Struct CommitTail 

pub struct CommitTail {
    pub client: Option<(String, u64)>,
    pub producer: Option<ProducerWrite>,
}

The commit-time bookkeeping the backend writes co-transactionally: a client session’s lmid upsert, a foreign session’s producer watermark. Folded into ONE backend call — not engine-driven steps — because the replicator’s divergence guard must measure the user statements’ captures BEFORE the lmid row lands in the capture buffer; only the backend can order that (REPLICATOR-INTERACTIVE-TXN-DESIGN.md §4.2).

The two fields are mutually exclusive by construction — the wire refuses a request carrying both (design 306 §3.4) — so a backend writes whichever is present without a precedence rule. That silent precedence is exactly what let the two hosts diverge before 306.

Fields

client: Option<(String, u64)>

producer: Option<ProducerWrite>