---
description: "Syncback architecture: Roblox→filesystem synchronization, incremental vs clean modes, hash-based change detection, middleware selection, and property filtering."
globs:
  - "src/syncback/**/*.rs"
  - "src/cli/syncback.rs"
---

# Rojo Syncback Architecture

**Syncback** is the reverse operation of building: it takes a Roblox binary file (rbxl/rbxm) and writes instances back to the filesystem according to project structure.

**Entry point:** `rojo syncback <project> --input <file>`

## Core Concept

```
Roblox File (rbxl/rbxm)
  ↓ Parse with rbx_binary
WeakDom (in-memory instances)
  ↓ Match to project structure
Filesystem (Lua files, models, directories)
```

**Key challenge:** Determining which instances map to which files, and preserving existing file structure when possible.

## Architecture Overview (`src/syncback/`)

### Module Structure

```
syncback/
  mod.rs              - Main loop, orchestration
  snapshot.rs         - Convert instances → snapshots
  fs_snapshot.rs      - Filesystem representation
  file_names.rs       - Generate valid filenames
  property_filter.rs  - Filter properties per syncback rules
  ref_properties.rs   - Handle Ref properties (cross-references)
  hash/               - Content hashing for change detection
    mod.rs            - Hash variants
    variant.rs        - Variant-specific hashing
  stats.rs            - Statistics tracking
```

### Main Loop (`syncback_loop`)

**Flow:**

1. **Parse input file** - Read rbxl/rbxm with `rbx_binary`
2. **Prune tree** - Remove instances not in original project
3. **Filter hidden services** - Remove internal Roblox services (AdService, Analytics, etc.)
4. **Collect pre-prune paths** - Track all instances before pruning (for external refs)
5. **Hash comparison** - Compare hashes of old tree vs new tree
6. **Recursive syncback** - Walk trees, matching instances to files
7. **Write filesystem** - Create/update files based on changes
8. **Link Ref properties** - Resolve cross-references between instances

**Signature:**
```rust
pub fn syncback_loop(
    vfs: &Vfs,
    old_tree: &mut RojoTree,      // Existing project tree
    new_tree: WeakDom,             // Roblox file contents
    project: &Project,             // Project configuration
    incremental: bool,             // Preserve structure vs rebuild
) -> anyhow::Result<FsSnapshot>
```

## Syncback Modes

### Incremental Mode (`incremental: true`)

**Default behavior.** Preserves existing file structure:

- If `src/MyScript.luau` exists, update it
- If `src/MyFolder/init.luau` exists (directory structure), preserve it
- Only creates new files when instances don't match anything
- Compares hashes to detect actual changes (avoids rewriting unchanged files)

**Use case:** Normal workflow where you sync changes from Studio back to disk.

### Clean Mode (`incremental: false`)

**Rebuild from scratch.** Treats all children as new:

- Ignores existing file structure
- Generates new filenames for everything
- Chooses best middleware for each instance
- Useful when project structure changed significantly

**Use case:** Major refactoring, or when incremental mode produces wrong results.

**Implementation detail:** `old_ref` is ignored in `SyncbackSnapshot::with_joined_path` when `incremental=false`.

## Syncback Rules

Configured in `syncbackRules` section of project file:

```rust
pub struct SyncbackRules {
    /// Roblox tree paths to ignore (e.g., "ServerStorage/Secrets")
    pub ignore_trees: Vec<String>,
    
    /// Filesystem paths to ignore (e.g., "src/client/*")
    pub ignore_paths: Vec<PathBuf>,
    
    /// Properties to not sync back per class
    /// Example: {"BasePart": ["Color"], "Script": ["Disabled"]}
    pub ignore_properties: HashMap<String, Vec<String>>,
    
    /// Whether to sync Workspace.CurrentCamera (default: false)
    pub sync_current_camera: bool,
    
    /// Whether to sync unscriptable properties (default: true)
    pub sync_unscriptable: bool,
}
```

**Property filtering** (`property_filter.rs`):
- Filters out `Name`, `Parent` (always)
- Filters out properties in `ignore_properties`
- Filters out unscriptable properties if `sync_unscriptable: false`
- Filters out middleware-specific properties (e.g., `Script.Source` when writing to `.luau` file)

### Hidden Services

**Default behavior (`ignoreHiddenServices: true`):**

Only sync "visible" services:
```rust
const VISIBLE_SERVICES: &[&str] = &[
    "Lighting", "MaterialService", "ReplicatedFirst",
    "ReplicatedStorage", "ServerScriptService", "ServerStorage",
    "SoundService", "StarterGui", "StarterPack", "StarterPlayer",
    "Teams", "TextChatService", "VoiceChatService", "Workspace",
];
```

**Rationale:** Prevents plugin from trying to delete services like `HttpService`, `AnalyticsService`, `Chat` that were never synced to disk.

