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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="zookeeperProgrammers.pdf"><img alt="PDF -icon" src="skin/images/pdfdoc.gif" class="skin"><br> - PDF</a> -</div> -<h1>ZooKeeper Programmer's Guide</h1> -<h3>Developing Distributed Applications that use ZooKeeper</h3> -<div id="front-matter"> -<div id="minitoc-area"> -<ul class="minitoc"> -<li> -<a href="#_introduction">Introduction</a> -</li> -<li> -<a href="#ch_zkDataModel">The ZooKeeper Data Model</a> -<ul class="minitoc"> -<li> -<a href="#sc_zkDataModel_znodes">ZNodes</a> -<ul class="minitoc"> -<li> -<a href="#sc_zkDataMode_watches">Watches</a> -</li> -<li> -<a href="#Data+Access">Data Access</a> -</li> -<li> -<a href="#Ephemeral+Nodes">Ephemeral Nodes</a> -</li> -<li> -<a href="#Sequence+Nodes+--+Unique+Naming">Sequence Nodes -- Unique Naming</a> -</li> -</ul> -</li> -<li> -<a href="#sc_timeInZk">Time in ZooKeeper</a> -</li> -<li> -<a href="#sc_zkStatStructure">ZooKeeper Stat Structure</a> -</li> -</ul> -</li> -<li> -<a href="#ch_zkSessions">ZooKeeper Sessions</a> -</li> -<li> -<a href="#ch_zkWatches">ZooKeeper Watches</a> -<ul class="minitoc"> -<li> -<a href="#sc_WatchSemantics">Semantics of Watches</a> -</li> -<li> -<a href="#sc_WatchGuarantees">What ZooKeeper Guarantees about Watches</a> -</li> -<li> -<a href="#sc_WatchRememberThese">Things to Remember about Watches</a> -</li> -</ul> -</li> -<li> -<a href="#sc_ZooKeeperAccessControl">ZooKeeper access control using ACLs</a> -<ul class="minitoc"> -<li> -<a href="#sc_ACLPermissions">ACL Permissions</a> -<ul class="minitoc"> -<li> -<a href="#sc_BuiltinACLSchemes">Builtin ACL Schemes</a> -</li> -<li> -<a href="#ZooKeeper+C+client+API">ZooKeeper C client API</a> -</li> -</ul> -</li> -</ul> -</li> -<li> -<a href="#sc_ZooKeeperPluggableAuthentication">Pluggable ZooKeeper authentication</a> -</li> -<li> -<a href="#ch_zkGuarantees">Consistency Guarantees</a> -</li> -<li> -<a href="#ch_bindings">Bindings</a> -<ul class="minitoc"> -<li> -<a href="#Java+Binding">Java Binding</a> -</li> -<li> -<a href="#C+Binding">C Binding</a> -<ul class="minitoc"> -<li> -<a href="#Installation">Installation</a> -</li> -<li> -<a href="#Building+Your+Own+C+Client">Building Your Own C Client</a> -</li> -</ul> -</li> -</ul> -</li> -<li> -<a href="#ch_guideToZkOperations">Building Blocks: A Guide to ZooKeeper Operations</a> -<ul class="minitoc"> -<li> -<a href="#sc_errorsZk">Handling Errors</a> -</li> -<li> -<a href="#sc_connectingToZk">Connecting to ZooKeeper</a> -</li> -<li> -<a href="#sc_readOps">Read Operations</a> -</li> -<li> -<a href="#sc_writeOps">Write Operations</a> -</li> -<li> -<a href="#sc_handlingWatches">Handling Watches</a> -</li> -<li> -<a href="#sc_miscOps">Miscelleaneous ZooKeeper Operations</a> -</li> -</ul> -</li> -<li> -<a href="#ch_programStructureWithExample">Program Structure, with Simple Example</a> -</li> -<li> -<a href="#ch_gotchas">Gotchas: Common Problems and Troubleshooting</a> -</li> -</ul> -</div> -</div> - - - - - - - -<a name="_introduction"></a> -<h2 class="h3">Introduction</h2> -<div class="section"> -<p>This document is a guide for developers wishing to create - distributed applications that take advantage of ZooKeeper's coordination - services. It contains conceptual and practical information.</p> -<p>The first four sections of this guide present higher level - discussions of various ZooKeeper concepts. These are necessary both for an - understanding of how ZooKeeper works as well how to work with it. It does - not contain source code, but it does assume a familiarity with the - problems associated with distributed computing. The sections in this first - group are:</p> -<ul> - -<li> - -<p> -<a href="#ch_zkDataModel">The ZooKeeper Data Model</a> -</p> - -</li> - - -<li> - -<p> -<a href="#ch_zkSessions">ZooKeeper Sessions</a> -</p> - -</li> - - -<li> - -<p> -<a href="#ch_zkWatches">ZooKeeper Watches</a> -</p> - -</li> - - -<li> - -<p> -<a href="#ch_zkGuarantees">Consistency Guarantees</a> -</p> - -</li> - -</ul> -<p>The next four sections provide practical programming - information. These are:</p> -<ul> - -<li> - -<p> -<a href="#ch_guideToZkOperations">Building Blocks: A Guide to ZooKeeper Operations</a> -</p> - -</li> - - -<li> - -<p> -<a href="#ch_bindings">Bindings</a> -</p> - -</li> - - -<li> - -<p> -<a href="#ch_programStructureWithExample">Program Structure, with Simple Example</a> - <em>[tbd]</em> -</p> - -</li> - - -<li> - -<p> -<a href="#ch_gotchas">Gotchas: Common Problems and Troubleshooting</a> -</p> - -</li> - -</ul> -<p>The book concludes with an <a href="#apx_linksToOtherInfo">appendix</a> containing links to other - useful, ZooKeeper-related information.