# ADR-0006: Undo snapshots and replay log

## Status

Accepted

## Context

Two related features address the same underlying need — making the
application safe and reproducible for learners:

1. **Accidental destruction.** A student typing `DROP TABLE
   Customers` and then realising what they did is a near-certain
   event in this audience. Without a recovery path, the experience
   is hostile and the learning moment is lost to panic.
2. **Replay and scripting.** A persistent record of every
   executed command is useful for tutorials, debugging, sharing
   reproducible problem reports, and rebuilding a project from a
   blank slate.

Both features are cheap to implement and high-leverage.

## Decision

### Undo snapshots

Before any destructive operation — `DROP`, `DELETE`, `TRUNCATE`,
schema-rebuild migrations, restore, etc. — the application takes a
snapshot of the database using SQLite's online backup API into a
ring buffer of recent snapshots (size to be tuned; initial target
N = 10).

An `undo` command (available in both modes as an app-level
command per ADR-0003 — no sigil) restores the most recent
snapshot. Each undo step is itself snapshotted to keep `redo`
possible.

**Undo requires confirmation.** Snapshots are taken only before
destructive operations, so the "current" state may include
non-destructive work (inserts, updates, schema additions) done
since the last snapshot. Restoring a snapshot therefore *can*
discard data the user has not been explicitly warned about.

Before restoring, the application displays a confirmation prompt
that includes:

- The snapshot's timestamp (local time, with a relative form such
  as "12 minutes ago").
- A short summary of the operation that triggered the snapshot
  (the command text, e.g. `DROP TABLE Customers`).
- A summary of changes that will be discarded if the undo
  proceeds — at minimum, counts of rows added/modified/deleted
  per table and any schema changes since the snapshot.

The user must explicitly confirm. A keyboard shortcut for
"confirm" is provided so power users are not slowed down, but
there is no flag to suppress the prompt — undo is rare enough,
and consequential enough, that the prompt is always shown.

### Replay log (`history.log`)

Every successfully executed command — DSL or SQL — is appended to
`history.log` in the project directory, one command per record,
with a timestamp and the resulting status. The format is
deliberately simple and human-readable so it can be hand-edited
and replayed.

The same format serves three purposes:

- A persistent input history surfaced via the TUI history feature.
- A scripting format: `.commands` files (or `history.log` itself)
  can be replayed via a `replay` command.
- A reproducible bug-report artifact when a project is shared.

The log is append-only during a session. It is **not** the
authoritative state of the project (that lives in `project.yaml`
+ `data/`, ADR-0004) — it is an audit and replay trail.

## Consequences

- Snapshots add modest overhead per destructive operation. The
  cost is bounded; learners care about safety far more than
  microseconds.
- The ring buffer size must be tuned later based on realistic
  database sizes; an initial value is fine for now.
- The replay log enables a future `replay` / scripting feature
  with no additional storage commitment.
- Tutorial authors gain a natural "starter script" format for
  exercises.

## Amendment 1 — Single-step undo: every-mutation snapshots, hybrid storage, batch granularity (2026-05-24)

The replay/journal half of this ADR (U3/U4) shipped via ADR-0034. This
amendment settles the **undo/snapshot half (U1/U2)** before
implementation, and **supersedes the original Decision's
"snapshots only before destructive operations" model** and its
confirmation rationale. Written with explicit user approval; the
implementation plan is `docs/plans/20260524-adr-0006-undo-snapshots.md`.
**Not yet implemented** at the time of writing — this records the
agreed design.

### Snapshot scope — every mutation (single-step undo)

The original Decision snapshots "before any destructive operation —
`DROP`, `DELETE`, `TRUNCATE`, schema-rebuild migrations, restore" and
explicitly treats inserts/updates/schema-additions as *non-destructive
work between snapshots*. That is **replaced**: a snapshot is taken
before **every** data/schema mutation — insert, update, delete, drop,
all DDL, and all SQL DML. Undo therefore behaves like a familiar
single-step "undo my last command" (Ctrl-Z), which is the right model
for this teaching environment (clarity over micro-optimisation, per
the project's "pedagogy wins ties" posture).

A consequence is that the original confirmation clause — "counts of
rows added/modified/deleted per table and any schema changes since the
snapshot" — **collapses**: with a snapshot per command there is no
intervening un-snapshotted work, so undo rolls back exactly one
command and the confirmation simply **names that command**. No
db-diff machinery is needed.

