feat: [Mnemosyne] add archive snapshot — point-in-time backup and restore

Implements snapshot_create, snapshot_list, snapshot_restore, and
snapshot_diff on MnemosyneArchive. Adds CLI subcommand
`mnemosyne snapshot create|list|restore|diff`. 17 new tests covering
all operations. Snapshots are stored as JSON files in
~/.hermes/mnemosyne/snapshots/ next to the archive.

Fixes #1268

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nexus/mnemosyne/archive.py

@@ -1105,6 +1105,175 @@ class MnemosyneArchive:
                 steps.append({"id": entry_id, "title": "[unknown]", "topics": []})
         return steps
 
+    # ─── Snapshot / Backup ────────────────────────────────────
+
+    def _snapshot_dir(self) -> Path:
+        """Return (and create) the snapshots directory next to the archive."""
+        d = self.path.parent / "snapshots"
+        d.mkdir(parents=True, exist_ok=True)
+        return d
+
+    @staticmethod
+    def _snapshot_filename(timestamp: str, label: str) -> str:
+        """Build a deterministic snapshot filename."""
+        safe_label = "".join(c if c.isalnum() or c in "-_" else "_" for c in label) if label else "snapshot"
+        return f"{timestamp}_{safe_label}.json"
+
+    def snapshot_create(self, label: str = "") -> dict:
+        """Serialize the current archive state to a timestamped snapshot file.
+
+        Args:
+            label: Human-readable label for the snapshot (optional).
+
+        Returns:
+            Dict with keys: snapshot_id, label, created_at, entry_count, path
+        """
+        now = datetime.now(timezone.utc)
+        timestamp = now.strftime("%Y%m%d_%H%M%S")
+        filename = self._snapshot_filename(timestamp, label)
+        snapshot_id = filename[:-5]  # strip .json
+        snap_path = self._snapshot_dir() / filename
+
+        payload = {
+            "snapshot_id": snapshot_id,
+            "label": label,
+            "created_at": now.isoformat(),
+            "entry_count": len(self._entries),
+            "archive_path": str(self.path),
+            "entries": [e.to_dict() for e in self._entries.values()],
+        }
+        with open(snap_path, "w") as f:
+            json.dump(payload, f, indent=2)
+
+        return {
+            "snapshot_id": snapshot_id,
+            "label": label,
+            "created_at": payload["created_at"],
+            "entry_count": payload["entry_count"],
+            "path": str(snap_path),
+        }
+
+    def snapshot_list(self) -> list[dict]:
+        """List available snapshots, newest first.
+
+        Returns:
+            List of dicts with keys: snapshot_id, label, created_at, entry_count, path
+        """
+        snap_dir = self._snapshot_dir()
+        snapshots = []
+        for snap_path in sorted(snap_dir.glob("*.json"), reverse=True):
+            try:
+                with open(snap_path) as f:
+                    data = json.load(f)
+                snapshots.append({
+                    "snapshot_id": data.get("snapshot_id", snap_path.stem),
+                    "label": data.get("label", ""),
+                    "created_at": data.get("created_at", ""),
+                    "entry_count": data.get("entry_count", len(data.get("entries", []))),
+                    "path": str(snap_path),
+                })
+            except (json.JSONDecodeError, OSError):
+                continue
+        return snapshots
+
+    def snapshot_restore(self, snapshot_id: str) -> dict:
+        """Restore the archive from a snapshot, replacing all current entries.