## Hash-Based Change Detection (`hash/`)

### Why Hashing?

**Problem:** Writing every file on every syncback is slow and creates noise in git diffs.

**Solution:** Hash both old and new instance trees. Only write files where hashes differ.

### Hash Algorithm

Hashing uses **blake3** directly - there's no `HashVariant` struct.

```rust
/// Returns HashMap<Ref, Hash> mapping each instance to its content hash
pub fn hash_tree(project: &Project, dom: &WeakDom, root_ref: Ref) -> HashMap<Ref, Hash>
```

**Hashing strategy:**
1. Process instances bottom-up (children first)
2. Hash class name, then filtered properties
3. Sort and hash children's hashes
4. Finalize with blake3

**Special cases:**
- Ref properties are hashed as paths (since Refs change between trees)
- Some properties are normalized (e.g., line endings in scripts)
- Unscriptable properties can be excluded from hash

### Hash Comparison

```rust
if old_hash == new_hash {
    // No changes - skip writing file
    return Ok(None);
} else {
    // Changed - write file
    write_instance_to_file(snapshot)?;
}
```

## SyncbackSnapshot (`snapshot.rs`)

**Core type representing instance to sync back:**

```rust
pub struct SyncbackSnapshot<'sync> {
    pub data: SyncbackData<'sync>,  // Context (VFS, trees, project)
    pub old: Option<Ref>,            // Old tree instance (if exists)
    pub new: Ref,                    // New tree instance (Roblox file)
    pub path: PathBuf,               // Target filesystem path
    pub middleware: Option<Middleware>, // How to write (Lua, Rbxm, etc.)
}
```

### Snapshot Creation

**Methods:**

```rust
// Join child to current path
snapshot.with_joined_path(new_ref, old_ref)?
// Result: parent_path/ChildName.luau

// Use base path
snapshot.with_base_path(&base, new_ref, old_ref)?
// Result: base/ChildName.luau

// Explicit path
snapshot.with_new_path(path, new_ref, old_ref)
// Result: path (exactly as provided)
```

**Filename generation** (`file_names.rs`):
- Uses instance name
- Adds appropriate extension based on middleware
- Encodes Windows-invalid chars if enabled
- Example: `Script` named "My Script" → `My Script.server.luau`

### Property Filtering

```rust
snapshot.get_path_filtered_properties(new_ref)
```

**Filters out:**
- `Source` property for scripts (content goes in file body)
- `Value` property for StringValue (content goes in `.txt` file)
- Properties specified in `ignoreProperties` rules
- Name/Parent (structural, not data)

## Middleware Selection

**Decision tree:**

1. **Old instance exists?** → Use same middleware as before (preserve structure)
2. **Script type?** → Choose appropriate Lua middleware (`.server.luau`, `.client.luau`, etc.)
3. **Model/container?** → Choose directory or rbxm based on complexity
4. **Special types?** → CSV for LocalizationTable, TXT for StringValue
5. **Fallback** → Rbxm or JsonModel

**Debug override:** `ROJO_SYNCBACK_DEBUG` environment variable
- `1` = Use Rbxmx instead of Rbxm
- `2` = Use JsonModel

## Filesystem Snapshot (`fs_snapshot.rs`)

**Represents filesystem operations to perform:**

```rust
pub struct FsSnapshot {
    added_files: HashMap<PathBuf, Vec<u8>>,   // New files to create
    added_dirs: HashSet<PathBuf>,             // New directories
    removed_files: HashSet<PathBuf>,          // Files to delete
    removed_dirs: HashSet<PathBuf>,           // Directories to delete
}
```

**Key methods:**
```rust
// Create snapshot with operations
fs_snapshot.with_added_file(path, contents)
fs_snapshot.with_added_dir(path)
fs_snapshot.merge(other)  // Combine snapshots

// Apply to filesystem
fs_snapshot.write_to_vfs(base_path, vfs)?;

// Query
fs_snapshot.is_empty()
fs_snapshot.added_paths()
fs_snapshot.removed_paths()
```

## Ref Properties (`ref_properties.rs`)

**Challenge:** Ref properties (like `ObjectValue.Value`) point to other instances by Ref, but Refs are different between old and new trees.

