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Log Replication

In this stage, you’ll replicate operations from the leader to followers in the cluster…

Logs

Section titled “Logs”

…

API

Section titled “API”

POST /raft/append-entries

Section titled “POST /raft/append-entries”

Extends the heartbeat RPC from the leader election stage to carry actual log entries:

POST /raft/append-entries
Content-Type: application/json


{
  "term": 3,
  "leader-id": "10.0.42.101:8080",
  "prev-log-index": 2,
  "prev-log-term": 2,
  "entries": [
    { "index": 3, "term": 3, "command": "PUT foo bar" },
    { "index": 4, "term": 3, "command": "DELETE baz" }
  ],
  "leader-commit": 2
}


----


200
Content-Type: application/json


{
  "term": 3,
  "success": true
}
  • prev-log-index / prev-log-term: index and term of the entry immediately preceding the new ones; follower rejects if it doesn’t match
  • entries: one or more log entries to append; empty for heartbeats as before
  • leader-commit: leader’s current commit index; follower advances its own commit index to min(leader-commit, index of last new entry)
  • success: false if the follower’s log didn’t match prev-log-index / prev-log-term; leader will decrement nextIndex and retry

Rough Notes

Section titled “Rough Notes”

External Contract

Section titled “External Contract”
  • PUT/DELETE return 200 only after the entry is committed to a majority
  • GET returns only committed, applied state
  • GET /log returns entries (from snapshot point onwards), commit-index, and last-applied
  • Redirect contract from leader election unchanged - following a redirect now has real semantics

Implementation

Section titled “Implementation”
  • Raft log replaces the WAL from stage 3
  • Leader appends client operations to log, replicates to followers via AppendEntries RPC
  • Entry committed when replicated to majority - leader advances commit index
  • Followers apply committed entries to state machine (KV store)
  • Log consistency checks (prevLogIndex, prevLogTerm) on every AppendEntries
  • Truncate conflicting uncommitted entries on followers
  • nextIndex and matchIndex tracking per follower
  • Fsync log entries before responding to client
  • Happy path only - no crashes/failures yet

Testing

Section titled “Testing”
  • Linearizability verified with porcupine
  • Requires history recording in the test framework (track operation start/end times and observed values across concurrent requests)

Resources

Section titled “Resources”