# systems

design patterns that compose the concrete pieces documented elsewhere —
queues, delivery semantics, cursors, batch/serving splits. the layer above any
specific database, language, or protocol.

the boundary against its neighbours:

- `storage/` and `protocols/` — concrete systems (redis, sqlite, turso,
  turbopuffer, atproto, MCP)
- `data-structures/` — implementation tradeoffs in reusable structures
- `languages/` — language idioms (python, zig)
- `operations/` — running and shipping what you built
- `systems/` — the patterns that compose those things

if a note is mostly about what to do when something is already deployed and
misbehaving, it belongs in `operations/`.

## topics

- [background-tasks](./background-tasks.md) — the celery→sidekiq→arq→docket
  lineage, why redis streams won over lists, perpetuals, scheduling future work
  (streams mechanics live in [storage/redis/streams](../storage/redis/streams.md))
- [at-least-once-forwarder](./at-least-once-forwarder.md) — outbox,
  ack-after-durable-commit, and a cursor pinned to min(inflight); why
  ack-on-receipt plus shedding is silent loss
- [serving-from-snapshots](./serving-from-snapshots.md) — build offline, verify
  against a manifest, adopt atomically; the overlay is authoritative, and batch
  cadence sets the freshness floor
- [change-timestamps](./change-timestamps.md) — "when we last wrote" and "when
  the content last changed" are different columns; pinning the wrong one turns
  a correctness gate into a measurement of write traffic
- [watermark-semantics](./watermark-semantics.md) — one cursor, one meaning;
  the cost of a gate scales with the quantum of the thing it waits on
- [prior-art-encodes-someone-elses-constraints](./prior-art-encodes-someone-elses-constraints.md)
  — a mature schema's complexity is evidence of its constraints, not of its
  correctness for you; take the primitives, leave the envelope