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-<?xml version="1.0" encoding="UTF-8"?>
-<!--
- Copyright 2002-2004 The Apache Software Foundation
-
- Licensed under the Apache License, Version 2.0 (the "License");
- you may not use this file except in compliance with the License.
- You may obtain a copy of the License at
-
- http://www.apache.org/licenses/LICENSE-2.0
-
- Unless required by applicable law or agreed to in writing, software
- distributed under the License is distributed on an "AS IS" BASIS,
- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- See the License for the specific language governing permissions and
- limitations under the License.
--->
-
-<!DOCTYPE article PUBLIC "-//OASIS//DTD Simplified DocBook XML V1.0//EN"
-"http://www.oasis-open.org/docbook/xml/simple/1.0/sdocbook.dtd">
-<article id="ar_Recipes">
- <title>ZooKeeper Recipes and Solutions</title>
-
- <articleinfo>
- <legalnotice>
- <para>Licensed under the Apache License, Version 2.0 (the "License");
- you may not use this file except in compliance with the License. You may
- obtain a copy of the License at <ulink
- url="http://www.apache.org/licenses/LICENSE-2.0">http://www.apache.org/licenses/LICENSE-2.0</ulink>.</para>
-
- <para>Unless required by applicable law or agreed to in writing,
- software distributed under the License is distributed on an "AS IS"
- BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
- implied. See the License for the specific language governing permissions
- and limitations under the License.</para>
- </legalnotice>
-
- <abstract>
- <para>This guide contains pseudocode and guidelines for using Zookeeper to
- solve common problems in Distributed Application Coordination. It
- discusses such problems as event handlers, queues, and locks..</para>
-
- <para>$Revision: 1.6 $ $Date: 2008/09/19 03:46:18 $</para>
- </abstract>
- </articleinfo>
-
- <section id="ch_recipes">
- <title>A Guide to Creating Higher-level Constructs with ZooKeeper</title>
-
- <para>In this article, you'll find guidelines for using
- ZooKeeper to implement higher order functions. All of them are conventions
- implemented at the client and do not require special support from
- ZooKeeper. Hopfully the community will capture these conventions in client-side libraries
- to ease their use and to encourage standardization.</para>
-
- <para>One of the most interesting things about ZooKeeper is that even
- though ZooKeeper uses <emphasis>asynchronous</emphasis> notifications, you
- can use it to build <emphasis>synchronous</emphasis> consistency
- primitives, such as queues and locks. As you will see, this is possible
- because ZooKeeper imposes an overall order on updates, and has mechanisms
- to expose this ordering.</para>
-
- <para>Note that the recipes below attempt to employ best practices. In
- particular, they avoid polling, timers or anything else that would result
- in a "herd effect", causing bursts of traffic and limiting
- scalability.</para>
-
- <para>There are many useful functions that can be imagined that aren't
- included here - revocable read-write priority locks, as just one example.
- And some of the constructs mentioned here - locks, in particular -
- illustrate certain points, even though you may find other constructs, such
- as event handles or queues, a more practical means of performing the same
- function. In general, the examples in this section are designed to
- stimulate thought.</para>
-
- <section id="sc_recipes_errorHandlingNote">
- <title>Important Note About Error Handling</title>
-
- <para>When implementing the recipes you must handle recoverable exceptions
- (see the <ulink url="https://cwiki.apache.org/confluence/display/ZOOKEEPER/FAQ">FAQ</ulink>).
