# Concepts
+ All `write/read` are operation logs
  + Logs have order: **`[A, B, C] != [B, A, C]`**
+ Nodes are called **`peers`**
+ Roles of **`peers`**: **`leader`**, **`candidate`**, **`follower`** and **`learner`**
+ Logs are only written by **`leader`** and replicated to **`followers`** and **`learners`**
+ Every **`leader`** has a **`term`**
  + Each **`term`** has at most one **`leader`**
+ Logs are identified by **`index`** and **`term`**
  + **`index`** is the position of a log entry
  + **`term`** is the term of the leadership for current **`leader`**

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