Merge pull request #499 from DFE-Digital/closedxml-adr

Added ADR for ClosedXML usage

docs/adrs/0012-use-closedxml-library-for-excel-file-manipulation.md

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+# 12. Use ClosedXML Library for Excel File Manipulation
+
+**Date**: 2024-09-13
+
+## Status
+
+Accepted
+
+## Context
+
+We need a reliable way to handle Excel file manipulation in our FIAT project. Various libraries were evaluated, including ClosedXML, EPPlus, and NPOI, each with its own pros and cons.
+
+- **ClosedXML** is widely used, open-source (MIT license), and a wrapper around OpenXML, making it a strong candidate due to its ease of use and low licensing risk. It has 75 million downloads and is widely accepted in the development community.
+- **EPPlus** is another popular choice but requires a commercial license for certain use cases, which incurs additional costs.
+- **NPOI** is an alternative, but there are concerns regarding its association with political issues.
+
+While ClosedXML is actively maintained, the authors have indicated that breaking changes are introduced in some versions, which implies some caution is required when upgrading.
+
+## Decision
+
+We will use the ClosedXML library for Excel file manipulation within FIAT. This decision is based on its:
+
+- Prevalence and wide community usage (15.5k daily downloads).
+- Open-source MIT license, which avoids potential licensing costs.
+- Simplified wrapper around OpenXML, making it easier for developers to interact with Excel files.
+
+Given that our use case is straightforward and doesn't rely on complex or bleeding-edge functionality, the risk of using ClosedXML is minimal. However, we will take care when upgrading to newer versions, reviewing the release notes for breaking changes.
+
+Should we encounter issues in the future, we will migrate to OpenXML, Microsoft's maintained package for manipulating Excel files. While OpenXML is more complex to use, it is a more stable and long-term solution should ClosedXML become unsuitable.
+
+## Consequences
+
+- The library offers a straightforward way to manipulate Excel files, reducing development complexity and time.
+- The open-source nature of the library ensures no additional licensing costs, unlike EPPlus.
+- Some risk of breaking changes when updating the library exists, so a strategy for version management and thorough testing during upgrades will be essential.
+
+This ADR will be revisited if major issues arise or if simpler alternatives (like directly using OpenXML) prove more beneficial in the future.