# Broker 到底是什麼？

根據POSA\[1]一書中給的Borker的定義：

> The Broker architectural pattern can be used to structure distributed software systems with decoupled components that interact by remote service invocations. **A broker component Is responsible for coordinating communication**, such as forwarding requests. as well as for transmitting results and exceptions.

**Broker 其實就是一種專門設計用來傳遞、聯絡各component 的一種軟體架構**。主要分成三個主體client、Broker、servers，有別於client-server架構，**Client觸發服務實質上是透過Broker(中間人)。**

Broker 誕生是為了應付一個持續增長的系統，然而為了維護性、安全性、彈性等原因才誕生出來的。而主要遇到的困難是：**1. 系統內的component能夠互相的溝通、2.系統運行時，component可以動態地被調整(上架、下架)、3.對client端隱藏後端的service.**

> Broker 特性 hint: 紀錄client與server的狀態，並且對client隱藏server 所具有的服務

**靜態視角(proxy可直接併入client、server ; Bridge可併入Broker )** 從Class diagram 來看Broker，大致上切分為三個角色，client、Broker、servers。彼此之間傳遞透過proxy，而broker 與其他broker則是透過bridge傳遞。

![Broker architectural class diagram](https://cdn-images-1.medium.com/max/1200/1*Xp1gnt6N83vaV99XXAXDYA.png)

**動態視角(下圖將proxy合併進client、server)** 首先Server 先將自己註冊進入Broker ，而Broker會將 service存取方式保留repo.中。

![img](https://cdn-images-1.medium.com/max/800/1*ymNZflz3WNspCfq_26I03g.png)

當Client透過proxy向Broker觸發某一個服務，Broker 再向Server取要結果後回傳給Client。

![](https://cdn-images-1.medium.com/max/800/1*iZS8nCsnkVBNwgspD8J2sg.png)

**總結** 市面上有很多種broker，而且有很多的變型。但總而言之，Broker 是用來幫忙client觸發服務的，並具有「紀錄client與server的狀態，且對client隱藏server 所具有的服務」的特性！。

**參考資料** \
\[1] Frank, Buschmann, et al. “Pattern-oriented software architecture: a system of patterns.” *Wiley, ISBN 0* 471.95889 (1996)


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