feed.clj

(def payment-codes-per-block 4096)

(defn create-payment-code-feed
  [db]
  (let [payment-code-feed (chan 10)]
    (go-loop []
             (let [block-id (allocate-block db)
                   block-start (* block-id payment-codes-per-block)]
               (-> (onto-chan payment-code-feed
                              (->>
                                (range block-start (+ block-start payment-codes-per-block))
                                (map sa/int->six-alpha))
                              ;; Don't close!
                              false)
                   ;; park here until all of that block has been consumed.
                   <!))
             ;; Loop forever.
             (recur))
    payment-code-feed))

Very nice, and I also like the related article
https://medium.com/@hlship/better-abstractions-with-core-async-1cd5fa509eca

A very minor nit - for someone else reading the code, I think it is more clear to omit the threading macro on line 9. It is more explicit that the parking is waiting for onto-chan to finish if the code is closer together :

(<! (onto-chan payment-code-feed
                             (->>
                                (range block-start (+ block-start payment-codes-per-block))
                                (map sa/int->six-alpha))
                              ;; Don't close!
                              false)))