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Subject svn commit: r993301 [45/49] - in /websites/staging/zookeeper/trunk/content: ./ doc/r3.5.2-alpha/ doc/r3.5.2-alpha/api/ doc/r3.5.2-alpha/api/org/ doc/r3.5.2-alpha/api/org/apache/ doc/r3.5.2-alpha/api/org/apache/zookeeper/ doc/r3.5.2-alpha/api/org/apache...
Date Thu, 21 Jul 2016 17:04:51 GMT
Added: websites/staging/zookeeper/trunk/content/doc/r3.5.2-alpha/zookeeperHierarchicalQuorums.html
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--- websites/staging/zookeeper/trunk/content/doc/r3.5.2-alpha/zookeeperHierarchicalQuorums.html (added)
+++ websites/staging/zookeeper/trunk/content/doc/r3.5.2-alpha/zookeeperHierarchicalQuorums.html Thu Jul 21 17:04:48 2016
@@ -0,0 +1,264 @@
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+<a class="dida" href="zookeeperHierarchicalQuorums.pdf"><img alt="PDF -icon" src="skin/images/pdfdoc.gif" class="skin"><br>
+        PDF</a>
+</div>
+<h1>Introduction to hierarchical quorums</h1>
+<div id="front-matter"></div>
+  
+
+  
+
+    
+<p>
+    This document gives an example of how to use hierarchical quorums. The basic idea is
+    very simple. First, we split servers into groups, and add a line for each group listing
+    the servers that form this group. Next we have to assign a weight to each server.  
+    </p>
+    
+    
+<p>
+    The following example shows how to configure a system with three groups of three servers
+    each, and we assign a weight of 1 to each server:
+    </p>
+    
+    
+<pre class="code">
+    group.1=1:2:3
+    group.2=4:5:6
+    group.3=7:8:9
+   
+    weight.1=1
+    weight.2=1
+    weight.3=1
+    weight.4=1
+    weight.5=1
+    weight.6=1
+    weight.7=1
+    weight.8=1
+    weight.9=1
+ 	</pre>
+
+	
+<p>    
+    When running the system, we are able to form a quorum once we have a majority of votes from
+    a majority of non-zero-weight groups. Groups that have zero weight are discarded and not
+    considered when forming quorums. Looking at the example, we are able to form a quorum once
+    we have votes from at least two servers from each of two different groups.
+    </p> 
+ 
+<p align="right">
+<font size="-2"></font>
+</p>
+</div>
+<!--+
+    |end content
+    +-->
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+</div>
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+</body>
+</html>

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--- websites/staging/zookeeper/trunk/content/doc/r3.5.2-alpha/zookeeperInternals.html (added)
+++ websites/staging/zookeeper/trunk/content/doc/r3.5.2-alpha/zookeeperInternals.html Thu Jul 21 17:04:48 2016
@@ -0,0 +1,793 @@
+<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
+<html>
+<head>
+<META http-equiv="Content-Type" content="text/html; charset=UTF-8">
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+<a class="dida" href="zookeeperInternals.pdf"><img alt="PDF -icon" src="skin/images/pdfdoc.gif" class="skin"><br>
+        PDF</a>
+</div>
+<h1>ZooKeeper Internals</h1>
+<div id="front-matter">
+<div id="minitoc-area">
+<ul class="minitoc">
+<li>
+<a href="#ch_Introduction">Introduction</a>
+</li>
+<li>
+<a href="#sc_atomicBroadcast">Atomic Broadcast</a>
+<ul class="minitoc">
+<li>
+<a href="#sc_guaranteesPropertiesDefinitions">Guarantees, Properties, and Definitions</a>
+</li>
+<li>
+<a href="#sc_leaderElection">Leader Activation</a>
+</li>
+<li>
+<a href="#sc_activeMessaging">Active Messaging</a>
+</li>
+<li>
+<a href="#sc_summary">Summary</a>
+</li>
+<li>
+<a href="#sc_comparisons">Comparisons</a>
+</li>
+</ul>
+</li>
+<li>
+<a href="#sc_quorum">Quorums</a>
+</li>
+<li>
+<a href="#sc_logging">Logging</a>
+<ul class="minitoc">
+<li>
+<a href="#sc_developerGuidelines">Developer Guidelines</a>
+<ul class="minitoc">
+<li>
+<a href="#sc_rightLevel">Logging at the Right Level</a>
+</li>
+<li>
+<a href="#sc_slf4jIdioms">Use of Standard slf4j Idioms</a>
+</li>
+</ul>
+</li>
+</ul>
+</li>
+</ul>
+</div>
+</div>
+  
+
+  
+
+  
+<a name="ch_Introduction"></a>
+<h2 class="h3">Introduction</h2>
+<div class="section">
+<p>This document contains information on the inner workings of ZooKeeper. 
