MongoDB's introduction of multi-document ACID transactions in version 4.0 marks a significant enhancement, allowing developers to manage complex applications with greater ease and data integrity. Historically, MongoDB has relied on its document model, which supports single-document atomicity, to meet most application needs, but the addition of multi-document transactions addresses more complex use cases, particularly in "System of Record" or "Line of Business" applications. These transactions provide a consistent view of data and enforce all-or-nothing execution, akin to what developers expect from relational databases, without sacrificing MongoDB's core benefits such as flexibility and distributed architecture. The development of these transactions has been a multi-year effort, improving MongoDB's capabilities without impacting performance for workloads that do not require them. This enhancement not only simplifies application logic but also aligns MongoDB with the transactional guarantees traditionally offered by relational databases, offering developers the ability to build applications faster and more efficiently while maintaining the flexibility of a non-relational database.