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This gist describes a technique for asynchronously fetching values from an external resource. This fetch may be expensive so we don't want to block the calling thread. Using a hash-set and watcher decouples the fetch of the value from the request for the value without callbacks. Looking at the difference of the before and after set values in the…
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| (def fetch-set (ref (hash-set))) | |
| (def resource-map (ref {})) | |
| (defn fetch-resource | |
| [resource-key] | |
| ;; perform some expensive operation that returns a value | |
| ) | |
| (defn request-fetch | |
| "Requests an asynchronous fetch of some resource." | |
| [fetch-set resource-key] | |
| (dosync | |
| (commute fetch-set conj resource-key) | |
| ;; return nil to indicate that a fetch was requested, the value isn't yet present | |
| nil)) | |
| (defn fetch-set-watcher | |
| "Asynchronously fetch the resources that have been added to the watched set." | |
| [_ _ old-set new-set] | |
| (doseq [resource-key (clojure.set/difference new-set old-set)] | |
| (future | |
| (let [resource (fetch-resource resource-key)] | |
| (dosync | |
| (when-not (get @resource-map resource-key) | |
| ;; alter so the view on resource-map is consistent between | |
| ;; the when-not and alter. Don't want to clobber an existing | |
| ;; value in the resource-map if it was added concurrently. | |
| (alter resource-map assoc resource-key resource)) | |
| ;; This removal will trigger the watcher again but will | |
| ;; be ignored because of the set difference. | |
| (commute fetch-set disj resource-key)))))) | |
| (add-watch fetch-set :async-fetch fetch-set-watcher) | |
| ;; Then deref & read resource-map to access the asynchronously fetched values | |
| (defn get-resource | |
| [resource-key] | |
| (if-let [existing-resource (get @resource-map resource-key)] | |
| existing-resource | |
| ;; request-fetch returns nil, which lets a caller easily check | |
| ;; if the resource was returned. | |
| (request-fetch resource-key))) |
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