我從開發計費系統中學到的5件事

{"type":"doc","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"半年前，我加入了Pleo，一家擁有大約1.5萬名客戶的FinTech初創公司。我的主要工作是負責進一步開發計費系統。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"剛加入時，我有點擔心計費系統能有多少事情要做？我們會不會在三四個個月後就無事可做呢？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"新客戶註冊，支付每月的訂閱費，就這樣了……對吧？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"是的，但實際上也不是。這只是最基本的場景。除了這個，還有大量的邊界情況和陷阱需要考慮。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"一個個找出這些邊界情況並不是件好差事，我希望有人能提供一份簡短的指南。於是，我寫了這篇文章。如果你正在考慮構建(甚至只是使用)計費系統，可以看一下。"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"不是所有貨幣都有小數位"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在設計數據庫表結構時，有一個普遍觀點是“永遠不要用浮點數來表示錢”。一些人建議使用"},{"type":"codeinline","content":[{"type":"text","text":"MONEY"}]},{"type":"text","text":"數據類型，另一些人則告訴你要使用"},{"type":"codeinline","content":[{"type":"text","text":"DECIMAL"}]},{"type":"text","text":"。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"對於計費系統來說，這兩種類型在某些情況下都是錯的。當然，在美國和大多數歐洲國家，貨幣是有小數位的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在日本，情況就不是這樣的——你不能收取2500.50日元的費用。除了日本，冰島的貨幣也沒有小數位。相反，伊拉克第納爾（伊拉克貨幣單位）有三個小數位。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"從技術上講，你可以認爲整數也是一種小數，但如果你沒有考慮到一些情況，有可能會給你的系統帶來麻煩。在構建計費系統時，你必須考慮足夠的擴展性，以便處理這些國家的貨幣，即使它們可能不是你當前關注的焦點。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在保存數值時，可以包含可能會使用到的小數位。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"例如，在大多數情況下："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"100美元可以存爲10000"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"2500日元存爲2500"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"2500冰島克朗存爲2500"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"100伊拉克第納爾存爲100000"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"以下是部分沒有小數位的貨幣。"}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/0f\/0fc8d7e0515a24cdc499f45ca8026cb2.jpeg","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"ISO 4217標準規定了世界各國的貨幣編碼，可以在"},{"type":"link","attrs":{"href":"https:\/\/en.wikipedia.org\/wiki\/ISO_4217#Active_codes","title":"","type":null},"content":[{"type":"text","text":"維基百科"}]},{"type":"text","text":"上找到其他更多內容。"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"票據一旦創建就無法取消"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"至少在歐洲大部分地區不能取消。在美國，取消未支付的票據是很常見的事情。然而，這在許多歐盟國家是不合法的。如果你因此犯了錯，它將成爲一個巨大的痛點！"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這屬於“標準會計”的一部分，但有時它確實很傷人，因爲錯誤總是會不可避免地發生。票據一旦創建，如果你想要取消，只能再創建一個全額的貸記單。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"什麼是貸記單？你可以把它看成是一種反向票據。貸記單是一種獨特的單據，就像發票一樣，它們也有自己的編號。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"因此，貸記單是一種可以用來將原始發票標記爲“已支付”的文件，即使沒有發生金錢轉移。"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"不是所有的計費系統都是合規的"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"當你進入一個新區域開發業務，你要做的不僅僅是爲你的產品和服務定價，還必須確保自己遵守各種規章制度——這可能比你想象的要微妙得多。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"很多公司從第三方SaaS公司購買服務，我們通常認爲這樣做是合規的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"但我發現情況並非總是如此(當然，我們有責任去核實)。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這裏有兩個例子："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"1.在歐盟，供應商必須在每次銷售前確保客戶的增值稅號碼(如果使用了)是正確的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"例如，Stripe會做一次增值稅信息檢查，但不做任何其他後續檢查，也不檢查與客戶詳細信息是否匹配。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"其他一些服務根本什麼都不檢查，所以請確保你是合規的！"}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/ef\/ef2e0d16142f78a2ab6d65d7ccfe7825.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"2.歐盟增值稅發票必須包含公司增值稅的具體信息。然而，很多發票生成器(包括Stripe公司)生成的票據甚至不包含增值稅信息。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"你可能需要找到一種方法，比如利用地址欄備註稅務相關信息。無論使用哪種方式，你都要確保自己是合規的！"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"當用戶變更了付費計劃該怎麼辦"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我原以爲讓一家公司成爲付費用戶是很容易的。他們註冊後，經過短暫的試用，就成爲付費用戶(“Evergreen”)，一切都很棒!"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/a0\/a02426483bd444bf856a2f9380cb47e7.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"理想情況下，一家公司開始使用產品，在後續的整個生命週期當中都不做付費計劃變更（直到退出）。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"與很多其他公司一樣，我們也提供了不同級別的服務，需要客戶預付一個月的使用費。因此，如果客戶對服務進行了升級或降級，我們要在未來的某個時刻修改他們的付費計劃。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/87\/871cb485cad8904c57102f5f6e7705bc.png","alt":null,"title":null,"style":[{"key":"width","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