382 lines
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382 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="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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<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 from the replica to the server.
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This is made concrete in an HTTP representation.</p>
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<p>The protocol builds on the model presented in the previous chapter, 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 server'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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<p>Each client is identified and authenticated with a "client_id key", known only to the server and to the replicas replicating the task history.</p>
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<h2 id="server"><a class="header" href="#server">Server</a></h2>
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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.</p>
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<ul>
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<li>versions: a set of {versionId: UUID, parentVersionId: UUID, historySegment: bytes}</li>
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<li>latestVersionId: UUID</li>
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<li>snapshotVersionId: UUID</li>
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<li>snapshot: bytes</li>
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</ul>
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<p>For each client, it stores a set of versions as well as the latest version ID, defaulting to the nil UUID.
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Each version has a version ID, a parent version ID, and a history segment (opaque data containing the operations for that version).
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The server should maintain the following invariants for each client:</p>
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<ol>
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<li>latestVersionId is nil or exists in the set of versions.</li>
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<li>Given versions v1 and v2 for a client, with v1.versionId != v2.versionId and v1.parentVersionId != nil, v1.parentVersionId != v2.parentVersionId.
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In other words, versions do not branch.</li>
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<li>If snapshotVersionId is nil, then there is a version with parentVersionId == nil.</li>
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<li>If snapshotVersionId is not nil, then there is a version with parentVersionId = snapshotVersionId.</li>
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</ol>
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<p>Note that versions form a linked list beginning with the latestVersionId stored for the client.
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This linked list need not continue back to a version with v.parentVersionId = nil.
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It may end at any point when v.parentVersionId is not found in the set of Versions.
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This observation allows the server to discard older versions.
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The third invariant prevents the server from discarding versions if there is no snapshot.
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The fourth invariant prevents the server from discarding versions newer than the snapshot.</p>
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<h2 id="data-formats"><a class="header" href="#data-formats">Data Formats</a></h2>
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<h3 id="encryption"><a class="header" href="#encryption">Encryption</a></h3>
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<p>The client configuration includes an encryption secret of arbitrary length and a clientId to identify itself.
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This section describes how that information is used to encrypt and decrypt data sent to the server (versions and snapshots).</p>
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<h4 id="key-derivation"><a class="header" href="#key-derivation">Key Derivation</a></h4>
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<p>The client derives the 32-byte encryption key from the configured encryption secret using PBKDF2 with HMAC-SHA256 and 100,000 iterations.
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The salt is the SHA256 hash of the 16-byte form of the client ID.</p>
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<h4 id="encryption-1"><a class="header" href="#encryption-1">Encryption</a></h4>
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<p>The client uses <a href="https://commondatastorage.googleapis.com/chromium-boringssl-docs/aead.h.html">AEAD</a>, with algorithm CHACHA20_POLY1305.
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The client should generate a random nonce, noting that AEAD is <em>not secure</em> if a nonce is used repeatedly for the same key.</p>
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<p>AEAD supports additional authenticated data (AAD) which must be provided for both open and seal operations.
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In this protocol, the AAD is always 17 bytes of the form:</p>
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<ul>
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<li><code>app_id</code> (byte) - always 1</li>
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<li><code>version_id</code> (16 bytes) - 16-byte form of the version ID associated with this data
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<ul>
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<li>for versions (AddVersion, GetChildVersion), the <em>parent</em> version_id</li>
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<li>for snapshots (AddSnapshot, GetSnapshot), the snapshot version_id</li>
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</ul>
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</li>
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</ul>
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<p>The <code>app_id</code> field is for future expansion to handle other, non-task data using this protocol.
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Including it in the AAD ensures that such data cannot be confused with task data.</p>
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<p>Although the AEAD specification distinguishes ciphertext and tags, for purposes of this specification they are considered concatenated into a single bytestring as in BoringSSL's <code>EVP_AEAD_CTX_seal</code>.</p>
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<h4 id="representation"><a class="header" href="#representation">Representation</a></h4>
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<p>The final byte-stream is comprised of the following structure:</p>
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<ul>
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<li><code>version</code> (byte) - format version (always 1)</li>
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<li><code>nonce</code> (12 bytes) - encryption nonce</li>
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<li><code>ciphertext</code> (remaining bytes) - ciphertext from sealing operation</li>
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</ul>
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<p>The <code>version</code> field identifies this data format, and future formats will have a value other than 1 in this position.</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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<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</li>
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<li>history segment</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 ID matches its stored latest parent ID.</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 set of versions for the client, the client's latest version ID is set to the new version ID.
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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 latest version ID.
