormant (a partition may hide a competitor),
//! * the local epoch changed, or
//! * the decision would come later than the settle period plus the grace margin
//! (other nodes report the proposal as expired by then).
//!
//! Otherwise it builds the snapshot at the base heads and commits, recording the new
//! epoch so every node can verify its own build against it.
//!
//! The task lives only in memory. If the coordinator dies, nobody decides and the
//! proposal expires.
use std::collections::HashMap;
use std::time::Duration;
use tracing::{info, warn};
use super::records::{self, Decision, Record};
use super::snapshot;
use super::Host;
use crate::store::StoreError;
const TICK: Duration = Duration::from_secs(1);
pub(crate) async fn run<H: Host>(host: H, id: String) {
info!(compaction = %id, "Coordinating compaction");
loop {
tokio::time::sleep(TICK).await;
match step(&host, &id).await {
Ok(true) => return,
Ok(false) => {}
Err(error) => {
warn!(compaction = %id, error = %format!("{error:#}"), "Coordinator step failed; retrying");
}
}
}
}
/// One evaluation. Returns true once the coordinator is done.
async fn step<H: Host>(host: &H, id: &str) -> Result<bool, StoreError> {
let wanted = id.to_owned();
let (record, all) = host
.read_doc(move |doc| (records::load(doc, &wanted), records::load_all(doc)))
.await;
let Some(record) = record else {
warn!(compaction = %id, "Compaction record vanished; stopping coordinator");
return Ok(true);
};
if record.decision.is_some() {
return Ok(true);
}
let now = records::unix_millis();
let grace = host.dormant_after().as_millis() as u64;
if let Some(reason) = abort_reason(host, id, &record, &all, now, grace).await {
decide_abort(host, id, reason).await?;
return Ok(true);
}
if now < record.proposal.settle_until {
return Ok(false);
}
commit(host, id, &record).await?;
Ok(true)
}
async fn abort_reason<H: Host>(
host: &H,
id: &str,
record: &Record,
all: &[(String, Record)],
now: u64,
grace: u64,
) -> Option<String> {
let proposal = &record.proposal;
if let Some(request) = &record.abort_requested {
return Some(format!("aborted by operator via {}", request.by));
}
if records::EpochRecord::from(host.current_epoch()) != proposal.from_epoch {
return Some("local epoch changed during settle".to_owned());
}
if let Some((other, _)) = all.iter().find(|(other, candidate)| {
other != id
&& candidate.proposal.from_epoch == proposal.from_epoch
&& candidate.phase(now, grace).is_open()
}) {
return Some(format!("competing compaction {other} for the same epoch"));
}
// Peers forgotten in the meantime are ignored: an operator decided they are gone.
let peers: HashMap<String, bool> = host
.peer_summaries()
.await
.into_iter()
.map(|peer| (peer.id, peer.active))
.collect();
let dormant: Vec<&str> = proposal
.peers_considered
.iter()
.filter(|peer| peers.get(peer.as_str()) == Some(&false))
.map(String::as_str)
.collect();
if !dormant.is_empty() {
return Some(format!(
"peers went dormant during settle: {}",
dormant.join(", ")
));
}
if now > proposal.settle_until.saturating_add(grace) {
return Some("decision deadline passed".to_owned());
}
None
}
async fn commit<H: Host>(host: &H, id: &str, record: &Record) -> Result<(), StoreError> {
let Some(request) = record.snapshot_request(id) else {
return decide_abort(
host,
id,
"unreadable epoch or base heads in proposal".into(),
)
.await;
};
// Clone under the read lock, build without any lock.
let copy = host.read_doc(|doc| doc.clone()).await;
let mut snapshot = match snapshot::build_detached(copy, request).await {
Ok((snapshot, _copy)) => snapshot,
Err(error) => {
return decide_abort(host, id, format!("snapshot build failed: {error}")).await;
}
};
let to_epoch = snapshot.epoch;
let snapshot_bytes = snapshot.doc.save().len() as u64;
let decision = Decision::Committed {
to_epoch: to_epoch.into(),
snapshot_bytes,
decided_at: records::unix_millis(),
};
write_decision(host, id, decision).await?;
// The local switch reuses it instead of building again.
host.compaction().store_prebuilt(id, snapshot);
info!(compaction = %id, to_epoch = %to_epoch, snapshot_bytes, "Compaction committed");
Ok(())
}
async fn decide_abort<H: Host>(host: &H, id: &str, reason: String) -> Result<(), StoreError> {
warn!(compaction = %id, reason = %reason, "Compaction aborted");
write_decision(
host,
id,
Decision::Aborted {
reason,
decided_at: records::unix_millis(),
},
)
.await
}
async fn write_decision<H: Host>(host: &H, id: &str, decision: Decision) -> Result<(), StoreError> {
let id = id.to_owned();
host.commit_doc(move |doc| records::write_decision(doc, &id, &decision))
.await
}
|