In
- particular, several of the recipes employ sequential ephemeral
- nodes. When creating a sequential ephemeral node there is an error case in
- which the create() succeeds on the server but the server crashes before
- returning the name of the node to the client. When the client reconnects its
- session is still valid and, thus, the node is not removed. The implication is
- that it is difficult for the client to know if its node was created or not. The
- recipes below include measures to handle this.</para>
- </section>
-
- <section id="sc_outOfTheBox">
- <title>Out of the Box Applications: Name Service, Configuration, Group
- Membership</title>
-
- <para>Name service and configuration are two of the primary applications
- of ZooKeeper. These two functions are provided directly by the ZooKeeper
- API.</para>
-
- <para>Another function directly provided by ZooKeeper is <emphasis>group
- membership</emphasis>. The group is represented by a node. Members of the
- group create ephemeral nodes under the group node. Nodes of the members
- that fail abnormally will be removed automatically when ZooKeeper detects
- the failure.</para>
- </section>
-
- <section id="sc_recipes_eventHandles">
- <title>Barriers</title>
-
- <para>Distributed systems use <emphasis>barriers</emphasis>
- to block processing of a set of nodes until a condition is met
- at which time all the nodes are allowed to proceed. Barriers are
- implemented in ZooKeeper by designating a barrier node. The
- barrier is in place if the barrier node exists. Here's the
- pseudo code:</para>
-
- <orderedlist>
- <listitem>
- <para>Client calls the ZooKeeper API's <emphasis
- role="bold">exists()</emphasis> function on the barrier node, with
- <emphasis>watch</emphasis> set to true.</para>
- </listitem>
-
- <listitem>
- <para>If <emphasis role="bold">exists()</emphasis> returns false,
the
- barrier is gone and the client proceeds</para>
- </listitem>
-
- <listitem>
- <para>Else, if <emphasis role="bold">exists()</emphasis> returns
true,
- the clients wait for a watch event from ZooKeeper for the barrier
- node.</para>
- </listitem>
-
- <listitem>
- <para>When the watch event is triggered, the client reissues the
- <emphasis role="bold">exists( )</emphasis> call, again waiting until
- the barrier node is removed.</para>
- </listitem>
- </orderedlist>
-
- <section id="sc_doubleBarriers">
- <title>Double Barriers</title>
-
- <para>Double barriers enable clients to synchronize the beginning and
- the end of a computation. When enough processes have joined the barrier,
- processes start their computation and leave the barrier once they have
- finished. This recipe shows how to use a ZooKeeper node as a
- barrier.</para>
-
- <para>The pseudo code in this recipe represents the barrier node as
- <emphasis>b</emphasis>. Every client process <emphasis>p</emphasis>
- registers with the barrier node on entry and unregisters when it is
- ready to leave. A node registers with the barrier node via the <emphasis
- role="bold">Enter</emphasis> procedure below, it waits until
- <emphasis>x</emphasis> client process register before proceeding with
- the computation. (The <emphasis>x</emphasis> here is up to you to
- determine for your system.)</para>
-
- <informaltable colsep="0" frame="none" rowsep="0">
- <tgroup cols="2">
- <tbody>
- <row>
- <entry align="center"><emphasis
- role="bold">Enter</emphasis></entry>
-
- <entry align="center"><emphasis
- role="bold">Leave</emphasis></entry>
- </row>
-
- <row>
- <entry align="left"><orderedlist>
- <listitem>
- <para>Create a name <emphasis><emphasis>n</emphasis>
=
- <emphasis>b</emphasis>+“/”+<emphasis>p</emphasis></emphasis></para>
- </listitem>
-
- <listitem>
- <para>Set watch: <emphasis
- role="bold">exists(<emphasis>b</emphasis>
+ ‘‘/ready’’,
- true)</emphasis></para>
- </listitem>
-
- <listitem>
- <para>Create child: <emphasis role="bold">create(
- <emphasis>n</emphasis>, EPHEMERAL)</emphasis></para>
- </listitem>
-
- <listitem>
- <para><emphasis role="bold">L = getChildren(b,
- false)</emphasis></para>
- </listitem>
-
- <listitem>
- <para>if fewer children in L than<emphasis>
- x</emphasis>, wait for watch event</para>
- </listitem>
-
- <listitem>
- <para>else <emphasis role="bold">create(b + ‘‘/ready’’,
- REGULAR)</emphasis></para>
- </listitem>
- </orderedlist></entry>
-
- <entry><orderedlist>
- <listitem>
- <para><emphasis role="bold">L = getChildren(b,
- false)</emphasis></para>
- </listitem>
-
- <listitem>
- <para>if no children, exit</para>
- </listitem>
-
- <listitem>
- <para>if <emphasis>p</emphasis> is only process node
in
- L, delete(n) and exit</para>
- </listitem>
-
- <listitem>
- <para>if <emphasis>p</emphasis> is the lowest process
- node in L, wait on highest process node in L</para>
- </listitem>
-
- <listitem>
- <para>else <emphasis
- role="bold">delete(<emphasis>n</emphasis>)
</emphasis>if
- still exists and wait on lowest process node in L</para>
- </listitem>
-
- <listitem>
- <para>goto 1</para>
- </listitem>
- </orderedlist></entry>
- </row>
- </tbody>
- </tgroup>
- </informaltable>
- <para>On entering, all processes watch on a ready node and
- create an ephemeral node as a child of the barrier node. Each process
- but the last enters the barrier and waits for the ready node to appear
- at line 5. The process that creates the xth node, the last process, will
- see x nodes in the list of children and create the ready node, waking up
- the other processes. Note that waiting processes wake up only when it is
- time to exit, so waiting is efficient.