+    So far, it discusses these topics:
+    </p>
+<ul>    
+
+<li>
+<p>
+<a href="#sc_atomicBroadcast">Atomic Broadcast</a>
+</p>
+</li>
+
+<li>
+<p>
+<a href="#sc_logging">Logging</a>
+</p>
+</li>
+
+</ul>
+</div>
+
+
+<a name="sc_atomicBroadcast"></a>
+<h2 class="h3">Atomic Broadcast</h2>
+<div class="section">
+<p>
+At the heart of ZooKeeper is an atomic messaging system that keeps all of the servers in sync.</p>
+<a name="sc_guaranteesPropertiesDefinitions"></a>
+<h3 class="h4">Guarantees, Properties, and Definitions</h3>
+<p>
+The specific guarantees provided by the messaging system used by ZooKeeper are the following:</p>
+<dl>
+
+
+<dt>
+<term>
+<em>Reliable delivery</em>
+</term>
+</dt>
+<dd>
+<p>If a message, m, is delivered 
+by one server, it will be eventually delivered by all servers.</p>
+</dd>
+
+
+<dt>
+<term>
+<em>Total order</em>
+</term>
+</dt>
+<dd>
+<p> If a message is 
+delivered before message b by one server, a will be delivered before b by all 
+servers. If a and b are delivered messages, either a will be delivered before b 
+or b will be delivered before a.</p>
+</dd>
+
+
+<dt>
+<term>
+<em>Causal order</em> 
+</term>
+</dt>
+<dd>
+<p>
+If a message b is sent after a message a has been delivered by the sender of b, 
+a must be ordered before b. If a sender sends c after sending b, c must be ordered after b.
+</p>
+</dd>
+
+
+</dl>
+<p>
+The ZooKeeper messaging system also needs to be efficient, reliable, and easy to 
+implement and maintain. We make heavy use of messaging, so we need the system to 
+be able to handle thousands of requests per second. Although we can require at 
+least k+1 correct servers to send new messages, we must be able to recover from 
+correlated failures such as power outages. When we implemented the system we had 
+little time and few engineering resources, so we needed a protocol that is 
+accessible to engineers and is easy to implement. We found that our protocol 
+satisfied all of these goals.
+
+</p>
+<p>
+Our protocol assumes that we can construct point-to-point FIFO channels between 
+the servers. While similar services usually assume message delivery that can 
+lose or reorder messages, our assumption of FIFO channels is very practical 
+given that we use TCP for communication. Specifically we rely on the following property of TCP:</p>
+<dl>
+
+
+<dt>
+<term>
+<em>Ordered delivery</em>
+</term>
+</dt>
+<dd>
+<p>Data is delivered in the same order it is sent and a message m is 
+delivered only after all messages sent before m have been delivered. 
+(The corollary to this is that if message m is lost all messages after m will be lost.)</p>
+</dd>
+
+
+<dt>
+<term>
+<em>No message after close</em>
+</term>
+</dt>
+<dd>
+<p>Once a FIFO channel is closed, no messages will be received from it.</p>
+</dd>
+
+
+</dl>
+<p>
+FLP proved that consensus cannot be achieved in asynchronous distributed systems 
+if failures are possible. To ensure we achieve consensus in the presence of failures 
+we use timeouts. However, we rely on times for liveness not for correctness. So, 
+if timeouts stop working (clocks malfunction for example) the messaging system may 
+hang, but it will not violate its guarantees.</p>
+<p>When describing the ZooKeeper messaging protocol we will talk of packets, 
+proposals, and messages:</p>
+<dl>
+
+<dt>
+<term>
+<em>Packet</em>
+</term>
+</dt>
+<dd>
+<p>a sequence of bytes sent through a FIFO channel</p>
+</dd>
+<dt>
+<term>
+<em>Proposal</em>
+</term>
+</dt>
+<dd>
+<p>a unit of agreement. Proposals are agreed upon by exchanging packets 
+with a quorum of ZooKeeper servers. Most proposals contain messages, however the 
+NEW_LEADER proposal is an example of a proposal that does not correspond to a message.</p>
+</dd>
+<dt>
+<term>
+<em>Message</em>
+</term>
+</dt>
+<dd>
+<p>a sequence of bytes to be atomically broadcast to all ZooKeeper 
+servers. A message put into a proposal and agreed upon before it is delivered.</p>
+</dd>
+
+
+</dl>
+<p>
+As stated above, ZooKeeper guarantees a total order of messages, and it also 
+guarantees a total order of proposals. ZooKeeper exposes the total ordering using
+a ZooKeeper transaction id (<em>zxid</em>). All proposals will be stamped with a zxid when 
+it is proposed and exactly reflects the total ordering. Proposals are sent to all 
+ZooKeeper servers and committed when a quorum of them acknowledge the proposal. 
