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|alternative credits - +--> -<div id="credit2"></div> -</div> -<!--+ - |end Menu - +--> -<!--+ - |start content - +--> -<div id="content"> -<div title="Portable Document Format" class="pdflink"> -<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 is 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> -</div> -<!--+ - |end content - +--> -<div class="clearboth"> </div> -</div> -<div id="footer"> -<!--+ - |start bottomstrip - +--> -<div class="lastmodified"> -<script type="text/javascript"><!-- -document.write("Last Published: " + document.lastModified); -// --></script> -</div> -<div class="copyright"> - Copyright © - <a href="http://www.apache.org/licenses/";>The Apache Software Foundation.</a> -</div> -<!--+ - |end bottomstrip - +--> -</div> -</body> -</html>
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|alternative credits - +--> -<div id="credit2"></div> -</div> -<!--+ - |end Menu - +--> -<!--+ - |start content - +--> -<div id="content"> -<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<#></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.<#></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><session_id></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<#></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><session_id></td> - <td>Statistics on each client connection, also - operations on those connections (such as - termination). 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|alternative credits - +--> -<div id="credit2"></div> -</div> -<!--+ - |end Menu - +--> -<!--+ - |start content - +--> -<div id="content"> -<div title="Portable Document Format" class="pdflink"> -<a class="dida" href="zookeeperObservers.pdf"><img alt="PDF -icon" src="skin/images/pdfdoc.gif" class="skin"><br> - 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 numbe 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 - +--> -<div class="clearboth"> </div> -</div> -<div id="footer"> -<!--+ - |start bottomstrip - +--> -<div class="lastmodified"> -<script type="text/javascript"><!-- -document.write("Last Published: " + document.lastModified); -// --></script> -</div> -<div class="copyright"> - Copyright © - <a href="http://www.apache.org/licenses/";>The Apache Software Foundation.</a> -</div> -<!--+ - |end bottomstrip - +--> -</div> -</body> -</html> http://git-wip-us.apache.org/repos/asf/zookeeper/blob/cf24deb2/docs/zookeeperObservers.pdf ---------------------------------------------------------------------- diff --git a/docs/zookeeperObservers.pdf b/docs/zookeeperObservers.pdf deleted file mode 100644 index 843e860..0000000 Binary files a/docs/zookeeperObservers.pdf and /dev/null differ http://git-wip-us.apache.org/repos/asf/zookeeper/blob/cf24deb2/docs/zookeeperOtherInfo.html ---------------------------------------------------------------------- diff --git a/docs/zookeeperOtherInfo.html b/docs/zookeeperOtherInfo.html deleted file mode 100644 index 24b594c..0000000 --- a/docs/zookeeperOtherInfo.html +++ /dev/null @@ -1,230 +0,0 @@ -<!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; 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