工作流框架Activiti中的事務和併發！詳細解析工作流中的異步和排他操作

{"type":"doc","content":[{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"事務和併發","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"異步操作","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"Activiti通過事務方式執行流程,可以根據需求定製","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Activiti處理事務:","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果觸發了Activiti的操作(開始流程,完成任務,觸發流程繼續執行),activiti會推進流程,直到每個分支都進入等待狀態","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"抽象的說,會從流程圖執行深度優先搜索,如果每個分支都遇到等待狀態,就會返回","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"等待狀態是稍後需要執行任務,Activiti會把當前狀態保存到數據庫中,然後等待下一次觸發","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"觸發可能來自外部,比如用戶任務或接收到一個消息,也可能來自Activiti本身(定時器事件)","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":"none"},"content":[{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/67/67d4572e056138a4ec14b6e4b8b7278a.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":false,"pastePass":false}}]},{"type":"listitem","attrs":{"listStyle":"none"},"content":[{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"流程包含用戶任務,服務任務和定時器事件完成用戶任務和校驗地址是在同一個工作單元中,兩者的成功和失敗是原子性的.意味着如果服務任務拋出異常,要回滾當前事務,這樣流程會退回到用戶任務,用戶任務就依然在數據庫裏這就是activiti默認的行爲.在(1)中應用或客戶端線程完成任務.這會執行服務,流程推進,直到遇到一個等待狀態,就是定時器(2),然後它會返回給調用者(3),並提交事務(如果事務是由Activiti開啓的)","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"有時需要自定義控制流程中事務的邊界,把業務邏輯包裹在一起.這就需要使用","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"異步執行:","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":"none"},"content":[{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/07/07bc5f31e176ed2ffc4bff30f31d5422.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":false,"pastePass":false}}]},{"type":"listitem","attrs":{"listStyle":"none"},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"完成了用戶任務,生成一個發票,把發票發送給客戶生成發票不在同一個工作單元內了.如果生成發票出錯不需要對用戶任務進行回滾Activiti實現的是完成用戶任務(1),提交事務,返回給調用者應用.然後在後臺的線程中,異步執行生成發票.後臺線程就是Activiti的Job執行器(一個線程池)週期對數據庫的Job進行掃描:當到達\"generate invoice\"任務,爲Activiti創建一個稍後執行的Job\"消息\",並保存到數據庫.Job會被Job執行器獲取並執行.也會給本地Job執行器一個提醒,告訴有一個新Job,來增加性能","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"要想使用這個特性,要使用","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"activiti:async=\"true\"","attrs":{}},{"type":"text","text":" 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(因爲其他事務可能在提交之前進行了修改,會在每個事務裏增加數據庫同一行的版本),這個時候,第一個提交的事務會成功,其他會因爲樂觀鎖異常導致失敗","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":2,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"這就解決了上面流程的問題:","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":3,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果多個分支同步到達並行匯聚,會假設都在登錄,並增加父流程的版本號(流程實例)然後嘗試提交","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":3,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第一個分支會成功提交,其他分支會因爲樂觀鎖導致失敗","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":3,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"因爲流程是被job觸發的,Activiti會嘗試在等待一段時間後嘗試執行同一個job,這段時間可以同步網關的狀態","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"Activiti樂觀鎖是一個很好的解決方案嗎?","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"樂觀鎖允許Activiti避免非一致性,確定流程不會\"堵在匯聚網關\":","attrs":{}},{"type":"text","text":" 或者所有分支都通過網關，或者數據庫中的job正在嘗試通過","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"雖然這是一個對於持久性和一致性的完美解決方案,但對於上層來說不一定是期望的行爲:","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":2,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"Activiti只會對同一個job重試估計次數(默認配置爲3).之後,job還會在數據庫裏,但是不會再重試了.意味着這個操作必須手工執行job的觸發","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":2,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"如果job有非事務方面的效果,不會因爲失敗的事務回滾:如果“預定演唱會門票”服務沒有與Activiti共享事務,重試job可能導致我們預定了過多門票","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"針對這些問題,在Activiti中推出了新的概念:排他job","attrs":{}}]}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":5},"content":[{"type":"text","text":"排他Job","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"對於一個流程實例,排他任務不能同時執行兩個","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"考慮上面的流程:如果我們把服務任務申請爲排他任務,JobExecutor會保證對應的job不會併發執行.","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"會保證無論什麼時候獲取一個流程實例的排他任務,都會把同一個流程實例的其他任務都取出來,放在同一個工作線程中執行.保證job是順序執行的","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"從activiti 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