2016/06/17/raft-consensus-single-node.html - kudu-site - Git at Google

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     <h1 class="entry-title">Using Raft Consensus on a Single Node</h1>
     <p class="meta">Posted 17 Jun 2016 by Mike Percy</p>
   </header>
   <div class="entry-content">
     <p>As Kudu marches toward its 1.0 release, which will include support for
 multi-master operation, we are working on removing old code that is no longer
 needed. One such piece of code is called LocalConsensus. Once LocalConsensus is
 removed, we will be using Raft consensus even on Kudu tables that have a
 replication factor of 1.</p>

 <!--more-->

 <p>Using Raft consensus in single-node cases is important for multi-master
 support because it will allow people to dynamically increase their Kudu
 cluster’s existing master server replication factor from 1 to many (3 or 5 are
 typical).</p>

 <h1 id="the-consensus-interface">The Consensus interface</h1>

 <p>In Kudu, the
 <a href="https://github.com/apache/incubator-kudu/blob/branch-0.9.x/src/kudu/consensus/consensus.h">Consensus</a>
 interface was created as an abstraction to allow us to build the plumbing
 around how a consensus implementation would interact with the underlying
 tablet. We were able to build out this “scaffolding” long before our Raft
 implementation was complete.</p>

 <p>The Consensus API has the following main responsibilities:</p>

 <ol>
   <li>Support acting as a Raft <code>LEADER</code> and replicate writes to a local
 write-ahead log (WAL) as well as followers in the Raft configuration. For
 each operation written to the leader, a Raft implementation must keep track
 of how many nodes have written a copy of the operation being replicated, and
 whether or not that constitutes a majority. Once a majority of the nodes
 have written a copy of the data, it is considered committed.</li>
   <li>Support acting as a Raft <code>FOLLOWER</code> by accepting writes from the leader and
 preparing them to be eventually committed.</li>
   <li>Support voting in and initiating leader elections.</li>
   <li>Support participating in and initiating configuration changes (such as going
 from a replication factor of 3 to 4).</li>
 </ol>

 <p>The first implementation of the Consensus interface was called LocalConsensus.
 LocalConsensus only supported acting as a leader of a single-node configuration
 (hence the name “local”). It could not replicate to followers, participate in
 elections, or change configurations. These limitations have led us to
 <a href="https://gerrit.cloudera.org/3350">remove</a> LocalConsensus from the code base
 entirely.</p>

 <p>Because Kudu has a full-featured Raft implementation, Kudu’s RaftConsensus
 supports all of the above functions of the Consensus interface.</p>

 <h1 id="using-a-single-node-raft-configuration">Using a Single-node Raft configuration</h1>

 <p>A common question on the Raft mailing lists is: “Is it even possible to use
 Raft on a single node?” The answer is yes.</p>

 <p>Fundamentally, Raft works by first electing a leader that is responsible for
 replicating write operations to the other members of the configuration. In
 order to elect a leader, Raft requires a (strict) majority of the voters to
 vote “yes” in an election. When there is only a single eligible node in the
 configuration, there is no chance of losing the election. Raft specifies that
 when starting an election, a node must first vote for itself and then contact
 the rest of the voters to tally their votes. If there is only a single node, no
 communication is required and an election succeeds instantaneously.</p>

 <p>So, when does it make sense to use Raft for a single node?</p>

 <p>It makes sense to do this when you want to allow growing the replication factor
 in the future. This is something that Kudu needs to support. When deploying
 Kudu, someone may wish to test it out with limited resources in a small
 environment. Eventually, they may wish to transition that cluster to be a
 staging or production environment, which would typically require the fault
 tolerance achievable with multi-node Raft. Without a consensus implementation
 that supports configuration changes, there would be no way to gracefully
 support this. Because single-node Raft supports dynamically adding an
 additional node to its configuration, it is possible to go from one replica to
 2 and then 3 replicas and end up with a fault-tolerant cluster without
 incurring downtime.</p>

 <h1 id="more-about-raft">More about Raft</h1>

 <p>To learn more about how Kudu uses Raft consensus, you may find the relevant
 <a href="https://github.com/apache/incubator-kudu/blob/master/docs/design-docs/README.md">design docs</a>
 interesting. In the future, we may also post more articles on the Kudu blog
 about how Kudu uses Raft to achieve fault tolerance.</p>

 <p>To learn more about the Raft protocol itself, please see the <a href="https://raft.github.io/">Raft consensus
 home page</a>. The design of Kudu’s Raft implementation
 is based on the extended protocol described in Diego Ongaro’s Ph.D.
 dissertation, which you can find linked from the above web site.</p>