+If a proposal contains a message, the message will be delivered when the proposal 
+is committed. Acknowledgement means the server has recorded the proposal to persistent storage. 
+Our quorums have the requirement that any pair of quorum must have at least one server 
+in common. We ensure this by requiring that all quorums have size (<em>n/2+1</em>) where 
+n is the number of servers that make up a ZooKeeper service.
+</p>
+<p>
+The zxid has two parts: the epoch and a counter. In our implementation the zxid 
+is a 64-bit number. We use the high order 32-bits for the epoch and the low order 
+32-bits for the counter. Because it has two parts represent the zxid both as a 
+number and as a pair of integers, (<em>epoch, count</em>). The epoch number represents a 
+change in leadership. Each time a new leader comes into power it will have its 
+own epoch number. We have a simple algorithm to assign a unique zxid to a proposal: 
+the leader simply increments the zxid to obtain a unique zxid for each proposal. 
+<em>Leadership activation will ensure that only one leader uses a given epoch, so our 
+simple algorithm guarantees that every proposal will have a unique id.</em>
+
+</p>
+<p>
+ZooKeeper messaging consists of two phases:</p>
+<dl>
+
+<dt>
+<term>
+<em>Leader activation</em>
+</term>
+</dt>
+<dd>
+<p>In this phase a leader establishes the correct state of the system 
+and gets ready to start making proposals.</p>
+</dd>
+
+
+<dt>
+<term>
+<em>Active messaging</em>
+</term>
+</dt>
+<dd>
+<p>In this phase a leader accepts messages to propose and coordinates message delivery.</p>
+</dd>
+
+</dl>
+<p>
+ZooKeeper is a holistic protocol. We do not focus on individual proposals, rather 
+look at the stream of proposals as a whole. Our strict ordering allows us to do this 
+efficiently and greatly simplifies our protocol. Leadership activation embodies 
+this holistic concept. A leader becomes active only when a quorum of followers 
+(The leader counts as a follower as well. You can always vote for yourself ) has synced 
+up with the leader, they have the same state. This state consists of all of the 
+proposals that the leader believes have been committed and the proposal to follow 
+the leader, the NEW_LEADER proposal. (Hopefully you are thinking to 
+yourself, <em>Does the set of proposals that the leader believes has been committed 
+included all the proposals that really have been committed?</em> The answer is <em>yes</em>. 
+Below, we make clear why.)
+</p>
+<a name="sc_leaderElection"></a>
+<h3 class="h4">Leader Activation</h3>
+<p>
+Leader activation includes leader election. We currently have two leader election 
+algorithms in ZooKeeper: LeaderElection and FastLeaderElection (AuthFastLeaderElection 
+is a variant of FastLeaderElection that uses UDP and allows servers to perform a simple
+form of authentication to avoid IP spoofing). ZooKeeper messaging doesn't care about the 
+exact method of electing a leader has long as the following holds:
+</p>
+<ul>
+
+
+<li>
+<p>The leader has seen the highest zxid of all the followers.</p>
+</li>
+
+<li>
+<p>A quorum of servers have committed to following the leader.</p>
+</li>
+
+
+</ul>
+<p>
+Of these two requirements only the first, the highest zxid amoung the followers 
+needs to hold for correct operation. The second requirement, a quorum of followers, 
+just needs to hold with high probability. We are going to recheck the second requirement, 
+so if a failure happens during or after the leader election and quorum is lost, 
+we will recover by abandoning leader activation and running another election.
+</p>
+<p>
+After leader election a single server will be designated as a leader and start 
+waiting for followers to connect. The rest of the servers will try to connect to 
+the leader. The leader will sync up with followers by sending any proposals they 
+are missing, or if a follower is missing too many proposals, it will send a full 
+snapshot of the state to the follower.
+</p>
+<p>
+There is a corner case in which a follower that has proposals, U, not seen 
+by a leader arrives. Proposals are seen in order, so the proposals of U will have a zxids 
+higher than zxids seen by the leader. The follower must have arrived after the 
+leader election, otherwise the follower would have been elected leader given that 
+it has seen a higher zxid. Since committed proposals must be seen by a quorum of 
+servers, and a quorum of servers that elected the leader did not see U, the proposals 
+of you have not been committed, so they can be discarded. When the follower connects 
+to the leader, the leader will tell the follower to discard U.
+</p>
+<p>
+A new leader establishes a zxid to start using for new proposals by getting the 
+epoch, e, of the highest zxid it has seen and setting the next zxid to use to be 
+(e+1, 0), fter the leader syncs with a follower, it will propose a NEW_LEADER 
+proposal. Once the NEW_LEADER proposal has been committed, the leader will activate 
+and start receiving and issuing proposals.
