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Parameter coercion
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Compiling middleware
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Validating route-trees
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TODO: Interceptors
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<!-- Title -->
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<h1>
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<a href="." >Compiling middleware</a>
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<section class="normal markdown-section">
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<h2 id="compiling-middleware">Compiling Middleware</h2>
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<p>The <a href="#meta-data-based-extensions">meta-data extensions</a> are a easy way to extend the system. Routes meta-data can be transformed into any shape (records, functions etc.) in route compilation, enabling fast access at request-time.</p>
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<p>Still, we can do better. As we know the exact route that interceptor/middleware is linked to, we can pass the (compiled) route information into the interceptor/middleware at creation-time. It can extract and transform relevant data just for it and pass it into the actual request-handler via a closure - yielding faster runtime processing.</p>
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<p>To do this we use <a href="#middleware-records">middleware records</a> <code>:gen</code> hook instead of the normal <code>:wrap</code>. <code>:gen</code> expects a function of <code>route-meta router-opts => wrap</code>. Middleware can also return <code>nil</code>, which effective unmounts the middleware. Why mount a <code>wrap-enforce-roles</code> middleware for a route if there are no roles required for it?</p>
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<p>To demonstrate the two approaches, below are response coercion middleware written as normal ring middleware function and as middleware record with <code>:gen</code>. These are the actual codes are from <a href="https://github.com/metosin/reitit/blob/master/src/reitit/coercion.cljc" target="_blank"><code>reitit.coercion</code></a>:</p>
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<h3 id="naive">Naive</h3>
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<ul>
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<li>Extracts the compiled route information on every request.</li>
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</ul>
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<pre><code class="lang-clj">(<span class="hljs-name"><span class="hljs-builtin-name">defn</span></span> wrap-coerce-response
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<span class="hljs-string">"Pluggable response coercion middleware.
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Expects a :coercion of type `reitit.coercion.protocol/Coercion`
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and :responses from route meta, otherwise does not mount."</span>
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[handler]
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(<span class="hljs-name"><span class="hljs-builtin-name">fn</span></span>
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([request]
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(<span class="hljs-name"><span class="hljs-builtin-name">let</span></span> [response (<span class="hljs-name">handler</span> request)
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method (<span class="hljs-symbol">:request-method</span> request)
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match (<span class="hljs-name">ring/get-match</span> request)
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responses (<span class="hljs-name"><span class="hljs-builtin-name">-></span></span> match <span class="hljs-symbol">:result</span> method <span class="hljs-symbol">:meta</span> <span class="hljs-symbol">:responses</span>)
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coercion (<span class="hljs-name"><span class="hljs-builtin-name">-></span></span> match <span class="hljs-symbol">:meta</span> <span class="hljs-symbol">:coercion</span>)
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opts (<span class="hljs-name"><span class="hljs-builtin-name">-></span></span> match <span class="hljs-symbol">:meta</span> <span class="hljs-symbol">:opts</span>)]
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(<span class="hljs-name"><span class="hljs-builtin-name">if</span></span> (<span class="hljs-name"><span class="hljs-builtin-name">and</span></span> coercion responses)
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(<span class="hljs-name"><span class="hljs-builtin-name">let</span></span> [coercers (<span class="hljs-name">response-coercers</span> coercion responses opts)
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coerced (<span class="hljs-name">coerce-response</span> coercers request response)]
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(<span class="hljs-name">coerce-response</span> coercers request (<span class="hljs-name">handler</span> request)))
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(<span class="hljs-name">handler</span> request))))
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([request respond raise]
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(<span class="hljs-name"><span class="hljs-builtin-name">let</span></span> [response (<span class="hljs-name">handler</span> request)
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method (<span class="hljs-symbol">:request-method</span> request)
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match (<span class="hljs-name">ring/get-match</span> request)
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responses (<span class="hljs-name"><span class="hljs-builtin-name">-></span></span> match <span class="hljs-symbol">:result</span> method <span class="hljs-symbol">:meta</span> <span class="hljs-symbol">:responses</span>)
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coercion (<span class="hljs-name"><span class="hljs-builtin-name">-></span></span> match <span class="hljs-symbol">:meta</span> <span class="hljs-symbol">:coercion</span>)
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opts (<span class="hljs-name"><span class="hljs-builtin-name">-></span></span> match <span class="hljs-symbol">:meta</span> <span class="hljs-symbol">:opts</span>)]
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(<span class="hljs-name"><span class="hljs-builtin-name">if</span></span> (<span class="hljs-name"><span class="hljs-builtin-name">and</span></span> coercion responses)
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(<span class="hljs-name"><span class="hljs-builtin-name">let</span></span> [coercers (<span class="hljs-name">response-coercers</span> coercion responses opts)
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coerced (<span class="hljs-name">coerce-response</span> coercers request response)]
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(<span class="hljs-name">handler</span> request #(<span class="hljs-name">respond</span> (<span class="hljs-name">coerce-response</span> coercers request %))))
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(<span class="hljs-name">handler</span> request respond raise))))))
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</code></pre>
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<h3 id="compiled">Compiled</h3>
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<ul>
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<li>Route information is provided via a closure</li>
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<li>Pre-compiled coercers</li>
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<li>Mounts only if <code>:coercion</code> and <code>:responses</code> are defined for the route</li>
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</ul>
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<pre><code class="lang-clj">(<span class="hljs-name"><span class="hljs-builtin-name">def</span></span> gen-wrap-coerce-response
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<span class="hljs-string">"Generator for pluggable response coercion middleware.
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Expects a :coercion of type `reitit.coercion.protocol/Coercion`
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and :responses from route meta, otherwise does not mount."</span>
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(<span class="hljs-name">middleware/create</span>
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{<span class="hljs-symbol">:name</span> :<span class="hljs-symbol">:coerce-response</span>
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<span class="hljs-symbol">:gen</span> (<span class="hljs-name"><span class="hljs-builtin-name">fn</span></span> [{<span class="hljs-symbol">:keys</span> [responses coercion opts]} _]
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(<span class="hljs-name"><span class="hljs-builtin-name">if</span></span> (<span class="hljs-name"><span class="hljs-builtin-name">and</span></span> coercion responses)
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(<span class="hljs-name"><span class="hljs-builtin-name">let</span></span> [coercers (<span class="hljs-name">response-coercers</span> coercion responses opts)]
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(<span class="hljs-name"><span class="hljs-builtin-name">fn</span></span> [handler]
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(<span class="hljs-name"><span class="hljs-builtin-name">fn</span></span>
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([request]
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(<span class="hljs-name">coerce-response</span> coercers request (<span class="hljs-name">handler</span> request)))
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([request respond raise]
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(<span class="hljs-name">handler</span> request #(<span class="hljs-name">respond</span> (<span class="hljs-name">coerce-response</span> coercers request %)) raise)))))))}))
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</code></pre>
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<p>The <code>:gen</code> -version has 50% less code, is easier to reason about and is 2-4x faster on basic perf tests.</p>
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