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<title>Synchronization Model - TaskChampion</title>
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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"><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" class="active"><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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<main>
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<h1 id="synchronization-model"><a class="header" href="#synchronization-model">Synchronization Model</a></h1>
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<p>The <a href="./taskdb.html">task database</a> also implements synchronization.
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Synchronization occurs between disconnected replicas, mediated by a server.
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The replicas never communicate directly with one another.
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The server does not have access to the task data; it sees only opaque blobs of data with a small amount of metadata.</p>
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<p>The synchronization process is a critical part of the task database's functionality, and it cannot function efficiently without occasional synchronization operations</p>
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<h2 id="operational-transforms"><a class="header" href="#operational-transforms">Operational Transforms</a></h2>
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<p>Synchronization is based on <a href="https://en.wikipedia.org/wiki/Operational_transformation">operational transformation</a>.
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This section will assume some familiarity with the concept.</p>
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<h2 id="state-and-operations"><a class="header" href="#state-and-operations">State and Operations</a></h2>
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<p>At a given time, the set of tasks in a replica's storage is the essential "state" of that replica.
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All modifications to that state occur via operations, as defined in <a href="./storage.html">Replica Storage</a>.
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We can draw a network, or graph, with the nodes representing states and the edges representing operations.
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For example:</p>
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<pre><code class="language-text"> o -- State: {abc-d123: 'get groceries', priority L}
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| -- Operation: set abc-d123 priority to H
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o -- State: {abc-d123: 'get groceries', priority H}
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</code></pre>
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<p>For those familiar with distributed version control systems, a state is analogous to a revision, while an operation is analogous to a commit.</p>
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<p>Fundamentally, synchronization involves all replicas agreeing on a single, linear sequence of operations and the state that those operations create.
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Since the replicas are not connected, each may have additional operations that have been applied locally, but which have not yet been agreed on.
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The synchronization process uses operational transformation to "linearize" those operations.</p>
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<p>This process is analogous (vaguely) to rebasing a sequence of Git commits.
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Critically, though, operations cannot merge; in effect, the only option is rebasing.
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Furthermore, once an operation has been sent to the server it cannot be changed; in effect, the server does not permit "force push".</p>
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<h3 id="sync-operations"><a class="header" href="#sync-operations">Sync Operations</a></h3>
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<p>The <a href="./storage.html">Replica Storage</a> model contains additional information in its operations that is not included in operations synchronized to other replicas.
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In this document, we will be discussing "sync operations" of the form</p>
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<ul>
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<li><code>Create(uuid)</code></li>
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<li><code>Delete(uuid)</code></li>
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<li><code>Update(uuid, property, value, timestamp)</code></li>
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</ul>
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<h3 id="versions"><a class="header" href="#versions">Versions</a></h3>
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<p>Occasionally, database states are given a name (that takes the form of a UUID).
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The system as a whole (all replicas) constructs a branch-free sequence of versions and the operations that separate each version from the next.
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The version with the nil UUID is implicitly the empty database.</p>
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<p>The server stores the operations to change a state from a "parent" version to a "child" version, and provides that information as needed to replicas.
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Replicas use this information to update their local task databases, and to generate new versions to send to the server.</p>
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<p>Replicas generate a new version to transmit local changes to the server.
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The changes are represented as a sequence of operations with the state resulting from the final operation corresponding to the version.
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In order to keep the versions in a single sequence, the server will only accept a proposed version from a replica if its parent version matches the latest version on the server.</p>
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<p>In the non-conflict case (such as with a single replica), then, a replica's synchronization process involves gathering up the operations it has accumulated since its last synchronization; bundling those operations into a version; and sending that version to the server.</p>
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<h3 id="replica-invariant"><a class="header" href="#replica-invariant">Replica Invariant</a></h3>
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<p>The replica's <a href="./storage.html">storage</a> contains the current state in <code>tasks</code>, the as-yet un-synchronized operations in <code>operations</code>, and the last version at which synchronization occurred in <code>base_version</code>.</p>
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<p>The replica's un-synchronized operations are already reflected in its local <code>tasks</code>, so the following invariant holds:</p>
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<blockquote>
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<p>Applying <code>operations</code> to the set of tasks at <code>base_version</code> gives a set of tasks identical
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to <code>tasks</code>.</p>
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</blockquote>
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<h3 id="transformation"><a class="header" href="#transformation">Transformation</a></h3>
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<p>When the latest version on the server contains operations that are not present in the replica, then the states have diverged.
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For example:</p>
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<pre><code class="language-text"> o -- version N
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w|\a
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o o
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x| \b
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o o
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y| \c
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o o -- replica's local state
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z|
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o -- version N+1
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</code></pre>
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<p>(diagram notation: <code>o</code> designates a state, lower-case letters designate operations, and versions are presented as if they were numbered sequentially)</p>
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<p>In this situation, the replica must "rebase" the local operations onto the latest version from the server and try again.
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This process is performed using operational transformation (OT).
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The result of this transformation is a sequence of operations based on the latest version, and a sequence of operations the replica can apply to its local task database to reach the same state
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Continuing the example above, the resulting operations are shown with <code>'</code>:</p>
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<pre><code class="language-text"> o -- version N
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w|\a
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o o
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x| \b
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o o
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y| \c
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o o -- replica's intermediate local state
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z| |w'
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o-N+1 o
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a'\ |x'
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o o
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b'\ |y'
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o o
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c'\|z'
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o -- version N+2
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</code></pre>
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<p>The replica applies w' through z' locally, and sends a' through c' to the server as the operations to generate version N+2.
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Either path through this graph, a-b-c-w'-x'-y'-z' or a'-b'-c'-w-x-y-z, must generate <em>precisely</em> the same final state at version N+2.
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Careful selection of the operations and the transformation function ensure this.</p>
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<p>See the comments in the source code for the details of how this transformation process is implemented.</p>
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<h2 id="synchronization-process"><a class="header" href="#synchronization-process">Synchronization Process</a></h2>
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<p>To perform a synchronization, the replica first requests the child version of <code>base_version</code> from the server (GetChildVersion).
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It applies that version to its local <code>tasks</code>, rebases its local <code>operations</code> as described above, and updates <code>base_version</code>.
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The replica repeats this process until the server indicates no additional child versions exist.
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If there are no un-synchronized local operations, the process is complete.</p>
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<p>Otherwise, the replica creates a new version containing its local operations, giving its <code>base_version</code> as the parent version, and transmits that to the server (AddVersion).
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In most cases, this will succeed, but if another replica has created a new version in the interim, then the new version will conflict with that other replica's new version and the server will respond with the new expected parent version.
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In this case, the process repeats.
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If the server indicates a conflict twice with the same expected base version, that is an indication that the replica has diverged (something serious has gone wrong).</p>
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<h2 id="servers"><a class="header" href="#servers">Servers</a></h2>
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<p>A replica depends on periodic synchronization for performant operation.
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Without synchronization, its list of pending operations would grow indefinitely, and tasks could never be expired.
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So all replicas, even "singleton" replicas which do not replicate task data with any other replica, must synchronize periodically.</p>
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<p>TaskChampion provides a <code>LocalServer</code> for this purpose.
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It implements the <code>get_child_version</code> and <code>add_version</code> operations as described, storing data on-disk locally.</p>
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