+</p>
+<p>
+It all sounds complicated but here are the basic rules of operation during leader 
+activation:
+</p>
+<ul>
+
+<li>
+<p>A follower will ACK the NEW_LEADER proposal after it has synced with the leader.</p>
+</li>
+
+<li>
+<p>A follower will only ACK a NEW_LEADER proposal with a given zxid from a single server.</p>
+</li>
+
+<li>
+<p>A new leader will COMMIT the NEW_LEADER proposal when a quorum of followers have ACKed it.</p>
+</li>
+
+<li>
+<p>A follower will commit any state it received from the leader when the NEW_LEADER proposal is COMMIT.</p>
+</li>
+
+<li>
+<p>A new leader will not accept new proposals until the NEW_LEADER proposal has been COMMITED.</p>
+</li>
+
+</ul>
+<p>
+If leader election terminates erroneously, we don't have a problem since the 
+NEW_LEADER proposal will not be committed since the leader will not have quorum. 
+When this happens, the leader and any remaining followers will timeout and go back 
+to leader election.
+</p>
+<a name="sc_activeMessaging"></a>
+<h3 class="h4">Active Messaging</h3>
+<p>
+Leader Activation does all the heavy lifting. Once the leader is coronated he can 
+start blasting out proposals. As long as he remains the leader no other leader can 
+emerge since no other leader will be able to get a quorum of followers. If a new 
+leader does emerge, 
+it means that the leader has lost quorum, and the new leader will clean up any 
+mess left over during her leadership activation.
+</p>
+<p>ZooKeeper messaging operates similar to a classic two-phase commit.</p>
+<img alt="" src="images/2pc.jpg"><p>
+All communication channels are FIFO, so everything is done in order. Specifically 
+the following operating constraints are observed:</p>
+<ul>
+
+
+<li>
+<p>The leader sends proposals to all followers using 
+the same order. Moreover, this order follows the order in which requests have been 
+received. Because we use FIFO channels this means that followers also receive proposals in order.
+</p>
+</li>
+
+
+<li>
+<p>Followers process messages in the order they are received. This 
+means that messages will be ACKed in order and the leader will receive ACKs from 
+followers in order, due to the FIFO channels. It also means that if message $m$ 
+has been written to non-volatile storage, all messages that were proposed before 
+$m$ have been written to non-volatile storage.</p>
+</li>
+
+
+<li>
+<p>The leader will issue a COMMIT to all followers as soon as a 
+quorum of followers have ACKed a message. Since messages are ACKed in order, 
+COMMITs will be sent by the leader as received by the followers in order.</p>
+</li>
+
+
+<li>
+<p>COMMITs are processed in order. Followers deliver a proposals 
+message when that proposal is committed.</p>
+</li>
+
+
+</ul>
+<a name="sc_summary"></a>
+<h3 class="h4">Summary</h3>
+<p>So there you go. Why does it work? Specifically, why does a set of proposals 
+believed by a new leader always contain any proposal that has actually been committed? 
+First, all proposals have a unique zxid, so unlike other protocols, we never have 
+to worry about two different values being proposed for the same zxid; followers 
+(a leader is also a follower) see and record proposals in order; proposals are 
+committed in order; there is only one active leader at a time since followers only 
+follow a single leader at a time; a new leader has seen all committed proposals 
+from the previous epoch since it has seen the highest zxid from a quorum of servers; 
+any uncommited proposals from a previous epoch seen by a new leader will be committed 
+by that leader before it becomes active.</p>
+<a name="sc_comparisons"></a>
+<h3 class="h4">Comparisons</h3>
+<p>
+Isn't this just Multi-Paxos? No, Multi-Paxos requires some way of assuring that 
+there is only a single coordinator. We do not count on such assurances. Instead 
+we use the leader activation to recover from leadership change or old leaders 
+believing they are still active.
+</p>
+<p>
+Isn't this just Paxos? Your active messaging phase looks just like phase 2 of Paxos? 
+Actually, to us active messaging looks just like 2 phase commit without the need to 
+handle aborts. Active messaging is different from both in the sense that it has 
+cross proposal ordering requirements. If we do not maintain strict FIFO ordering of 
+all packets, it all falls apart. Also, our leader activation phase is different from 
+both of them. In particular, our use of epochs allows us to skip blocks of uncommitted
+proposals and to not worry about duplicate proposals for a given zxid.
+</p>
+</div>
+
+
+<a name="sc_quorum"></a>
+<h2 class="h3">Quorums</h2>
+<div class="section">
+<p>
+Atomic broadcast and leader election use the notion of quorum to guarantee a consistent
+view of the system. By default, ZooKeeper uses majority quorums, which means that every
+voting that happens in one of these protocols requires a majority to vote on. One example is
+acknowledging a leader proposal: the leader can only commit once it receives an
+acknowledgement from a quorum of servers.
