Polish the design document of ServiceComb Pack
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# Saga Pack Design
[](design_zh.md)
+
+##BackGround Introduction
+The following illustration shows a typical distributed transaction call, in which a user requests a distributed service call, and the initial service calls two participating services in sequence (Service A, Service B). When Service A executes successfully and Service B hits a problem, our distributed transaction need to call service A's compensation operation to ensure the consistency of the distributed transaction (a single transaction fails and the entire distributed transaction needs to be rolled back), because there is no connection between the two participating services. A coordinator is therefore required to assist with related recovery.
+
+
+
+In the process of performing compensation for distributed transactions, we can divide them into two different sets of compensation methods depending on the compensation execution:
+* Imperfect Compensation (Saga) - The compensation operation leaves traces of the original transaction operation before, and in general we set the cancellation state in the original transaction record.
+* Perfect Compensation (TCC) - The compensation operation thoroughly cleans up the original transaction operation before, and generally does not retain the original transaction transaction, and the user is unaware of the state information before the transaction cancels.
+
## Overview
Pack contains two components: *alpha* and *omega*. Alpha is the pack leader and backed by database to make sure transaction events stored permanently while omega is the pack worker and embedded inside services to intercept transaction invocation and report events to alpha.
@@ -15,6 +25,43 @@
+## System Architecture
+We can learn more about the relationship between Alpha and Omega modules under the Pack system architecture diagram.
+
+
+
+The entire architecture is divided into three parts, one is the Alpha Coordinator (which supports multiple instances to provide highly available support), the other is Omega injected into the microservice instance, and the interaction protocol between Alpha and Omega, which currently supports SagaSaga And TCC two kinds of distributed transaction coordination protocol implementation.
+
+### Omega
+
+Omega contains modules related to analyzing user distributed transaction logic:
+
+* Transaction Annotation: The user adds these labels to their business code to describe the information related to the distributed transaction, so that Omega can handle the processing in accordance with the coordination requirements of the distributed transaction. If you extend your own distributed transactions, you can also do so by defining your own transaction dimensions.
+
+* Transaction Interceptor: In this module we use AOP to intercept user-labeled code to add relevant business logic code, to obtain information related to distributed transactions and local transaction execution, and uses transaction transport module sends events with Alpha.
+
+* Transaction Context: The transaction context provides a means for Omega to pass transaction call information, and with the previously mentioned global transaction ID and the correspondence of local transaction IDs, Alpha can easily retrieve all local transaction event information related to a distributed transaction.
+
+* Transaction Executor: The transaction executor is primarily a module designed to handle transaction call timeouts. Because the connection between Alpha and Omega may be unreliable, it is difficult for the Alpha side to determine whether the Omega local transaction execution timeout is caused by the alpha and Omega's own call, so the transaction executor is designed to monitor omega's local performance simplifies Omega's timeout. The default implementation of Omega is to call the transaction method directly, and Alpha's background service determines whether the transaction execution time times timeout by scanning the event table.
+
+* Transaction Callback: When Omega establishes a connection with Alpha, it registers with Alpha, and when alpha needs to be coordinated, it calls the Omega-registered callback method directly to communicate. Since microservice instances will start and stop frequently in cloud-based scenarios, we cannot assume that Alpha will always be able to find transaction callbacks on the original registration, so we recommend that the microservice instances be stateless, so that Alpha can only communicate with the corresponding Omega based on the service name.
+
+### Transport
+
+Transaction Transport: The transaction transport module is responsible for communication between Omega and Alpha, and in the specific implementation process, Pack defines the transaction interaction methods of TCC and Saga by defining the relevant Grpc description interface file, as well as the events associated with the interaction. We enabled mutual calls between Omega and Alpha with the help of the two-way flow interface provided by Grpc. Omega and Alpha's transmissions are based on Grpc multilingual support and provide the foundation for a multilingual version of Omega.
