https://cloud.google.com/firestore/docs

Firestore is a NoSQL document database built for automatic scaling

Firestore supports multi-document, ACID-compliant transactions

Firestore provides real-time updates by letting clients listen to a document and use queries to get real-time updates. You provide a callback that immediately creates a document snapshot with the current contents of a single document.

Firestore doesn't support join operations, inequality filtering on multiple properties, or filtering on data that is based on results of a subquery.

Firestore is optimized for OLTP

If your app needs both OLTP and OLAP systems, use Firestore as your OLTP system and incrementally sync or export Firestore data to BigQuery for analysis.

Firestore prioritizes durability and availability over latency by doing cross-region or cross-zone synchronous writes. If your app demands consistent sub–10 millisecond latency when reading or writing data, consider using an in-memory database like Memorystore for Redis.

firestore is just a next generation of datastore

Firestore in Datastore mode

https://cloud.google.com/datastore/docs/firestore-or-datastore

Firestore in Datastore mode uses Datastore system behavior but accesses Firestore's storage layer, removing the following Datastore limitations:

• Eventual consistency: Datastore queries become strongly consistent unless you explicitly request eventual consistency.
• Queries in transactions are no longer required to be ancestor queries.
• Transactions are no longer limited to 25 entity groups.
• Writes to an entity group are no longer limited to 1 per second.

Datastore mode disables Firestore features that are not compatible with Datastore:

• The project will accept Datastore API requests and deny Firestore API requests.
• The project will use Datastore indexes instead of Firestore indexes.