</p> -<p>Most of information in this document is written to be accessible as - stand-alone reference material. However, before starting your first - ZooKeeper application, you should probably at least read the chaptes on - the <a href="#ch_zkDataModel">ZooKeeper Data Model</a> and <a href="#ch_guideToZkOperations">ZooKeeper Basic Operations</a>. Also, - the <a href="#ch_programStructureWithExample">Simple Programmming - Example</a> <em>[tbd]</em> is helpful for understanding the basic - structure of a ZooKeeper client application.</p> -</div> - - -<a name="ch_zkDataModel"></a> -<h2 class="h3">The ZooKeeper Data Model</h2> -<div class="section"> -<p>ZooKeeper has a hierarchal name space, much like a distributed file - system. The only difference is that each node in the 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. Paths to nodes are - always expressed as canonical, absolute, slash-separated paths; there are - no relative reference. Any unicode character can be used in a path subject - to the following constraints:</p> -<ul> - -<li> - -<p>The null character (\u0000) cannot be part of a path name. (This - causes problems with the C binding.)</p> - -</li> - - -<li> - -<p>The following characters can't be used because they don't - display well, or render in confusing ways: \u0001 - \u0019 and \u007F - - \u009F.</p> - -</li> - - -<li> - -<p>The following characters are not allowed: \ud800 -uF8FFF, - \uFFF0 - uFFFF.</p> - -</li> - - -<li> - -<p>The "." character can be used as part of another name, but "." - and ".." cannot alone be used to indicate a node along a path, - because ZooKeeper doesn't use relative paths. The following would be - invalid: "/a/b/./c" or "/a/b/../c".</p> - -</li> - - -<li> - -<p>The token "zookeeper" is reserved.</p> - -</li> - -</ul> -<a name="sc_zkDataModel_znodes"></a> -<h3 class="h4">ZNodes</h3> -<p>Every node in a ZooKeeper tree is referred to as a - <em>znode</em>. Znodes maintain a stat structure that - includes version numbers for data changes, acl changes. The stat - structure also has timestamps. The version number, together with the - timestamp, allows ZooKeeper to validate the cache and to coordinate - 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. And when a client performs an update or a delete, - it must supply the version of the data of the znode it is changing. If - the version it supplies doesn't match the actual version of the data, - the update will fail. (This behavior can be overridden. For more - information see... )<em>[tbd...]</em> -</p> -<div class="note"> -<div class="label">Note</div> -<div class="content"> - -<p>In distributed application engineering, the word - <em>node</em> can refer to a generic host machine, a - server, a member of an ensemble, a client process, etc. In the ZooKeeper - documentation, <em>znodes</em> refer to the data nodes. - <em>Servers</em> refer to machines that make up the - ZooKeeper service; <em>quorum peers</em> refer to the - servers that make up an ensemble; client refers to any host or process - which uses a ZooKeeper service.</p> - -</div> -</div> -<p> A znode is the main abstraction a programmer needs to be aware of. Znodes have - several characteristics that are worth mentioning here.</p> -<a name="sc_zkDataMode_watches"></a> -<h4>Watches</h4> -<p>Clients can set watches on znodes. Changes to that znode trigger - the watch and then clear the watch. When a watch triggers, ZooKeeper - sends the client a notification. More information about watches can be - found in the section - <a href="#ch_zkWatches">ZooKeeper Watches</a>.</p> -<a name="Data+Access"></a> -<h4>Data Access</h4> -<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 was not designed to be a general database or large - object store. Instead, it manages coordination data. This data can - come in the form of configuration, status information, rendezvous, etc. - A common property of the various forms of coordination data is that - they are relatively small: measured in kilobytes. - The ZooKeeper client and the server implementations have sanity checks - to ensure that znodes have less than 1M of data, but the data should - be much less than that on average. Operating on relatively large data - sizes will cause some operations to take much more time than others and - will affect the latencies of some operations because of the extra time - needed to move more data over the network and onto storage media. If - large data storage is needed, the usually pattern of dealing with such - data is to store it on a bulk storage system, such as NFS or HDFS, and - store pointers to the storage locations in ZooKeeper.