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taskchampion/snapshots.html
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taskchampion/snapshots.html
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<!DOCTYPE HTML>
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<!-- Book generated using mdBook -->
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<title>Snapshots - 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"><strong aria-hidden="true">2.2.1.</strong> Synchronization Model</a></li><li class="chapter-item expanded "><a href="snapshots.html" class="active"><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="plans.html"><strong aria-hidden="true">2.2.4.</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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link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
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</script>
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<div id="content" class="content">
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<main>
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<h1 id="snapshots"><a class="header" href="#snapshots">Snapshots</a></h1>
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<p>The basic synchronization model described in the previous page has a few shortcomings:</p>
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<ul>
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<li>servers must store an ever-increasing quantity of versions</li>
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<li>a new replica must download all versions since the beginning in order to derive the current state</li>
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</ul>
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<p>Snapshots allow TaskChampion to avoid both of these issues.
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A snapshot is a copy of the task database at a specific version.
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It is created by a replica, encrypted, and stored on the server.
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A new replica can simply download a recent snapshot and apply any additional versions synchronized since that snapshot was made.
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Servers can delete and reclaim space used by older versions, as long as newer snapshots are available.</p>
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<h2 id="snapshot-heuristics"><a class="header" href="#snapshot-heuristics">Snapshot Heuristics</a></h2>
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<p>A server implementation must answer a few questions:</p>
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<ul>
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<li>How often should snapshots be made?</li>
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<li>When can versions be deleted?</li>
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<li>When can snapshots be deleted?</li>
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</ul>
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<p>A critical invariant is that at least one snapshot must exist for any database that does not have a child of the nil version.
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This ensures that a new replica can always derive the latest state.</p>
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<p>Aside from that invariant, the server implementation can vary in its answers to these questions, with the following considerations:</p>
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<p>Snapshots should be made frequently enough that a new replica can initialize quickly.</p>
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<p>Existing replicas will fail to synchronize if they request a child version that has been deleted.
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This failure can cause data loss if the replica had local changes.
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It's conceivable that replicas may not sync for weeks or months if, for example, they are located on a home computer while the user is on holiday.</p>
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<h2 id="requesting-new-snapshots"><a class="header" href="#requesting-new-snapshots">Requesting New Snapshots</a></h2>
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<p>The server requests snapshots from replicas, indicating an urgency for the request.
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Some replicas, such as those running on PCs or servers, can produce a snapshot even at low urgency.
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Other replicas, in more restricted environments such as mobile devices, will only produce a snapshot at high urgency.
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This saves resources in these restricted environments.</p>
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<p>A snapshot must be made on a replica with no unsynchronized operations.
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As such, it only makes sense to request a snapshot in response to a successful AddVersion request.</p>
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<h2 id="handling-deleted-versions"><a class="header" href="#handling-deleted-versions">Handling Deleted Versions</a></h2>
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<p>When a replica requests a child version, the response must distinguish two cases:</p>
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<ol>
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<li>No such child version exists because the replica is up-to-date.</li>
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<li>No such child version exists because it has been deleted, and the replica must re-initialize itself.</li>
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</ol>
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<p>The details of this logic are covered in the <a href="./sync-protocol.html">Server-Replica Protocol</a>.</p>
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</main>
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