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Update 4.storage-service.md #950

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Sep 10, 2021
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4 changes: 4 additions & 0 deletions docs-2.0/1.introduction/3.nebula-graph-architecture/4.storage-service.md
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Expand Up @@ -204,7 +204,9 @@ pId = vid % numParts + 1;

Raft 多副本的方式与 HDFS 多副本的方式是不同的，Raft 基于“多数派”投票，因此副本数量不能是偶数。

<!--
Listener：这是一种特殊的 Raft 角色，并不参与投票，也不能用于多副本的数据一致性。在Nebula Graph中，其作用是从 Leader 读取 Raft-wal，并向ElasticSearch集群同步。
-->

### Multi Group Raft

Expand All @@ -228,13 +230,15 @@ Nebula Graph中，每个分片都是串行写日志，为了提高吞吐，写

例如无锁CAS操作需要之前的WAL全部提交后才能执行，如果一个批量写入的WAL里包含了CAS类型的WAL，就需要拆分成粒度更小的几个组，还要保证这几组WAL串行提交。

<!--
### Listener 角色

Listener 角色的存在主要是为了应对**扩容**，扩容时新增的机器需要很长时间去同步数据，如果以 follower 角色同步数据，会导致整个集群的高可用性下降。

新增Listener角色后，会写入command类型的WAL，leader在写WAL时如果发现有`add learner`的command，会将learner加入自己的peers，并将它标记为Listener，在统计多数派的时候，不会算上Listener，但是日志还是会照常发送给它们，Listener本身也不会主动发起选举。

Raft listener进程还可以从Leader获取数据后，然后将它们写入Elasticsearch集群，以便实现全文搜索。详情请参见[部署Raft Listener](../../4.deployment-and-installation/6.deploy-text-based-index/3.deploy-listener.md)。
-->

### leader切换（Transfer Leadership）

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