- </para>
-
- <para>On exit, you can't use a flag such as <emphasis>ready</emphasis>
- because you are watching for process nodes to go away. By using
- ephemeral nodes, processes that fail after the barrier has been entered
- do not prevent correct processes from finishing. When processes are
- ready to leave, they need to delete their process nodes and wait for all
- other processes to do the same.</para>
-
- <para>Processes exit when there are no process nodes left as children of
- <emphasis>b</emphasis>. However, as an efficiency, you can use the
- lowest process node as the ready flag. All other processes that are
- ready to exit watch for the lowest existing process node to go away, and
- the owner of the lowest process watches for any other process node
- (picking the highest for simplicity) to go away. This means that only a
- single process wakes up on each node deletion except for the last node,
- which wakes up everyone when it is removed.</para>
- </section>
- </section>
-
- <section id="sc_recipes_Queues">
- <title>Queues</title>
-
- <para>Distributed queues are a common data structure. To implement a
- distributed queue in ZooKeeper, first designate a znode to hold the queue,
- the queue node. The distributed clients put something into the queue by
- calling create() with a pathname ending in "queue-", with the
- <emphasis>sequence</emphasis> and <emphasis>ephemeral</emphasis>
flags in
- the create() call set to true. Because the <emphasis>sequence</emphasis>
- flag is set, the new pathnames will have the form
- _path-to-queue-node_/queue-X, where X is a monotonic increasing number. A
- client that wants to be removed from the queue calls ZooKeeper's <emphasis
- role="bold">getChildren( )</emphasis> function, with
- <emphasis>watch</emphasis> set to true on the queue node, and begins
- processing nodes with the lowest number. The client does not need to issue
- another <emphasis role="bold">getChildren( )</emphasis> until it exhausts
- the list obtained from the first <emphasis role="bold">getChildren(
- )</emphasis> call. If there are are no children in the queue node, the
- reader waits for a watch notification to check the queue again.</para>
-
- <note>
- <para>There now exists a Queue implementation in ZooKeeper
- recipes directory. This is distributed with the release --
- src/recipes/queue directory of the release artifact.
- </para>
- </note>
-
- <section id="sc_recipes_priorityQueues">
- <title>Priority Queues</title>
-
- <para>To implement a priority queue, you need only make two simple
- changes to the generic <ulink url="#sc_recipes_Queues">queue
- recipe</ulink> . First, to add to a queue, the pathname ends with
- "queue-YY" where YY is the priority of the element with lower numbers
- representing higher priority (just like UNIX). Second, when removing
- from the queue, a client uses an up-to-date children list meaning that
- the client will invalidate previously obtained children lists if a watch
- notification triggers for the queue node.</para>
- </section>
- </section>
-
- <section id="sc_recipes_Locks">
- <title>Locks</title>
-
- <para>Fully distributed locks that are globally synchronous, meaning at
- any snapshot in time no two clients think they hold the same lock. These
- can be implemented using ZooKeeeper. As with priority queues, first define
- a lock node.</para>
-
- <note>
- <para>There now exists a Lock implementation in ZooKeeper
- recipes directory. This is distributed with the release --
- src/recipes/lock directory of the release artifact.
