Akka Distributed Data使用了CRDTs（Conflict Free Replicated Data Type）
所有数据项都通过直接复制或基于gossip协议传播到所有节点或具有特定角色（role）的节点
还可以对读、写的一致性进行细粒度的控制

An ActorSystem is a heavyweight structure that will allocate 1…N Threads, 
so create one per logical application

Akka Cluster provides a fault-tolerant decentralized peer-to-peer based Cluster Membership Service with no single point of failure or single point of bottleneck. 
It does this using gossip protocols and an automatic failure detector.

Event Sourcing  事件溯源 是一种架构模式
事件驱动，业务由事件驱动来完成
事件是一等公民，系统的数据以事件为基础，事件保存在某种存储上
业务数据只是一些由事件产生的视图

Akka2.6开始，akka-remote 不推荐使用，akka-cluster-client 被 Deprecated 
使用 Akka Cluster 机制进行集群内消息通信，如 akka-cluster-sharding akka-cluster-distributed-data akka-cluster-pubsub 
Akka Cluster 之间（集群外）使用 Akka gRPC 通信
Akka Cluster之外推荐使用Akka gRPC

ClassicCluster Client is deprecated in favor of using Akka gRPC. 
It is not advised to build new applications with Cluster Client, and existing users should migrate.

gRPC（Google Remote Procedure Calls）
gRPC是一个高性能、开源、通用的RPC框架，面向移动和HTTP/2设计，基于Protocol Buffers的RPC框架
基于HTTP/2标准设计，支持 双向流、头部压缩、单TCP连接上的多复用请求等特性 ，省电且省空间

招行年报 2%的人掌握80%资产
头部0.07%富豪群体，持有31.54%的资产，人均资产 2780.43万，
中部2.05%中产群体，持有50.58%的资产，人均资产153.31万，
占总数97.88%的一般客户，持有的资产仅有17.87%，人均资产1.14万元

Since Akka enforces parental supervision every actor is supervised and (potentially) the supervisor of its children, 
it is advisable to familiarize with Actor Systems, supervision and handling exceptions as well as Actor References, Paths and Addresses.

Using ask will send a message to the receiving Actor as with tell, 
and the receiving actor must reply with sender() ! reply in order to complete the returned Future with a value.
The ask operation involves creating an internal actor for handling this reply, 
which needs to have a timeout after which it is destroyed in order not to leak resources