Struct Follower 

pub struct Follower { /* private fields */ }

The twin’s loop over one source into one set of stores.

Implementations

impl Follower

pub fn new(stores: Arc<ServerStores>, source: Box<dyn LogSource>) -> Self

A follower of source into stores.

pub fn with_volume(self, volume: Arc<dyn Volume>) -> Self

Stage snapshots on volume instead of the box’s own filesystem (the tests’ capped volume, twin-resync-space 2026-10-06).

pub fn sharing_failures(self, failures: Arc<Mutex<Option<Failing>>>) -> Self

Share the run of failures with the follower before this one (the supervisor’s re-dial starts a new follower on the same primary: the run goes on).

pub fn with_fetch_bytes(self, bytes: u64) -> Self

How many blob bytes one fetch asks for (tests ask for tiny fetches to exercise the restart-mid-range path).

pub fn applied_seq(&self) -> u64

The twin’s own head.

pub fn step(&self) -> Result<Progress>

One poll: head, the gap rule, fetch-append-apply until level.

A poll that reached the primary and then failed says so in the status (2026-09-21, lane T4). Only the head() call used to set a status on failure. Everything after it — append_at on a full volume, an apply that will not go in, a fetch that dies mid-range — propagated with ?, and the STATUS WAS LEFT EXACTLY AS THE LAST GOOD POLL SET IT: following, often caught_up, with an empty why. run logged one warn and polled again. So the shape a twin’s life actually ends in — its data volume filling — read as a healthy, current copy on both twin status and the primary’s heartbeat, for as long as the box stayed up. A twin’s whole job is to be trustworthy at the moment nobody has time to check.

pub fn run(&self, poll: Duration, stop: &AtomicBool) -> Ended

Poll until stop; an unreachable primary is retried every poll, a snapshot-needed stops the loop and leaves the status saying so. What it returns says why it returned (Ended).

pub fn maintenance_beat(&self)

One poll while this twin grows its own set: the status says maintenance FIRST (so the report on this very poll carries the word), then the primary is asked for its head only — nothing is fetched, appended or applied, so nothing touches the unmounted set.