+</p>
+<p>
+If we extract the properties that we really need from our use of majorities, we have that we only
+need to guarantee that groups of processes used to validate an operation by voting (e.g., acknowledging
+a leader proposal) pairwise intersect in at least one server. Using majorities guarantees such a property.
+However, there are other ways of constructing quorums different from majorities. For example, we can assign
+weights to the votes of servers, and say that the votes of some servers are more important. To obtain a quorum,
+we get enough votes so that the sum of weights of all votes is larger than half of the total sum of all weights.    
+</p>
+<p>
+A different construction that uses weights and is useful in wide-area deployments (co-locations) is a hierarchical
+one. With this construction, we split the servers into disjoint groups and assign weights to processes. To form 
+a quorum, we have to get a hold of enough servers from a majority of groups G, such that for each group g in G,
+the sum of votes from g is larger than half of the sum of weights in g. Interestingly, this construction enables
+smaller quorums. If we have, for example, 9 servers, we split them into 3 groups, and assign a weight of 1 to each
+server, then we are able to form quorums of size 4. Note that two subsets of processes composed each of a majority
+of servers from each of a majority of groups necessarily have a non-empty intersection. It is reasonable to expect
+that a majority of co-locations will have a majority of servers available with high probability. 
+</p>
+<p>
+With ZooKeeper, we provide a user with the ability of configuring servers to use majority quorums, weights, or a 
+hierarchy of groups.
+</p>
+</div>
+
+
+<a name="sc_logging"></a>
+<h2 class="h3">Logging</h2>
+<div class="section">
+<p>
+Zookeeper uses 
+<a href="http://www.slf4j.org/index.html">slf4j</a> as an abstraction layer for logging. 
+<a href="http://logging.apache.org/log4j">log4j</a> in version 1.2 is chosen as the final logging implementation for now.
+For better embedding support, it is planned in the future to leave the decision of choosing the final logging implementation to the end user.
+Therefore, always use the slf4j api to write log statements in the code, but configure log4j for how to log at runtime.
+Note that slf4j has no FATAL level, former messages at FATAL level have been moved to ERROR level. 
+For information on configuring log4j for
+ZooKeeper, see the <a href="zookeeperAdmin.html#sc_logging">Logging</a> section 
+of the <a href="zookeeperAdmin.html">ZooKeeper Administrator's Guide.</a>
+
+
+</p>
+<a name="sc_developerGuidelines"></a>
+<h3 class="h4">Developer Guidelines</h3>
+<p>Please follow the  
+<a href="http://www.slf4j.org/manual.html">slf4j manual</a> when creating log statements within code.
+Also read the
+<a href="http://www.slf4j.org/faq.html#logging_performance">FAQ on performance</a>
+, when creating log statements. Patch reviewers will look for the following:</p>
+<a name="sc_rightLevel"></a>
+<h4>Logging at the Right Level</h4>
+<p>
+There are several levels of logging in slf4j. 
+It's important to pick the right one. In order of higher to lower severity:</p>
+<ol>
+   
+<li>
+<p>ERROR level designates error events that might still allow the application to continue running.</p>
+</li>
+   
+<li>
+<p>WARN level designates potentially harmful situations.</p>
+</li>
+   
+<li>
+<p>INFO level designates informational messages that highlight the progress of the application at coarse-grained level.</p>
+</li>
+   
+<li>
+<p>DEBUG Level designates fine-grained informational events that are most useful to debug an application.</p>
+</li>
+   
+<li>
+<p>TRACE Level designates finer-grained informational events than the DEBUG.</p>
+</li>
+
+</ol>
+<p>
+ZooKeeper is typically run in production such that log messages of INFO level 
+severity and higher (more severe) are output to the log.</p>
+<a name="sc_slf4jIdioms"></a>
+<h4>Use of Standard slf4j Idioms</h4>
+<p>
+<em>Static Message Logging</em>
+</p>
+<pre class="code">
+LOG.debug("process completed successfully!");
+</pre>
+<p>
+However when creating parameterized messages are required, use formatting anchors.
+</p>
+<pre class="code">
+LOG.debug("got {} messages in {} minutes",new Object[]{count,time});    
+</pre>
+<p>
+<em>Naming</em>
+</p>
+<p>
+Loggers should be named after the class in which they are used.
+</p>
+<pre class="code">
+public class Foo {
+    private static final Logger LOG = LoggerFactory.getLogger(Foo.class);
+    ....
