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72 lines
3.5 KiB
Markdown
72 lines
3.5 KiB
Markdown
# Compiling Middleware
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The [dynamic extensions](dynamic_extensions.md) is a easy way to extend the system. To enable fast lookups into route data, we can compile them into any shape (records, functions etc.) we want, enabling fast access at request-time.
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But, we can do much better. As we know the exact route that middleware/interceptor is linked to, we can pass the (compiled) route information into the middleware/interceptor at creation-time. It can do local reasoning: extract and transform relevant data just for it and pass it into the actual request-handler via a closure - yielding much faster runtime processing. It can also decide not to mount itself by returning `nil`. Why mount a `wrap-enforce-roles` middleware for a route if there are no roles required for it?
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To enable this we use [middleware records](data_driven_middleware.md) `:compile` key instead of the normal `:wrap`. `:compile` expects a function of `route-data router-opts => ?wrap`.
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To demonstrate the two approaches, below are response coercion middleware written as normal ring middleware function and as middleware record with `:compile`.
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## Normal Middleware
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* Reads the compiled route information on every request.
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```clj
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(defn wrap-coerce-response
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"Middleware for pluggable response coercion.
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Expects a :coercion of type `reitit.coercion/Coercion`
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and :responses from route data, otherwise will do nothing."
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[handler]
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(fn
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([request]
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(let [response (handler request)
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method (:request-method request)
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match (ring/get-match request)
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responses (-> match :result method :data :responses)
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coercion (-> match :data :coercion)
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opts (-> match :data :opts)]
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(if (and coercion responses)
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(let [coercers (response-coercers coercion responses opts)]
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(coerce-response coercers request response))
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response)))
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([request respond raise]
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(let [method (:request-method request)
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match (ring/get-match request)
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responses (-> match :result method :data :responses)
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coercion (-> match :data :coercion)
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opts (-> match :data :opts)]
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(if (and coercion responses)
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(let [coercers (response-coercers coercion responses opts)]
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(handler request #(respond (coerce-response coercers request %))))
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(handler request respond raise))))))
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```
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## Compiled Middleware
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* Route information is provided via a closure
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* Pre-compiled coercers
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* Mounts only if `:coercion` and `:responses` are defined for the route
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* Also defines spec for the route data `:responses` for the [route data validation](route_data_validation.md).
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```clj
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(require '[reitit.spec :as rs])
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(def coerce-response-middleware
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"Middleware for pluggable response coercion.
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Expects a :coercion of type `reitit.coercion/Coercion`
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and :responses from route data, otherwise does not mount."
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{:name ::coerce-response
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:spec ::rs/responses
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:compile (fn [{:keys [coercion responses]} opts]
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(if (and coercion responses)
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(let [coercers (coercion/response-coercers coercion responses opts)]
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(fn [handler]
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(fn
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([request]
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(coercion/coerce-response coercers request (handler request)))
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([request respond raise]
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(handler request #(respond (coercion/coerce-response coercers request %)) raise)))))))})
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```
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The latter has 50% less code, is easier to reason about and is much faster.
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