Google Git
Sign in
apache / zookeeper / branch-3.2 / . / docs / bookkeeperOverview.html
blob: 28931631e5df995cf41ea9a7adc8e96275b0f652 [file] [log] [blame]
<!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">
<meta content="Apache Forrest" name="Generator">
<meta name="Forrest-version" content="0.8">
<meta name="Forrest-skin-name" content="pelt">
<title>BookKeeper overview</title>
<link type="text/css" href="skin/basic.css" rel="stylesheet">
<link media="screen" type="text/css" href="skin/screen.css" rel="stylesheet">
<link media="print" type="text/css" href="skin/print.css" rel="stylesheet">
<link type="text/css" href="skin/profile.css" rel="stylesheet">
<script src="skin/getBlank.js" language="javascript" type="text/javascript"></script><script src="skin/getMenu.js" language="javascript" type="text/javascript"></script><script src="skin/fontsize.js" language="javascript" type="text/javascript"></script>
<link rel="shortcut icon" href="images/favicon.ico">
</head>
<body onload="init()">
<script type="text/javascript">ndeSetTextSize();</script>
<div id="top">
<!--+
|breadtrail
+-->
<div class="breadtrail">
<a href="http://www.apache.org/">Apache</a> &gt; <a href="http://hadoop.apache.org/">Hadoop</a> &gt; <a href="http://hadoop.apache.org/zookeeper/">ZooKeeper</a><script src="skin/breadcrumbs.js" language="JavaScript" type="text/javascript"></script>
</div>
<!--+
|header
+-->
<div class="header">
<!--+
|start group logo
+-->
<div class="grouplogo">
<a href="http://hadoop.apache.org/"><img class="logoImage" alt="Hadoop" src="images/hadoop-logo.jpg" title="Apache Hadoop"></a>
</div>
<!--+
|end group logo
+-->
<!--+
|start Project Logo
+-->
<div class="projectlogo">
<a href="http://hadoop.apache.org/zookeeper/"><img class="logoImage" alt="ZooKeeper" src="images/zookeeper_small.gif" title="ZooKeeper: distributed coordination"></a>
</div>
<!--+
|end Project Logo
+-->
<!--+
|start Search
+-->
<div class="searchbox">
<form action="http://www.google.com/search" method="get" class="roundtopsmall">
<input value="hadoop.apache.org" name="sitesearch" type="hidden"><input onFocus="getBlank (this, 'Search the site with google');" size="25" name="q" id="query" type="text" value="Search the site with google">&nbsp;
<input name="Search" value="Search" type="submit">
</form>
</div>
<!--+
|end search
+-->
<!--+
|start Tabs
+-->
<ul id="tabs">
<li>
<a class="unselected" href="http://hadoop.apache.org/zookeeper/">Project</a>
</li>
<li>
<a class="unselected" href="http://wiki.apache.org/hadoop/ZooKeeper">Wiki</a>
</li>
<li class="current">
<a class="selected" href="index.html">ZooKeeper 3.2 Documentation</a>
</li>
</ul>
<!--+
|end Tabs
+-->
</div>
</div>
<div id="main">
<div id="publishedStrip">
<!--+
|start Subtabs
+-->
<div id="level2tabs"></div>
<!--+
|end Endtabs
+-->
<script type="text/javascript"><!--
document.write("Last Published: " + document.lastModified);
// --></script>
</div>
<!--+
|breadtrail
+-->
<div class="breadtrail">
&nbsp;
</div>
<!--+
|start Menu, mainarea
+-->
<!--+
|start Menu
+-->
<div id="menu">
<div onclick="SwitchMenu('menu_1.1', 'skin/')" id="menu_1.1Title" class="menutitle">Overview</div>
<div id="menu_1.1" class="menuitemgroup">
<div class="menuitem">
<a href="index.html">Welcome</a>
</div>
<div class="menuitem">