+
+### Alpha
+
+In order to realize its transactional coordination function, Alpha first needs to receive omega-uploaded events through *Transaction Transport*, and in the *Event Store* module, Alpha uses the Event API to provide event query services to the outside world. Alpha scans the execution event information for distributed transactions through the *Event Scanner*, identifies the time-out transaction, and sends instructions to Omega to complete the transaction coordination. Because Alpha Coordination provides a highly available architecture in a multi-instance approach, this requires *Alpha Cluster Manger* to manage the coordination before alpha cluster instances. Users can monitor the execution of distributed transactions by managing terminals.
+
+* Event Store: Alpha's event storage is currently built on top of the database. To reduce the complexity of system implementations, the highly available architecture of alpha clusters is based on database clusters. To improve the query efficiency of the database, we divide the data store into an online library and an archive library based on the global transaction performance of the event, store the outstanding distributed transaction events in the online library, and store the completed distributed transaction events in the archive library.
+
+* Event API: which exposes as a Restful Event Query Service. This module feature is first applied in the acceptance test of the pack, through the event API acceptance test code can be easily understood the events received internally by Alpha. Acceptance tests verify that the relevant transaction coordination functions are correct by simulating various distributed transaction execution exceptions (errors or timeouts) compared to transaction events received by Alpha.
+
+* Management Console: the management terminal provides a statistical analysis of the performance of a distributed transaction by accessing the Rest service provided by the Event API, and can track the execution of a single global transaction to find out the cause of the failure of the transaction.
+
+* Alpha Cluster Manager: which is responsible for alpha instance registration, managing the execution of individual services in Alpha, and providing Omega with a list of service services that are up-to-date. The cluster manager user can easily implement the start-stop operation of the Alpha service instance and the rolling upgrade capability of the Alpha service instance.
+
## Workflow Saga
In Saga workflow, the sub transaction need to provide the compensation method. If something is wrong, the Coordinator will send the command to the omega to do the forward or backward recovery.
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# Saga Pack 设计文档
[](design.md)
+## 业务背景介绍
