6000 字 ｜Redis 分佈式鎖｜從青銅到鑽石的演進方案

{"type":"doc","content":[{"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":"上篇我們講到如何用本地內存和 Redis","attrs":{}},{"type":"link","attrs":{"href":"https://xie.infoq.cn/article/3325c0e17956a9f65485038ba","title":"","type":null},"content":[{"type":"text","text":"https://xie.infoq.cn/article/3325c0e17956a9f65485038ba","attrs":{}}]},{"type":"text","text":" 做緩存來增強系統的性能，另外探討了加鎖解決緩存擊穿的問題。但是","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"本地加鎖","attrs":{}}],"attrs":{}},{"type":"text","text":"的方式在分佈式的場景下就不適用了，所以本文我們來探討下如何引入","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"分佈式鎖","attrs":{}}],"attrs":{}},{"type":"text","text":"解決本地鎖的問題。","attrs":{}}]},{"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":"本篇主要內容如下：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/52/5216847c25537a393c7479477a31ee65.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":"本文的代碼和教程已經同步到 Github 和 碼雲。","attrs":{}}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"Github","attrs":{}},{"type":"text","text":": 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10 W 個請求，每個微服務接收 2.5 W 個請求，假如緩存失效了，每個微服務在訪問數據庫時加鎖，通過鎖（synchronzied 或 lock）來鎖住自己的線程資源，從而防止緩存擊穿。","attrs":{}}]},{"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":"這是一種本地加鎖的方式，在分佈式情況下會帶來數據不一致的問題：比如服務 A 獲取數據後，更新緩存 key =100，服務 B 不受服務 A 的鎖限制，併發去更新緩存 key = 99，最後的結果可能是 99 或 100，但這是一種未知的狀態，與期望結果不一致。流程圖如下所示：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/fd/fd4450687098dbd6a460180813617aca.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"二、什麼是分佈式鎖","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"基於上面本地鎖的問題，我們需要一種支持分佈式集羣環境下的鎖：查詢 DB 時，只有一個線程能訪問，其他線程都需要等待第一個線程釋放鎖資源後，才能繼續執行。","attrs":{}}]},{"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":"可以把鎖看成房門外的一把鎖，所有併發線程比作人，他們都想進入房間，房間內只能有一個人進入。當有人進入後，將門反鎖，其他人必須等待，直到進去的人出來。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/9b/9b8ce6f7c9e59ac2dc74083904b99e5a.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":"我們來看下分佈式鎖的基本原理，如下圖所示：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/01/0164f68825cfd430d46b2ec89085d2d3.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":"我們來分析下上圖的分佈式鎖：","attrs":{}}]},{"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":"1.前端將 10W 的高併發請求轉發給四個題目微服務。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"2.每個微服務處理 2.5 W 個請求。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"3.每個處理請求的線程在執行業務之前，需要先搶佔鎖。可以理解爲“佔坑”。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"4.獲取到鎖的線程在執行完業務後，釋放鎖。可以理解爲“釋放坑位”。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"5.未獲取到的線程需要等待鎖釋放。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"6.釋放鎖後，其他線程搶佔鎖。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"7.重複執行步驟 4、5、6。","attrs":{}}]}]}],"attrs":{}},{"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":"大白話解釋：所有請求的線程都去同一個地方","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"“佔坑”","attrs":{}}],"attrs":{}},{"type":"text","text":"，如果有坑位，就執行業務邏輯，沒有坑位，就需要其他線程釋放“坑位”。這個坑位是所有線程可見的，可以把這個坑位放到 Redis 緩存或者數據庫，這篇講的就是如何用 Redis 做","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"“分佈式坑位”","attrs":{}}],"attrs":{}},{"type":"text","text":"。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"三、Redis 的 SETNX","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Redis 作爲一個公共可訪問的地方，正好可以作爲“佔坑”的地方。","attrs":{}}]},{"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":"用 Redis 實現分佈式鎖的幾種方案，我們都是用 SETNX 命令（設置 key 等於某 value）。只是高階方案傳的參數個數不一樣，以及考慮了異常情況。","attrs":{}}]},{"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":"我們來看下這個命令，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"SETNX","attrs":{}}],"attrs":{}},{"type":"text","text":"是","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"set If not exist","attrs":{}}],"attrs":{}},{"type":"text","text":"的簡寫。意思就是當 key 不存在時，設置 key 的值，存在時，什麼都不做。","attrs":{}}]},{"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 Redis 命令行中是這樣執行的：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"sh"},"content":[{"type":"text","text":"set NX\n","attrs":{}}]},{"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":"我們可以進到 redis 容器中來試下 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"SETNX","attrs":{}}],"attrs":{}},{"type":"text","text":" 命令。","attrs":{}}]},{"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":"先進入容器：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"sh"},"content":[{"type":"text","text":"docker exec -it redid-cli\n","attrs":{}}]},{"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":"然後執行 SETNX 命令：將 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"wukong","attrs":{}}],"attrs":{}},{"type":"text","text":" 這個 key 對應的 value 設置成 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"1111","attrs":{}}],"attrs":{}},{"type":"text","text":"。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"sh"},"content":[{"type":"text","text":"set wukong 1111 NX\n","attrs":{}}]},{"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":"返回 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"OK","attrs":{}}],"attrs":{}},{"type":"text","text":"，表示設置成功。重複執行該命令，返回 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"nil","attrs":{}}],"attrs":{}},{"type":"text","text":"表示設置失敗。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/a1/a198a8a2f7aa0c427e144c8c18814055.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"四、青銅方案","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們先用 Redis 的 SETNX 命令來實現最簡單的分佈式鎖。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"3.1 青銅原理","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們來看下流程圖：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/18/18d5123f916c6ababeb423c148affd63.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":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"多個併發線程都去 Redis 中申請鎖，也就是執行 setnx 命令，假設線程 A 執行成功，說明當前線程 A 獲得了。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"其他線程執行 setnx 命令都會是失敗的，所以需要等待線程 A 釋放鎖。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"線程 A 執行完自己的業務後，刪除鎖。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"其他線程繼續搶佔鎖，也就是執行 setnx 命令。因爲線程 A 已經刪除了鎖，所以又有其他線程可以搶佔到鎖了。","attrs":{}}]}]}],"attrs":{}},{"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":"代碼示例如下，Java 中 setnx 命令對應的代碼爲 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"setIfAbsent","attrs":{}}],"attrs":{}},{"type":"text","text":"。","attrs":{}}]},{"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":"setIfAbsent 方法的第一個參數代表 key，第二個參數代表值。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"// 1.先搶佔鎖\nBoolean lock = redisTemplate.opsForValue().setIfAbsent(\"lock\", \"123\");\nif(lock) {\n // 2.搶佔成功，執行業務\n List typeEntityListFromDb = getDataFromDB();\n // 3.解鎖\n redisTemplate.delete(\"lock\");\n return typeEntityListFromDb;\n} else {\n // 4.休眠一段時間\n sleep(100);\n // 5.搶佔失敗，等待鎖釋放\n return getTypeEntityListByRedisDistributedLock();\n}\n","attrs":{}}]},{"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":"一個小問題：那爲什麼需要休眠一段時間？","attrs":{}}]},{"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":"因爲該程序存在遞歸調用，可能會導致棧空間益處。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"3.2 青銅方案的缺陷","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"青銅之所以叫青銅，是因爲它是最初級的，肯定會帶來很多問題。","attrs":{}}]},{"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","marks":[{"type":"strong","attrs":{}}],"text":"設想一種家庭場景","attrs":{}},{"type":"text","text":"：晚上小空一個人開鎖進入了房間，打開了電燈💡，然後突然","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"斷電","attrs":{}}],"attrs":{}},{"type":"text","text":"了，小空想開門出去，但是找不到門鎖位置，那小明就進不去了，外面的人也進不來。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/4a/4aa6c3ca920106b029107932fdaa61f2.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":"從技術的角度看：setnx 佔鎖成功，業務代碼出現異常或者服務器宕機，沒有執行刪除鎖的邏輯，就造成了死鎖。","attrs":{}}]},{"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":"那如何規避這個風險呢？","attrs":{}}]},{"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":"設置鎖的自動過期時間，過一段時間後，自動刪除鎖，這樣其他線程就能獲取到鎖了。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"四、白銀方案","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"4.1 生活中的例子","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"上面提到的青銅方案會有死鎖問題，那我們就用上面的規避風險的方案來設計下，也就是我們的白銀方案。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/a9/a9401e7ef4f7600feda21e51e9ae45ff.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":"還是生活中的例子：小空開鎖成功後，給這款智能鎖設置了一個沙漏倒計時⏳，沙漏完後，門鎖自動打開。即使房間突然斷電，過一段時間後，鎖會自動打開，其他人就可以進來了。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"4.2 技術原理圖","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"和青銅方案不同的地方在於，在佔鎖成功後，設置鎖的過期時間，這兩步是分步執行的。如下圖所示：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/0b/0b2f822de7e63bee6490f3071d8847c7.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"4.3 示例代碼","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"清理 redis key 的代碼如下","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"// 在 10s 以後，自動清理 lock\nredisTemplate.expire(\"lock\", 10, TimeUnit.SECONDS);\n","attrs":{}}]},{"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":"完整代碼如下：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"// 1.先搶佔鎖\nBoolean lock = redisTemplate.opsForValue().setIfAbsent(\"lock\", \"123\");\nif(lock) {\n // 2.在 10s 以後，自動清理 lock\n redisTemplate.expire(\"lock\", 10, TimeUnit.SECONDS);\n // 3.搶佔成功，執行業務\n List typeEntityListFromDb = getDataFromDB();\n // 4.解鎖\n redisTemplate.delete(\"lock\");\n return typeEntityListFromDb;\n}\n","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"4.4 白銀方案的缺陷","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"白銀方案看似解決了線程異常或服務器宕機造成的鎖未釋放的問題，但還是存在其他問題：","attrs":{}}]},{"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":"因爲佔鎖和設置過期時間是分兩步執行的，所以如果在這兩步之間發生了異常，則鎖的過期時間根本就沒有設置成功。","attrs":{}}]},{"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":"所以和青銅方案有一樣的問題：鎖永遠不能過期。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"五、黃金方案","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"5.1 原子指令","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"上面的白銀方案中，佔鎖和設置鎖過期時間是分步兩步執行的，這個時候，我們可以聯想到什麼：事務的原子性（Atom）。","attrs":{}}]},{"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","marks":[{"type":"strong","attrs":{}}],"text":"原子性","attrs":{}},{"type":"text","text":"：多條命令要麼都成功執行，要麼都不執行。","attrs":{}}]},{"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":"將兩步放在一步中執行：佔鎖+設置鎖過期時間。","attrs":{}}]},{"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":"Redis 正好支持這種操作：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"sh"},"content":[{"type":"text","text":"# 設置某個 key 的值並設置多少毫秒或秒 過期。\nset PX NX\n或\nset EX NX\n","attrs":{}}]},{"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":"然後可以通過如下命令查看 key 的變化","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"sh"},"content":[{"type":"text","text":"ttl \n","attrs":{}}]},{"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":"下面演示下如何設置 key 並設置過期時間。注意：執行命令之前需要先刪除 key，可以通過客戶端或命令刪除。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"sh"},"content":[{"type":"text","text":"# 設置 key=wukong，value=1111，過期時間=5000ms\nset wukong 1111 PX 5000 NX\n# 查看 key 的狀態\nttl wukong\n","attrs":{}}]},{"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":"執行結果如下圖所示：每運行一次 ttl 命令，就可以看到 wukong 的過期時間就會減少。最後會變爲 -2（已過期）。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/5e/5e7973ce1267106b07079ba971678558.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"5.2 技術原理圖","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"黃金方案和白銀方案的不同之處：獲取鎖的時候，也需要設置鎖的過期時間，這是一個原子操作，要麼都成功執行，要麼都不執行。如下圖所示：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/ef/eff1c75b8a475e8b2f4ec5e9aa134ebb.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"5.3 示例代碼","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"設置 lock 的值等於 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"123","attrs":{}}],"attrs":{}},{"type":"text","text":"，過期時間爲 10 秒。如果 10 秒 以後，lock 還存在，則清理 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搶佔鎖","attrs":{}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/e4/e49510e7799ee1c104a365722af07704.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":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"用戶 A 先搶佔到了鎖，並設置了這個鎖 10 秒以後自動開鎖，鎖的編號爲 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"123","attrs":{}}],"attrs":{}},{"type":"text","text":"。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"10 秒以後，A 還在執行任務，此時鎖被自動打開了。","attrs":{}}]}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"5.4.2 用戶 B 搶佔鎖","attrs":{}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/9b/9b11a91dd57a8d77269b5041f9066d8c.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":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"用戶 B 看到房間的鎖打開了，於是搶佔到了鎖，設置鎖的編號爲 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"123","attrs":{}}],"attrs":{}},{"type":"text","text":"，並設置了過期時間 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"10 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","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"123","attrs":{}}],"attrs":{}},{"type":"text","text":"的鎖。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"用戶 B 還在執行任務，發現鎖已經被打開了。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"用戶 B 非常生氣：","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"我還沒執行完任務呢，鎖怎麼開了？","attrs":{}}]}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"5.4.3 用戶 C 搶佔鎖","attrs":{}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/92/9235c2481212b49fad066b98f2c4ba22.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":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"用戶 B 的鎖被 A 主動打開後，A 離開房間，B 還在執行任務。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"用戶 C 搶佔到鎖，C 開始執行任務。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"因房間內只允許一個用戶執行任務，所以用戶 B 和 用戶 C 執行任務產生了衝突。","attrs":{}}]}]}],"attrs":{}},{"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":"從上面的案例中我們可以知道，因爲用戶 A 處理任務所需要的時間","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"大於","attrs":{}},{"type":"text","text":"鎖自動清理（開鎖）的時間，所以在自動開鎖後，又有其他用戶搶佔到了鎖。當用戶 A 完成任務後，會把其他用戶搶佔到的鎖給主動打開。","attrs":{}}]},{"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":"**這裏爲什麼會打開別人的鎖？**因爲鎖的編號都叫做 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"“123”","attrs":{}}],"attrs":{}},{"type":"text","text":"，用戶 A 只認鎖 id，看見編號爲 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"“123”","attrs":{}}],"attrs":{}},{"type":"text","text":"的鎖就開，結果把用戶 B 的鎖打開了，此時用戶 B 還未執行完任務，當然生氣了。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"六、鉑金方案","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"6.1 生活中的例子","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"上面的黃金方案的缺陷也很好解決，給每個鎖設置不同的編號不就好了～","attrs":{}}]},{"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":"如下圖所示，B 搶佔的鎖是藍色的，和 A 搶佔到綠色鎖不一樣。這樣就不會被 A 打開了。","attrs":{}}]},{"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":"做了個動圖，方便理解：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/35/35d4b476b8b820b4e01c96ae9d067b34.gif","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":"靜態圖更高清，可以看看：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/28/28a5d629a2619b93f889ead5776539e6.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"6.2 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代碼示例","attrs":{}}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"// 1.生成唯一 id\nString uuid = UUID.randomUUID().toString();\n// 2. 搶佔鎖\nBoolean lock = redisTemplate.opsForValue().setIfAbsent(\"lock\", uuid, 10, TimeUnit.SECONDS);\nif(lock) {\n System.out.println(\"搶佔成功：\" + uuid);\n // 3.搶佔成功，執行業務\n List typeEntityListFromDb = getDataFromDB();\n // 4.獲取當前鎖的值\n String lockValue = redisTemplate.opsForValue().get(\"lock\");\n // 5.如果鎖的值和設置的值相等，則清理自己的鎖\n if(uuid.equals(lockValue)) {\n System.out.println(\"清理鎖：\" + lockValue);\n redisTemplate.delete(\"lock\");\n }\n return typeEntityListFromDb;\n} else {\n System.out.println(\"搶佔失敗，等待鎖釋放\");\n // 4.休眠一段時間\n sleep(100);\n // 5.搶佔失敗，等待鎖釋放\n return getTypeEntityListByRedisDistributedLock();\n}\n","attrs":{}}]},{"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":"1.生成隨機唯一 id，給鎖加上唯一值。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"2.搶佔鎖，並設置過期時間爲 10 s，且鎖具有隨機唯一 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