| // Copyright 2015 The Vanadium Authors. All rights reserved. |
| // Use of this source code is governed by a BSD-style |
| // license that can be found in the LICENSE file. |
| |
| This is an implemetation of the Raft agreement protocol. Each raft |
| member maintains a log of commands in the same order. All commands |
| go through a single master. The master keeps track of the commit point, |
| i.e., the point in the log where a quorum of servers have stored the log. |
| Each server can apply the commands up to the commit point. When a client |
| starts a member it provides a callback for applying the commands as they are |
| committed. It is up to the application to make sure commands are idempotent. |
| For example, it can remember the last command applied and not let any old |
| ones be reapplied. |
| |
| Raft members use the file system to persist their log records across crashes. |
| We're currently not syncing after writing each record to the disk to |
| speed things up. Because of the idempotent callback, this will work as long |
| as a member of the quorum survives each master reelection but I may have to |
| eventualy rethink this. |
| |
| The leader sends heartbeat messages at a fixed interval (hb). Each follower will |
| trigger a new election if it hasn't heard from the leader in an interval 2.x * hb, |
| where x is a random number between 0 and 1. The random interval reduces but does |
| not eliminate the likelihood of two elections starting simultaneously. |
| |
| The VDL protocol is internal, i.e., it is just for raft members to talk to each |
| other. |