+
+        Args:
+            snapshot_id: The snapshot_id returned by snapshot_create / snapshot_list.
+
+        Returns:
+            Dict with keys: snapshot_id, restored_count, previous_count
+
+        Raises:
+            FileNotFoundError: If no snapshot with that ID exists.
+        """
+        snap_dir = self._snapshot_dir()
+        snap_path = snap_dir / f"{snapshot_id}.json"
+        if not snap_path.exists():
+            raise FileNotFoundError(f"Snapshot not found: {snapshot_id}")
+
+        with open(snap_path) as f:
+            data = json.load(f)
+
+        previous_count = len(self._entries)
+        self._entries = {}
+        for entry_data in data.get("entries", []):
+            entry = ArchiveEntry.from_dict(entry_data)
+            self._entries[entry.id] = entry
+
+        self._save()
+        return {
+            "snapshot_id": snapshot_id,
+            "restored_count": len(self._entries),
+            "previous_count": previous_count,
+        }
+
+    def snapshot_diff(self, snapshot_id: str) -> dict:
+        """Compare a snapshot against the current archive state.
+
+        Args:
+            snapshot_id: The snapshot_id to compare against current state.
+
+        Returns:
+            Dict with keys:
+              - snapshot_id: str
+              - added: list of {id, title} — in current, not in snapshot
+              - removed: list of {id, title} — in snapshot, not in current
+              - modified: list of {id, title, snapshot_hash, current_hash}
+              - unchanged: int — count of identical entries
+
+        Raises:
+            FileNotFoundError: If no snapshot with that ID exists.
+        """
+        snap_dir = self._snapshot_dir()
+        snap_path = snap_dir / f"{snapshot_id}.json"
+        if not snap_path.exists():
+            raise FileNotFoundError(f"Snapshot not found: {snapshot_id}")
+
+        with open(snap_path) as f:
+            data = json.load(f)
+
+        snap_entries: dict[str, dict] = {}
+        for entry_data in data.get("entries", []):
+            snap_entries[entry_data["id"]] = entry_data
+
+        current_ids = set(self._entries.keys())
+        snap_ids = set(snap_entries.keys())
+
+        added = []
+        for eid in current_ids - snap_ids:
+            e = self._entries[eid]
+            added.append({"id": e.id, "title": e.title})
+
+        removed = []
+        for eid in snap_ids - current_ids:
+            snap_e = snap_entries[eid]
+            removed.append({"id": snap_e["id"], "title": snap_e.get("title", "")})
+
+        modified = []
+        unchanged = 0
+        for eid in current_ids & snap_ids:
+            current_hash = self._entries[eid].content_hash
+            snap_hash = snap_entries[eid].get("content_hash")
+            if current_hash != snap_hash:
+                modified.append({
+                    "id": eid,
+                    "title": self._entries[eid].title,
+                    "snapshot_hash": snap_hash,
+                    "current_hash": current_hash,
+                })
+            else:
+                unchanged += 1
+
+        return {
+            "snapshot_id": snapshot_id,
+            "added": sorted(added, key=lambda x: x["title"]),
+            "removed": sorted(removed, key=lambda x: x["title"]),
+            "modified": sorted(modified, key=lambda x: x["title"]),
+            "unchanged": unchanged,
+        }
+
     def rebuild_links(self, threshold: Optional[float] = None) -> int:
         """Recompute all links from scratch.
 

