One PostgreSQL logical-replication slot feeds the engine, which demuxes each change by table and dispatches it to the targets bound to that table — an indexed in-memory replica, an HTTP-sync target, and a replication fan-out target. A change is acked back to the source only once every target on the source confirms it is durable. Make one target slow and watch the ACK hold back.
The engine holds an in-memory change buffer per pipeline and advances the source ACK only when
all targets confirm durability — so a slow or backpressured target (here, HTTP sync) holds the
ACK back without losing changes. See concepts/ordering-and-delivery.