⚡ Bolt: Offload blocking I/O operations to worker threads in async FastAPI routes

[anchapin]

💡 What: Offloaded blocking synchronous operations (generator.generate, Path.exists, Path.read_bytes) in FastAPI endpoints render_pdf and render_resume_pdf to worker threads using anyio.to_thread.run_sync and functools.partial.
🎯 Why: FastAPI endpoints defined with async def run directly on the main async event loop. Executing blocking, CPU-bound tasks (like LaTeX PDF compilation in generator.generate) and synchronous file I/O operations on the main thread blocked the event loop. This caused a major concurrency bottleneck, completely starving the server from handling other incoming requests while a PDF was being generated or read.
📊 Impact: Significantly improves the API's concurrent request handling capabilities. The event loop is no longer blocked during expensive PDF generations and file reads, avoiding total server freezes under load.
🔬 Measurement: Verify by running load tests against the /v1/render/pdf or /v1/resumes/{id}/render/pdf endpoints. You should observe much better concurrent processing times without the server pausing entirely during individual request execution. Run test suite via python -m pytest to confirm everything works as expected.

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PR created automatically by Jules for task 18003314079667917910 started by @anchapin

Summary by Sourcery

Offload blocking PDF generation and file I/O in async FastAPI PDF rendering endpoints to worker threads to improve concurrency and responsiveness.

Enhancements:

• Run synchronous PDF generation and filesystem operations in render_pdf and render_resume_pdf via worker threads instead of the main event loop.

Documentation:

• Document the performance impact and best practices for offloading blocking I/O in FastAPI async endpoints in the Bolt engineering notes.

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[sourcery-ai]

Reviewer's Guide

Async FastAPI PDF rendering endpoints now offload blocking PDF generation and filesystem I/O to worker threads via anyio.to_thread.run_sync, improving event loop responsiveness, and the performance notes in .jules/bolt.md are updated with this pattern and its rationale.

Sequence diagram for async PDF rendering with worker thread offloading

sequenceDiagram
    actor Client
    participant FastAPI as FastAPIApp
    participant render_pdf as render_pdf_endpoint
    participant EventLoop as AsyncEventLoop
    participant AnyIO as anyio_to_thread
    participant Worker as WorkerThread
    participant Generator as TemplateGenerator
    participant FS as FileSystem

    Client->>FastAPI: HTTP POST /v1/render/pdf
    FastAPI->>EventLoop: schedule render_pdf
    EventLoop->>render_pdf: execute async handler

    render_pdf->>AnyIO: run_sync(generate_func)
    AnyIO->>Worker: execute generator.generate
    Worker->>Generator: generate(variant, output_format, output_path)
    Generator-->>Worker: PDF written to output.pdf
    Worker-->>AnyIO: return
    AnyIO-->>render_pdf: await result

    render_pdf->>AnyIO: run_sync(output_pdf.exists)
    AnyIO->>Worker: execute Path.exists
    Worker->>FS: check output_pdf
    FS-->>Worker: exists
    Worker-->>AnyIO: return bool
    AnyIO-->>render_pdf: exists
    render_pdf->>render_pdf: validate exists or raise HTTPException

    render_pdf->>AnyIO: run_sync(output_pdf.read_bytes)
    AnyIO->>Worker: execute Path.read_bytes
    Worker->>FS: read output_pdf bytes
    FS-->>Worker: content bytes
    Worker-->>AnyIO: return content
    AnyIO-->>render_pdf: content bytes

    render_pdf-->>FastAPI: Response(content=content, media_type=application/pdf)
    FastAPI-->>Client: HTTP 200 PDF response

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File-Level Changes

Change	Details	Files
Offload blocking PDF generation and file I/O from async FastAPI endpoints to worker threads using anyio.to_thread.run_sync and functools.partial.	Wrap generator.generate invocations in partial with the appropriate keyword arguments and execute them via await anyio.to_thread.run_sync(...) instead of calling synchronously in async routes. Replace direct Path.exists() checks with exists = await anyio.to_thread.run_sync(output_pdf.exists) and branch on the resulting boolean in async endpoints. Replace direct Path.read_bytes() calls with content = await anyio.to_thread.run_sync(output_pdf.read_bytes) to avoid blocking the event loop during file reads in async handlers.	api/main.py
Document the performance learning about offloading blocking I/O in FastAPI and the chosen implementation pattern.	Add a new dated note describing how blocking CPU-bound work and filesystem I/O in async FastAPI endpoints degrade concurrency. Record the action guideline to use anyio.to_thread.run_sync for synchronous work and functools.partial for passing keyword arguments to run_sync.	.jules/bolt.md

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[sourcery-ai]

Hey - I've left some high level feedback:

• The render_pdf and render_resume_pdf endpoints now share nearly identical generate_func and to_thread.run_sync logic; consider extracting a small helper (e.g., async generate_pdf_to_path(...) or a generic async run_blocking(...)) to reduce duplication and keep future changes to the offloading behavior in one place.
• Since anyio.to_thread.run_sync accepts *args/**kwargs, you could simplify the calls by passing arguments directly instead of constructing a functools.partial, which would make the offloading sites a bit more readable.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The `render_pdf` and `render_resume_pdf` endpoints now share nearly identical `generate_func` and `to_thread.run_sync` logic; consider extracting a small helper (e.g., `async generate_pdf_to_path(...)` or a generic `async run_blocking(...)`) to reduce duplication and keep future changes to the offloading behavior in one place.
- Since `anyio.to_thread.run_sync` accepts `*args`/`**kwargs`, you could simplify the calls by passing arguments directly instead of constructing a `functools.partial`, which would make the offloading sites a bit more readable.

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