634aaadb73
This abandons field-by-field compatibility with the TaskWarrior TDB2 format, which wasn't a sustainable strategy anyway. Instead, tasks are represented as a TaskMap with custom key formats. In this commit, there are only a few allowed keys, with room to grow. Replica returns convenience wrappers Task (read-only) and TaskMut (read-write) with getters and setters to make modifying tasks easier.
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Replica Storage
Each replica has a storage backend.
The interface for this backend is given in crate::taskstorage::TaskStorage and TaskStorageTxn.
The storage is transaction-protected, with the expectation of a serializable isolation level. The storage contains the following information:
tasks: a set of tasks, indexed by UUIDbase_version: the number of the last version sync'd from the server (a single integer)operations: all operations performed since base_versionworking_set: a mapping from integer -> UUID, used to keep stable small-integer indexes into the tasks for users' convenience. This data is not synchronized with the server and does not affect any consistency guarantees.
Tasks
The tasks are stored as an un-ordered collection, keyed by task UUID. Each task in the database has represented by a key-value map. See Tasks for details on the content of that map.
Operations
Every change to the task database is captured as an operation. In other words, operations act as deltas between database states. Operations are crucial to synchronization of replicas, using a technique known as Operational Transforms.
Each operation has one of the forms
Create(uuid)Delete(uuid)Update(uuid, property, value, timestamp)
The Create form creates a new task. It is invalid to create a task that already exists.
Similarly, the Delete form deletes an existing task. It is invalid to delete a task that does not exist.
The Update form updates the given property of the given task, where property and value are both strings.
Value can also be None to indicate deletion of a property.
It is invalid to update a task that does not exist.
The timestamp on updates serves as additional metadata and is used to resolve conflicts.