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Merge pull request #19 from jpfuentes2/master 

Fix some docs/internals typos and add raft link
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Armon Dadgar 2014-04-03 10:39:37 -07:00
parent 12c202efaf 7f8b17ae2f
commit 392c2d2f60
1 changed files with 10 additions and 10 deletions
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website/source/docs/internals/consensus.html.markdown
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@ -21,12 +21,12 @@ to learn about them without having to go spelunking through the source code.
## Raft Protocol Overview ## Raft Protocol Overview
Raft is a relatively new consensus algorithm that is based on Paxos, Raft is a relatively new consensus algorithm that is based on Paxos,
but is designed to have fewer states and a simpler more understandable but is designed to have fewer states and a simpler, more understandable
algorithm. There are a few key terms to know when discussing Raft: algorithm. There are a few key terms to know when discussing Raft:
* Log - The primary unit of work in a Raft system is a log entry. The problem * Log - The primary unit of work in a Raft system is a log entry. The problem
of consistency can be decomposed into a *replicated log*. A log is a an ordered of consistency can be decomposed into a *replicated log*. A log is an ordered
seequence of entries. We consider the log consistent if all members agree on sequence of entries. We consider the log consistent if all members agree on
the entries and their order. the entries and their order.
* FSM - [Finite State Machine](http://en.wikipedia.org/wiki/Finite-state_machine). * FSM - [Finite State Machine](http://en.wikipedia.org/wiki/Finite-state_machine).
@ -51,7 +51,7 @@ The leader is responsible for ingesting new log entries, replicating to follower
and managing when an entry is considered committed. and managing when an entry is considered committed.
Raft is a complex protocol, and will not be covered here in detail. For the full Raft is a complex protocol, and will not be covered here in detail. For the full
specification, we recommend reading the paper. We will attempt to provide a high specification, we recommend reading the [paper](https://ramcloud.stanford.edu/wiki/download/attachments/11370504/raft.pdf). We will attempt to provide a high
level description, which may be useful for building a mental picture. level description, which may be useful for building a mental picture.
Raft nodes are always in one of three states: follower, candidate or leader. All Raft nodes are always in one of three states: follower, candidate or leader. All
@ -73,7 +73,7 @@ and in Consul's case, we use [LMDB](http://symas.com/mdb/) to maintain cluster s
An obvious question relates to the unbounded nature of a replicated log. Raft provides An obvious question relates to the unbounded nature of a replicated log. Raft provides
a mechanism by which the current state is snapshotted, and the log is compacted. Because a mechanism by which the current state is snapshotted, and the log is compacted. Because
of the FSM abstraction, restoring the state of the FSM must result in the same state of the FSM abstraction, restoring the state of the FSM must result in the same state
as a reply of old logs. This allows Raft to capture the FSM state at a point in time, as a replay of old logs. This allows Raft to capture the FSM state at a point in time,
and then remove all the logs that were used to reach that state. This is performed automatically and then remove all the logs that were used to reach that state. This is performed automatically
without user intervention, and prevents unbounded disk usage as well as minimizing without user intervention, and prevents unbounded disk usage as well as minimizing
time spent replaying logs. One of the advantages of using LMDB is that it allows Consul time spent replaying logs. One of the advantages of using LMDB is that it allows Consul
@ -81,7 +81,7 @@ to continue accepting new transactions even while old state is being snapshotted
preventing any availability issues. preventing any availability issues.
Lastly, there is the issue of updating the peer set when new servers are joining Lastly, there is the issue of updating the peer set when new servers are joining
or existing servers are leaving. As long as a quorum of nodes are available, this or existing servers are leaving. As long as a quorum of nodes is available, this
is not an issue as Raft provides mechanisms to dynamically update the peer set. is not an issue as Raft provides mechanisms to dynamically update the peer set.
If a quorum of nodes is unavailable, then this becomes a very challenging issue. If a quorum of nodes is unavailable, then this becomes a very challenging issue.
For example, suppose there are only 2 peers, A and B. The quorum size is also For example, suppose there are only 2 peers, A and B. The quorum size is also
@ -153,8 +153,8 @@ recommended deployment is either 3 or 5 servers.
<td>2</td> <td>2</td>
<td>0</td> <td>0</td>
</tr> </tr>
<tr> <tr class="warning">
<td><b>3</b></td> <td>3</td>
<td>2</td> <td>2</td>
<td>1</td> <td>1</td>
</tr> </tr>
@ -163,8 +163,8 @@ recommended deployment is either 3 or 5 servers.
<td>3</td> <td>3</td>
<td>1</td> <td>1</td>
</tr> </tr>
<tr> <tr class="warning">
<td><b>5</b></td> <td>5</td>
<td>3</td> <td>3</td>
<td>2</td> <td>2</td>
</tr> </tr>