In RavenDB all actions performed on documents are fully ACID (Atomicity, Consistency, Isolation, Durability).
The indexing mechanism is built on a BASE model (Basically Available, Soft state, Eventual consistency).
RavenDB has a built-in transaction support. A storage engine used by RavenDB under the hood (Esent / Voron) supports ACID transactional model.
ACID for document operations
An each document operation or a batch of operations applied to a set of documents sent in a single HTTP request will execute in a single transaction. The ACID properties of RavenDB for standard transactions:
Atomicity - All operations are atomic. Either they succeed or fail, not midway operation. In particular, operations on multiple documents will all happen atomically, all the way or none at all.
Consistency and Isolation / Consistency of Scans - In a single transaction, all operations operate under snapshot isolation. Even if you access multiple documents, you'll get all of their state as it was in the beginning of the request.
Visibility - All transactions are immediately made available on commit. Thus, if a transaction is commit after updating two docs, you'll always see the updates to those two docs at the same time. (That is, you either see the updates to both, or you don't see the update to either one).
Durability - If an operation has completed successfully, it was fsync'ed to disk. Reads will never return any data that hasn't been flushed to disk.
The transaction model is different when indexes are involved, because indexes are BASE, not ACID. Then the following constraints are applied to query operations:
Basically Available - Index query results will be always available, but they might be stale.
Soft state - The state of the system could change over the time because some amount of time is needed to perform the indexing. This is an incremental operation the less documents remains to index, the more accurate index results we have.
Eventual consistency - The database will eventually become consistent once it stops receiving new documents and the indexing operation finishes.
RavenDB also is able to perform distributed transactions by implementing two-phase commit (2PC) transactions. More about DTC transaction support you will find here.