⚡ Bolt: Offload blocking I/O to anyio in FastAPI routes

[anchapin]

💡 What: Wrapped blocking synchronous operations (generator.generate, Path.read_bytes, Path.exists, generator._compile_pdf, etc.) in the api/main.py FastAPI routes with anyio.to_thread.run_sync (and functools.partial when arguments were needed).
🎯 Why: FastAPI's async def routes run on the main event loop. Any blocking synchronous call in these routes (like running pdflatex or reading files) stalls the entire application, making it unable to process other incoming concurrent requests. This causes significant performance degradation under load.
📊 Impact: Considerably improves concurrent performance by offloading CPU-bound or blocking I/O tasks to worker threads, freeing up the FastAPI event loop to handle hundreds of concurrent requests simultaneously.
🔬 Measurement: Run load tests using locust or wrk against the /v1/render/pdf or /v1/cover-letter API endpoints to observe improved throughput and decreased P99 latencies under concurrent load.

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PR created automatically by Jules for task 15037805867948622709 started by @anchapin

Summary by Sourcery

Offload blocking PDF generation and file I/O in FastAPI resume and cover-letter endpoints to background threads using anyio to prevent blocking the event loop and improve concurrency.

Enhancements:

• Wrap synchronous PDF generation, LaTeX compilation, and file read/write operations in FastAPI routes with anyio.to_thread.run_sync to avoid blocking the async event loop.
• Adjust resume YAML serialization to generate the YAML string in memory and write it via a non-blocking threaded file write.
• Document the performance lesson about using anyio for blocking I/O in FastAPI in .jules/bolt.md for future optimizations.

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[sourcery-ai]

Reviewer's Guide

Offloads blocking PDF generation and file I/O in FastAPI routes to worker threads using anyio, and documents this performance pattern in the Bolt playbook.

Sequence diagram for offloading blocking PDF rendering in FastAPI route

sequenceDiagram
    actor Client
    participant FastAPIRoute as FastAPIRoute_render_pdf
    participant Anyio as AnyioToThread
    participant Generator as TemplateGenerator
    participant FS as FileSystem
    participant PdfEngine as PdfCompilation

    Client->>FastAPIRoute: HTTP POST /v1/render/pdf
    FastAPIRoute->>FastAPIRoute: Create temp_path and output_pdf

    FastAPIRoute->>Anyio: run_sync(generator.generate, variant, pdf, output_pdf)
    Anyio->>Generator: generate(variant, output_format, output_path)
    Generator->>PdfEngine: invoke pdflatex
    PdfEngine-->>Generator: write PDF to output_pdf
    Generator-->>Anyio: return
    Anyio-->>FastAPIRoute: await completion

    FastAPIRoute->>Anyio: run_sync(output_pdf.exists)
    Anyio->>FS: exists(output_pdf)
    FS-->>Anyio: bool
    Anyio-->>FastAPIRoute: exists_result
    FastAPIRoute->>FastAPIRoute: Raise HTTP 500 if exists_result is False

    FastAPIRoute->>Anyio: run_sync(output_pdf.read_bytes)
    Anyio->>FS: read_bytes(output_pdf)
    FS-->>Anyio: pdf_bytes
    Anyio-->>FastAPIRoute: pdf_bytes

    FastAPIRoute-->>Client: HTTP 200 Response(pdf_bytes, application/pdf)

Loading

File-Level Changes

Change	Details	Files
Offload blocking PDF generation and file I/O in async FastAPI routes to anyio thread workers.	Wrap synchronous generator.generate calls in anyio.to_thread.run_sync with functools.partial to pass arguments Run Path.exists and Path.read_bytes for generated PDFs via anyio.to_thread.run_sync to avoid blocking the event loop Offload generator._compile_pdf and subsequent PDF file reads in the cover letter endpoint to anyio.to_thread.run_sync Generate YAML as a string and write it via Path.write_text wrapped in anyio.to_thread.run_sync in the resume PDF endpoint	api/main.py
Document the anyio pattern for handling blocking I/O in FastAPI routes in the Bolt performance guide.	Add a new dated section describing the performance issue caused by blocking operations on the FastAPI event loop Recommend wrapping blocking I/O-bound tasks with anyio.to_thread.run_sync and functools.partial when needed	.jules/bolt.md

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[sourcery-ai]

Hey - I've left some high level feedback:

• You now have several repeated anyio.to_thread.run_sync(functools.partial(...)) call sites; consider extracting a small helper (e.g. run_in_thread(fn, *args, **kwargs)) to reduce duplication and make future changes to threading behavior centralized.
• For the quick Path.exists and Path.read_bytes checks, you might consider grouping related filesystem work into a single threaded function (e.g. generate-and-read) to avoid multiple thread hops per request and keep the I/O pattern easier to reason about.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- You now have several repeated `anyio.to_thread.run_sync(functools.partial(...))` call sites; consider extracting a small helper (e.g. `run_in_thread(fn, *args, **kwargs)`) to reduce duplication and make future changes to threading behavior centralized.
- For the quick `Path.exists` and `Path.read_bytes` checks, you might consider grouping related filesystem work into a single threaded function (e.g. generate-and-read) to avoid multiple thread hops per request and keep the I/O pattern easier to reason about.

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