# Engine-level features

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{% tab title="End users" %}
These pages explain what CrossWatch means by **Watchlist**, **History**, and **Ratings**.

You’ll only need this when a plan looks confusing.

Jump to:

* [Watchlist](/blueprint-architecture/sync/engine-level-features/watchlist.md)
* [History](/blueprint-architecture/sync/engine-level-features/history.md)
* [Ratings](/blueprint-architecture/sync/engine-level-features/ratings.md)

Related:

* Pair feature toggles: [Configure Pairs](/crosswatch/configure-pairs.md)
* How plans are built: [One-way sync](/blueprint-architecture/orchestrator/one-way-sync.md), [Two-way sync](/blueprint-architecture/orchestrator/two-way-sync.md)
  {% endtab %}

{% tab title="Power users" %}
These are the shared feature semantics used by the orchestrator across providers.

**Core code:** `cw_platform/orchestrator/_pairs_oneway.py`, `cw_platform/orchestrator/_pairs_twoway.py`

### Shared contract (every feature)

1. **Build snapshot** (`build_index`).
2. **Plan** (`adds/removes` or `upserts/unrates`).
3. **Apply** (`add/remove` on the destination).
4. **Persist** (baselines + checkpoints in `state.json`).

Related runtime pages:

* Snapshot rules: [Snapshots](/blueprint-architecture/orchestrator/snapshots.md)
* Diff logic: [Planner](/blueprint-architecture/orchestrator/planner.md)
* Writes + chunking: [Applier](/blueprint-architecture/orchestrator/applier.md)
* Baseline persistence: [State.json](/blueprint-architecture/orchestrator/state-json.md)

### Canonical keys and ID tokens

Everything is keyed by a **canonical key** derived from IDs.

Common tokens:

* `imdb:tt...`
* `tmdb:...`
* `tvdb:...`

Typed tokens are used to reduce season/episode duplication when keys differ.

### Index semantics: `present` vs `delta`

Providers declare how their snapshot behaves:

* `present`: full list each run.
* `delta`: incremental reads. The orchestrator merges baseline + current delta.

`delta` providers rely more on state (watermarks / checkpoints) for correctness.
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