+下图展示了一个典型的分布式事务调用， 用户请求触发分布式服务调用， 初始服务会顺序调用两个参与服务（服务A，服务B）。当服务A执行成功，而服务B执行出现了问题，我们这个分布式事务调用需要调用服务A的补偿操作来确保分布式事务的一致性（单个事务失败，整个分布式事务需要进行回滚），由于这两个参与服务之间没有联系，因此需要一个协调器来帮助进行相关的恢复。
+
+分布式事务的在执行补偿的过程中，我们可以根据补偿执行的不同将其分成两组不同的补偿方式：
+* 不完美补偿（Saga） - 补偿操作会留下之前原始事务操作的痕迹，一般来说我们是会在原始事务记录中设置取消状态。
+* 完美补偿（TCC) - 补偿操作会彻底清理之前的原始事务操作，一般来说是不会保留原始事务交易记录，用户是感知不到事务取消之前的状态信息的。
+
## 概览
Pack中包含两个组件，即 **alpha** 和 **omega**。
* alpha充当协调者的角色，主要负责对事务的事件进行持久化存储以及协调子事务的状态，使其得以最终与全局事务的状态保持一致。
@@ -17,6 +24,40 @@
+## Pack的架构图
+我们可以从下图进一步了解ServiceComb Pack架构下，Alpha与Omega内部各模块之间的关系图。
+
+整个架构分为三个部分，一个是Alpha协调器（支持多个实例提供高可用支持），另外一个就是注入到微服务实例中的Omega，以及Alpha与Omega之间的交互协议， 目前Pack支持Saga 以及TCC两种分布式事务协调协议实现。
+
+### Omega
+Omega包含了与分析用户分布式事务逻辑相关的模块 事务注解模块（Transaction Annotation） 以及 事务拦截器（Transaction Interceptor）； 分布式事务执行相关的事务上下文（Transaction Context），事务回调（Transaction Callback) ，事务执行器 （Transaction Executor）；以及负责与Alpha进行通讯的事务传输（Transaction Transport）模块。
+
+事务注解模块是分布式事务的用户界面，用户将这些标注添加到自己的业务代码之上用以描述与分布式事务相关的信息，这样Omega就可以按照分布式事务的协调要求进行相关的处理。如果大家扩展自己的分布式事务，也可以通过定义自己的事务标注来实现。
+
+事务拦截器这个模块我们可以借助AOP手段，在用户标注的代码基础上添加相关的拦截代码，获取到与分布式事务以及本地事务执行相关的信息，并借助事务传输模块与Alpha进行通讯传递事件。
+
+事务上下文为Omega内部提供了一个传递事务调用信息的一个手段，借助前面提到的全局事务ID以及本地事务ID的对应关系，Alpha可以很容易检索到与一个分布式事务相关的所有本地事务事件信息。
+
+事务执行器主要是为了处理事务调用超时设计的模块。由于Alpha与Omega之间的连接有可能不可靠，Alpha端很难判断Omega本地事务执行超时是由Alpha与Omega直接的网络引起的还是Omega自身调用的问题，因此设计了事务执行器来监控Omega的本地的执行情况，简化Omega的超时操作。目前Omega的缺省实现是直接调用事务方法，由Alpha的后台服务通过扫描事件表的方式来确定事务执行时间是否超时。
+
+事务回调 在Omega与Alpha建立连接的时候就会向Alpha进行注册，当Alpha需要进行相关的协调操作的时候，会直接调用Omega注册的回调方法进行通信。 由于微服务实例在云化场景启停会很频繁，我们不能假设Alpha一直能找到原有注册上的事务回调， 因此我们建议微服务实例是无状态的，这样Alpha只需要根据服务名就能找到对应的Omega进行通信。
+
+### Transport
+
+事务传输模块负责Omega与Alpha之间的通讯，在具体的实现过程中，Pack通过定义相关的Grpc描述接口文件定义了TCC 以及Saga的事务交互方法， 同时也定义了与交互相关的事件。我们借助了Grpc所提供的双向流操作接口实现了Omega与Alpha之间的相互调用。 Omega和Alpha的传输建立在Grpc多语言支持的基础上，为实现多语言版本的Omega奠定了基础。
+
+### Alpha
+
+Alpha为了实现其事务协调的功能，首先需要通过事务传输（Transaction Transport）接收Omega上传的事件， 并将事件存在事件存储（Event Store）模块中，Alpha通过事件API (Event API）对外提供事件查询服务。Alpha会通过事件扫描器（Event Scanner）对分布式事务的执行事件信息进行扫描分析，识别超时的事务，并向Omega发送相关的指令来完成事务协调的工作。由于Alpha协调是采用多个实例的方式对外提供高可用架构， 这就需要Alpha集群管理器（Alpha Cluster Manger）来管理Alpha集群实例之前的协调。用户可以通过管理终端（Manage console）对分布式事务的执行情况进行监控。
+
+目前Alpha的事件存储是构建在数据库基础之上的。为了降低系统实现的复杂程度，Alpha集群的高可用架构是建立在数据库集群基础之上的。 为了提高数据库的查询效率，我们会根据事件的全局事务执行情况的装将数据存储分成了在线库以及存档库，将未完成的分布式事务事件存储在在线库中， 将已经完成的分布式事务事件存储在存档库中。
+
+事件API是Alpha对外暴露的Restful事件查询服务。 这模块功能首先应用在Pack的验收测试中，通过事件API验收测试代码可以很方便的了解Alpha内部接收的事件。验收测试通过模拟各种分布式事务执行异常情况（错误或者超时），比对Alpha接收到的事务事件来验证相关的其事务协调功能是否正确。
+
+管理终端通过访问事件API提供的Rest服务，向用户提供是分布式事务执行情况的统计分析，并且可以追踪单个全局事务的执行情况，找出事务的失败的原因。
+
+Alpha集群管理器负责Alpha实例注册工作，管理Alpha中单个服务的执行情况， 并且为Omega提供一个及时更新的服务列表。 通过集群管理器用户可以轻松实现Alpha服务实例的启停操作，以及Alpha服务实例的滚动升级功能。
+
## Saga 具体处理流程
Saga处理场景是要求相关的子事务提供事务处理函数同时也提供补偿函数。Saga协调器alpha会根据事务的执行情况向omega发送相关的指令，确定是否向前重试或者向后恢复。
### 成功场景
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