nexus/mnemosyne/cli.py

@@ -6,7 +6,8 @@ Provides: mnemosyne ingest, mnemosyne search, mnemosyne link, mnemosyne stats,
           mnemosyne tag, mnemosyne untag, mnemosyne retag,
           mnemosyne timeline, mnemosyne neighbors, mnemosyne path,
           mnemosyne touch, mnemosyne decay, mnemosyne vitality,
-          mnemosyne fading, mnemosyne vibrant
+          mnemosyne fading, mnemosyne vibrant,
+          mnemosyne snapshot create|list|restore|diff
 """
 
 from __future__ import annotations
@@ -303,6 +304,68 @@ def cmd_fading(args):
         print()
 
 
+def cmd_snapshot(args):
+    archive = MnemosyneArchive()
+    if args.snapshot_cmd == "create":
+        result = archive.snapshot_create(label=args.label or "")
+        print(f"Snapshot created: {result['snapshot_id']}")
+        print(f"  Label:   {result['label'] or '(none)'}")
+        print(f"  Entries: {result['entry_count']}")
+        print(f"  Path:    {result['path']}")
+    elif args.snapshot_cmd == "list":
+        snapshots = archive.snapshot_list()
+        if not snapshots:
+            print("No snapshots found.")
+            return
+        for s in snapshots:
+            print(f"[{s['snapshot_id']}]")
+            print(f"  Label:   {s['label'] or '(none)'}")
+            print(f"  Created: {s['created_at']}")
+            print(f"  Entries: {s['entry_count']}")
+            print()
+    elif args.snapshot_cmd == "restore":
+        try:
+            result = archive.snapshot_restore(args.snapshot_id)
+        except FileNotFoundError as e:
+            print(str(e))
+            sys.exit(1)
+        print(f"Restored from snapshot: {result['snapshot_id']}")
+        print(f"  Entries restored: {result['restored_count']}")
+        print(f"  Previous count:   {result['previous_count']}")
+    elif args.snapshot_cmd == "diff":
+        try:
+            diff = archive.snapshot_diff(args.snapshot_id)
+        except FileNotFoundError as e:
+            print(str(e))
+            sys.exit(1)
+        print(f"Diff vs snapshot: {diff['snapshot_id']}")
+        print(f"  Added   ({len(diff['added'])}):    ", end="")
+        if diff["added"]:
+            print()
+            for e in diff["added"]:
+                print(f"    + [{e['id'][:8]}] {e['title']}")
+        else:
+            print("none")
+        print(f"  Removed ({len(diff['removed'])}):    ", end="")
+        if diff["removed"]:
+            print()
+            for e in diff["removed"]:
+                print(f"    - [{e['id'][:8]}] {e['title']}")
+        else:
+            print("none")
+        print(f"  Modified({len(diff['modified'])}):    ", end="")
+        if diff["modified"]:
+            print()
+            for e in diff["modified"]:
+                print(f"    ~ [{e['id'][:8]}] {e['title']}")
+        else:
+            print("none")
+        print(f"  Unchanged: {diff['unchanged']}")
+    else:
+        print(f"Unknown snapshot subcommand: {args.snapshot_cmd}")
+        sys.exit(1)
+
+
 def cmd_vibrant(args):
     archive = MnemosyneArchive()
     results = archive.vibrant(limit=args.limit)
@@ -401,10 +464,23 @@ def main():
     vb = sub.add_parser("vibrant", help="Show most alive entries (highest vitality)")
     vb.add_argument("-n", "--limit", type=int, default=10, help="Max entries to show")
 
+    sn = sub.add_parser("snapshot", help="Point-in-time backup and restore")
+    sn_sub = sn.add_subparsers(dest="snapshot_cmd")
+    sn_create = sn_sub.add_parser("create", help="Create a new snapshot")
+    sn_create.add_argument("--label", default="", help="Human-readable label for the snapshot")
+    sn_sub.add_parser("list", help="List available snapshots")
+    sn_restore = sn_sub.add_parser("restore", help="Restore archive from a snapshot")
+    sn_restore.add_argument("snapshot_id", help="Snapshot ID to restore")
+    sn_diff = sn_sub.add_parser("diff", help="Show what changed since a snapshot")
+    sn_diff.add_argument("snapshot_id", help="Snapshot ID to compare against")
+
     args = parser.parse_args()
     if not args.command:
         parser.print_help()
         sys.exit(1)
+    if args.command == "snapshot" and not args.snapshot_cmd:
+        sn.print_help()
+        sys.exit(1)
 
     dispatch = {
         "stats": cmd_stats,
@@ -430,6 +506,7 @@ def main():
         "vitality": cmd_vitality,
         "fading": cmd_fading,
         "vibrant": cmd_vibrant,
+        "snapshot": cmd_snapshot,
     }
     dispatch[args.command](args)
 

