Module chain_subscribe 

Chain subscription: replay-from-sequence plus live events (ADR-103 D).

ChainManager::subscribe returns a ChainSubscription. It yields the retained events from from_seq, then live events, with no gap and no duplicate between the two:

1. under the chain lock, subscribe creates the live receiver and records the snapshot end (the chain’s next sequence); every append publishes to the hub under that same lock, so they cannot interleave;
2. the replay is then paged in short lock holds (REPLAY_PAGE events, at most REPLAY_SCAN scanned per hold) over [from_seq, end);
3. live events below end are dropped (belt and braces).

A deep from_seq therefore never clones the chain under the lock, and a from_seq older than the replay window is refused with a typed error.

Live delivery uses a bounded tokio::sync::broadcast. send never blocks, so a stalled subscriber cannot slow an append; it falls behind, the oldest items are overwritten, and its next read yields ChainItem::Lagged carrying resume_from, the sequence to resubscribe from to recover. The subscription holds only a Weak reference to the manager and ends when the manager is dropped.

Structs

ChainFilter

Which events a subscription yields.

ChainSubscription

A replay-then-live stream of chain events.

Enums

ChainItem

One item of a subscription.

SubscribeError

Why a subscription was refused.

Constants

CHAIN_SUBSCRIBE_CAPACITY

Live buffer per subscriber (events). A subscriber slower than this many events behind sees ChainItem::Lagged.

DEFAULT_MAX_REPLAY_WINDOW

Default replay window: the furthest back (in events behind the head) a subscriber may start. Adjust with ChainManager::set_max_replay_window.