<a href="zookeeperOver.html">Overview</a>
</div>
<div class="menuitem">
<a href="zookeeperStarted.html">Getting Started</a>
</div>
<div class="menuitem">
<a href="releasenotes.html">Release Notes</a>
</div>
</div>
<div onclick="SwitchMenu('menu_1.2', 'skin/')" id="menu_1.2Title" class="menutitle">Developer</div>
<div id="menu_1.2" class="menuitemgroup">
<div class="menuitem">
<a href="api/index.html">API Docs</a>
</div>
<div class="menuitem">
<a href="zookeeperProgrammers.html">Programmer's Guide</a>
</div>
<div class="menuitem">
<a href="javaExample.html">Java Example</a>
</div>
<div class="menuitem">
<a href="zookeeperTutorial.html">Barrier and Queue Tutorial</a>
</div>
<div class="menuitem">
<a href="recipes.html">Recipes</a>
</div>
</div>
<div onclick="SwitchMenu('menu_selected_1.3', 'skin/')" id="menu_selected_1.3Title" class="menutitle" style="background-image: url('skin/images/chapter_open.gif');">BookKeeper</div>
<div id="menu_selected_1.3" class="selectedmenuitemgroup" style="display: block;">
<div class="menuitem">
<a href="bookkeeperStarted.html">Getting started</a>
</div>
<div class="menupage">
<div class="menupagetitle">Overview</div>
</div>
<div class="menuitem">
<a href="bookkeeperConfig.html">Setup guide</a>
</div>
<div class="menuitem">
<a href="bookkeeperProgrammer.html">Programmer's guide</a>
</div>
</div>
<div onclick="SwitchMenu('menu_1.4', 'skin/')" id="menu_1.4Title" class="menutitle">Admin &amp; Ops</div>
<div id="menu_1.4" class="menuitemgroup">
<div class="menuitem">
<a href="zookeeperAdmin.html">Administrator's Guide</a>
</div>
<div class="menuitem">
<a href="zookeeperQuotas.html">Quota Guide</a>
</div>
<div class="menuitem">
<a href="zookeeperJMX.html">JMX</a>
</div>
</div>
<div onclick="SwitchMenu('menu_1.5', 'skin/')" id="menu_1.5Title" class="menutitle">Contributor</div>
<div id="menu_1.5" class="menuitemgroup">
<div class="menuitem">
<a href="zookeeperInternals.html">ZooKeeper Internals</a>
</div>
</div>
<div onclick="SwitchMenu('menu_1.6', 'skin/')" id="menu_1.6Title" class="menutitle">Miscellaneous</div>
<div id="menu_1.6" class="menuitemgroup">
<div class="menuitem">
<a href="http://wiki.apache.org/hadoop/ZooKeeper">Wiki</a>
</div>
<div class="menuitem">
<a href="http://wiki.apache.org/hadoop/ZooKeeper/FAQ">FAQ</a>
</div>
<div class="menuitem">
<a href="http://hadoop.apache.org/zookeeper/mailing_lists.html">Mailing Lists</a>
</div>
</div>
<div id="credit"></div>
<div id="roundbottom">
<img style="display: none" class="corner" height="15" width="15" alt="" src="skin/images/rc-b-l-15-1body-2menu-3menu.png"></div>
<!--+
|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="bookkeeperOverview.pdf"><img alt="PDF -icon" src="skin/images/pdfdoc.gif" class="skin"><br>
PDF</a>
</div>
<h1>BookKeeper overview</h1>
<div id="minitoc-area">
<ul class="minitoc">
<li>
<a href="#bk_Overview">BookKeeper overview</a>
<ul class="minitoc">
<li>
<a href="#bk_basicComponents">Basic elements</a>
</li>
<li>
<a href="#bk_moreDetail">In slightly more detail...</a>
</li>
</ul>
</li>
</ul>
</div>
<a name="N10009"></a><a name="bk_Overview"></a>
<h2 class="h3">BookKeeper overview</h2>
<div class="section">
<p>This document explains basic concepts of BookKeeper. We start by discussing
the basic elements of BookKeeper, and next we discuss how they work together.