- </para>
- </note>
-
- <para>Clients wishing to obtain a lock do the following:</para>
-
- <orderedlist>
- <listitem>
- <para>Call <emphasis role="bold">create( )</emphasis> with a pathname
- of "_locknode_/guid-lock-" and the <emphasis>sequence</emphasis> and
- <emphasis>ephemeral</emphasis> flags set. The <emphasis>guid</emphasis>
- is needed in case the create() result is missed. See the note below.</para>
- </listitem>
-
- <listitem>
- <para>Call <emphasis role="bold">getChildren( )</emphasis> on the
lock
- node <emphasis>without</emphasis> setting the watch flag (this is
- important to avoid the herd effect).</para>
- </listitem>
-
- <listitem>
- <para>If the pathname created in step <emphasis
- role="bold">1</emphasis> has the lowest sequence number suffix, the
- client has the lock and the client exits the protocol.</para>
- </listitem>
-
- <listitem>
- <para>The client calls <emphasis role="bold">exists( )</emphasis>
with
- the watch flag set on the path in the lock directory with the next
- lowest sequence number.</para>
- </listitem>
-
- <listitem>
- <para>if <emphasis role="bold">exists( )</emphasis> returns false,
go
- to step <emphasis role="bold">2</emphasis>. Otherwise, wait for a
- notification for the pathname from the previous step before going to
- step <emphasis role="bold">2</emphasis>.</para>
- </listitem>
- </orderedlist>
-
- <para>The unlock protocol is very simple: clients wishing to release a
- lock simply delete the node they created in step 1.</para>
-
- <para>Here are a few things to notice:</para>
-
- <itemizedlist>
- <listitem>
- <para>The removal of a node will only cause one client to wake up
- since each node is watched by exactly one client. In this way, you
- avoid the herd effect.</para>
- </listitem>
- </itemizedlist>
-
- <itemizedlist>
- <listitem>
- <para>There is no polling or timeouts.</para>
- </listitem>
- </itemizedlist>
-
- <itemizedlist>
- <listitem>
- <para>Because of the way you implement locking, it is easy to see the
- amount of lock contention, break locks, debug locking problems,
- etc.</para>
- </listitem>
- </itemizedlist>
-
- <section id="sc_recipes_GuidNote">
- <title>Recoverable Errors and the GUID</title>
- <itemizedlist>
- <listitem>
- <para>If a recoverable error occurs calling <emphasis role="bold">create()</emphasis>
the
- client should call <emphasis role="bold">getChildren()</emphasis> and check
for a node
- containing the <emphasis>guid</emphasis> used in the path name.
- This handles the case (noted <ulink url="#sc_recipes_errorHandlingNote">above</ulink>)
of
- the create() succeeding on the server but the server crashing before returning the name
- of the new node.</para>
- </listitem>
- </itemizedlist>
- </section>
-
- <section>
- <title>Shared Locks</title>
-
- <para>You can implement shared locks by with a few changes to the lock
- protocol:</para>
-
- <informaltable colsep="0" frame="none" rowsep="0">
- <tgroup cols="2">
- <tbody>
- <row>
- <entry align="center"><emphasis role="bold">Obtaining a read
- lock:</emphasis></entry>
-
- <entry align="center"><emphasis role="bold">Obtaining a write
- lock:</emphasis></entry>
- </row>
-
- <row>
- <entry align="left"><orderedlist>
- <listitem>
- <para>Call <emphasis role="bold">create( )</emphasis>
to
- create a node with pathname
- "<filename>guid-/read-</filename>". This is the
- lock node use later in the protocol. Make sure to set both
- the <emphasis>sequence</emphasis> and
- <emphasis>ephemeral</emphasis> flags.</para>
- </listitem>
-
- <listitem>
- <para>Call <emphasis role="bold">getChildren( )</emphasis>
- on the lock node <emphasis>without</emphasis> setting the
- <emphasis>watch</emphasis> flag - this is important, as it
- avoids the herd effect.</para>
- </listitem>
-
- <listitem>
