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[09/17] zookeeper git commit: ZOOKEEPER-3155: Remove Forrest XMLs and their build process from the …
http://git-wip-us.apache.org/repos/asf/zookeeper/blob/cf24deb2/docs/zookeeperOver.html
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-<ul class="minitoc">
-<li>
-<a href="#ch_DesignOverview">ZooKeeper: A Distributed Coordination Service for Distributed
- Applications</a>
-<ul class="minitoc">
-<li>
-<a href="#sc_designGoals">Design Goals</a>
-</li>
-<li>
-<a href="#sc_dataModelNameSpace">Data model and the hierarchical namespace</a>
-</li>
-<li>
-<a href="#Nodes+and+ephemeral+nodes">Nodes and ephemeral nodes</a>
-</li>
-<li>
-<a href="#Conditional+updates+and+watches">Conditional updates and watches</a>
-</li>
-<li>
-<a href="#Guarantees">Guarantees</a>
-</li>
-<li>
-<a href="#Simple+API">Simple API</a>
-</li>
-<li>
-<a href="#Implementation">Implementation</a>
-</li>
-<li>
-<a href="#Uses">Uses</a>
-</li>
-<li>
-<a href="#Performance">Performance</a>
-</li>
-<li>
-<a href="#Reliability">Reliability</a>
-</li>
-<li>
-<a href="#The+ZooKeeper+Project">The ZooKeeper Project</a>
-</li>
-</ul>
-</li>
-</ul>
-</div>
-</div>
-
-
-
-
-
-<a name="ch_DesignOverview"></a>
-<h2 class="h3">ZooKeeper: A Distributed Coordination Service for Distributed
- Applications</h2>
-<div class="section">
-<p>ZooKeeper is a distributed, open-source coordination service for
- distributed applications. It exposes a simple set of primitives that
- distributed applications can build upon to implement higher level services
- for synchronization, configuration maintenance, and groups and naming. It
- is designed to be easy to program to, and uses a data model styled after
- the familiar directory tree structure of file systems. It runs in Java and
- has bindings for both Java and C.</p>
-<p>Coordination services are notoriously hard to get right. They are
- especially prone to errors such as race conditions and deadlock. The
- motivation behind ZooKeeper is to relieve distributed applications the
- responsibility of implementing coordination services from scratch.</p>
-<a name="sc_designGoals"></a>
-<h3 class="h4">Design Goals</h3>
-<p>
-<strong>ZooKeeper is simple.</strong> ZooKeeper
- allows distributed processes to coordinate with each other through a
- shared hierarchal namespace which is organized similarly to a standard
- file system. The name space consists of data registers - called znodes,
- in ZooKeeper parlance - and these are similar to files and directories.
- Unlike a typical file system, which is designed for storage, ZooKeeper
- data is kept in-memory, which means ZooKeeper can acheive high
- throughput and low latency numbers.</p>
-<p>The ZooKeeper implementation puts a premium on high performance,
- highly available, strictly ordered access. The performance aspects of
- ZooKeeper means it can be used in large, distributed systems. The
- reliability aspects keep it from being a single point of failure. The
- strict ordering means that sophisticated synchronization primitives can
- be implemented at the client.</p>
-<p>
-<strong>ZooKeeper is replicated.</strong> Like the
- distributed processes it coordinates, ZooKeeper itself is intended to be
- replicated over a sets of hosts called an ensemble.</p>
-<table class="ForrestTable" cellspacing="1" cellpadding="4">
-<tr>
-<td>ZooKeeper Service</td>
-</tr>
-<tr>
-<td>
-
- <img alt="" src="images/zkservice.jpg">
-
- </td>
-</tr>
-</table>
-<p>The servers that make up the ZooKeeper service must all know about
- each other. They maintain an in-memory image of state, along with a
- transaction logs and snapshots in a persistent store. As long as a