</p>
<a name="N10012"></a><a name="bk_basicComponents"></a>
<h3 class="h4">Basic elements</h3>
<p>
BookKeeper uses four basic elements:
</p>
<ul>
<li>
<p>
<strong>Ledger</strong>: A ledger is a sequence of entries, and each entry is a sequence of bytes. Entries are
written sequentially to a ledger and at most once. Consequently, ledgers have an append-only semantics;
</p>
</li>
<li>
<p>
<strong>BookKeeper client</strong>: A client runs along with a BookKeeper application, and it enables applications
to execute operations on ledgers, such as creating a ledger and writing to it;
</p>
</li>
<li>
<p>
<strong>Bookie</strong>: A bookie is a BookKeeper storage server. Bookies store the content of ledgers. For any given
ledger L, we call an <em>ensemble</em> the group of bookies storing the content of L. For performance, we store on
each bookie of an ensemble only a fragment of a ledger. That is, we stripe when writing entries to a ledger such that
each entry is written to sub-group of bookies of the ensemble.
</p>
</li>
<li>
<p>
<strong>Metadata storage service</strong>: BookKeeper requires a metadata storage service to store information related
to ledgers and available bookies. We currently use ZooKeeper for such a task.
</p>
</li>
</ul>
<a name="N10046"></a><a name="bk_moreDetail"></a>
<h3 class="h4">In slightly more detail...</h3>
<p> BookKeeper implements highly available logs, and it has been designed with write-ahead logging in mind. Besides high availability
due to the replicated nature of the service, it provides high throughput due to striping. As we write entries in a subset of bookies of an
ensemble and rotate writes across available quorums, we are able to increase throughput with the number of servers for both reads and writes.
Scalability is a property that is possible to achieve in this case due to the use of quorums. Other replication techniques, such as
state-machine replication, do not enable such a property.
</p>
<p> An application first creates a ledger before writing to bookies through a local BookKeeper client instance. To
create a ledger, an application has to specify which kind of ledger it wants to use: self-verifying or generic. Self-verifying
includes a digest on every entry, which enables a reduction on the degree of replication. Generic ledgers do not store a digest
along with entries at the cost of using more bookies.
</p>
<p> Upon creating a ledger, a BookKeeper clients writes metadata about the ledger to ZooKeeper. A given client first creates
a znode named "L" as a child of "/ledger" with the SEQUENCE flag. ZooKeeper consequently assigns a unique sequence number to the
node, naming the node "/Lx", where x is the sequence number assigned. We use this sequence number as the identifier of the ledger.
This identifier is necessary when opening a ledger. We also store the ensemble composition so that readers know which set of bookies
of access for a given ledger.
</p>
<p>
Each ledger currently has a single writer. This writer has to execute a close ledger operation before any other client can read
from it. If the writer of a ledger does not close a ledger properly because, for example, it has crashed before having the
opportunity of closing the ledger, then the next client that tries to open a ledger executes an procedure to recover it. As closing a ledger
consists essentially of writing the last entry written to a ledger to ZooKeeper, the recovery procedure simply finds the last entry
written correctly and writes it to ZooKeeper in the form of a close znode as a child of "/Lx", where x is the identifier of the ledger.
</p>
<p>
Note that currently this recovery procedure is executed automatically upon trying to open a ledger and no explicit action is necessary.
Although two clients may try to recover a ledger concurrently, only one will succeed, the first one that is able to create the close znode
for the ledger.
</p>
</div>
<p align="right">
<font size="-2"></font>
</p>
</div>
<!--+
|end content
+-->
<div class="clearboth">&nbsp;</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 &copy;
2008 <a href="http://www.apache.org/licenses/">The Apache Software Foundation.</a>
</div>
<!--+
|end bottomstrip
+-->
</div>
</body>
</html>