+    public Foo() {
+       LOG.info("constructing Foo");
+</pre>
+<p>
+<em>Exception handling</em>
+</p>
+<pre class="code">
+try {
+  // code
+} catch (XYZException e) {
+  // do this
+  LOG.error("Something bad happened", e);
+  // don't do this (generally)
+  // LOG.error(e);
+  // why? because "don't do" case hides the stack trace
+ 
+  // continue process here as you need... recover or (re)throw
+}
+</pre>
+</div>
+
+
+<p align="right">
+<font size="-2"></font>
+</p>
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+<div title="Portable Document Format" class="pdflink">
+<a class="dida" href="zookeeperJMX.pdf"><img alt="PDF -icon" src="skin/images/pdfdoc.gif" class="skin"><br>
+        PDF</a>
+</div>
+<h1>ZooKeeper JMX</h1>
+<div id="front-matter">
+<div id="minitoc-area">
+<ul class="minitoc">
+<li>
+<a href="#ch_jmx">JMX</a>
+</li>
+<li>
+<a href="#ch_starting">Starting ZooKeeper with JMX enabled</a>
+</li>
+<li>
+<a href="#ch_console">Run a JMX console</a>
+</li>
+<li>
+<a href="#ch_reference">ZooKeeper MBean Reference</a>
+</li>
+</ul>
+</div>
+</div>
+  
+
+  
+
+  
+<a name="ch_jmx"></a>
+<h2 class="h3">JMX</h2>
+<div class="section">
+<p>Apache ZooKeeper has extensive support for JMX, allowing you
+    to view and manage a ZooKeeper serving ensemble.</p>
+<p>This document assumes that you have basic knowledge of
+    JMX. See <a href="http://java.sun.com/javase/technologies/core/mntr-mgmt/javamanagement/">
+    Sun JMX Technology</a> page to get started with JMX.
+    </p>
+<p>See the <a href="http://java.sun.com/javase/6/docs/technotes/guides/management/agent.html">
+    JMX Management Guide</a> for details on setting up local and
+    remote management of VM instances. By default the included
+    <em>zkServer.sh</em> supports only local management -
+    review the linked document to enable support for remote management
+    (beyond the scope of this document).
+    </p>
+</div>
+
+  
+<a name="ch_starting"></a>
+<h2 class="h3">Starting ZooKeeper with JMX enabled</h2>
+<div class="section">
+<p>The class
+      <em>org.apache.zookeeper.server.quorum.QuorumPeerMain</em>
+      will start a JMX manageable ZooKeeper server. This class
+      registers the proper MBeans during initalization to support JMX
+      monitoring and management of the
+      instance. See <em>bin/zkServer.sh</em> for one
+      example of starting ZooKeeper using QuorumPeerMain.</p>
+</div>
+
+  
+<a name="ch_console"></a>
+<h2 class="h3">Run a JMX console</h2>
+<div class="section">
+<p>There are a number of JMX consoles available which can connect
+      to the running server. For this example we will use Sun's
+      <em>jconsole</em>.</p>
+<p>The Java JDK ships with a simple JMX console
+      named <a href="http://java.sun.com/developer/technicalArticles/J2SE/jconsole.html">jconsole</a>
+      which can be used to connect to ZooKeeper and inspect a running
+      server. Once you've started ZooKeeper using QuorumPeerMain
+      start <em>jconsole</em>, which typically resides in
+      <em>JDK_HOME/bin/jconsole</em>
+</p>
+<p>When the "new connection" window is displayed either connect
+      to local process (if jconsole started on same host as Server) or
+      use the remote process connection.</p>
+<p>By default the "overview" tab for the VM is displayed (this
+      is a great way to get insight into the VM btw). Select
+      the "MBeans" tab.</p>
+<p>You should now see <em>org.apache.ZooKeeperService</em>
+      on the left hand side. Expand this item and depending on how you've
+      started the server you will be able to monitor and manage various
+      service related features.</p>
+<p>Also note that ZooKeeper will register log4j MBeans as
+    well. In the same section along the left hand side you will see
+    "log4j". Expand that to manage log4j through JMX. Of particular
+    interest is the ability to dynamically change the logging levels
+    used by editing the appender and root thresholds. Log4j MBean
+    registration can be disabled by passing
+    <em>-Dzookeeper.jmx.log4j.disable=true</em> to the JVM
+    when starting ZooKeeper.
