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    # 2/8 SRE 讀書會共筆 --- 場地提供:PIXNET - 誠徵 [SRE 服務可靠度工程師](https://gist.github.com/jnlin/8329bea1c1ed92a1924da7e032304261),有興趣的朋友可聯繫 applyjob@pixnet.tw # 第十九章:Load Balancing at the Frontend - by 91APP 黃冠維 Frontend Load Balancer - 流量要怎麼從用戶傳導到 Load Balancer ## 負載平衡的一些考量 * 再強大的硬體都有極限 * 負載平衡系統 用來決定 哪些機器處理某個請求 * 架構考量 / 技術考量 / 使用者流量的屬性 搜尋請求 - 快速反應 影片上傳 - 大頻寬 * 使用 DNS 來進行負載平衡 * DNS 是最簡單、最有效的負載平衡機制 * 一個 Load Balancer 有很多個 IP,根據流量來作延展 AWS 的 ELB,最多解析 8 個 IP 壓測工具,如果解出的 IP 是同一個,Load Balancer 很快就被打掛了 假設全世界的使用者,都會指到不同 IP 使用傳統 ELB,如果預期會有大量流量,需要開 support ticket 給 AWS,作 warm up 的工作 * 使用虛擬 IP 處理負載平衡 (NLB) * 對用戶來說,解析到同一個 IP * 對封包來說,要怎麼知道要傳的目的地機器? id(packet) mod N * 用 MAC address 的解法 * 用 NAT 的解法 * 用 GRE 封包包裝的解法 => 可以跨機房 # 第二十章:Load Balancing in the Datacenter - by 91APP 黃冠維 * 理想狀況:每一台 server 一樣的 loading * 實際狀況:每一台 loading 有差別 如果沒有平均,那 1000 台就沒辦法正常的 Load Balance ## 流量控制、健康管理 健康、拒絕連接、跛腳鴨狀態 例如:Server 升級的時候,可以宣告自己是「跛腳鴨狀態」,拒絕新連線的連入 ## 子集劃分 Healthy Check,Server數太多的時候,不是好的分散式系統 例如:一個 Load Balancer 節點,負責三十個 Server 的 Healthy Check,前端 Connection 太多 建連線成本很大 用 Random 的結果,會不夠平均 套了神秘的演算法,就可以平均分配囉! 寫 Python 程式來驗證 用 random 是 C(300, 90) 的概念 ## 連線分配 * 輪詢 (Simple Round-Robin) * 子集太小 * 每個 Request 成本不同 * 不同等級的後端 Server * 其他因素 * Least-Loaded Round-Robin * Server 很快回錯誤,比正常消化快,Load Balancer 會一直丟給這個有問題的 Server * 不知道其他 Load Balancer 有丟給節點的數量 * Weighted Round-Robin * 混合各種系統健康速度,來作加權,決定要丟給哪個節點 ## 討論 * jnlin (PIXNET):遇過 DDoS 攻擊,後來用 DNS 來解決,把從越南、中國來的攻擊流量進行阻擋或 Filter * Rick (91APP):可以用 Route53 的 GeoDNS 來解決 * 某些店被競品競爭,會被 DDoS * Raix (PIXNET):Virtual IP 如何 Route 到特定的機器 * 硬體可以做得到 * 小明:有沒有一個方式,可以在各國有自己的 8.8.8.8 # 第二十一章:Handling overload - by PIXNET Dennis ## 系統總是會過載 * 服務降級 * 只搜尋小部分的回傳結果 * 存取非即時的 Cache 資料 * QPS 陷阱 * 限制服務端用量 * 針對「異常」,返回錯誤訊息 * 對服務配置額度 * 服務端節流機制 * 配額不足回應過多,應自己限制 request * Request 可以分重要性 * 重要性可以繼承 * 資源利用信號 * 以 CPU 為指標 * 由執行器負載均值來決定 * 過載錯誤處理 * 大部分資源過載 * 停止 retry * 小部分資源過載 * 快速拒絕,快速 retry * 可能是 load balancer 不完美造成的 * Retry 的次數 * 次數限制 < 3 次 * 觀察 retry 比例 < 10% * 過載情況下,不需要 retry * 由可用資源拒絕 將過載極限值盡量提升到很難發生的程度 正瑋:降級服務,有人曾經作過這樣設計嗎? Rick (91APP):作開關,例如廣告拿掉。 Rick (91APP):有在做容量 Control 嗎? 正瑋:跟容量規劃有點像 Rick (91APP):nginx 有內建,API Gateway 有類似的機制

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