- majority of the servers are available, the ZooKeeper service will be
- available.</p>
-<p>Clients connect to a single ZooKeeper server. The client maintains
- a TCP connection through which it sends requests, gets responses, gets
- watch events, and sends heart beats. If the TCP connection to the server
- breaks, the client will connect to a different server.</p>
-<p>
-<strong>ZooKeeper is ordered.</strong> ZooKeeper
- stamps each update with a number that reflects the order of all
- ZooKeeper transactions. Subsequent operations can use the order to
- implement higher-level abstractions, such as synchronization
- primitives.</p>
-<p>
-<strong>ZooKeeper is fast.</strong> It is
- especially fast in "read-dominant" workloads. ZooKeeper applications run
- on thousands of machines, and it performs best where reads are more
- common than writes, at ratios of around 10:1.</p>
-<a name="sc_dataModelNameSpace"></a>
-<h3 class="h4">Data model and the hierarchical namespace</h3>
-<p>The name space provided by ZooKeeper is much like that of a
- standard file system. A name is a sequence of path elements separated by
- a slash (/). Every node in ZooKeeper's name space is identified by a
- path.</p>
-<table class="ForrestTable" cellspacing="1" cellpadding="4">
-<tr>
-<td>ZooKeeper's Hierarchical Namespace</td>
-</tr>
-<tr>
-<td>
-
- <img alt="" src="images/zknamespace.jpg">
-
- </td>
-</tr>
-</table>
-<a name="Nodes+and+ephemeral+nodes"></a>
-<h3 class="h4">Nodes and ephemeral nodes</h3>
-<p>Unlike is standard file systems, each node in a ZooKeeper
- namespace can have data associated with it as well as children. It is
- like having a file-system that allows a file to also be a directory.
- (ZooKeeper was designed to store coordination data: status information,
- configuration, location information, etc., so the data stored at each
- node is usually small, in the byte to kilobyte range.) We use the term
- <em>znode</em> to make it clear that we are talking about
- ZooKeeper data nodes.</p>
-<p>Znodes maintain a stat structure that includes version numbers for
- data changes, ACL changes, and timestamps, to allow cache validations
- and coordinated updates. Each time a znode's data changes, the version
- number increases. For instance, whenever a client retrieves data it also
- receives the version of the data.</p>
-<p>The data stored at each znode in a namespace is read and written
- atomically. Reads get all the data bytes associated with a znode and a
- write replaces all the data. Each node has an Access Control List (ACL)
- that restricts who can do what.</p>
-<p>ZooKeeper also has the notion of ephemeral nodes. These znodes
- exists as long as the session that created the znode is active. When the
- session ends the znode is deleted. Ephemeral nodes are useful when you
- want to implement <em>[tbd]</em>.</p>
-<a name="Conditional+updates+and+watches"></a>
-<h3 class="h4">Conditional updates and watches</h3>
-<p>ZooKeeper supports the concept of <em>watches</em>.
- Clients can set a watch on a znodes. A watch will be triggered and
- removed when the znode changes. When a watch is triggered the client
- receives a packet saying that the znode has changed. And if the
- connection between the client and one of the Zoo Keeper servers is
- broken, the client will receive a local notification. These can be used
- to <em>[tbd]</em>.</p>
-<a name="Guarantees"></a>
-<h3 class="h4">Guarantees</h3>
-<p>ZooKeeper is very fast and very simple. Since its goal, though, is
- to be a basis for the construction of more complicated services, such as
- synchronization, it provides a set of guarantees. These are:</p>
-<ul>
-
-<li>