+    </p>
+</div>
+
+  
+<a name="ch_reference"></a>
+<h2 class="h3">ZooKeeper MBean Reference</h2>
+<div class="section">
+<p>This table details JMX for a server participating in a
+    replicated ZooKeeper ensemble (ie not standalone). This is the
+    typical case for a production environment.</p>
+<table class="ForrestTable" cellspacing="1" cellpadding="4">
+<caption>MBeans, their names and description</caption>
+      
+<title>MBeans, their names and description</title>
+
+      
+          
+<tr>
+            
+<th>MBean</th>
+            <th>MBean Object Name</th>
+            <th>Description</th>
+          
+</tr>
+        
+          
+<tr>
+            
+<td>Quorum</td>
+            <td>ReplicatedServer_id&lt;#&gt;</td>
+            <td>Represents the Quorum, or Ensemble - parent of all
+            cluster members. Note that the object name includes the
+            "myid" of the server (name suffix) that your JMX agent has
+            connected to.</td>
+          
+</tr>
+          
+<tr>
+            
+<td>LocalPeer|RemotePeer</td>
+            <td>replica.&lt;#&gt;</td>
+            <td>Represents a local or remote peer (ie server
+            participating in the ensemble). Note that the object name
+            includes the "myid" of the server (name suffix).</td>
+          
+</tr>
+          
+<tr>
+            
+<td>LeaderElection</td>
+            <td>LeaderElection</td>
+            <td>Represents a ZooKeeper cluster leader election which is
+            in progress. Provides information about the election, such as
+            when it started.</td>
+          
+</tr>
+          
+<tr>
+            
+<td>Leader</td>
+            <td>Leader</td>
+            <td>Indicates that the parent replica is the leader and
+            provides attributes/operations for that server. Note that
+            Leader is a subclass of ZooKeeperServer, so it provides
+            all of the information normally associated with a
+            ZooKeeperServer node.</td>
+          
+</tr>
+          
+<tr>
+            
+<td>Follower</td>
+            <td>Follower</td>
+            <td>Indicates that the parent replica is a follower and
+            provides attributes/operations for that server. Note that
+            Follower is a subclass of ZooKeeperServer, so it provides
+            all of the information normally associated with a
+            ZooKeeperServer node.</td>
+          
+</tr>
+          
+<tr>
+            
+<td>DataTree</td>
+            <td>InMemoryDataTree</td>
+            <td>Statistics on the in memory znode database, also
+            operations to access finer (and more computationally
+            intensive) statistics on the data (such as ephemeral
+            count). InMemoryDataTrees are children of ZooKeeperServer
+            nodes.</td>
+          
+</tr>
+          
+<tr>
+            
+<td>ServerCnxn</td>
+            <td>&lt;session_id&gt;</td>
+            <td>Statistics on each client connection, also
+            operations on those connections (such as
+            termination). Note the object name is the session id of
+            the connection in hex form.</td>
+          
+</tr>
+    
+</table>
+<p>This table details JMX for a standalone server. Typically
+    standalone is only used in development situations.</p>
+<table class="ForrestTable" cellspacing="1" cellpadding="4">
+<caption>MBeans, their names and description</caption>
+      
+<title>MBeans, their names and description</title>
+
+      
+          
+<tr>
+            
+<th>MBean</th>
+            <th>MBean Object Name</th>
+            <th>Description</th>
+          
+</tr>
+        
+          
+<tr>
+            
+<td>ZooKeeperServer</td>
+            <td>StandaloneServer_port&lt;#&gt;</td>
+            <td>Statistics on the running server, also operations
+            to reset these attributes. Note that the object name
+            includes the client port of the server (name
+            suffix).</td>
+          
+</tr>
+          
+<tr>
+            
+<td>DataTree</td>
+            <td>InMemoryDataTree</td>
+            <td>Statistics on the in memory znode database, also
+            operations to access finer (and more computationally
+            intensive) statistics on the data (such as ephemeral
+            count).</td>
+          
+</tr>
+          
+<tr>
+            
+<td>ServerCnxn</td>
+            <td>&lt;session_id&gt;</td>
+            <td>Statistics on each client connection, also
+            operations on those connections (such as
+            termination). Note the object name is the session id of
+            the connection in hex form.</td>
+          
+</tr>
+    
+</table>
+</div>
+
+
+<p align="right">
+<font size="-2"></font>
+</p>
+</div>
+<!--+
+    |end content
+    +-->
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+document.write("Last Published: " + document.lastModified);
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+</div>
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+         2008-2013 <a href="http://www.apache.org/licenses/">The Apache Software Foundation.</a>
+</div>
+<!--+
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+</body>
+</html>

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+        PDF</a>
+</div>
+<h1>ZooKeeper Observers</h1>
+<div id="front-matter">
+<div id="minitoc-area">
+<ul class="minitoc">
+<li>
+<a href="#ch_Introduction">Observers: Scaling ZooKeeper Without Hurting Write Performance
+      </a>
+</li>
+<li>
+<a href="#sc_UsingObservers">How to use Observers</a>
+</li>
+<li>
+<a href="#ch_UseCases">Example use cases</a>
+</li>
+</ul>
+</div>
+</div>
+  
+
+  
+
+  
+<a name="ch_Introduction"></a>
+<h2 class="h3">Observers: Scaling ZooKeeper Without Hurting Write Performance
+      </h2>
+<div class="section">
+<p>
+      Although ZooKeeper performs very well by having clients connect directly
+      to voting members of the ensemble, this architecture makes it hard to
+      scale out to huge numbers of clients. The problem is that as we add more
+      voting members, the write performance drops. This is due to the fact that
+      a write operation requires the agreement of (in general) at least half the
+      nodes in an ensemble and therefore the cost of a vote can increase
+      significantly as more voters are added.