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	<article>
	<header>
	<h1 class="entry-title">Using Raft Consensus on a Single Node</h1>
	<p class="meta">Posted 17 Jun 2016 by Mike Percy</p>
	</header>
	<div class="entry-content">
	<p>As Kudu marches toward its 1.0 release, which will include support for
	multi-master operation, we are working on removing old code that is no longer
	needed. One such piece of code is called LocalConsensus. Once LocalConsensus is
	removed, we will be using Raft consensus even on Kudu tables that have a
	replication factor of 1.</p>

	<!--more-->

	<p>Using Raft consensus in single-node cases is important for multi-master
	support because it will allow people to dynamically increase their Kudu
	cluster’s existing master server replication factor from 1 to many (3 or 5 are
	typical).</p>

	<h1 id="the-consensus-interface">The Consensus interface</h1>

	<p>In Kudu, the
	<a href="https://github.com/apache/incubator-kudu/blob/branch-0.9.x/src/kudu/consensus/consensus.h">Consensus</a>
	interface was created as an abstraction to allow us to build the plumbing
	around how a consensus implementation would interact with the underlying
	tablet. We were able to build out this “scaffolding” long before our Raft
	implementation was complete.</p>

	<p>The Consensus API has the following main responsibilities:</p>

	<ol>
	<li>Support acting as a Raft <code>LEADER</code> and replicate writes to a local
	write-ahead log (WAL) as well as followers in the Raft configuration. For
	each operation written to the leader, a Raft implementation must keep track
	of how many nodes have written a copy of the operation being replicated, and
	whether or not that constitutes a majority. Once a majority of the nodes
	have written a copy of the data, it is considered committed.</li>
	<li>Support acting as a Raft <code>FOLLOWER</code> by accepting writes from the leader and
	preparing them to be eventually committed.</li>
	<li>Support voting in and initiating leader elections.</li>
	<li>Support participating in and initiating configuration changes (such as going
	from a replication factor of 3 to 4).</li>
	</ol>

	<p>The first implementation of the Consensus interface was called LocalConsensus.
	LocalConsensus only supported acting as a leader of a single-node configuration
	(hence the name “local”). It could not replicate to followers, participate in
	elections, or change configurations. These limitations have led us to
	<a href="https://gerrit.cloudera.org/3350">remove</a> LocalConsensus from the code base
	entirely.</p>

	<p>Because Kudu has a full-featured Raft implementation, Kudu’s RaftConsensus
	supports all of the above functions of the Consensus interface.</p>

	<h1 id="using-a-single-node-raft-configuration">Using a Single-node Raft configuration</h1>

	<p>A common question on the Raft mailing lists is: “Is it even possible to use
	Raft on a single node?” The answer is yes.</p>

	<p>Fundamentally, Raft works by first electing a leader that is responsible for
	replicating write operations to the other members of the configuration. In
	order to elect a leader, Raft requires a (strict) majority of the voters to
	vote “yes” in an election. When there is only a single eligible node in the
	configuration, there is no chance of losing the election. Raft specifies that
	when starting an election, a node must first vote for itself and then contact
	the rest of the voters to tally their votes. If there is only a single node, no
	communication is required and an election succeeds instantaneously.</p>

	<p>So, when does it make sense to use Raft for a single node?</p>

	<p>It makes sense to do this when you want to allow growing the replication factor
	in the future. This is something that Kudu needs to support. When deploying
	Kudu, someone may wish to test it out with limited resources in a small
	environment. Eventually, they may wish to transition that cluster to be a
	staging or production environment, which would typically require the fault
	tolerance achievable with multi-node Raft. Without a consensus implementation
	that supports configuration changes, there would be no way to gracefully
	support this. Because single-node Raft supports dynamically adding an
	additional node to its configuration, it is possible to go from one replica to
	2 and then 3 replicas and end up with a fault-tolerant cluster without
	incurring downtime.</p>

	<h1 id="more-about-raft">More about Raft</h1>

	<p>To learn more about how Kudu uses Raft consensus, you may find the relevant
	<a href="https://github.com/apache/incubator-kudu/blob/master/docs/design-docs/README.md">design docs</a>
	interesting. In the future, we may also post more articles on the Kudu blog
	about how Kudu uses Raft to achieve fault tolerance.</p>

	<p>To learn more about the Raft protocol itself, please see the <a href="https://raft.github.io/">Raft consensus
	home page</a>. The design of Kudu’s Raft implementation
	is based on the extended protocol described in Diego Ongaro’s Ph.D.
	dissertation, which you can find linked from the above web site.</p>

	</div>
	</article>


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