-
-<p>Sequential Consistency - Updates from a client will be applied
- in the order that they were sent.</p>
-
-</li>
-
-
-<li>
-
-<p>Atomicity - Updates either succeed or fail. No partial
- results.</p>
-
-</li>
-
-
-<li>
-
-<p>Single System Image - A client will see the same view of the
- service regardless of the server that it connects to.</p>
-
-</li>
-
-</ul>
-<ul>
-
-<li>
-
-<p>Reliability - Once an update has been applied, it will persist
- from that time forward until a client overwrites the update.</p>
-
-</li>
-
-</ul>
-<ul>
-
-<li>
-
-<p>Timeliness - The clients view of the system is guaranteed to
- be up-to-date within a certain time bound.</p>
-
-</li>
-
-</ul>
-<p>For more information on these, and how they can be used, see
- <em>[tbd]</em>
-</p>
-<a name="Simple+API"></a>
-<h3 class="h4">Simple API</h3>
-<p>One of the design goals of ZooKeeper is provide a very simple
- programming interface. As a result, it supports only these
- operations:</p>
-<dl>
-
-<dt>
-<term>create</term>
-</dt>
-<dd>
-<p>creates a node at a location in the tree</p>
-</dd>
-
-
-<dt>
-<term>delete</term>
-</dt>
-<dd>
-<p>deletes a node</p>
-</dd>
-
-
-<dt>
-<term>exists</term>
-</dt>
-<dd>
-<p>tests if a node exists at a location</p>
-</dd>
-
-
-<dt>
-<term>get data</term>
-</dt>
-<dd>
-<p>reads the data from a node</p>
-</dd>
-
-
-<dt>
-<term>set data</term>
-</dt>
-<dd>
-<p>writes data to a node</p>
-</dd>
-
-
-<dt>
-<term>get children</term>
-</dt>
-<dd>
-<p>retrieves a list of children of a node</p>
-</dd>
-
-
-<dt>
-<term>sync</term>
-</dt>
-<dd>
-<p>waits for data to be propagated</p>
-</dd>
-
-</dl>
-<p>For a more in-depth discussion on these, and how they can be used
- to implement higher level operations, please refer to
- <em>[tbd]</em>
-</p>
-<a name="Implementation"></a>
-<h3 class="h4">Implementation</h3>
-<p>
-<a href="#fg_zkComponents">ZooKeeper Components</a> shows the high-level components
- of the ZooKeeper service. With the exception of the request processor,
- each of
- the servers that make up the ZooKeeper service replicates its own copy
- of each of components.</p>
-<table class="ForrestTable" cellspacing="1" cellpadding="4">
-<tr>
-<td>ZooKeeper Components</td>
-</tr>
-<tr>
-<td>
-
- <img alt="" src="images/zkcomponents.jpg">
-
- </td>
-</tr>
-</table>
-<p>The replicated database is an in-memory database containing the
- entire data tree. Updates are logged to disk for recoverability, and
- writes are serialized to disk before they are applied to the in-memory
- database.</p>
-<p>Every ZooKeeper server services clients. Clients connect to
- exactly one server to submit irequests. Read requests are serviced from
- the local replica of each server database. Requests that change the
- state of the service, write requests, are processed by an agreement
- protocol.</p>
-<p>As part of the agreement protocol all write requests from clients
- are forwarded to a single server, called the
- <em>leader</em>. The rest of the ZooKeeper servers, called
- <em>followers</em>, receive message proposals from the
- leader and agree upon message delivery. The messaging layer takes care
- of replacing leaders on failures and syncing followers with
- leaders.</p>
-<p>ZooKeeper uses a custom atomic messaging protocol. Since the
- messaging layer is atomic, ZooKeeper can guarantee that the local
- replicas never diverge. When the leader receives a write request, it
- calculates what the state of the system is when the write is to be
- applied and transforms this into a transaction that captures this new
- state.</p>
-<a name="Uses"></a>
-<h3 class="h4">Uses</h3>
-<p>The programming interface to ZooKeeper is deliberately simple.
