Show notes for Apropos on January 18, 2019.

Jan-18-2019-Show-Notes.md

Topics

Clojure 1.10 Release

• Changes.md
• Error Printing in REPL guide
• 1.10 Release blog announcement

2018 Retrospective

2019 Prospective

• State of Clojure Community Survey 2019

Audience question: Google Closure vs Webpack vs Rollup

At the REPL

Implement a key-value database that stores to a file for durability.

Complications

How can this be made transactional?

How to make it thread-safe?

How to make it safe across processes?

Solution 1

(def db (atom {}))

(add-watch db ::saver (fn [_ _ old new]
                        (spit "dbsave.edn" (pr-str new))))

(defn read-db []
  (clojure.edn/read-string (slurp "dbsave.edn")))

(defn restore-db []
  (reset! db (read-db)))

Solution 2

We wanted the writes to be thread-safe.

(def db (atom {}))

(def die? false)

(def t (doto (Thread. (fn []
                        (loop [old-state @db]
                          (when (not die?)
                            (Thread/sleep 1000)
                            (let [new-state @db]
                              (when (not= old-state new-state)
                                (spit "dbsave.edn" (pr-str new-state)))
                              (recur new-state))))))
         (.start)))

(defn read-db []
  (clojure.edn/read-string (slurp "dbsave.edn")))

(defn restore-db []
  (reset! db (read-db)))