nexus/mnemosyne/tests/test_snapshots.py

@@ -0,0 +1,240 @@
+"""Tests for Mnemosyne snapshot (point-in-time backup/restore) feature."""
+
+from __future__ import annotations
+
+import json
+import tempfile
+from pathlib import Path
+
+import pytest
+
+from nexus.mnemosyne.archive import MnemosyneArchive
+from nexus.mnemosyne.ingest import ingest_event
+
+
+def _make_archive(tmp_dir: str) -> MnemosyneArchive:
+    path = Path(tmp_dir) / "archive.json"
+    return MnemosyneArchive(archive_path=path, auto_embed=False)
+
+
+# ─── snapshot_create ─────────────────────────────────────────────────────────
+
+def test_snapshot_create_returns_metadata():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        ingest_event(archive, title="Alpha", content="First entry", topics=["a"])
+        ingest_event(archive, title="Beta", content="Second entry", topics=["b"])
+
+        result = archive.snapshot_create(label="before-bulk-op")
+
+        assert result["entry_count"] == 2
+        assert result["label"] == "before-bulk-op"
+        assert "snapshot_id" in result
+        assert "created_at" in result
+        assert "path" in result
+        assert Path(result["path"]).exists()
+
+
+def test_snapshot_create_no_label():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        ingest_event(archive, title="Gamma", content="Third entry", topics=[])
+
+        result = archive.snapshot_create()
+
+        assert result["label"] == ""
+        assert result["entry_count"] == 1
+        assert Path(result["path"]).exists()
+
+
+def test_snapshot_file_contains_entries():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        e = ingest_event(archive, title="Delta", content="Fourth entry", topics=["d"])
+        result = archive.snapshot_create(label="check-content")
+
+        with open(result["path"]) as f:
+            data = json.load(f)
+
+        assert data["entry_count"] == 1
+        assert len(data["entries"]) == 1
+        assert data["entries"][0]["id"] == e.id
+        assert data["entries"][0]["title"] == "Delta"
+
+
+def test_snapshot_create_empty_archive():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        result = archive.snapshot_create(label="empty")
+        assert result["entry_count"] == 0
+        assert Path(result["path"]).exists()
+
+
+# ─── snapshot_list ───────────────────────────────────────────────────────────
+
+def test_snapshot_list_empty():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        assert archive.snapshot_list() == []
+
+
+def test_snapshot_list_returns_all():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        ingest_event(archive, title="One", content="c1", topics=[])
+        archive.snapshot_create(label="first")
+        ingest_event(archive, title="Two", content="c2", topics=[])
+        archive.snapshot_create(label="second")
+
+        snapshots = archive.snapshot_list()
+        assert len(snapshots) == 2
+        labels = {s["label"] for s in snapshots}
+        assert "first" in labels
+        assert "second" in labels
+
+
+def test_snapshot_list_metadata_fields():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        archive.snapshot_create(label="meta-check")
+        snapshots = archive.snapshot_list()
+        s = snapshots[0]
+        for key in ("snapshot_id", "label", "created_at", "entry_count", "path"):
+            assert key in s
+
+
+def test_snapshot_list_newest_first():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        archive.snapshot_create(label="a")
+        archive.snapshot_create(label="b")
+        snapshots = archive.snapshot_list()
+        # Filenames sort lexicographically; newest (b) should be first
+        # (filenames include timestamp so alphabetical = newest-last;
+        # snapshot_list reverses the glob order → newest first)
+        assert len(snapshots) == 2
+        # Both should be present; ordering is newest first
+        ids = [s["snapshot_id"] for s in snapshots]
+        assert ids == sorted(ids, reverse=True)
+
+
+# ─── snapshot_restore ────────────────────────────────────────────────────────
+
+def test_snapshot_restore_replaces_entries():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        ingest_event(archive, title="Kept", content="original content", topics=["orig"])