</p> -<a name="Ephemeral+Nodes"></a> -<h4>Ephemeral Nodes</h4> -<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. Because of this behavior - ephemeral znodes are not allowed to have children.</p> -<a name="Sequence+Nodes+--+Unique+Naming"></a> -<h4>Sequence Nodes -- Unique Naming</h4> -<p>When creating a znode you can also request that - ZooKeeper append a monotonically increasing counter to the end - of path. This counter is unique to the parent znode. The - counter has a format of %010d -- that is 10 digits with 0 - (zero) padding (the counter is formatted in this way to - simplify sorting), i.e. "<path>0000000001". See - <a href="recipes.html#sc_recipes_Queues">Queue - Recipe</a> for an example use of this feature. Note: the - counter used to store the next sequence number is a signed int - (4bytes) maintained by the parent node, the counter will - overflow when incremented beyond 2147483647 (resulting in a - name "<path>-2147483648").</p> -<a name="sc_timeInZk"></a> -<h3 class="h4">Time in ZooKeeper</h3> -<p>ZooKeeper tracks time multiple ways:</p> -<ul> - -<li> - -<p> -<strong>Zxid</strong> -</p> - - -<p>Every change to the ZooKeeper state receives a stamp in the - form of a <em>zxid</em> (ZooKeeper Transaction Id). - This exposes the total ordering of all changes to ZooKeeper. Each - change will have a unique zxid and if zxid1 is smaller than zxid2 - then zxid1 happened before zxid2.</p> - -</li> - - -<li> - -<p> -<strong>Version numbers</strong> -</p> - - -<p>Every change to a node will cause an increase to one of the - version numbers of that node. The three version numbers are version - (number of changes to the data of a znode), cversion (number of - changes to the children of a znode), and aversion (number of changes - to the ACL of a znode).</p> - -</li> - - -<li> - -<p> -<strong>Ticks</strong> -</p> - - -<p>When using multi-server ZooKeeper, servers use ticks to define - timing of events such as status uploads, session timeouts, - connection timeouts between peers, etc. The tick time is only - indirectly exposed through the minimum session timeout (2 times the - tick time); if a client requests a session timeout less than the - minimum session timeout, the server will tell the client that the - session timeout is actually the minimum session timeout.</p> - -</li> - - -<li> - -<p> -<strong>Real time</strong> -</p> - - -<p>ZooKeeper doesn't use real time, or clock time, at all except - to put timestamps into the stat structure on znode creation and - znode modification.</p> - -</li> - -</ul> -<a name="sc_zkStatStructure"></a> -<h3 class="h4">ZooKeeper Stat Structure</h3> -<p>The Stat structure for each znode in ZooKeeper is made up of the - following fields:</p> -<ul> - -<li> - -<p> -<strong>czxid</strong> -</p> - - -<p>The zxid of the change that caused this znode to be - created.</p> - -</li> - - -<li> - -<p> -<strong>mzxid</strong> -</p> - - -<p>The zxid of the change that last modified this znode.</p> - -</li> - - -<li> - -<p> -<strong>pzxid</strong> -</p> - - -<p>The zxid of the change that last modified children of this znode.</p> - -</li> - - -<li> - -<p> -<strong>ctime</strong> -</p> - - -<p>The time in milliseconds from epoch when this znode was - created.</p> - -</li> - - -<li> - -<p> -<strong>mtime</strong> -</p> - - -<p>The time in milliseconds from epoch when this znode was last - modified.</p> - -</li> - - -<li> - -<p> -<strong>version</strong> -</p> - - -<p>The number of changes to the data of this znode.</p> - -</li> - - -<li> - -<p> -<strong>cversion</strong> -</p> - - -<p>The number of changes to the children of this znode.</p> - -</li> - - -<li> - -<p> -<strong>aversion</strong> -</p> - - -<p>The number of changes to the ACL of this znode.</p> - -</li> - - -<li> - -<p> -<strong>ephemeralOwner</strong> -</p> - - -<p>The session id of the owner of this znode if the znode is an - ephemeral node. If it is not an ephemeral node, it will be - zero.</p> - -</li> - - -<li> - -<p> -<strong>dataLength</strong> -</p> - - -<p>The length of the data field of this znode.</p> - -</li> - - -<li> - -<p> -<strong>numChildren</strong> -</p> - - -<p>The number of children of this znode.