A comparison of transaction models in distributed document databases highlights the differences between Couchbase, MongoDB, Google Cloud Firestore, and Fauna. Couchbase's transactions are client-side only, with poor consistency levels, generality, and performance, but adequate ease-of-use. MongoDB offers better consistency levels, generality, and performance, but poor theoretical consistency due to the complexity of its configuration options. Google Cloud Firestore has a strong consistency model, good generality, and performance, but limited query flexibility and data model strangeness. Fauna provides a high level of consistency, strict serializability, and performance, with a comprehensive standard library within its FQL API, making it an ideal choice for developers seeking a document database with strong transactional capabilities.