+        snap = archive.snapshot_create(label="pre-change")
+
+        # Mutate archive after snapshot
+        ingest_event(archive, title="New entry", content="post-snapshot", topics=["new"])
+        assert archive.count == 2
+
+        result = archive.snapshot_restore(snap["snapshot_id"])
+
+        assert result["restored_count"] == 1
+        assert result["previous_count"] == 2
+        assert archive.count == 1
+        entry = list(archive._entries.values())[0]
+        assert entry.title == "Kept"
+
+
+def test_snapshot_restore_persists_to_disk():
+    with tempfile.TemporaryDirectory() as tmp:
+        path = Path(tmp) / "archive.json"
+        archive = _make_archive(tmp)
+        ingest_event(archive, title="Persisted", content="should survive reload", topics=[])
+        snap = archive.snapshot_create(label="persist-test")
+
+        ingest_event(archive, title="Transient", content="added after snapshot", topics=[])
+        archive.snapshot_restore(snap["snapshot_id"])
+
+        # Reload from disk
+        archive2 = MnemosyneArchive(archive_path=path, auto_embed=False)
+        assert archive2.count == 1
+        assert list(archive2._entries.values())[0].title == "Persisted"
+
+
+def test_snapshot_restore_missing_raises():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        with pytest.raises(FileNotFoundError):
+            archive.snapshot_restore("nonexistent_snapshot_id")
+
+
+# ─── snapshot_diff ───────────────────────────────────────────────────────────
+
+def test_snapshot_diff_no_changes():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        ingest_event(archive, title="Stable", content="unchanged content", topics=[])
+        snap = archive.snapshot_create(label="baseline")
+
+        diff = archive.snapshot_diff(snap["snapshot_id"])
+
+        assert diff["added"] == []
+        assert diff["removed"] == []
+        assert diff["modified"] == []
+        assert diff["unchanged"] == 1
+
+
+def test_snapshot_diff_detects_added():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        ingest_event(archive, title="Original", content="existing", topics=[])
+        snap = archive.snapshot_create(label="before-add")
+        ingest_event(archive, title="Newcomer", content="added after", topics=[])
+
+        diff = archive.snapshot_diff(snap["snapshot_id"])
+
+        assert len(diff["added"]) == 1
+        assert diff["added"][0]["title"] == "Newcomer"
+        assert diff["removed"] == []
+        assert diff["unchanged"] == 1
+
+
+def test_snapshot_diff_detects_removed():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        e1 = ingest_event(archive, title="Will Be Removed", content="doomed", topics=[])
+        ingest_event(archive, title="Survivor", content="stays", topics=[])
+        snap = archive.snapshot_create(label="pre-removal")
+        archive.remove(e1.id)
+
+        diff = archive.snapshot_diff(snap["snapshot_id"])
+
+        assert len(diff["removed"]) == 1
+        assert diff["removed"][0]["title"] == "Will Be Removed"
+        assert diff["added"] == []
+        assert diff["unchanged"] == 1
+
+
+def test_snapshot_diff_detects_modified():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        e = ingest_event(archive, title="Mutable", content="original content", topics=[])
+        snap = archive.snapshot_create(label="pre-edit")
+        archive.update_entry(e.id, content="updated content", auto_link=False)
+
+        diff = archive.snapshot_diff(snap["snapshot_id"])
+
+        assert len(diff["modified"]) == 1
+        assert diff["modified"][0]["title"] == "Mutable"
+        assert diff["modified"][0]["snapshot_hash"] != diff["modified"][0]["current_hash"]
+        assert diff["added"] == []
+        assert diff["removed"] == []
+
+
+def test_snapshot_diff_missing_raises():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        with pytest.raises(FileNotFoundError):
+            archive.snapshot_diff("no_such_snapshot")
+
+
+def test_snapshot_diff_includes_snapshot_id():
+    with tempfile.TemporaryDirectory() as tmp:
+        archive = _make_archive(tmp)
+        snap = archive.snapshot_create(label="id-check")
+        diff = archive.snapshot_diff(snap["snapshot_id"])
+        assert diff["snapshot_id"] == snap["snapshot_id"]