Clojure core.async. How to download lazily using go block inside open?

Question

This is a continuation of my previous question. How to create a lazy piecemeal sequence in clojure?

I want to download data from a database in parts. First, I load the first 500 lines, and then send a query to select from the next 500 lines, and so on, until I get all the data from the server.

I wrote the code:

(jdbc/atomic conn
 (with-open [cursor (jdbc/fetch-lazy conn [sql_query])]
   (let [lazyseq (jdbc/cursor->lazyseq cursor)
         counter (atom 1)]
     (swap! lazyseq_maps assoc :session_id {:get_next? (chan 1) :over_500 (chan 1) :data []})
     (>!! (:get_next? (:session_id @lazyseq_maps)) true)
     (go
       (doseq [row lazyseq]
         (swap! counter inc)
         (when (<! (:get_next? (:session_id @lazyseq_maps)))
           (swap! lazyseq_maps update-in [:session_id :data] conj row)
           (if (not= 0 (mod @counter 500))
             (>! (:get_next? (:session_id @lazyseq_maps)) true)
             (>! (:over_500 (:session_id @lazyseq_maps)) true))))
        ;
        (close! (:get_next? (:session_id @lazyseq_maps)))
        (close! (:over_500 (:session_id @lazyseq_maps)))
        (.close conn))

     (when (<!! (:over_500 (:session_id @lazyseq_maps))) {:message "over 500 rows"
                                                          :id :session_id
                                                          :data (:data (:session_id @lazyseq_maps))}))))

I take strings using a dose loop. When the doseq has passed 500 lines, I leave the loop (when (<! (:get_next? (:session_id @lazyseq_maps)))and wait for the signal from the outside to get the next 500 lines.

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Answer 1

(go ...) , (with-open ...).

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(go (with-open ...))

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Answer 2

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