- <para>If there are no children with a pathname starting
- with "<filename>write-</filename>" and having a lower
- sequence number than the node created in step <emphasis
- role="bold">1</emphasis>, the client has the lock and can
- exit the protocol. </para>
- </listitem>
-
- <listitem>
- <para>Otherwise, call <emphasis role="bold">exists(
- )</emphasis>, with <emphasis>watch</emphasis> flag,
set on
- the node in lock directory with pathname staring with
- "<filename>write-</filename>" having the next lowest
- sequence number.</para>
- </listitem>
-
- <listitem>
- <para>If <emphasis role="bold">exists( )</emphasis>
- returns <emphasis>false</emphasis>, goto step <emphasis
- role="bold">2</emphasis>.</para>
- </listitem>
-
- <listitem>
- <para>Otherwise, wait for a notification for the pathname
- from the previous step before going to step <emphasis
- role="bold">2</emphasis></para>
- </listitem>
- </orderedlist></entry>
-
- <entry><orderedlist>
- <listitem>
- <para>Call <emphasis role="bold">create( )</emphasis>
to
- create a node with pathname
- "<filename>guid-/write-</filename>". This is the
- lock node spoken of later in the protocol. Make sure to
- set both <emphasis>sequence</emphasis> and
- <emphasis>ephemeral</emphasis> flags.</para>
- </listitem>
-
- <listitem>
- <para>Call <emphasis role="bold">getChildren( )
- </emphasis> on the lock node <emphasis>without</emphasis>
- setting the <emphasis>watch</emphasis> flag - this is
- important, as it avoids the herd effect.</para>
- </listitem>
-
- <listitem>
- <para>If there are no children with a lower sequence
- number than the node created in step <emphasis
- role="bold">1</emphasis>, the client has the lock and the
- client exits the protocol.</para>
- </listitem>
-
- <listitem>
- <para>Call <emphasis role="bold">exists( ),</emphasis>
- with <emphasis>watch</emphasis> flag set, on the node with
- the pathname that has the next lowest sequence
- number.</para>
- </listitem>
-
- <listitem>
- <para>If <emphasis role="bold">exists( )</emphasis>
- returns <emphasis>false</emphasis>, goto step <emphasis
- role="bold">2</emphasis>. Otherwise, wait for a
- notification for the pathname from the previous step
- before going to step <emphasis
- role="bold">2</emphasis>.</para>
- </listitem>
- </orderedlist></entry>
- </row>
- </tbody>
- </tgroup>
- </informaltable>
-
- <para>Notes:</para>
-
- <itemizedlist>
- <listitem>
- <para>It might appear that this recipe creates a herd effect:
- when there is a large group of clients waiting for a read
- lock, and all getting notified more or less simultaneously
- when the "<filename>write-</filename>" node with the lowest
- sequence number is deleted. In fact. that's valid behavior:
- as all those waiting reader clients should be released since
- they have the lock. The herd effect refers to releasing a
- "herd" when in fact only a single or a small number of
- machines can proceed.</para>
- </listitem>
- </itemizedlist>
-
- <itemizedlist>
- <listitem>
- <para>See the <ulink url="#sc_recipes_GuidNote">note for Locks</ulink>
on how to use the guid in the node.</para>
- </listitem>
- </itemizedlist>
-
- </section>
-
- <section id="sc_revocableSharedLocks">
- <title>Revocable Shared Locks</title>
-
- <para>With minor modifications to the Shared Lock protocol, you make
- shared locks revocable by modifying the shared lock protocol:</para>
-
- <para>In step <emphasis role="bold">1</emphasis>, of both obtain
reader
- and writer lock protocols, call <emphasis role="bold">getData(
- )</emphasis> with <emphasis>watch</emphasis> set, immediately after
the
- call to <emphasis role="bold">create( )</emphasis>. If the client
- subsequently receives notification for the node it created in step
- <emphasis role="bold">1</emphasis>, it does another <emphasis