</p> - -</li> - - -</ul> -</div> - - -<a name="ch_zkSessions"></a> -<h2 class="h3">ZooKeeper Sessions</h2> -<div class="section"> -<p>A ZooKeeper client establishes a session with the ZooKeeper - service by creating a handle to the service using a language - binding. Once created, the handle starts of in the CONNECTING state - and the client library tries to connect to one of the servers that - make up the ZooKeeper service at which point it switches to the - CONNECTED state. During normal operation will be in one of these - two states. If an unrecoverable error occurs, such as session - expiration or authentication failure, or if the application explicitly - closes the handle, the handle will move to the CLOSED state. - The following figure shows the possible state transitions of a - ZooKeeper client:</p> -<img alt="" src="images/state_dia.jpg"><p>To create a client session the application code must provide - a connection string containing a comma separated list of host:port pairs, - each corresponding to a ZooKeeper server (e.g. "127.0.0.1:4545" or - "127.0.0.1:3000,127.0.0.1:3001,127.0.0.1:3002"). The ZooKeeper - client library will pick an arbitrary server and try to connect to - it. If this connection fails, or if the client becomes - disconnected from the server for any reason, the client will - automatically try the next server in the list, until a connection - is (re-)established.</p> -<p> -<strong>Added in 3.2.0</strong>: An - optional "chroot" suffix may also be appended to the connection - string. This will run the client commands while interpreting all - paths relative to this root (similar to the unix chroot - command). If used the example would look like: - "127.0.0.1:4545/app/a" or - "127.0.0.1:3000,127.0.0.1:3001,127.0.0.1:3002/app/a" where the - client would be rooted at "/app/a" and all paths would be relative - to this root - ie getting/setting/etc... "/foo/bar" would result - in operations being run on "/app/a/foo/bar" (from the server - perspective). This feature is particularly useful in multi-tenant - environments where each user of a particular ZooKeeper service - could be rooted differently. This makes re-use much simpler as - each user can code his/her application as if it were rooted at - "/", while actual location (say /app/a) could be determined at - deployment time.</p> -<p>When a client gets a handle to the ZooKeeper service, - ZooKeeper creates a ZooKeeper session, represented as a 64-bit - number, that it assigns to the client. If the client connects to a - different ZooKeeper server, it will send the session id as a part - of the connection handshake. As a security measure, the server - creates a password for the session id that any ZooKeeper server - can validate.The password is sent to the client with the session - id when the client establishes the session. The client sends this - password with the session id whenever it reestablishes the session - with a new server.</p> -<p>One of the parameters to the ZooKeeper client library call - to create a ZooKeeper session is the session timeout in - milliseconds. The client sends a requested timeout, the server - responds with the timeout that it can give the client. The current - implementation requires that the timeout be a minimum of 2 times - the tickTime (as set in the server configuration) and a maximum of - 20 times the tickTime. The ZooKeeper client API allows access to - the negotiated timeout.</p> -<p>When a client (session) becomes partitioned from the ZK - serving cluster it will begin searching the list of servers that - were specified during session creation. Eventually, when - connectivity between the client and at least one of the servers is - re-established, the session will either again transition to the - "connected" state (if reconnected within the session timeout - value) or it will transition to the "expired" state (if - reconnected after the session timeout). It is not advisable to - create a new session object (a new ZooKeeper.class or zookeeper - handle in the c binding) for disconnection. The ZK client library - will handle reconnect for you. In particular we have heuristics - built into the client library to handle things like "herd effect", - etc... Only create a new session when you are notified of session - expiration (mandatory).