Add doc

Author: beebeb

Course4/database_transaction.md

@@ -212,8 +212,8 @@ START TRANSACTION;
 COMMIT;
 ```
 
-2. 如果不允许超卖的话，则在事务的过程中，注意事务还未结束，检查quantity是否小于0， 小于0则说明超卖了，立即回滚事务，撤销当前的订单。
-
+2. 如果不允许超卖的话，则在事务的过程中，注意事务还未结束，检查quantity是否小于0， 小于0则说明超卖了，立即回滚事务，撤销当前的订单
+。
 ```
 select quantity from inventory_test where product_id=1
 

Course5/2021-09-23-06-17-24.png

@@ -0,0 +1,47 @@
+#  1. 在Micro Service体系下如何开发业务
+
+
+## 2. 事务脚本以及领域模型
+
+## 2.1 事务脚本
+
+Organize the business logic as a collection of procedural transaction scripts, one for each type of request.
+
+![](2021-09-23-06-17-24.png)
+
+## 领域模型
+
+Organize the business logic as an object model consisting of classes that have state and behavior.
+
+![](2021-09-23-06-20-00.png)
+
+
+## 3.  领域驱动（DDD）
+
+* Entity
+* Value Object
+* Subdomains
+* Factory
+* Repository
+* Aggregate
+* Service
+
+### 3.1 Aggregate是什么含义
+
+Organize a domain model as a collection of aggregates, each of which is a graph of objects that can be treated as a unit.
+
+![](2021-09-23-06-37-56.png)
+
+#### 3.1.1 Aggregate的特征
+
+Ensure that an aggregate is a self-contained unit that can enforce its invariants.
+
+* 避免跨服务的Object reference, 即是多个Aggregate通过外键的值相互关联，而不是对象的引用。
+  ![](2021-09-23-06-42-05.png)
+* 一个事务只能更新创建和更新单个Aggregate,Aggregate满足Microservice的事务模型约束。
+
+### 3.2 领域事件
+
+An aggregate publishes a domain event when it’s created or undergoes some other significant change.
+
+## 4. 示例

Course5/2021-09-23-06-20-00.png

@@ -0,0 +1,41 @@
+CQRS
+====
+
+Command query responsibility segregation
+
+## Command
+
+CUD
+
+## Query
+
+R
+
+1. API composition
+2. Dedicated data view
+
+
+OrderHistoryService
+
+handler  -> order.created
+         -> customer.updated
+         -> ...
+
+ElasticSearch
+
+
+### Issues
+
+1. Handle duplicated messages
+    1.1 
+        PROCESSED_EVENT table
+        eventId
+        maxEventId
+    1.2 
+        幂等性
+
+## API Gateway
+
+1. API composition
+2. Edge function(Authentication, Authorization, Rate limit, Monitor)
+3. Proxy