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312 lines
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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" class="active"><strong aria-hidden="true">4.1.</strong> Selection</a></li><li><a href="../common_patterns/on_demand_inputs.html"><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="#selection" id="selection">Selection</a></h1>
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<p>The "selection" (or "firewall") pattern is when you have a query Qsel that reads from some
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other Qbase and extracts some small bit of information from Qbase that it returns.
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In particular, Qsel does not combine values from other queries. In some sense,
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then, Qsel is redundant -- you could have just extracted the information
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the information from Qbase yourself, and done without the salsa machinery. But
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Qsel serves a role in that it limits the amount of re-execution that is required
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when Qbase changes.</p>
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<h2><a class="header" href="#example-the-base-query" id="example-the-base-query">Example: the base query</a></h2>
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<p>For example, imagine that you have a query <code>parse</code> that parses the input text of a request
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and returns a <code>ParsedResult</code>, which contains a header and a body:</p>
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<pre><code class="language-rust ignore">#[derive(Clone, Debug, PartialEq, Eq)]
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struct ParsedResult {
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header: Vec<ParsedHeader>,
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body: String,
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}
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#[derive(Clone, Debug, PartialEq, Eq)]
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struct ParsedHeader {
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key: String,
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value: String,
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}
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#[salsa::query_group(Request)]
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trait RequestParser {
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/// The base text of the request.
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#[salsa::input]
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fn request_text(&self) -> String;
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/// The parsed form of the request.
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fn parse(&self) -> ParsedResult;
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}
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</code></pre>
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<h2><a class="header" href="#example-a-selecting-query" id="example-a-selecting-query">Example: a selecting query</a></h2>
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<p>And now you have a number of derived queries that only look at the header.
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For example, one might extract the "content-type' header:</p>
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<pre><code class="language-rust ignore">#[salsa::query_group(Request)]
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trait RequestUtil: RequestParser {
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fn content_type(&self) -> Option<String>;
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}
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fn content_type(db: &impl RequestUtil) -> Option<String> {
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db.parse()
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.header
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.iter()
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.find(|header| header.key == "content-type")
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.map(|header| header.value.clone())
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}
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</code></pre>
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<h2><a class="header" href="#why-prefer-a-selecting-query" id="why-prefer-a-selecting-query">Why prefer a selecting query?</a></h2>
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<p>This <code>content_type</code> query is an instance of the <em>selection</em> pattern. It only
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"selects" a small bit of information from the <code>ParsedResult</code>. You might not have
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made it a query at all, but instead made it a method on <code>ParsedResult</code>.</p>
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<p>But using a query for <code>content_type</code> has an advantage: now if there are downstream
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queries that only depend on the <code>content_type</code> (or perhaps on other headers extracted
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via a similar pattern), those queries will not have to be re-executed when the request
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changes <em>unless</em> the content-type header changes. Consider the dependency graph:</p>
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<pre><code class="language-text">request_text --> parse --> content_type --> (other queries)
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</code></pre>
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<p>When the <code>request_text</code> changes, we are always going to have to re-execute <code>parse</code>.
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If that produces a new parsed result, we are <em>also</em> going to re-execute <code>content_type</code>.
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But if the result of <code>content_type</code> has not changed, then we will <em>not</em> re-execute
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the other queries.</p>
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<h2><a class="header" href="#more-levels-of-selection" id="more-levels-of-selection">More levels of selection</a></h2>
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<p>In fact, in our example we might consider introducing another level of selection.
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Instead of having <code>content_type</code> directly access the results of <code>parse</code>, it might be better
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to insert a selecting query that just extracts the header:</p>
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<pre><code class="language-rust ignore">#[salsa::query_group(Request)]
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trait RequestUtil: RequestParser {
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fn header(&self) -> Vec<ParsedHeader>;
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fn content_type(&self) -> Option<String>;
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}
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fn header(db: &impl RequestUtil) -> Vec<ParsedHeader> {
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db.parse().header.clone()
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}
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fn content_type(db: &impl RequestUtil) -> Option<String> {
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db.header()
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.iter()
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.find(|header| header.key == "content-type")
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.map(|header| header.value.clone())
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}
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</code></pre>
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<p>This will result in a dependency graph like so:</p>
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<pre><code class="language-text">request_text --> parse --> header --> content_type --> (other queries)
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</code></pre>
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<p>The advantage of this is that changes that only effect the "body" or
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only consume small parts of the request will
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not require us to re-execute <code>content_type</code> at all. This would be particularly
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valuable if there are a lot of dependent headers.</p>
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<h2><a class="header" href="#a-note-on-cloning-and-efficiency" id="a-note-on-cloning-and-efficiency">A note on cloning and efficiency</a></h2>
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<p>In this example, we used common Rust types like <code>Vec</code> and <code>String</code>,
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and we cloned them quite frequently. This will work just fine in Salsa,
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but it may not be the most efficient choice. This is because each clone
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is going to produce a deep copy of the result. As a simple fix, you
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might convert your data structures to use <code>Arc</code> (e.g., <code>Arc<Vec<ParsedHeader>></code>),
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which makes cloning cheap.</p>
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