wilhuff

The backend is almost entirely custom, though many common low-level open-source components are used pervasively. For example, for components written in C++ we use Abseil, Google Test, etc.

The SDKs are all open source and make use of other open source components:

  * Android: https://github.com/firebase/firebase-android-sdk/tree/master/firebase-firestore
  * iOS: https://github.com/firebase/firebase-ios-sdk/tree/master/Firestore
  * Web: https://github.com/firebase/firebase-js-sdk/tree/master/packages/firestore

For example, the iOS SDK uses LevelDB for local key-value storage and gRPC for communication with the backend.

It's probably better said that Cloud Firestore and Cloud Spanner share common infrastructure.

For example, Cloud Firestore uses TrueTime to provide similarly strong consistency guarantees to Cloud Spanner (see https://cloud.google.com/spanner/docs/true-time-external-con...)

I'm not aware of any whitepapers describing the listen feature that provides real-time updates.