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265 lines
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265 lines
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<ol class="chapter"><li class="chapter-item expanded "><a href="installation.html"><strong aria-hidden="true">1.</strong> Installation</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="running-sync-server.html"><strong aria-hidden="true">1.1.</strong> Running the Sync Server</a></li></ol></li><li class="chapter-item expanded "><a href="internals.html"><strong aria-hidden="true">2.</strong> Internal Details</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="data-model.html"><strong aria-hidden="true">2.1.</strong> Data Model</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="storage.html" class="active"><strong aria-hidden="true">2.1.1.</strong> Replica Storage</a></li><li class="chapter-item expanded "><a href="taskdb.html"><strong aria-hidden="true">2.1.2.</strong> Task Database</a></li><li class="chapter-item expanded "><a href="tasks.html"><strong aria-hidden="true">2.1.3.</strong> Tasks</a></li></ol></li><li class="chapter-item expanded "><a href="sync.html"><strong aria-hidden="true">2.2.</strong> Synchronization and the Sync Server</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="sync-model.html"><strong aria-hidden="true">2.2.1.</strong> Synchronization Model</a></li><li class="chapter-item expanded "><a href="snapshots.html"><strong aria-hidden="true">2.2.2.</strong> Snapshots</a></li><li class="chapter-item expanded "><a href="sync-protocol.html"><strong aria-hidden="true">2.2.3.</strong> Server-Replica Protocol</a></li><li class="chapter-item expanded "><a href="encryption.html"><strong aria-hidden="true">2.2.4.</strong> Encryption</a></li><li class="chapter-item expanded "><a href="http.html"><strong aria-hidden="true">2.2.5.</strong> HTTP Implementation</a></li><li class="chapter-item expanded "><a href="object-store.html"><strong aria-hidden="true">2.2.6.</strong> Object-Store Implementation</a></li><li class="chapter-item expanded "><a href="plans.html"><strong aria-hidden="true">2.2.7.</strong> Planned Functionality</a></li></ol></li></ol></li></ol> </div>
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<h1 class="menu-title">TaskChampion</h1>
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<h1 id="replica-storage"><a class="header" href="#replica-storage">Replica Storage</a></h1>
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<p>Each replica has a storage backend.
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The interface for this backend is given in <code>crate::taskstorage::Storage</code> and <code>StorageTxn</code>.</p>
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<p>The storage is transaction-protected, with the expectation of a serializable isolation level.
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The storage contains the following information:</p>
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<ul>
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<li><code>tasks</code>: a set of tasks, indexed by UUID</li>
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<li><code>base_version</code>: the number of the last version sync'd from the server (a single integer)</li>
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<li><code>operations</code>: all operations performed since base_version</li>
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<li><code>working_set</code>: 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.</li>
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</ul>
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<h2 id="tasks"><a class="header" href="#tasks">Tasks</a></h2>
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<p>The tasks are stored as an un-ordered collection, keyed by task UUID.
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Each task in the database has represented by a key-value map.
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See <a href="./tasks.html">Tasks</a> for details on the content of that map.</p>
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<h2 id="operations"><a class="header" href="#operations">Operations</a></h2>
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<p>Every change to the task database is captured as an operation.
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In other words, operations act as deltas between database states.
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Operations are crucial to synchronization of replicas, described in <a href="./sync-model.html">Synchronization Model</a>.</p>
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<p>Operations are entirely managed by the replica, and some combinations of operations are described as "invalid" here.
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A replica must not create invalid operations, but should be resilient to receiving invalid operations during a synchronization operation.</p>
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<p>Each operation has one of the forms </p>
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<ul>
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<li><code>Create(uuid)</code></li>
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<li><code>Delete(uuid, oldTask)</code></li>
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<li><code>Update(uuid, property, oldValue, newValue, timestamp)</code></li>
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<li><code>UndoPoint()</code></li>
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</ul>
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<p>The Create form creates a new task.
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It is invalid to create a task that already exists.</p>
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<p>Similarly, the Delete form deletes an existing task.
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It is invalid to delete a task that does not exist.
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The <code>oldTask</code> property contains the task data from before it was deleted.</p>
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<p>The Update form updates the given property of the given task, where the property and values are strings.
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The <code>oldValue</code> gives the old value of the property (or None to create a new property), while <code>newValue</code> gives the new value (or None to delete a property).
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It is invalid to update a task that does not exist.
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The timestamp on updates serves as additional metadata and is used to resolve conflicts.</p>
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<h3 id="application"><a class="header" href="#application">Application</a></h3>
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<p>Each operation can be "applied" to a task database in a natural way:</p>
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<ul>
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<li>Applying <code>Create</code> creates a new, empty task in the task database.</li>
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<li>Applying <code>Delete</code> deletes a task, including all of its properties, from the task database.</li>
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<li>Applying <code>Update</code> modifies the properties of a task.</li>
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<li>Applying <code>UndoPoint</code> does nothing.</li>
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</ul>
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<h3 id="undo"><a class="header" href="#undo">Undo</a></h3>
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<p>Each operation also contains enough information to reverse its application:</p>
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<ul>
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<li>Undoing <code>Create</code> deletes a task.</li>
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<li>Undoing <code>Delete</code> creates a task, including all of the properties in <code>oldTask</code>.</li>
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<li>Undoing <code>Update</code> modifies the properties of a task, reverting to <code>oldValue</code>.</li>
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<li>Undoing <code>UndoPoint</code> does nothing.</li>
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</ul>
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<p>The <code>UndoPoint</code> operation serves as a marker of points in the operation sequence to which the user might wish to undo.
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For example, creation of a new task with several properities involves several operations, but is a single step from the user's perspective.
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An "undo" command reverses operations, removing them from the operations sequence, until it reaches an <code>UndoPoint</code> operation.</p>
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<h3 id="synchronizing-operations"><a class="header" href="#synchronizing-operations">Synchronizing Operations</a></h3>
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<p>After operations are synchronized to the server, they can no longer be undone.
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As such, the <a href="./sync-model.html">synchronization model</a> uses simpler operations.
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Replica operations are converted to sync operations as follows:</p>
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<ul>
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<li><code>Create(uuid)</code> -> <code>Create(uuid)</code> (no change)</li>
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<li><code>Delete(uuid, oldTask)</code> -> <code>Delete(uuid)</code></li>
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<li><code>Update(uuid, property, oldValue, newValue, timestamp)</code> -> <code>Update(uuid, property, newValue, timestamp)</code></li>
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<li><code>UndoPoint()</code> -> Ø (dropped from operation sequence)</li>
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</ul>
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<p>Once a sequence of operations has been synchronized, there is no need to store those operations on the replica.
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The current implementation deletes operations at that time.
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An alternative approach is to keep operations for existing tasks, and provide access to those operations as a "history" of modifications to the task.</p>
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