- role="bold">getData( )</emphasis> on that node, with
- <emphasis>watch</emphasis> set and looks for the string "unlock", which
- signals to the client that it must release the lock. This is because,
- according to this shared lock protocol, you can request the client with
- the lock give up the lock by calling <emphasis role="bold">setData()
- </emphasis> on the lock node, writing "unlock" to that node.</para>
-
- <para>Note that this protocol requires the lock holder to consent to
- releasing the lock. Such consent is important, especially if the lock
- holder needs to do some processing before releasing the lock. Of course
- you can always implement <emphasis>Revocable Shared Locks with Freaking
- Laser Beams</emphasis> by stipulating in your protocol that the revoker
- is allowed to delete the lock node if after some length of time the lock
- isn't deleted by the lock holder.</para>
- </section>
- </section>
-
- <section id="sc_recipes_twoPhasedCommit">
- <title>Two-phased Commit</title>
-
- <para>A two-phase commit protocol is an algorithm that lets all clients in
- a distributed system agree either to commit a transaction or abort.</para>
-
- <para>In ZooKeeper, you can implement a two-phased commit by having a
- coordinator create a transaction node, say "/app/Tx", and one child node
- per participating site, say "/app/Tx/s_i". When coordinator creates the
- child node, it leaves the content undefined. Once each site involved in
- the transaction receives the transaction from the coordinator, the site
- reads each child node and sets a watch. Each site then processes the query
- and votes "commit" or "abort" by writing to its respective node. Once the
- write completes, the other sites are notified, and as soon as all sites
- have all votes, they can decide either "abort" or "commit". Note that a
- node can decide "abort" earlier if some site votes for "abort".</para>
-
- <para>An interesting aspect of this implementation is that the only role
- of the coordinator is to decide upon the group of sites, to create the
- ZooKeeper nodes, and to propagate the transaction to the corresponding
- sites. In fact, even propagating the transaction can be done through
- ZooKeeper by writing it in the transaction node.</para>
-
- <para>There are two important drawbacks of the approach described above.
- One is the message complexity, which is O(n²). The second is the
- impossibility of detecting failures of sites through ephemeral nodes. To
- detect the failure of a site using ephemeral nodes, it is necessary that
- the site create the node.</para>
-
- <para>To solve the first problem, you can have only the coordinator
- notified of changes to the transaction nodes, and then notify the sites
- once coordinator reaches a decision. Note that this approach is scalable,
- but it's is slower too, as it requires all communication to go through the
- coordinator.</para>
-
- <para>To address the second problem, you can have the coordinator
- propagate the transaction to the sites, and have each site creating its
- own ephemeral node.</para>
- </section>
-
- <section id="sc_leaderElection">
- <title>Leader Election</title>
-
- <para>A simple way of doing leader election with ZooKeeper is to use the
- <emphasis role="bold">SEQUENCE|EPHEMERAL</emphasis> flags when creating
- znodes that represent "proposals" of clients. The idea is to have a znode,
- say "/election", such that each znode creates a child znode "/election/guid-n_"
- with both flags SEQUENCE|EPHEMERAL. With the sequence flag, ZooKeeper
- automatically appends a sequence number that is greater than any one
- previously appended to a child of "/election". The process that created
- the znode with the smallest appended sequence number is the leader.