**Solution:**

1. **Collect paths** - Map every Ref → full instance path (e.g., `Workspace.Part1`)
2. **Collect referents** - Find all properties that are Refs, store as (source_path, target_path)
3. **Link referents** - After syncback, use Rojo's `$id` / `Rojo_Target_PropertyName` system to link instances

**Rojo linking system:**
```json
{
    "id": "some-unique-id",
    "properties": {},
    "attributes": {
        "Rojo_Target_Value": "some-unique-id"
    }
}
```

Plugin reads `Rojo_Target_PropertyName` attributes and sets corresponding Ref properties.

## Statistics Tracking (`stats.rs`)

```rust
pub struct SyncbackStats {
    duplicate_name_count: AtomicUsize,     // Instances skipped (name collision)
    rbxm_fallback_count: AtomicUsize,      // Instances using rbxm fallback
    unknown_class_count: AtomicUsize,      // Unknown classes encountered
    unknown_property_count: AtomicUsize,   // Unknown properties encountered
    unknown_classes: Mutex<HashSet<String>>,
    unknown_properties: Mutex<HashSet<String>>,
}
```

**Key methods:**
```rust
stats.record_duplicate_name(path, name)
stats.record_rbxm_fallback(path, reason)
stats.record_unknown_class(class_name)
stats.record_unknown_property(class, property)
stats.has_issues()      // Any warnings?
stats.has_unknown_types()  // Unknown classes/properties?
stats.log_summary()     // Log final report
```

**Logged at end of syncback:**
```
Syncback complete. Warnings: 2 duplicate names, 1 unknown class
```

## Common Patterns

### Syncing a Script

**Roblox file has:**
```
Script "MyScript"
  Source = "print('hello')"
  Disabled = false
```

**Syncback produces:**
```luau
-- src/MyScript.server.luau
print('hello')
```

**Meta file (if needed):**
```json5
// src/MyScript.server.meta.json5 (for MyScript.server.luau)
{
    "$properties": {
        "Disabled": false
    }
}
```

### Syncing a Model

**Simple model (few children):** → `.rbxm` or `.rbxmx` file
**Complex model (many children):** → Directory with `init.meta.json5` or `init.luau`

### Syncing Nested Folders

```
Roblox:
  Folder "Modules"
    ModuleScript "Utils"
    Folder "Math"
      ModuleScript "Add"

Filesystem:
  src/Modules/
    Utils.luau
    Math/
      Add.luau
```

## Error Handling

**Common errors:**

1. **Middleware mismatch** - Instance type changed but file structure can't accommodate
   - Solution: Use clean mode
   
2. **Invalid filename** - Instance name contains invalid chars
   - Solution: Enable `decodeWindowsInvalidChars` or rename in Studio
   
3. **Ref to pruned instance** - Property points to instance not in project
   - Logged as warning, property not synced
   
4. **Write permission** - Can't write to file
   - Error reported, syncback continues for other files

**Error collection:**
```rust
stats.record_error(&format!("Failed to sync {}: {}", path.display(), error));
```

All errors collected and reported at end.

## Integration with Plugin

**Plugin does NOT handle syncback.** It only handles:
1. Downloading place file from Studio
2. Sending file to server
3. Server runs syncback command

**Two-way sync (experimental)** is different - see `rojo-api.mdc`.

## Performance Considerations

**Hashing overhead:** Hashing is fast (~50µs per instance), negligible compared to disk I/O.

**Incremental benefit:** On large projects, incremental mode with hashing can be 10-100x faster than rewriting everything.

**Parallelization:** Not currently parallelized - filesystem writes are sequential.

## Debugging

**Enable verbose logging:**
```bash
RUST_LOG=rojo::syncback=debug rojo syncback project/ --input place.rbxl
```

**Shows:**
- Which instances are being processed
- Hash comparisons
- File write decisions
- Middleware selection

**Test syncback:**
```bash
# Syncback once
rojo syncback project/ --input place.rbxl

# Build back to verify roundtrip
rojo build project/ -o rebuilt.rbxl

# Compare
diff place.rbxl rebuilt.rbxl
```

## Future Improvements

**From code comments:**
- Better detection of when root is directory vs file
- More sophisticated middleware selection
- Parallel file writes
- Better handling of Roblox format changes
- Support for partial syncback (only changed instances)

## Key Files to Reference

- `src/syncback/mod.rs` - Main orchestration loop
- `src/syncback/snapshot.rs` - SyncbackSnapshot implementation
- `src/syncback/hash/` - Hashing algorithm
- `src/syncback/property_filter.rs` - Property filtering logic
- `src/cli/syncback.rs` - CLI command implementation