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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>history segment.</li>
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</ul>
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<p>The response is either a version (success, <em>not-found</em>, or <em>gone</em>, as determined by the first of the following to apply:</p>
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<ul>
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<li>If a version with parentVersionId equal to the requested parentVersionId exists, it is returned.</li>
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<li>If the requested parentVersionId is the nil UUID ..
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<ul>
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<li>..and snapshotVersionId is nil, the response is <em>not-found</em> (the client has no versions).</li>
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<li>..and snapshotVersionId is not nil, the response is <em>gone</em> (the first version has been deleted).</li>
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</ul>
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</li>
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<li>If a version with versionId equal to the requested parentVersionId exists, the response is <em>not-found</em> (the client is up-to-date)</li>
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<li>Otherwise, the response is <em>gone</em> (the requested version has been deleted).</li>
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</ul>
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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</li>
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<li>snapshot data (opaque to the server)</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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<h2 id="http-representation"><a class="header" href="#http-representation">HTTP Representation</a></h2>
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<p>The transactions above are realized for an HTTP server at <code><origin></code> using the HTTP requests and responses described here.
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The <code>origin</code> <em>should</em> be an HTTPS endpoint on general principle, but nothing in the functonality or security of the protocol depends on connection encryption.</p>
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<p>The replica identifies itself to the server using a <code>client_id</code> in the form of a UUID.
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This value is passed with every request in the <code>X-Client-Id</code> header, in its dashed-hex format.</p>
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<h3 id="addversion-1"><a class="header" href="#addversion-1">AddVersion</a></h3>
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<p>The request is a <code>POST</code> to <code><origin>/v1/client/add-version/<parentVersionId></code>.
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The request body contains the history segment, optionally encoded using any encoding supported by actix-web.
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The content-type must be <code>application/vnd.taskchampion.history-segment</code>.</p>
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<p>The success response is a 200 OK with an empty body.
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The new version ID appears in the <code>X-Version-Id</code> header.
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If included, a snapshot request appears in the <code>X-Snapshot-Request</code> header with value <code>urgency=low</code> or <code>urgency=high</code>.</p>
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<p>On conflict, the response is a 409 CONFLICT with an empty body.
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The expected parent version ID appears in the <code>X-Parent-Version-Id</code> header.</p>
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<p>Other error responses (4xx or 5xx) may be returned and should be treated appropriately to their meanings in the HTTP specification.</p>
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<h3 id="getchildversion-1"><a class="header" href="#getchildversion-1">GetChildVersion</a></h3>
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<p>The request is a <code>GET</code> to <code><origin>/v1/client/get-child-version/<parentVersionId></code>.</p>
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<p>The response is determined as described above.
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The <em>not-found</em> response is 404 NOT FOUND.
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The <em>gone</em> response is 410 GONE.
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Neither has a response body.</p>
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<p>On success, the response is a 200 OK.
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The version's history segment is returned in the response body, with content-type <code>application/vnd.taskchampion.history-segment</code>.
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The version ID appears in the <code>X-Version-Id</code> header.
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The response body may be encoded, in accordance with any <code>Accept-Encoding</code> header in the request.</p>
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<p>On failure, a client should treat a 404 NOT FOUND as indicating that it is up-to-date.
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Clients should treat a 410 GONE as a synchronization error.
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If the client has pending changes to send to the server, based on a now-removed version, then those changes cannot be reconciled and will be lost.
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The client should, optionally after consulting the user, download and apply the latest snapshot.</p>
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<h3 id="addsnapshot-1"><a class="header" href="#addsnapshot-1">AddSnapshot</a></h3>
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<p>The request is a <code>POST</code> to <code><origin>/v1/client/add-snapshot/<versionId></code>.
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The request body contains the snapshot data, optionally encoded using any encoding supported by actix-web.
|
|
The content-type must be <code>application/vnd.taskchampion.snapshot</code>.</p>
|
|
<p>If the version is invalid, as described above, the response should be 400 BAD REQUEST.
|
|
The server response should be 200 OK on success.</p>
|
|
<h3 id="getsnapshot-1"><a class="header" href="#getsnapshot-1">GetSnapshot</a></h3>
|
|
<p>The request is a <code>GET</code> to <code><origin>/v1/client/snapshot</code>.</p>
|
|
<p>The response is a 200 OK.
|
|
The snapshot is returned in the response body, with content-type <code>application/vnd.taskchampion.snapshot</code>.
|
|
The version ID appears in the <code>X-Version-Id</code> header.
|
|
The response body may be encoded, in accordance with any <code>Accept-Encoding</code> header in the request.</p>
|
|
<p>After downloading and decrypting a snapshot, a client must replace its entire local task database with the content of the snapshot.
|
|
Any local operations that had not yet been synchronized must be discarded.
|
|
After the snapshot is applied, the client should begin the synchronization process again, starting from the snapshot version.</p>
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