- </para>
-
- <para>That's not all, though. It is important to watch for failures of the
- leader, so that a new client arises as the new leader in the case the
- current leader fails. A trivial solution is to have all application
- processes watching upon the current smallest znode, and checking if they
- are the new leader when the smallest znode goes away (note that the
- smallest znode will go away if the leader fails because the node is
- ephemeral). But this causes a herd effect: upon a failure of the current
- leader, all other processes receive a notification, and execute
- getChildren on "/election" to obtain the current list of children of
- "/election". If the number of clients is large, it causes a spike on the
- number of operations that ZooKeeper servers have to process. To avoid the
- herd effect, it is sufficient to watch for the next znode down on the
- sequence of znodes. If a client receives a notification that the znode it
- is watching is gone, then it becomes the new leader in the case that there
- is no smaller znode. Note that this avoids the herd effect by not having
- all clients watching the same znode. </para>
-
- <para>Here's the pseudo code:</para>
-
- <para>Let ELECTION be a path of choice of the application. To volunteer to
- be a leader: </para>
-
- <orderedlist>
- <listitem>
- <para>Create znode z with path "ELECTION/guid-n_" with both SEQUENCE and
- EPHEMERAL flags;</para>
- </listitem>
-
- <listitem>
- <para>Let C be the children of "ELECTION", and i be the sequence
- number of z;</para>
- </listitem>
-
- <listitem>
- <para>Watch for changes on "ELECTION/guid-n_j", where j is the largest
- sequence number such that j < i and n_j is a znode in C;</para>
- </listitem>
- </orderedlist>
-
- <para>Upon receiving a notification of znode deletion: </para>
-
- <orderedlist>
- <listitem>
- <para>Let C be the new set of children of ELECTION; </para>
- </listitem>
-
- <listitem>
- <para>If z is the smallest node in C, then execute leader
- procedure;</para>
- </listitem>
-
- <listitem>
- <para>Otherwise, watch for changes on "ELECTION/guid-n_j", where j is the
- largest sequence number such that j < i and n_j is a znode in C;
- </para>
- </listitem>
- </orderedlist>
-
- <para>Notes:</para>
-
- <itemizedlist>
- <listitem>
- <para>Note that the znode having no preceding znode on the list of
- children does not imply that the creator of this znode is aware that it is
- the current leader. Applications may consider creating a separate znode
- to acknowledge that the leader has executed the leader procedure. </para>
- </listitem>
- </itemizedlist>
-
- <itemizedlist>
- <listitem>
- <para>See the <ulink url="#sc_recipes_GuidNote">note for Locks</ulink>
on how to use the guid in the node.</para>
- </listitem>
- </itemizedlist>
-
- </section>
- </section>
-</article>
http://git-wip-us.apache.org/repos/asf/zookeeper/blob/4607a3e1/src/docs/src/documentation/content/xdocs/site.xml
----------------------------------------------------------------------
diff --git a/src/docs/src/documentation/content/xdocs/site.xml b/src/docs/src/documentation/content/xdocs/site.xml
deleted file mode 100644
index 614fa6c..0000000
--- a/src/docs/src/documentation/content/xdocs/site.xml
+++ /dev/null
@@ -1,97 +0,0 @@
-<?xml version="1.0"?>
-<!--
- Copyright 2002-2004 The Apache Software Foundation
-
- Licensed under the Apache License, Version 2.0 (the "License");
- you may not use this file except in compliance with the License.
- You may obtain a copy of the License at
-
- http://www.apache.org/licenses/LICENSE-2.0
-
- Unless required by applicable law or agreed to in writing, software
- distributed under the License is distributed on an "AS IS" BASIS,
- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- See the License for the specific language governing permissions and
- limitations under the License.
--->
-
-<!--
-Forrest site.xml
-
-This file contains an outline of the site's information content. It is used to:
-- Generate the website menus (though these can be overridden - see docs)
-- Provide semantic, location-independent aliases for internal 'site:' URIs, eg
-<link href="site:changes"> links to changes.html (or ../changes.html if in
- subdir).
-- Provide aliases for external URLs in the external-refs section. Eg, <link
- href="ext:cocoon"> links to http://xml.apache.org/cocoon/
-
-See http://forrest.apache.org/docs/linking.html for more info.
