[AS-325] Tech note: Choosing org boundaries for subgraphs

[ilan-bel]

Summary

Adds docs/choosing-subgraph-boundaries.md. Four boundary models (by team / by domain / by data source / by usage pattern), a decision flow, sizing heuristics, and the reorganization sequence — sized to be the natural follow-on to the monolith-to-federation guide referenced in the ticket.

Tracks AS-325.

Note

The ticket calls out apollographql.com/docs/graphos/schema-design/guides/from-monolith as the natural neighbor for this content; interim home is here while apollographql/docs is archived and platform-docs needs SAML.

Test plan

• Renders cleanly (the ASCII decision flow especially)
• Cross-links to monolith-to-federation guide and existing TNs resolve

🤖 Generated with Claude Code

[apollo-solutions-reviewer]

Content is solid and well-organized; the four boundary models, decision flow, and sizing heuristics read clearly and the external links (monolith guide, TN0012, TN0024, TN0036, Contracts) all resolve. A few items before this fully satisfies AS-325:

• Broken internal link: the "See also" entry ./advanced-schema-design.md (AS-323) does not exist in this repo. Remove it or land it once AS-323 merges.
• AC gap: the ticket requires the new page be linked from the monolith migration guide's Step 3. That guide lives on apollographql.com/docs, so this PR cannot satisfy that AC from this repo. The PR note acknowledges the interim home; consider filing a follow-up to add the back-link once the docs site is writable, so the AC is tracked rather than silently dropped.
• AC gap: "reviewed by at least one SE who has worked on federation adoption" has no recorded approval yet.
• Depth vs ticket scope: the implementation guidance asked for team topologies (stream-aligned vs platform), monorepo vs polyrepo organization, and entity vs field ownership. The boundary models cover the core well, but those specific subtopics are thin or absent. Optional to expand here vs a follow-on.

None of this is blocking the prose quality; flagging so the AC is closed honestly.

[apollo-solutions-reviewer]

Broken link: ./advanced-schema-design.md does not exist in this repo (AS-323 not merged). Remove this entry or gate it on AS-323 landing.

[apollo-solutions-reviewer]

Verified at 4923bfd: the ./advanced-schema-design.md entry is no longer an active link. It is stashed in an HTML comment at the bottom of the file (<!-- Re-add once AS-323 lands: ... -->), so there is no broken link to render and it is gated on AS-323 landing as suggested. This thread can be resolved.

[apollo-solutions-reviewer]

Re-reviewed at 4923bfd. The one substantive item from the prior pass is resolved: the broken ./advanced-schema-design.md (AS-323) link no longer renders; it is gated behind an HTML comment to be re-added once AS-323 lands. All external links (monolith guide, TN0012, TN0024, TN0036, Contracts) resolve, and the four boundary models, decision flow, sizing heuristics, and reorganization sequence are sound and well-organized. Approving. The remaining notes from the earlier review (the monolith-guide back-link living in apollographql.com/docs, the SE sign-off AC, and optional depth on team-topologies / monorepo-vs-polyrepo / entity-vs-field ownership) are AC-tracking and follow-on items, not blockers to this content.