如何获得法拉第/扫描以遍历整个DynamoDB表？

如果它适合内存，则可以使用...

整个方案的关键是使用:limit 99映射以及一些:span-reqs {:max 1}映射来设置opts映射。该:span-reqs映射是完全不起眼的我，但它似乎是背后是什么概念“页面大小”真正的驱动程序。我已经建立了一个10元素表，例如...

;; This only works on the whole table because the table is small!!!!
(far/scan
  common/client-opts
  "users.robert.kuhar.wtf_far"
  {:return #{:part_key :sort_key :note}})
=>
[{:part_key "456", :sort_key "fha.abs", :note "\"456\",\"fha.abs\" created 2016-12-08T21:32:20.789Z."}
 {:part_key "456", :sort_key "fha.rank", :note "\"456\",\"fha.rank\" created 2016-12-08T21:32:20.789Z."}
 {:part_key "456", :sort_key "fha.raw", :note "\"456\",\"fha.raw\" created 2016-12-08T21:32:20.789Z."}
 {:part_key "456", :sort_key "fha.true-color", :note "\"456\",\"fha.true-color\" created 2016-12-08T21:32:20.789Z."}
 {:part_key "456", :sort_key "soil.ssurgo", :note "\"456\",\"soil.ssurgo\" created 2016-12-08T21:32:20.789Z."}
 {:part_key "123", :sort_key "fha.abs", :note "\"123\",\"fha.abs\" created 2016-12-08T21:24:30.139Z."}
 {:part_key "123", :sort_key "fha.rank", :note "\"123\",\"fha.rank\" created 2016-12-08T21:24:30.139Z"}
 {:part_key "123", :sort_key "fha.raw", :note "\"123\",\"fha.raw\" created 2016-12-08T21:24:30.139Z."}
 {:part_key "123", :sort_key "fha.true-color", :note "\"123\",\"fha.true-color\" created 2016-12-08T21:24:30.139Z."}
 {:part_key "123", :sort_key "soil.ssurgo", :note "\"123\",\"soil.ssurgo\" created 2016-12-08T21:24:30.139Z."}]

如果我想一次浏览这4个元素，则初始调用为...

(far/scan
  common/client-opts
  "users.robert.kuhar.wtf_far"
  {:return #{:part_key :sort_key :note}
   :limit 4
   :span-reqs {:max 1}})
=>
[{:part_key "456", :sort_key "fha.abs", :note "\"456\",\"fha.abs\" created 2016-12-08T21:32:20.789Z."}
 {:part_key "456", :sort_key "fha.rank", :note "\"456\",\"fha.rank\" created 2016-12-08T21:32:20.789Z."}
 {:part_key "456", :sort_key "fha.raw", :note "\"456\",\"fha.raw\" created 2016-12-08T21:32:20.789Z."}
 {:part_key "456", :sort_key "fha.true-color", :note "\"456\",\"fha.true-color\" created 2016-12-08T21:32:20.789Z."}]

所有后续通话都需要在:last-prim-kvs {:part_key "xxx" :sort_key "yyy"}opts映射中设置，以告诉法拉第在哪里接听。对于第二页，呼叫就像...

(far/scan
  common/client-opts
  "users.robert.kuhar.wtf_far"
  {:return #{:part_key :sort_key :note}
   :limit 4
   :span-reqs {:max 1}
   :last-prim-kvs {:part_key "456" :sort_key "fha.true-color"}})
=>
[{:part_key "456", :sort_key "soil.ssurgo", :note "\"456\",\"soil.ssurgo\" created 2016-12-08T21:32:20.789Z."}
 {:part_key "123", :sort_key "fha.abs", :note "\"123\",\"fha.abs\" created 2016-12-08T21:24:30.139Z."}
 {:part_key "123", :sort_key "fha.rank", :note "\"123\",\"fha.rank\" created 2016-12-08T21:24:30.139Z"}
 {:part_key "123", :sort_key "fha.raw", :note "\"123\",\"fha.raw\" created 2016-12-08T21:24:30.139Z."}]

我的10元素表的最后一页是...

(far/scan
  common/client-opts
  "users.robert.kuhar.wtf_far"
  {:return #{:part_key :sort_key :note}
   :limit 4
   :span-reqs {:max 1}
   :last-prim-kvs {:part_key "123" :sort_key "fha.raw"}})
=>
[{:part_key "123", :sort_key "fha.true-color", :note "\"123\",\"fha.true-color\" created 2016-12-08T21:24:30.139Z."}
 {:part_key "123", :sort_key "soil.ssurgo", :note "\"123\",\"soil.ssurgo\" created 2016-12-08T21:24:30.139Z."}]

即使我要了4个元素，也只有2个元素。尝试超出此范围/扫描始终是空的。

(far/scan
  common/client-opts
  "users.robert.kuhar.wtf_far"
  {:return #{:part_key :sort_key :note}
   :limit 4
   :span-reqs {:max 1}
   :last-prim-kvs {:part_key "123" :sort_key "soil.ssurgo"}})
=> []

因此，只要一切都适合内存，就可以做到端到端。

(loop [accum []
       page (far/scan 
              client-opts 
              "users.robert.kuhar.wtf_far" 
              {:limit 4 
               :span-reqs {:max 1}})]
  (if (empty? page)
    accum
    (let [last-on-page (last page)
          last-part-key (:part_key last-on-page)
          last-sort-key (:sort_key last-on-page)]
      (recur
        (into accum page)
        (far/scan
          client-opts
          "users.robert.kuhar.wtf_far"
          {:limit 4
           :span-reqs {:max 1}
           :last-prim-kvs {:part_key last-part-key :sort_key last-sort-key}})))))
=>
[{:part_key "456", :sort_key "fha.abs", :note "\"456\",\"fha.abs\" created 2016-12-08T21:32:20.789Z."}
 ...
 {:part_key "123", :sort_key "soil.ssurgo", :note "\"123\",\"soil.ssurgo\" created 2016-12-08T21:24:30.139Z."}]

我认为在“如何才能获得法拉第/扫描来遍历整个DynamoDB表？”的情况下，这是一个令人遗憾的最终答案。是它不能。您需要手工构建它。