The unit of data that flows from a Scriba.Source through the engine to a Scriba.Target.

A source decodes its native event into this shape and wraps it in a Broadway.Message. Handlers see the decoded :data and the engine routes events to workers by hashing :stream_id.

:position is a global, monotonic, numeric ordinal

This is the strongest assumption Scriba makes about a source, so it is worth stating rather than leaving implicit. :position must increase across the whole stream of events a projection consumes — not per stream — and comparisons on it must mean what they appear to mean. Four things rest on it:

• Dedup. An event counts as already applied when its position is at or below its stream's committed cursor.
• Cursor monotonicity. GREATEST in SQL keeps a cursor from moving backwards, which requires positions that order.
• The watermark. "Every event up to N is accounted for" is only meaningful if positions can be reasoned about contiguously.
• Resume. :start_from hands a position back to the source and expects everything at or below it to be filtered out.

Commanded's global event_number satisfies all four, which is also what makes the cursor carry-over in MIGRATION.md work: both libraries track the same number.

A store whose position is not a single increasing integer — EventStoreDB's commit/prepare pairs, say, or a vector clock — does not fit this shape, and making it fit is a design question rather than an adapter detail. Nothing is being built for that case speculatively; if you have such a store, open an issue and it can be designed against a real one. Scriba.Target carries the same reasoning for targets.