Implementing Raw Node Interface should be a 4-tier title

doc/project2-RaftKV.md

@@ -28,19 +28,21 @@ This part can be broken down into 3 steps, including:
 
 `raft.Raft` in `raft/raft.go` provides the core of the Raft algorithm including message handling, driving the logic clock, etc. For more implementation guides, please check `raft/doc.go` which contains an overview design and what these `MessageTypes` are responsible for.
 
-#### Leader election
+#### Leader election (project2aa)
 
 To implement leader election, you may want to start with `raft.Raft.tick()` which is used to advance the internal logical clock by a single tick and hence drive the election timeout or heartbeat timeout. You don’t need to care about the message sending and receiving logic now. If you need to send out a message,  just push it to `raft.Raft.msgs` and all messages the raft received will be passed to `raft.Raft.Step()`. The test code will get the messages from `raft.Raft.msgs` and pass response messages through `raft.Raft.Step()`. The `raft.Raft.Step()` is the entrance of message handling, you should handle messages like `MsgRequestVote`, `MsgHeartbeat` and their response. And please also implement test stub functions and get them called properly like `raft.Raft.becomeXXX` which used to update the raft internal state when the raft’s role changes.
 
 You can run `make project2aa` to test the implementation, and see some hints at the end of this part.
 
-#### Log replication
+#### Log replication (project2ab)
 
 To implement log replication, you may want to start with handling `MsgAppend` and `MsgAppendResponse` on both the sender and receiver side. Checkout `raft.RaftLog` in `raft/log.go` which is a helper struct that help you manage the raft log, in here you also need to interact with the upper application by the `Storage` interface defined in `raft/storage.go` to get the persisted data like log entries and snapshot.
 
 You can run `make project2ab` to test the implementation, and see some hints at the end of this part.
 
-### Implement the raw node interface
+### Raw node interface 
+
+#### Implement the raw node interface (project2ac)
 
 `raft.RawNode` in `raft/rawnode.go` is the interface we interact with the upper application, `raft.RawNode` contains `raft.Raft` and provide some wrapper functions like `RawNode.Tick()`and `RawNode.Step()`. It also provides `RawNode.Propose()` to let the upper application propose new raft logs.