### Confirmation rationale

The original justified the always-on prompt by "undo is rare and
consequential." Under single-step undo, undo is *more frequent*, but
the prompt is **kept** anyway, now justified by "the prompt names the
exact command being undone." There is still no flag to suppress it.
`undo` and `redo` each confirm (`Y` confirms; `N`/`Esc` cancels),
mirroring the existing `rebuild` modal. The redo prompt names the
command that will be re-applied.

### Snapshot mechanism — hybrid db + text (reconciles ADR-0015)

The original specifies SQLite's online backup API. Since then,
ADR-0015 made `playground.db` a *derived* artifact with
`project.yaml` + `data/*.csv` as the authoritative source, committed
last for crash recovery. A db-only restore is therefore no longer
sufficient on its own. The agreed mechanism is a **hybrid
whole-project snapshot**:

- the database is copied via the **online backup API** (honouring this
  ADR; it is also the only safe way to copy a live database), **and**
- `project.yaml` + `data/*.csv` are copied as inert files.

Undo **restores all three directly** — no rebuild, no re-derivation —
re-establishing a consistent `(db, yaml, csv)` triple. This satisfies
both this ADR (the backup API *is* used) and ADR-0015 (text remains
authoritative). The snapshot is staged *before* the mutation's
transaction and finalised into the ring *after* the database commit,
preserving ADR-0015 §6's commit-db-last ordering; a rolled-back
operation leaves no snapshot.

### Storage and lifetime — persisted ring, N = 50

Snapshots are **persisted on disk** under the project in a
`.snapshots/` directory and survive quit (undo works after reopening).
The ring keeps the most recent **N = 50** snapshots (the original's
N = 10 is raised, since single-step undo means N counts *commands*;
still a single tunable constant), evicting the oldest on overflow.
`.snapshots/` is added to the `.gitignore` template, **excluded from
`export`** (like `playground.db` and `history.log`), and on the
temp-project cleanup allowlist so an otherwise-empty temp carrying a
snapshots directory remains safely deletable.

### Redo

`redo` is supported (as the original states). New semantics are
pinned: **the redo stack is discarded on any new mutation** (standard
linear undo/redo). Each undo pushes the pre-undo state so redo can
restore it.

### Batch operations — one undo step; `import` excluded

A single user command that runs many sub-operations — `replay` today,
and any future in-project batch command — records **one** boundary
snapshot for the whole batch (not one per sub-command), via a
Begin/EndBatch worker primitive that suppresses per-command staging and
finalises a single ring entry only if ≥1 mutation actually ran. This is
a performance win (a long `history.log` replay is one database copy,
not N) and the consistent reading of "one undo step per user command."

`import` is **outside** the undo model entirely: per ADR-0015 §11 it
creates a *new* project and switches to it, leaving the current project
untouched on disk, so there is nothing to snapshot and it takes no undo
step (the new project simply starts with an empty ring). Project-switch
navigation undo ("go back to the previous project") is a separate,
out-of-scope mechanism — the prior project is intact and reachable via
`load` / `--resume`.

### Disable switch

A `--no-undo` CLI flag turns snapshotting off entirely (zero
per-command overhead), as a hardware escape hatch should per-command
snapshots prove too heavy. When set, `undo` / `redo` report that undo
is turned off. CLI-only for v1 (no in-app toggle).

### Consequences

- Per-mutation snapshotting costs one database backup + a text copy per
  command; a bulk paste of N inserts makes N snapshots. Bounded by the
  N = 50 ring and the `--no-undo` escape hatch; the ADR-0015 "batch"
  command remains the future remedy, and a hardlink/copy-on-write dedup
  of unchanged files between consecutive snapshots is a possible future
  optimisation (not v1).
- 50 × (database + text) of persisted snapshots can reach tens of MB
  for larger projects — an accepted, bounded cost.
- `Cargo.toml` gains the `backup` feature on `rusqlite`.
- The Phase-3 N/A matrix row ("auto-snapshot fires for SQL DML the same
  as DSL") becomes non-vacuous: the snapshot hook lives in the worker
  dispatch and covers DSL and SQL mutations uniformly.