- With it, however, you can implement higher order operations, such as
- synchronizations primitives, group membership, ownership, etc. Some
- distributed applications have used it to: <em>[tbd: add uses from
- white paper and video presentation.]</em> For more information, see
- <em>[tbd]</em>
-</p>
-<a name="Performance"></a>
-<h3 class="h4">Performance</h3>
-<p>ZooKeeper is designed to be highly performant. But is it? The
- results of the ZooKeeper's development team at Yahoo! Research indicate
- that it is. (See <a href="#fg_zkPerfRW">ZooKeeper Throughput as the Read-Write Ratio Varies</a>.) It is especially high
- performance in applications where reads outnumber writes, since writes
- involve synchronizing the state of all servers. (Reads outnumbering
- writes is typically the case for a coordination service.)</p>
-<table class="ForrestTable" cellspacing="1" cellpadding="4">
-<tr>
-<td>ZooKeeper Throughput as the Read-Write Ratio Varies</td>
-</tr>
-<tr>
-<td>
-
- <img alt="" src="images/zkperfRW-3.2.jpg">
-
- </td>
-</tr>
-</table>
-<p>The figure <a href="#fg_zkPerfRW">ZooKeeper Throughput as the Read-Write Ratio Varies</a> is a throughput
- graph of ZooKeeper release 3.2 running on servers with dual 2Ghz
- Xeon and two SATA 15K RPM drives. One drive was used as a
- dedicated ZooKeeper log device. The snapshots were written to
- the OS drive. Write requests were 1K writes and the reads were
- 1K reads. "Servers" indicate the size of the ZooKeeper
- ensemble, the number of servers that make up the
- service. Approximately 30 other servers were used to simulate
- the clients. The ZooKeeper ensemble was configured such that
- leaders do not allow connections from clients.</p>
-<div class="note">
-<div class="label">Note</div>
-<div class="content">
-<p>In version 3.2 r/w performance improved by ~2x
- compared to the <a href="http://zookeeper.apache.org/docs/r3.1.1/zookeeperOver.html#Performance">previous
- 3.1 release</a>.</p>
-</div>
-</div>
-<p>Benchmarks also indicate that it is reliable, too. <a href="#fg_zkPerfReliability">Reliability in the Presence of Errors</a> shows how a deployment responds to
- various failures. The events marked in the figure are the
- following:</p>
-<ol>
-
-<li>
-
-<p>Failure and recovery of a follower</p>
-
-</li>
-
-
-<li>
-
-<p>Failure and recovery of a different follower</p>
-
-</li>
-
-
-<li>
-
-<p>Failure of the leader</p>
-
-</li>
-
-
-<li>
-
-<p>Failure and recovery of two followers</p>
-
-</li>
-
-
-<li>
-
-<p>Failure of another leader</p>
-
-</li>
-
-</ol>
-<a name="Reliability"></a>
-<h3 class="h4">Reliability</h3>
-<p>To show the behavior of the system over time as
- failures are injected we ran a ZooKeeper service made up of
- 7 machines. We ran the same saturation benchmark as before,
- but this time we kept the write percentage at a constant
- 30%, which is a conservative ratio of our expected
- workloads.
- </p>
-<table class="ForrestTable" cellspacing="1" cellpadding="4">
-<tr>
-<td>Reliability in the Presence of Errors</td>
-</tr>
-<tr>
-<td>
-
- <img alt="" src="images/zkperfreliability.jpg">
-
- </td>
-</tr>
-</table>
-<p>The are a few important observations from this graph. First, if
- followers fail and recover quickly, then ZooKeeper is able to sustain a
- high throughput despite the failure. But maybe more importantly, the
- leader election algorithm allows for the system to recover fast enough
- to prevent throughput from dropping substantially. In our observations,
- ZooKeeper takes less than 200ms to elect a new leader. Third, as
- followers recover, ZooKeeper is able to raise throughput again once they
- start processing requests.</p>
-<a name="The+ZooKeeper+Project"></a>
-<h3 class="h4">The ZooKeeper Project</h3>
-<p>ZooKeeper has been
- <a href="https://cwiki.apache.org/confluence/display/ZOOKEEPER/PoweredBy">
- successfully used
- </a>
- in many industrial applications. It is used at Yahoo! as the
- coordination and failure recovery service for Yahoo! Message
- Broker, which is a highly scalable publish-subscribe system
- managing thousands of topics for replication and data
- delivery. It is used by the Fetching Service for Yahoo!
- crawler, where it also manages failure recovery. A number of
- Yahoo! advertising systems also use ZooKeeper to implement
- reliable services.
- </p>
-<p>All users and developers are encouraged to join the
- community and contribute their expertise. See the
- <a href="http://zookeeper.apache.org/">
- Zookeeper Project on Apache
- </a>
- for more information.
- </p>
-</div>
-
-<p align="right">
-<font size="-2"></font>
-</p>
-</div>
-<!--+
- |end content
- +-->
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-</div>
-<div class="copyright">
- Copyright ©
- <a href="http://www.apache.org/licenses/">The Apache Software Foundation.</a>
-</div>
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http://git-wip-us.apache.org/repos/asf/zookeeper/blob/cf24deb2/docs/zookeeperOver.pdf
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