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0006. Route anchors: the port names the side, the neighbour fixes the point

Status: accepted (SVG export change, October 2026)

Context

Section titled “Context”

The proposal defines compass ports (api.east) as the ends of a route. Taken literally, a port is the centre of a side. Real sheets rarely join centres: in the first converted architecture sheet, 12 of 13 arrows are straight lines that leave and enter boxes wherever the two boxes overlap, and the last one bends once. Fractional ports (api.north@25%) would add syntax and still need numbers the author has to work out.

Decision

Section titled “Decision”

A port names a side. The exact point on that side comes from the neighbouring point of the route:

  1. With waypoints, each side anchors at the projection of its neighbouring waypoint onto the side, clamped to the side.
  2. Two facing parallel sides without a waypoint between them anchor at the centre of the overlap of their extents, so the connector is straight. When the extents do not overlap, each side uses its own centre.
  3. Sides that do not face each other leave from their centres; perpendicular sides get one elbow.
  4. auto picks the side facing the neighbouring point, judged relative to the box proportions (the distance is divided by the half width or half height), so a wide band next to a small box uses its east or west side.

Relations without a route use auto on both ends and no waypoints.

Evidence

Section titled “Evidence”

Applied to the original sheet, rule 2 reproduces every straight arrow: NextJS (x 260..460) to the API gateway (x 240..700) gives x = 360; the gateway to the BFF (x 260..420) gives x = 340; Temporal (x 470..700) to the runtime band (x 240..780) gives x = 585; the LLM gateway (x 240..540) to the three providers gives x = 285, 390 and 495. All match the hand-drawn coordinates. The external clients arrow needs one waypoint, via [(1035, 48)], and rule 1 lands it at (1035, 530), the original end point.

Alternatives considered

Section titled “Alternatives considered”
  • Fractional ports: more syntax, numbers the author must compute, no gain for the common case.
  • Centre-to-centre lines clipped at the box border: diagonals everywhere on a sheet drawn with straight orthogonal arrows.
  • Full orthogonal routing with obstacle avoidance: belongs to the layout engine, not to a manual exporter.

Consequences

Section titled “Consequences”
  • Most sheets need no route statements; waypoints are reserved for the few bends.
  • Moving a box in the editor keeps straight connectors straight as long as the extents still overlap, which matches how authors expect drag to behave.
  • The rule is implemented once in routePoints() (@meltemi/diagrams-layout) and documented in docs/language/svg-export.md; the layout engine must keep it for placed elements.