Working notes — 05 data-and-databases projects.mdSUPPORTING MATERIAL
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Working notes — 05 data-and-databases projects.md (2026-09-22)

Scope: five section projects + one animated SVG (migration-both-ways.svg). Everything below verified by web search this session, 2026-09-22.

AWS facts checked

  • Free tier changed 2025-07-15. Accounts created on/after that date get a credit pool (Free Plan) instead of the old 12-month allowances (750 h/month RDS/EC2 micro etc.). Older accounts keep the legacy shape. Always Free unchanged: Lambda 1M invocations/mo, DynamoDB 25 GB, S3 5 GB. Source: aws.amazon.com free tier pages + coverage of the July 2025 revamp.
  • RDS automated backups: retention 1–35 days; daily snapshot plus transaction logs shipped ~every 5 minutes; PITR to any second up to the latest restorable time; restore always creates a new instance — never in place. Automated snapshots can be copied to manual to outlive retention. Source: docs.aws.amazon.com RDS UserGuide (WorkingWithAutomatedBackups), aws.amazon.com/rds/features/backup.
  • Aurora Serverless v2 scale-to-zero: announced 2024-11-20; min 0 ACU with auto-pause (idle 5 min–24 h), resume typically <15 s, storage still billed while paused; some configs prevent pausing (RDS Proxy, logical replication, global database). Source: AWS what's-new 2024-11 + Aurora UserGuide aurora-serverless-v2-auto-pause.
  • RDS Blue/Green supports PostgreSQL (Aurora + RDS for PostgreSQL, since Oct 2023). Green follows blue via logical replication for major upgrades; DDL on blue degrades replication; since 2024-11-22 minor-version B/G uses physical replication and green is read-only. So B/G is for upgrades and maintenance rehearsal, not the app-shape expand/contract migration. Source: AWS what's-new 2023-10, blue-green-deployments-considerations doc.

No prices quoted anywhere in projects.md (spec rule; only checked shapes, not numbers).

Design decisions

  • Project order rises: model → change the model live → measure it → make writes safe under concurrency → survive losing all of it.
  • Prompt-sequence requirement "review a schema a model proposed" lives in project 1, with a planted flaw so the review is an instrument, not a compliment generator (differs from the README's Request 1: the plant and the propose→review loop are the project here).
  • Project 4 deliberately separates isolation (lost update, READ COMMITTED) from atomicity (kill -9 between paired writes) — the folk cure "wrap it in a transaction" treats the wrong disease and the project makes that visible.
  • Project 5 is pitched junior (pg_dump/restore/diff, minutes measured); deep PITR/replication remain senior per the section's "not covered here".
  • Diagram: add → backfill → switch → remove with a dashed warm wall before REMOVE; green reversal rail under steps 1–3, blocked stub under step 4; one paced dot runs up → down → up past the wall and fades (SMIL only, frame one static-correct, loop restarts clean because the dot fades out).
  • migration-phases.svg (staff tier 19) is a cost curve; no overlap.

Text-width bookkeeping for the SVG

Budgets used: 12.5px sans ≈ 6.8 px/ch, 11px sans ≈ 6.0, 11px mono ≈ 6.6, 10px mono + 1.2 ls ≈ 7.2. Widest strings per 150px box kept ≤ 132px; caption lines ≤ 640px. Rendered in chrome-headless-shell at t≈0 and t≈6 s and inspected before committing to the reference.