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260 lines
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260 lines
13 KiB
HTML
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<ol class="chapter"><li><a href="../about_salsa.html"><strong aria-hidden="true">1.</strong> About salsa</a></li><li><a href="../how_to_use.html"><strong aria-hidden="true">2.</strong> How to use Salsa</a></li><li><a href="../how_salsa_works.html"><strong aria-hidden="true">3.</strong> How Salsa works</a></li><li><a href="../common_patterns.html"><strong aria-hidden="true">4.</strong> Common patterns</a></li><li><ol class="section"><li><a href="../common_patterns/selection.html"><strong aria-hidden="true">4.1.</strong> Selection</a></li><li><a href="../common_patterns/on_demand_inputs.html" class="active"><strong aria-hidden="true">4.2.</strong> On-demand (Lazy) inputs</a></li></ol></li><li><a href="../videos.html"><strong aria-hidden="true">5.</strong> YouTube videos</a></li><li><a href="../plumbing.html"><strong aria-hidden="true">6.</strong> Plumbing</a></li></ol>
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<h1 class="menu-title">Salsa</h1>
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<main>
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<h1><a class="header" href="#on-demand-lazy-inputs" id="on-demand-lazy-inputs">On-Demand (Lazy) Inputs</a></h1>
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<p>Salsa input queries work best if you can easily provide all of the inputs upfront.
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However sometimes the set of inputs is not known beforehand.</p>
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<p>A typical example is reading files from disk.
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While it is possible to eagerly scan a particular directory and create an in-memory file tree in a salsa input query, a more straight-forward approach is to read the files lazily.
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That is, when someone requests the text of a file for the first time:</p>
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<ol>
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<li>Read the file from disk and cache it.</li>
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<li>Setup a file-system watcher for this path.</li>
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<li>Invalidate the cached file once the watcher sends a change notification.</li>
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</ol>
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<p>This is possible to achieve in salsa, using a derived query and <code>report_synthetic_read</code> and <code>invalidate</code> queries.
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The setup looks roughly like this:</p>
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<pre><code class="language-rust ignore">#[salsa::query_group(VfsDatabaseStorage)]
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trait VfsDatabase: salsa::Database + FileWatcher {
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fn read(&self, path: PathBuf) -> String;
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}
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trait FileWatcher {
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fn watch(&self, path: &Path);
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fn did_change_file(&mut self, path: &Path);
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}
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fn read(db: &impl salsa::Database, path: PathBuf) -> String {
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db.salsa_runtime()
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.report_synthetic_read(salsa::Durability::LOW);
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db.watch(&path);
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std::fs::read_to_string(&path).unwrap_or_default()
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}
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#[salsa::database(VfsDatabaseStorage)]
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struct MyDatabase { ... }
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impl FileWatcher for MyDatabase {
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fn watch(&self, path: &Path) { ... }
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fn did_change_file(&mut self, path: &Path) {
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self.query_mut(ReadQuery).invalidate(path);
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}
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}
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</code></pre>
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<ul>
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<li>We declare the query as a derived query (which is the default).</li>
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<li>In the query implementation, we don't call any other query and just directly read file from disk.</li>
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<li>Because the query doesn't read any inputs, it will be assigned a <code>HIGH</code> durability by default, which we override with <code>report_synthetic_read</code>.</li>
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<li>The result of the query is cached, and we must call <code>invalidate</code> to clear this cache.</li>
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</ul>
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