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281 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"><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" class="active"><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="server-replica-protocol"><a class="header" href="#server-replica-protocol">Server-Replica Protocol</a></h1>
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<p>The server-replica protocol is defined abstractly in terms of request/response transactions.</p>
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<p>The protocol builds on the model presented in the previous chapters, and in particular on the synchronization process.</p>
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<h2 id="clients"><a class="header" href="#clients">Clients</a></h2>
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<p>From the protocol's perspective, replicas accessing the same task history are indistinguishable, so this protocol uses the term "client" to refer generically to all replicas replicating a single task history.</p>
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<h2 id="server"><a class="header" href="#server">Server</a></h2>
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<p>A server implements the requests and responses described below.
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Where the logic is implemented depends on the specific implementation of the protocol.</p>
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<p>For each client, the server is responsible for storing the task history, in the form of a branch-free sequence of versions.
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It also stores the latest snapshot, if any exists.
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From the server's perspective, snapshots and versions are opaque byte sequences.</p>
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<h2 id="version-invariant"><a class="header" href="#version-invariant">Version Invariant</a></h2>
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<p>The following invariant must always hold:</p>
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<blockquote>
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<p>All versions are linked by parent-child relationships to form a single chain.
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That is, each version must have no more than one parent and one child, and no more than one version may have zero parents or zero children.</p>
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</blockquote>
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<h2 id="data-formats"><a class="header" href="#data-formats">Data Formats</a></h2>
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<p>Task data sent to the server is encrypted by the client, using the scheme described in the "Encryption" chapter.</p>
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<h3 id="version"><a class="header" href="#version">Version</a></h3>
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<p>The decrypted form of a version is a JSON array containing operations in the order they should be applied.
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Each operation has the form <code>{TYPE: DATA}</code>, for example:</p>
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<ul>
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<li><code>[{"Create":{"uuid":"56e0be07-c61f-494c-a54c-bdcfdd52d2a7"}}]</code></li>
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<li><code>[{"Delete":{"uuid":"56e0be07-c61f-494c-a54c-bdcfdd52d2a7"}}]</code></li>
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<li><code>[{"Update":{"uuid":"56e0be07-c61f-494c-a54c-bdcfdd52d2a7","property":"prop","value":"v","timestamp":"2021-10-11T12:47:07.188090948Z"}}]</code></li>
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<li><code>[{"Update":{"uuid":"56e0be07-c61f-494c-a54c-bdcfdd52d2a7","property":"prop","value":null,"timestamp":"2021-10-11T12:47:07.188090948Z"}}]</code> (to delete a property)</li>
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</ul>
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<p>Timestamps are in RFC3339 format with a <code>Z</code> suffix.</p>
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<h3 id="snapshot"><a class="header" href="#snapshot">Snapshot</a></h3>
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<p>The decrypted form of a snapshot is a JSON object mapping task IDs to task properties.
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For example (pretty-printed for clarity):</p>
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<pre><code class="language-json">{
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"56e0be07-c61f-494c-a54c-bdcfdd52d2a7": {
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"description": "a task",
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"priority": "H"
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},
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"4b7ed904-f7b0-4293-8a10-ad452422c7b3": {
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"description": "another task"
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}
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}
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</code></pre>
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<h2 id="transactions"><a class="header" href="#transactions">Transactions</a></h2>
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<p>All interactions between the client and server are defined in terms of request/response transactions, as described here.</p>
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<h3 id="addversion"><a class="header" href="#addversion">AddVersion</a></h3>
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<p>The AddVersion transaction requests that the server add a new version to the client's task history.
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The request contains the following;</p>
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<ul>
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<li>parent version ID, and</li>
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<li>encrypted version data.</li>
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</ul>
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<p>The server determines whether the new version is acceptable, atomically with respect to other requests for the same client.
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If it has no versions for the client, it accepts the version.
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If it already has one or more versions for the client, then it accepts the version only if the given parent version has no children, thereby maintaining the version invariant.</p>
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<p>If the version is accepted, the server generates a new version ID for it.
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The version is added to the chain of versions for the client, and the new version ID is returned in the response to the client.
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The response may also include a request for a snapshot, with associated urgency.</p>
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<p>If the version is not accepted, the server makes no changes, but responds to the client with a conflict indication containing the ID of the version which has no children.
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The client may then "rebase" its operations and try again.
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Note that if a client receives two conflict responses with the same parent version ID, it is an indication that the client's version history has diverged from that on the server.</p>
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<h3 id="getchildversion"><a class="header" href="#getchildversion">GetChildVersion</a></h3>
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<p>The GetChildVersion transaction is a read-only request for a version.
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The request consists of a parent version ID.
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The server searches its set of versions for a version with the given parent ID.
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If found, it returns the version's</p>
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<ul>
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<li>version ID,</li>
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<li>parent version ID (matching that in the request), and</li>
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<li>encrypted version data.</li>
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</ul>
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<p>If not found, it returns an indication that no such version exists.</p>
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<h3 id="addsnapshot"><a class="header" href="#addsnapshot">AddSnapshot</a></h3>
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<p>The AddSnapshot transaction requests that the server store a new snapshot, generated by the client.
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The request contains the following:</p>
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<ul>
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<li>version ID at which the snapshot was made, and</li>
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<li>encrypted snapshot data.</li>
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</ul>
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<p>The server should validate that the snapshot is for an existing version and is newer than any existing snapshot.
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It may also validate that the snapshot is for a "recent" version (e.g., one of the last 5 versions).
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If a snapshot already exists for the given version, the server may keep or discard the new snapshot but should return a success indication to the client.</p>
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<p>The server response is empty.</p>
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<h3 id="getsnapshot"><a class="header" href="#getsnapshot">GetSnapshot</a></h3>
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<p>The GetSnapshot transaction requests that the server provide the latest snapshot.
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The response contains the snapshot version ID and the snapshot data, if those exist.</p>
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