doc: add textual description of the raft.Apply process (#511)

* doc: add textual description of the raft.Apply process

* Add more details of how follower updates its commitIndex

Co-authored-by: Matt Keeler <mkeeler@users.noreply.github.com>

commitment.go

@@ -6,7 +6,7 @@ import (
 )
 
 // Commitment is used to advance the leader's commit index. The leader and
-// replication goroutines report in newly written entries with Match(), and
+// replication goroutines report in newly written entries with match(), and
 // this notifies on commitCh when the commit index has advanced.
 type commitment struct {
 	// protects matchIndexes and commitIndex

docs/apply.md

@@ -1,7 +1,8 @@
 # Raft Apply
 
-Apply is the primary operation provided by raft.
-
+Apply is the primary operation provided by raft. A client calls `raft.Apply` to apply
+a command to the FSM. A command will first be commited, i.e., durably stored on a
+quorum of raft nodes. Then, the committed command is applied to fsm.
 
 This sequence diagram shows the steps involved in a `raft.Apply` operation. Each box
 across the top is a separate thread. The name in the box identifies the state of the peer
@@ -23,7 +24,7 @@ sequenceDiagram
    leadermain->>leadermain: store logs to disk
  
    leadermain-)leaderreplicate: triggerCh
-   leaderreplicate-->>followermain: AppendEntries RPC
+   leaderreplicate-->>followermain: Transport.AppendEntries RPC
  
    followermain->>followermain: store logs to disk
  
@@ -43,3 +44,73 @@ sequenceDiagram
    end
 
 ```
+
+Following is the description of each step as shown in the above diagram
+
+1. The raft node handles the `raft.Apply` call by creating a new log entry and send the entry
+to the `applyCh` channel.
+
+2. If the node is not a leader, the method will return an error of `ErrNotLeader`. Otherwise,
+the main loop of the leader node calls `raft.dispatchLogs` to write the log entry locally.
+
+3. `raft.dispatchLogs` also sends a notification to the `f.triggerCh` of each follower (`map[ServerID]*followerReplication`) to start replicating log entries to the followers.
+
+4. For each follower, the leader has started a long running routine (`replicate`) to
+replicates log entries. On receiving a log entry to the `triggerCh`, the `replicate`
+routine makes the `Transport.AppendEntries` RPC call to do the replication. The log entries
+to be replicated are from the follower's nextIndex to min(nextIndex + maxAppendEntries, 
+leader's lastIndex). Another parameter to AppendEntries is the LeaderCommitIndex. Following
+is some examples:
+
+```
+AppenEntries(Log: 1..5, LeaderCommitIndex: 0)    // Replicating log entries 1..5, 
+                                                 // the leader hasn't committed any log entry;
+AppendEntries(Log: 6..8, LeaderCommitIndex: 4)   // Replicating log entries 6..8,
+                                                 // log 0..4 are committed after the leader receives
+                                                 // a quorum of responses
+AppendEntries(Log: 9, LeaderCommitIndex: 8)      // Replicating log entry 9,
+                                                 // log 5..8 are committed.
+AppendEntries(Log: , LeaderCommitIndex: 9)       // no new log, bumping the commit index
+                                                 // to let the follower stay up to date of the
+                                                 // latest committed entries
+```
+
+5. The follower which receives the `appendEntries` RPC calls invokes `raft.appendEntries` to handle
+the request. It appends any new entries to the local log store.
+
+6. In the same method on the follower as step 5, if the LeaderCommitIndex > this follower's
+commitIndex, the follower updates it's commitIndex to min(LeaderCommitIndex, index of its last
+log entries). In the first `AppendEntries` call of the above example, the follower won't
+update it's commitIndex, because LeaderCommitIndex is 0. The last RPC call doesn't contain
+any new log, whereas the follower will update its commitIndex to 9.
+
+Further, the follower start `processLogs` to send all the committed entries that haven't been
+applied to fsm (`fsmMutateCh <- batch`). Otherwise (i.e., `commitIndex <= lastApplied`),
+the appendEntries RPC call returns success.
+
+Therefore, it's possible that a very small window of time exists when all followers have
+committed the log to disk, the write has been realized in the FSM of the leader but the
+followers have not yet applied the log to their FSM.
+
+7. The peer applies the commited entries to the FSM.
+
+8. If all went well, the follower responds success (`resp.Success = true`) to the 
+`appendEntries` RPC call.
+
+9. On receiving the successful response from `Transport.AppendEntries`, the leader needs to
+update the fsm based on the replicated log entries. Specifically, the leader finds the
+highest log entry index that has been replicated to a quorum of the servers (
+`if quorumMatchIndex > c.commitIndex`), update `commitIndex` to that index, and
+notify through the `commitCh` channel.
+
+10. The leader receives the notification on the  `r.leaderState.commitCh` channel and starts
+grouping the entries that can be applied to the fsm.
+
+11. `processLogs` applies all the committed entries that haven't been applied by batching the log entries and forwarding them through the `fsmMutateCh` channel to fsm.
+
+12. The actual place applying the commited log entries is in the main loop of `runFSM()`.
+
+13. After the log entries that contains the client req are applied to the fsm, the fsm
+module will set the reponses to the client request (`req.future.respond(nil)`). From the
+client's point of view, the future returned by `raft.Apply` should now be unblocked and
+calls to `Error()` or `Response()` should return the data at this point.