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|
//! The coordinator: the node that received the proposal counts the votes (`vote`) and
//! writes the decision.
//!
//! It commits as soon as a majority of the electorate voted for the remake. It aborts
//! if, before that:
//! * an operator requested an abort,
//! * the local epoch changed,
//! * a competing remake of the same epoch committed,
//! * a majority is out of reach (too many members voted for a competing remake), or
//! * the voting period (`settleSecs`) passed without a majority.
//!
//! Committing builds the snapshot at the base heads and records 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 (its votes become void).
use std::time::Duration;
use tracing::{info, warn};
use super::Host;
use super::records::{self, Decision, Record};
use super::snapshot;
use super::vote;
use crate::document::store::StoreError;
const TICK: Duration = Duration::from_secs(1);
pub(crate) async fn run<H: Host>(host: H, id: String) {
info!(remake = %id, "Coordinating remake");
loop {
tokio::time::sleep(TICK).await;
match step(&host, &id).await {
Ok(true) => return,
Ok(false) => {}
Err(error) => {
warn!(remake = %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!(remake = %id, "Remake 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) {
decide_abort(host, id, reason).await?;
return Ok(true);
}
let tally = vote::tally(id, &record, &all, now, grace);
if tally.reached() {
info!(remake = %id, votes = %tally, "Majority reached");
commit(host, id, &record).await?;
return Ok(true);
}
if tally.out_of_reach() {
decide_abort(host, id, format!("a majority is out of reach: {tally}")).await?;
return Ok(true);
}
if now >= record.proposal.settle_until {
decide_abort(
host,
id,
format!("no majority in the voting period: {tally}"),
)
.await?;
return Ok(true);
}
Ok(false)
}
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 the vote".to_owned());
}
if let Some((other, _)) = all.iter().find(|(other, candidate)| {
other != id
&& candidate.proposal.from_epoch == proposal.from_epoch
&& candidate.phase(now, grace) == records::Phase::Committed
}) {
return Some(format!("competing remake {other} committed"));
}
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.remake().store_prebuilt(id, snapshot);
info!(remake = %id, to_epoch = %to_epoch, snapshot_bytes, "Remake committed");
Ok(())
}
async fn decide_abort<H: Host>(host: &H, id: &str, reason: String) -> Result<(), StoreError> {
warn!(remake = %id, reason = %reason, "Remake 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
}
|