--->
-
-<site label="ZooKeeper" href="" xmlns="http://apache.org/forrest/linkmap/1.0">
-
- <docs label="Overview">
- <welcome label="Welcome" href="index.html" />
- <overview label="Overview" href="zookeeperOver.html" />
- <started label="Getting Started" href="zookeeperStarted.html" />
- <relnotes label="Release Notes" href="ext:relnotes" />
- </docs>
-
- <docs label="Developer">
- <api label="API Docs" href="ext:api/index" />
- <program label="Programmer's Guide" href="zookeeperProgrammers.html" />
- <javaEx label="Java Example" href="javaExample.html" />
- <barTutor label="Barrier and Queue Tutorial" href="zookeeperTutorial.html" />
- <recipes label="Recipes" href="recipes.html" />
- </docs>
-
- <docs label="Admin & Ops">
- <admin label="Administrator's Guide" href="zookeeperAdmin.html" />
- <quota label="Quota Guide" href="zookeeperQuotas.html" />
- <jmx label="JMX" href="zookeeperJMX.html" />
- <observers label="Observers Guide" href="zookeeperObservers.html" />
- <reconfig label="Dynamic Reconfiguration" href="zookeeperReconfig.html" />
- </docs>
-
- <docs label="Contributor">
- <internals label="ZooKeeper Internals" href="zookeeperInternals.html" />
- </docs>
-
- <docs label="Miscellaneous">
- <wiki label="Wiki" href="ext:wiki" />
- <faq label="FAQ" href="ext:faq" />
- <lists label="Mailing Lists" href="ext:lists" />
- <!--<other label="Other Info" href="zookeeperOtherInfo.html" />-->
- </docs>
-
-
-
- <external-refs>
- <site href="http://zookeeper.apache.org/"/>
- <lists href="http://zookeeper.apache.org/mailing_lists.html"/>
- <releases href="http://zookeeper.apache.org/releases.html">
- <download href="#Download" />
- </releases>
- <jira href="http://zookeeper.apache.org/issue_tracking.html"/>
- <wiki href="https://cwiki.apache.org/confluence/display/ZOOKEEPER" />
- <faq href="https://cwiki.apache.org/confluence/display/ZOOKEEPER/FAQ" />
- <zlib href="http://www.zlib.net/" />
- <lzo href="http://www.oberhumer.com/opensource/lzo/" />
- <gzip href="http://www.gzip.org/" />
- <cygwin href="http://www.cygwin.com/" />
- <osx href="http://www.apple.com/macosx" />
- <relnotes href="releasenotes.html" />
- <api href="api/">
- <started href="overview-summary.html#overview_description" />
- <index href="index.html" />
- <org href="org/">
- <apache href="apache/">
- <zookeeper href="zookeeper/">
- </zookeeper>
- </apache>
- </org>
- </api>
- </external-refs>
-
-</site>
http://git-wip-us.apache.org/repos/asf/zookeeper/blob/4607a3e1/src/docs/src/documentation/content/xdocs/tabs.xml
----------------------------------------------------------------------
diff --git a/src/docs/src/documentation/content/xdocs/tabs.xml b/src/docs/src/documentation/content/xdocs/tabs.xml
deleted file mode 100644
index af6c447..0000000
--- a/src/docs/src/documentation/content/xdocs/tabs.xml
+++ /dev/null
@@ -1,36 +0,0 @@
-<?xml version="1.0"?>
-<!--
- Copyright 2002-2004 The Apache Software Foundation
-
- Licensed under the Apache License, Version 2.0 (the "License");
- you may not use this file except in compliance with the License.
- You may obtain a copy of the License at
-
- http://www.apache.org/licenses/LICENSE-2.0
-
- Unless required by applicable law or agreed to in writing, software
- distributed under the License is distributed on an "AS IS" BASIS,
- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- See the License for the specific language governing permissions and
- limitations under the License.
--->
-
-<!DOCTYPE tabs PUBLIC "-//APACHE//DTD Cocoon Documentation Tab V1.0//EN"
- "http://forrest.apache.org/dtd/tab-cocoon-v10.dtd">
-
-<tabs software="ZooKeeper"
- title="ZooKeeper"
- copyright="The Apache Software Foundation"
- xmlns:xlink="http://www.w3.org/1999/xlink">
-
- <!-- The rules are:
- @dir will always have /index.html added.
- @href is not modified unless it is root-relative and obviously specifies a
- directory (ends in '/'), in which case /index.html will be added
- -->
-
- <tab label="Project" href="http://zookeeper.apache.org/" />
- <tab label="Wiki" href="https://cwiki.apache.org/confluence/display/ZOOKEEPER/" />
- <tab label="ZooKeeper 3.6 Documentation" dir="" />
-
-</tabs>
|