Scratch Project Architecture

Project File Format

Modern Scratch (3.x) projects are stored as .sb3 files, which are essentially ZIP archives containing JSON metadata, SVG/PNG assets, and optional audio files. When Scratch is embedded into other systems, the save process often involves asynchronous blob creation and upload to a backend API.

Implications for Large Deployments

In environments where multiple students work on shared workstations or cloud-based Scratch instances, race conditions and incomplete uploads can easily occur if network latency is high or storage quotas are poorly enforced. The risk escalates in kiosks or LMS integrations where multiple save triggers overlap.

Diagnosing Save/Load Corruption

Step 1: Validate Project Integrity

Rename the .sb3 file to .zip and inspect its contents. Missing asset files or malformed project.json entries point to interrupted saves.

unzip MyProject.sb3 -d project_contents
jq . project_contents/project.json

Step 2: Check Backend Storage Logs

Look for incomplete file writes, API timeout errors, or partial multipart uploads in your server logs.

Step 3: Analyze Network Conditions

Run packet captures to detect packet loss or high latency during save events. Even small interruptions can corrupt binary assets embedded in .sb3 files.

Common Pitfalls

• Saving projects over unstable Wi-Fi connections without retry logic.
• Embedding Scratch in iframes without handling postMessage completion events.
• Allowing simultaneous save requests from multiple browser tabs for the same project.
• Not enforcing file size limits before initiating uploads.

Step-by-Step Fixes

1. Implement Transactional Save Logic

Ensure your integration writes project files to temporary storage first, verifies checksum integrity, and only then commits to the final location.

// Pseudo-code for safe save
saveTemp(file)
if (verifyChecksum(file)) {
    commit(file)
} else {
    retrySave()
}

2. Add Client-Side Save Confirmation

Modify the Scratch embed or wrapper to emit a save-complete event only after receiving a backend acknowledgment.

3. Enable Asset Pre-Compression

Large PNG or WAV files increase upload risk. Convert audio to MP3 and images to JPEG where possible before packaging.

4. Use IndexedDB for Local Recovery

For browser-based Scratch, use IndexedDB to store interim project states in case of network failures.

5. Throttle Concurrent Saves

Prevent overlapping save processes by disabling save UI until the current operation completes.

Best Practices for Prevention

• Automate nightly integrity checks on stored .sb3 files.
• Educate instructors to periodically download local copies of student projects.
• Implement server-side project versioning to allow rollback after corruption.
• Use service workers to detect and retry failed uploads silently.
• Stress-test save/load performance under simulated classroom network conditions.

Conclusion

Scratch project corruption in enterprise or educational deployments is not merely a user error—it is often a consequence of architectural oversights in file handling and network resilience. By implementing transactional saves, robust recovery mechanisms, and proactive monitoring, IT teams can maintain a seamless creative environment for large groups of users.

FAQs

1. Can antivirus software cause Scratch save corruption?

Yes. Aggressive real-time scanning can lock temporary files mid-write, causing incomplete .sb3 archives.

2. How can I detect asset loss in Scratch projects automatically?

Parse the project.json and verify that every referenced asset exists in the archive with matching file size.

3. Does Scratch offline editor have the same risks?

Offline editor reduces network-related corruption, but local storage failures or sudden shutdowns can still cause incomplete saves.

4. Can CDN caching interfere with project loads?

Yes. If your LMS serves .sb3 files through a CDN, stale cache entries can cause users to load outdated or incomplete projects.

5. Should I compress assets before import or after export?

Before import is better, as it reduces save time and lowers the risk of mid-upload failure due to large file size.