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docs/design_notes.md

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+% SPDX-FileCopyrightText: 2023 James R. Barlow
+% SPDX-License-Identifier: CC-BY-SA-4.0
+
+# Design notes
+
+## Why doesn\'t OCRmyPDF use PyTesseract?
+
+PyTesseract is a Python wrapper around the Tesseract OCR engine. When
+OCRmyPDF was first written, PyTesseract used ABI bindings to call the
+Tesseract library. This was not a good fit for OCRmyPDF because ABI
+bindings can be fragile.
+
+PyTesseract has since evolved calling the Tesseract executable,
+abandoning the ABI approach and using the CLI instead, just like
+OCRmyPDF does. If it were written from scratch today, OCRmyPDF might use
+PyTesseract.
+
+PyTesseract has more features don\'t particularly need PDF output, but
+less features than OCRmyPDF\'s API for creating PDFs.
+
+## What is `executor()`?
+
+OCRmyPDF uses a custom concurrent executor which can support either
+threads or processes with the same interface. This is useful because
+OCRmyPDF can use either threads or processes to parallelize work,
+whichever is more appropriate for the task at hand.
+
+The interface is currently private and subject to change. In particular,
+if experiments with asyncio and anyio are successful, the interface will
+change.