+    </p>
+<p>
+      We have introduced a new type of ZooKeeper node called
+      an <em>Observer</em> which helps address this problem and
+      further improves ZooKeeper's scalability. Observers are non-voting members
+      of an ensemble which only hear the results of votes, not the agreement
+      protocol that leads up to them. Other than this simple distinction,
+      Observers function exactly the same as Followers - clients may connect to
+      them and send read and write requests to them. Observers forward these
+      requests to the Leader like Followers do, but they then simply wait to
+      hear the result of the vote. Because of this, we can increase the number
+      of Observers as much as we like without harming the performance of votes.
+    </p>
+<p>
+      Observers have other advantages. Because they do not vote, they are not a
+      critical part of the ZooKeeper ensemble. Therefore they can fail, or be
+      disconnected from the cluster, without harming the availability of the
+      ZooKeeper service. The benefit to the user is that Observers may connect
+      over less reliable network links than Followers. In fact, Observers may be
+      used to talk to a ZooKeeper server from another data center. Clients of
+      the Observer will see fast reads, as all reads are served locally, and
+      writes result in minimal network traffic as the number of messages
+      required in the absence of the vote protocol is smaller.
+    </p>
+</div>
+  
+<a name="sc_UsingObservers"></a>
+<h2 class="h3">How to use Observers</h2>
+<div class="section">
+<p>Setting up a ZooKeeper ensemble that uses Observers is very simple,
+    and requires just two changes to your config files. Firstly, in the config
+    file of every node that is to be an Observer, you must place this line:
+    </p>
+<pre class="code">
+      peerType=observer
+    </pre>
+<p>
+      This line tells ZooKeeper that the server is to be an Observer. Secondly,
+      in every server config file, you must add :observer to the server
+      definition line of each Observer. For example:
+    </p>
+<pre class="code">
+      server.1:localhost:2181:3181:observer
+    </pre>
+<p>
+      This tells every other server that server.1 is an Observer, and that they
+      should not expect it to vote. This is all the configuration you need to do
+      to add an Observer to your ZooKeeper cluster. Now you can connect to it as
+      though it were an ordinary Follower. Try it out, by running:</p>
+<pre class="code">
+      bin/zkCli.sh -server localhost:2181
+    </pre>
+<p>
+      where localhost:2181 is the hostname and port number of the Observer as
+      specified in every config file. You should see a command line prompt
+      through which you can issue commands like <em>ls</em> to query
+      the ZooKeeper service.
+    </p>
+</div>
+  
+  
+<a name="ch_UseCases"></a>
+<h2 class="h3">Example use cases</h2>
+<div class="section">
+<p>
+      Two example use cases for Observers are listed below. In fact, wherever
+      you wish to scale the number of clients of your ZooKeeper ensemble, or
+      where you wish to insulate the critical part of an ensemble from the load
+      of dealing with client requests, Observers are a good architectural
+      choice.
+    </p>
+<ul>
+      
+<li>
+	
+<p> As a datacenter bridge: Forming a ZK ensemble between two
+	datacenters is a problematic endeavour as the high variance in latency
+	between the datacenters could lead to false positive failure detection
+	and partitioning. However if the ensemble runs entirely in one
+	datacenter, and the second datacenter runs only Observers, partitions
+	aren't problematic as the ensemble remains connected. Clients of the
+	Observers may still see and issue proposals.</p>
+      
+</li>
+      
+<li>
+	
+<p>As a link to a message bus: Some companies have expressed an
+	interest in using ZK as a component of a persistent reliable message
+	bus. Observers would give a natural integration point for this work: a
+	plug-in mechanism could be used to attach the stream of proposals an
+	Observer sees to a publish-subscribe system, again without loading the
+	core ensemble.
+	</p>
+      
+</li>
+    
+</ul>
+</div>
+
+<p align="right">
+<font size="-2"></font>
+</p>
+</div>
+<!--+
+    |end content
+    +-->
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+</div>
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+</html>

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--- websites/staging/zookeeper/trunk/content/doc/r3.5.2-alpha/zookeeperOtherInfo.html (added)
+++ websites/staging/zookeeper/trunk/content/doc/r3.5.2-alpha/zookeeperOtherInfo.html Thu Jul 21 17:04:48 2016
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Added: websites/staging/zookeeper/trunk/content/doc/r3.5.2-alpha/zookeeperOtherInfo.pdf
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