Ledger is a distributed storage system for Fuchsia.
Each application (or more precisely, each [component]) running on behalf of a particular user has a separate data store provided and managed by Ledger, and vended to a client application by Fuchsia framework through its component context.
The data store for the particular component/user combination is private - not accessible to other apps of the same user, and not accessible to other users of the same app.
Each data store is transparently synchronized across devices of its user through a cloud provider. Any data operations are made offline-first with no coordination with the cloud. If concurrent modifications result in a data conflict, the conflict is resolved using an app-configurable merge policy.
Each data store is organized into collections exposing a key-value store API called pages. Page API supports storing data of arbitrary size, atomic changes across multiple keys, snapshots and modification observers.
Ledger should be used by software storing data that is scoped to a single user, and that needs to be synced on all of the users's devices. Ledger should also be used to store data that needs to be restored after a user resets a device.
The exception is if the data needs to be processed in the cloud or on a device not owned by the user. Because Ledger transfers opaque encrypted data, the data would need to be exchanged via a different channel.
There is a computation and disk space cost to storing data in Ledger, so if the data does not need to be persisted and synced, do not use Ledger. For example, Ledger is not the optimal for local caches.
Ledger is available to any software running under the Modular framework.
Documentation for using Ledger:
Documentation for integrating with Ledger in client apps:
Documentation for developing Ledger: