深入淺出kubernetes之WorkQueue詳解

{"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":"WorkQueue稱爲工作隊列，Kubernetes的WorkQueue隊列與普通FIFO（先進先出，First-In, First-Out）隊列相比，實現略顯複雜，它的主要功能在於標記和去重，並支持如下特性。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"有序"},{"type":"text","text":"：按照添加順序處理元素（item）。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"去重"},{"type":"text","text":"：相同元素在同一時間不會被重複處理，例如一個元素在處理之前被添加了多次，它只會被處理一次。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"併發性"},{"type":"text","text":"：多生產者和多消費者。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"標記機制"},{"type":"text","text":"：支持標記功能，標記一個元素是否被處理，也允許元素在處理時重新排隊。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"通知機制"},{"type":"text","text":"：ShutDown方法通過信號量通知隊列不再接收新的元素，並通知metric goroutine退出。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"延遲"},{"type":"text","text":"：支持延遲隊列，延遲一段時間後再將元素存入隊列。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"限速"},{"type":"text","text":"：支持限速隊列，元素存入隊列時進行速率限制。限制一個元素被重新排隊（Reenqueued）的次數。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"Metric"},{"type":"text","text":"：支持metric監控指標，可用於Prometheus監控。"}]}]}]},{"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":"WorkQueue支持3種隊列，並提供了3種接口，不同隊列實現可應對不同的使用場景，分別介紹如下。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"Interface"},{"type":"text","text":"：FIFO隊列接口，先進先出隊列，並支持去重機制。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"DelayingInterface"},{"type":"text","text":"：延遲隊列接口，基於Interface接口封裝，延遲一段時間後再將元素存入隊列。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"RateLimitingInterface"},{"type":"text","text":"：限速隊列接口，基於DelayingInterface接口封裝，支持元素存入隊列時進行速率限制。"}]}]}]},{"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":"size","attrs":{"size":16}},{"type":"strong"}],"text":"FIFO隊列"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"FIFO隊列支持最基本的隊列方法，例如插入元素、獲取元素、獲取隊列長度等。另外，WorkQueue中的限速及延遲隊列都基於Interface接口實現，其提供如下方法："}]},{"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"}],"text":"代碼路徑：vendor/k8s.io/client-go/util/workqueue/queue.go"}]},{"type":"codeblock","attrs":{"lang":"go"},"content":[{"type":"text","text":"type Interface interface {\n Add(item interface{})\n Len() int\n Get() (item interface{}, shutdown bool)\n Done(item interface{})\n ShutDown()\n ShuttingDown() bool\n}"}]},{"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":"FIFO隊列Interface方法說明如下。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"Add"},{"type":"text","text":"：給隊列添加元素（item），可以是任意類型元素。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"Len"},{"type":"text","text":"：返回當前隊列的長度。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"Get"},{"type":"text","text":"：獲取隊列頭部的一個元素。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"Done"},{"type":"text","text":"：標記隊列中該元素已被處理。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"ShutDown"},{"type":"text","text":"：關閉隊列。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"ShuttingDown"},{"type":"text","text":"：查詢隊列是否正在關閉。"}]}]}]},{"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":"FIFO隊列數據結構如下："}]},{"type":"codeblock","attrs":{"lang":"go"},"content":[{"type":"text","text":"type Type struct {\n queue []t\n dirty set\n processing set\n cond *sync.Cond\n shuttingDown bool\n metrics queueMetrics\n unfinishedWorkUpdatePeriod time.Duration\n clock clock.Clock\n}"}]},{"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":"FIFO隊列數據結構中最主要的字段有queue、dirty和processing。其中queue字段是實際存儲元素的地方，它是slice結構的，用於保證元素有序；dirty字段非常關鍵，除了能保證去重，還能保證在處理一個元素之前哪怕其被添加了多次（併發情況下），但也只會被處理一次；processing 字段用於標記機制，標記一個元素是否正在被處理。應根據WorkQueue的特性理解源碼的實現，FIFO存儲過程如圖5-9所示。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/c2/c26387cb10ca582ba63b2243afeac1bd.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":"center","origin":null},"content":[{"type":"text","text":"圖5-9  FIFO存儲過程"}]},{"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":"通過Add方法往FIFO隊列中分別插入1、2、3這3個元素，此時隊列中的queue和dirty字段分別存有1、2、3元素，processing字段爲空。然後通過Get方法獲取最先進入的元素（也就是1元素），此時隊列中的queue和dirty字段分別存有2、3元素，而1元素會被放入processing字段中，表示該元素正在被處理。最後，當我們處理完1元素時，通過Done方法標記該元素已經被處理完成，此時隊列中的processing字段中的1元素會被刪除。"}]},{"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":"如圖5-9所示，這是FIFO隊列的存儲流程，在正常的情況下，FIFO隊列運行在併發場景下。高併發下如何保證在處理一個元素之前哪怕其被添加了多次，但也只會被處理一次？下面進行講解，FIFO併發存儲過程如圖5-10所示。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/c7/c7ad23223a54b105e45c4107e747d3ca.png","alt":null,"title":null,"style":null,"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":"center","origin":null},"content":[{"type":"text","text":" 圖5-10  FIFO併發存儲過程"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":"center","origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如圖5-10所示，在併發場景下，假設goroutine A通過Get方法獲取1元素，1元素被添加到processing字段中，同一時間，goroutine B通過Add方法插入另一個1元素，此時在processing字段中已經存在相同的元素，所以後面的1元素並不會被直接添加到queue字段中，當前FIFO隊列中的dirty字段中存有1、2、3元素，processing字段存有1元素。在goroutine A通過Done方法標記處理完成後，如果dirty字段中存有1元素，則將1元素追加到queue字段中的尾部。需要注意的是，dirty和processing字段都是用Hash Map數據結構實現的，所以不需要考慮無序，只保證去重即可。"}]},{"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":"size","attrs":{"size":16}},{"type":"strong"}],"text":"延遲隊列"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"延遲隊列，基於FIFO隊列接口封裝，在原有功能上增加了AddAfter方法，其原理是延遲一段時間後再將元素插入FIFO隊列。延遲隊列數據結構如下："}]},{"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"}],"text":"代碼路徑：vendor/k8s.io/client-go/util/workqueue/delaying_queue.go"}]},{"type":"codeblock","attrs":{"lang":"go"},"content":[{"type":"text","text":"type DelayingInterface interface {\n Interface\n AddAfter(item interface{}, duration time.Duration)\n}\n\ntype delayingType struct {\n Interface\n clock clock.Clock\n stopCh chan struct{}\n heartbeat clock.Ticker\n waitingForAddCh chan *waitFor\n metrics retryMetrics\n deprecatedMetrics retryMetrics\n}"}]},{"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":"AddAfter方法會插入一個item（元素）參數，並附帶一個duration（延遲時間）參數，該duration參數用於指定元素延遲插入FIFO隊列的時間。如果duration小於或等於0，會直接將元素插入FIFO隊列中。"}]},{"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":"delayingType結構中最主要的字段是waitingForAddCh，其默認初始大小爲1000，通過AddAfter方法插入元素時，是非阻塞狀態的，只有當插入的元素大於或等於1000時，延遲隊列纔會處於阻塞狀態。waitingForAddCh字段中的數據通過goroutine運行的waitingLoop函數持久運行。延遲隊列運行原理如圖5-11所示。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/b1/b16ca44c74aa2070e943d261867da534.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":"center","origin":null},"content":[{"type":"text","text":"圖5-11  延遲隊列運行原理"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":"center","origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如圖5-11所示，將元素1放入waitingForAddCh字段中，通過waitingLoop函數消費元素數據。當元素的延遲時間不大於當前時間時，說明還需要延遲將元素插入FIFO隊列的時間，此時將該元素放入優先隊列（waitForPriorityQueue）中。當元素的延遲時間大於當前時間時，則將該元素插入FIFO隊列中。另外，還會遍歷優先隊列（waitForPriorityQueue）中的元素，按照上述邏輯驗證時間。"}]},{"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":"size","attrs":{"size":16}},{"type":"strong"}],"text":"限速隊列"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"限速隊列，基於延遲隊列和FIFO隊列接口封裝，限速隊列接口（RateLimitingInterface）在原有功能上增加了AddRateLimited、Forget、NumRequeues方法。限速隊列的重點不在於RateLimitingInterface接口，而在於它提供的4種限速算法接口（RateLimiter）。其原理是，限速隊列利用延遲隊列的特性，延遲某個元素的插入時間，達到限速目的。RateLimiter數據結構如下："}]},{"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"}],"text":"代碼路徑：vendor/k8s.io/client-go/util/workqueue/default_rate_limiters.go"}]},{"type":"codeblock","attrs":{"lang":"go"},"content":[{"type":"text","text":"type RateLimiter interface {\n When(item interface{}) time.Duration\n Forget(item interface{})\n NumRequeues(item interface{}) int\n}"}]},{"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":"限速隊列接口方法說明如下。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"When"},{"type":"text","text":"：獲取指定元素應該等待的時間。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"Forget"},{"type":"text","text":"：釋放指定元素，清空該元素的排隊數。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"NumRequeues"},{"type":"text","text":"：獲取指定元素的排隊數。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/60/60dfa0099a827284df3b3e9a42e1aa36.png","alt":null,"title":null,"style":null,"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":"下面會分別詳解WorkQueue提供的4種限速算法，應對不同的場景，這4種限速算法分別如下。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"令牌桶算法（BucketRateLimiter）。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"排隊指數算法（ItemExponentialFailureRateLimiter）。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"計數器算法（ItemFastSlowRateLimiter）。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"混合模式（MaxOfRateLimiter），將多種限速算法混合使用。"}]}]}]},{"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"}],"text":"1. 令牌桶算法"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"令牌桶算法是通過Go語言的第三方庫golang.org/x/time/rate實現的。令牌桶算法內部實現了一個存放token（令牌）的“桶”，初始時“桶”是空的，token會以固定速率往“桶”裏填充，直到將其填滿爲止，多餘的token會被丟棄。每個元素都會從令牌桶得到一個token，只有得到token的元素才允許通過（accept），而沒有得到token的元素處於等待狀態。令牌桶算法通過控制發放token來達到限速目的。令牌桶算法原理如圖5-12所示。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/6a/6a049ef23bcdc8b542bed2782b0d82a7.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":"center","origin":null},"content":[{"type":"text","text":"圖5-12  令牌桶算法原理"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":"center","origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"WorkQueue在默認的情況下會實例化令牌桶，代碼示例如下："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"go"},"content":[{"type":"text","text":"rate.NewLimiter(rate.Limit(10), 100)"}]},{"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":"在實例化rate.NewLimiter後，傳入r和b兩個參數，其中r參數表示每秒往“桶”裏填充的token數量，b參數表示令牌桶的大小（即令牌桶最多存放的token數量）。我們假定r爲10，b爲100。假設在一個限速週期內插入了1000個元素，通過r.Limiter.Reserve().Delay函數返回指定元素應該等待的時間，那麼前b（即100）個元素會被立刻處理，而後面元素的延遲時間分別爲item100/100ms、item101/200ms、item102/300ms、item103/400ms，以此類推。"}]},{"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"}],"text":"2. 排隊指數算法"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"排隊指數算法將相同元素的排隊數作爲指數，排隊數增大，速率限制呈指數級增長，但其最大值不會超過maxDelay。元素的排隊數統計是有限速週期的，一個限速週期是指從執行AddRateLimited方法到執行完Forget方法之間的時間。如果該元素被Forget方法處理完，則清空排隊數。排隊指數算法的核心實現代碼示例如下："}]},{"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"}],"text":"代碼路徑：vendor/k8s.io/client-go/util/workqueue/default_rate_limiters.go"}]},{"type":"codeblock","attrs":{"lang":"go"},"content":[{"type":"text","text":"r.failures[item] = r.failures[item] + 1\nbackoff := float64(r.baseDelay.Nanoseconds()) * math.Pow(2, float64(exp))\nif backoff > math.MaxInt64 {\n return r.maxDelay\n}"}]},{"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":"該算法提供了3個主要字段：failures、baseDelay、maxDelay。其中，failures字段用於統計元素排隊數，每當AddRateLimited方法插入新元素時，會爲該字段加1；另外，baseDelay字段是最初的限速單位（默認爲5ms），maxDelay字段是最大限速單位（默認爲1000s）。排隊指數增長趨勢如圖5-13所示。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/34/34c1cfab9dfb032236df58ab11d95ce5.png","alt":null,"title":"","style":[{"key":"width","value":"50%"},{"key":"bordertype","value":"none"}],"href":"","fromPaste":false,"pastePass":false}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":"center","origin":null},"content":[{"type":"text","text":"圖5-13  排隊指數增長趨勢"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":"center","origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"限速隊列利用延遲隊列的特性，延遲多個相同元素的插入時間，達到限速目的。"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/ad/adf83563816720d3b0dca4b34491293a.png","alt":null,"title":null,"style":null,"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":"我們假定baseDelay是1 * time.Millisecond，maxDelay是1000 * time.Second。假設在一個限速週期內通過AddRateLimited方法插入10個相同元素，那麼第1個元素會通過延遲隊列的AddAfter方法插入並設置延遲時間爲1ms（即baseDelay），第2個相同元素的延遲時間爲2ms，第3個相同元素的延遲時間爲4ms，第4個相同元素的延遲時間爲8ms，第5個相同元素的延遲時間爲16ms……第10個相同元素的延遲時間爲512ms，最長延遲時間不超過1000s（即maxDelay）。"}]},{"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"}],"text":"3. 計數器算法"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"計數器算法是限速算法中最簡單的一種，其原理是：限制一段時間內允許通過的元素數量，例如在1分鐘內只允許通過100個元素，每插入一個元素，計數器自增1，當計數器數到100的閾值且還在限速週期內時，則不允許元素再通過。但WorkQueue在此基礎上擴展了fast和slow速率。"}]},{"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":"計數器算法提供了4個主要字段：failures、fastDelay、slowDelay及maxFastAttempts。其中，failures字段用於統計元素排隊數，每當AddRateLimited方法插入新元素時，會爲該字段加1；而fastDelay和slowDelay字段是用於定義fast、slow速率的；另外，maxFastAttempts字段用於控制從fast速率轉換到slow速率。計數器算法核心實現的代碼示例如下："}]},{"type":"codeblock","attrs":{"lang":"text"},"content":[{"type":"text","text":"r.failures[item] = r.failures[item] + 1\nif r.failures[item] <= r.maxFastAttempts {\n return r.fastDelay\n}\nreturn r.slowDelay"}]},{"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":"假設fastDelay是5 * time.Millisecond，slowDelay是10 * time.Second，maxFastAttempts是3。在一個限速週期內通過AddRateLimited方法插入4個相同的元素，那麼前3個元素使用fastDelay定義的fast速率，當觸發maxFastAttempts字段時，第4個元素使用slowDelay定義的slow速率。"}]},{"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"}],"text":"4. 混合模式"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"混合模式是將多種限速算法混合使用，即多種限速算法同時生效。例如，同時使用排隊指數算法和令牌桶算法，代碼示例如下："}]},{"type":"codeblock","attrs":{"lang":"go"},"content":[{"type":"text","text":"func DefaultControllerRateLimiter() RateLimiter {\n return NewMaxOfRateLimiter(\n NewItemExponentialFailureRateLimiter(5*time.Millisecond, 1000*time.Second),\n &BucketRateLimiter{Limiter: rate.NewLimiter(rate.Limit(10), 100)},\n )\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/14/144c8a519a432e8d1af766f9fb5cb652.jpeg","alt":null,"title":"","style":[{"key":"width","value":"25%"},{"key":"bordertype","value":"none"}],"href":"https://u.jd.com/3K5DNZ","fromPaste":false,"pastePass":false}},{"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":"本文選自《Kubernetes源碼剖析》一書"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"鄭東旭著"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"電子工業出版社出版"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"購買鏈接：https://u.jd.com/3K5DNZ"}]}]}