CodeParkSingle-document writes are atomic, and multi-document ACID transactions are supported on replica sets since 4.0 and on sharded clusters since 4.2
Writes to a single document have always been atomic. Atomic means the write fully happens or does not happen at all. Multi-document ACID transactions arrived in MongoDB 4.0 for replica sets and in 4.2 for sharded clusters.
"MongoDB has no transactions; only single-document operations are atomic" was true only before 4.0. "Transactions work on standalone servers but not on replica sets" is backwards: transactions do not work on a standalone server, only on replica sets and sharded clusters. "Transactions exist but cannot span more than one collection" is wrong: one transaction can change documents in many collections.
Transactions cost more than single-document writes. So good schema design still tries to keep related changes in one document.
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