⚡ Bolt: Fix blocking I/O in async endpoint#6
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- Offload synchronous file I/O and SQLAlchemy database operations to thread pool using `asyncio.to_thread`. - Prevents blocking the main event loop during image uploads and DB commits. - Improves concurrency for the `create_issue` endpoint.
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Pull request overview
This PR refactors the create_issue endpoint to offload blocking I/O operations to separate threads using asyncio.to_thread(), preventing the FastAPI event loop from blocking during file writes and database commits.
Key Changes:
- Wrapped file I/O operations in a
save_file()function executed viaasyncio.to_thread() - Wrapped database operations in a
save_to_db()function executed viaasyncio.to_thread()
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| def save_to_db(): | ||
| new_issue = Issue( | ||
| description=description, | ||
| category=category, | ||
| image_path=image_path, | ||
| source="web" | ||
| ) | ||
| db.add(new_issue) | ||
| db.commit() | ||
| db.refresh(new_issue) | ||
| return new_issue | ||
|
|
||
| new_issue = await asyncio.to_thread(save_to_db) |
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The database session db is not thread-safe and should not be used across threads. SQLAlchemy sessions are bound to the thread that created them. When save_to_db() executes in a separate thread via asyncio.to_thread(), it uses the db session that was created in the main event loop thread, which can lead to undefined behavior, connection errors, or data corruption.
Instead, create a new database session within the save_to_db() function that will be used exclusively in that thread. You can import SessionLocal from backend.database and instantiate a new session inside the function, ensuring proper closure with a try-finally block.
| # Offload blocking file I/O to a thread | ||
| def save_file(): | ||
| with open(image_path, "wb") as buffer: | ||
| shutil.copyfileobj(image.file, buffer) |
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The image.file object (a SpooledTemporaryFile from UploadFile) is being accessed in a separate thread. FastAPI's UploadFile is designed to work with async code and may have issues when the underlying file object is accessed from a different thread than where the UploadFile was created.
A safer approach is to read the file contents in the main async context first, then pass the bytes to the thread for writing. Alternatively, ensure the file object's position is properly managed and consider using image.file.read() before passing to the thread.
| # Offload blocking file I/O to a thread | |
| def save_file(): | |
| with open(image_path, "wb") as buffer: | |
| shutil.copyfileobj(image.file, buffer) | |
| # Read the uploaded file in the async context to avoid accessing | |
| # the underlying UploadFile.file from a separate thread. | |
| image_data = await image.read() | |
| # Offload blocking file I/O to a thread | |
| def save_file(): | |
| with open(image_path, "wb") as buffer: | |
| buffer.write(image_data) |
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⚡ Bolt: Offload blocking I/O in async endpoint
💡 What:
Refactored the
create_issueendpoint inbackend/main.pyto run blocking file write operations and synchronous database commits in a separate thread usingasyncio.to_thread.🎯 Why:
The original implementation performed synchronous file I/O (
shutil.copyfileobj) and database operations (db.commit) directly within anasync defendpoint. In FastAPI, this blocks the main event loop, preventing the server from handling other concurrent requests while these operations complete.📊 Impact:
Prevents the server from freezing during file uploads or database waits. While difficult to measure with small loads/files, this is a critical architectural fix for scalability and responsiveness under load.
🔬 Measurement:
Verified with a concurrent benchmark script (
benchmark.py) sending 3 simultaneous requests. The server handled them successfully without errors, and the logic now adheres to FastAPI best practices forasyncendpoints.PR created automatically by Jules for task 7293520402018783653 started by @RohanExploit