One push subscription, as a value (spec/01 §4).

A source that supports :push sends its subscriber two messages, both tagged with the reference subscribe/2 answered:

{:macaw_source, ref, {:schema, schema}}      # first, and again when the schema changes
{:macaw_source, ref, {:delivery, delivery}}  # an increment: the rows new since the last

schema_message/2 and delivery_message/2 build them. The subscriber holds a Push in its state: receive/2 takes each message (a schema is cached, a delivery buffered), and drain/2 merges everything buffered since the last pulse at once, so a burst of pushes between two pulses costs one merge. held/1 is the delivery as it stands.

• The handshake sends the schema first. A delivery names its schema's version; one naming a version not yet cached falls back to describe (drain/2's function, the Macaw.Data.Schema.Cache.resolve/4 describe function), asked once and cached.
• An increment is merged by the window field: its rows after what is held are appended; its late rows, at or before that, are kept unless the very row is held already; the rows stay ordered by the window field, the served window grown to cover them. A schema with no window field keeps the newest limit rows, marking the delivery capped when it drops any.

It is not a process: the process that subscribes receives the messages in its mailbox and passes them here. Ported from the logic of visualize's Visualize.Feed.Monitor, whose message tag was :visualize_feed.