</p> -<p>Session expiration is managed by the ZooKeeper cluster - itself, not by the client. When the ZK client establishes a - session with the cluster it provides a "timeout" value detailed - above. This value is used by the cluster to determine when the - client's session expires. Expirations happens when the cluster - does not hear from the client within the specified session timeout - period (i.e. no heartbeat). At session expiration the cluster will - delete any/all ephemeral nodes owned by that session and - immediately notify any/all connected clients of the change (anyone - watching those znodes). At this point the client of the expired - session is still disconnected from the cluster, it will not be - notified of the session expiration until/unless it is able to - re-establish a connection to the cluster. The client will stay in - disconnected state until the TCP connection is re-established with - the cluster, at which point the watcher of the expired session - will receive the "session expired" notification.</p> -<p>Example state transitions for an expired session as seen by - the expired session's watcher:</p> -<ol> - -<li> -<p>'connected' : session is established and client - is communicating with cluster (client/server communication is - operating properly)</p> -</li> - -<li> -<p>.... client is partitioned from the - cluster</p> -</li> - -<li> -<p>'disconnected' : client has lost connectivity - with the cluster</p> -</li> - -<li> -<p>.... time elapses, after 'timeout' period the - cluster expires the session, nothing is seen by client as it is - disconnected from cluster</p> -</li> - -<li> -<p>.... time elapses, the client regains network - level connectivity with the cluster</p> -</li> - -<li> -<p>'expired' : eventually the client reconnects to - the cluster, it is then notified of the - expiration</p> -</li> - -</ol> -<p>Another parameter to the ZooKeeper session establishment - call is the default watcher. Watchers are notified when any state - change occurs in the client. For example if the client loses - connectivity to the server the client will be notified, or if the - client's session expires, etc... This watcher should consider the - initial state to be disconnected (i.e. before any state changes - events are sent to the watcher by the client lib). In the case of - a new connection, the first event sent to the watcher is typically - the session connection event.</p> -<p>The session is kept alive by requests sent by the client. If - the session is idle for a period of time that would timeout the - session, the client will send a PING request to keep the session - alive. This PING request not only allows the ZooKeeper server to - know that the client is still active, but it also allows the - client to verify that its connection to the ZooKeeper server is - still active. The timing of the PING is conservative enough to - ensure reasonable time to detect a dead connection and reconnect - to a new server.</p> -<p> - Once a connection to the server is successfully established - (connected) there are basically two cases where the client lib generates - connectionloss (the result code in c binding, exception in Java -- see - the API documentation for binding specific details) when either a synchronous or - asynchronous operation is performed and one of the following holds: - </p> -<ol> - -<li> -<p>The application calls an operation on a session that is no - longer alive/valid</p> -</li> - -<li> -<p>The ZooKeeper client disconnects from a server when there - are pending operations to that server, i.e., there is a pending asynchronous call. - </p> -</li> - -</ol> -<p> -<strong>Added in 3.2.0 -- SessionMovedException</strong>. There is an internal - exception that is generally not seen by clients called the SessionMovedException. - This exception occurs because a request was received on a connection for a session - which has been reestablished on a different server. The normal cause of this error is - a client that sends a request to a server, but the network packet gets delayed, so - the client times out and connects to a new server. When the delayed packet arrives at - the first server, the old server detects that the session has moved, and closes the - client connection. Clients normally do not see this error since they do not read - from those old connections. (Old connections are usually closed.) One situation in which this - condition can be seen is when two clients try to reestablish the same connection using - a saved session id and password. One of the clients will reestablish the connection - and the second client will be disconnected (causing the pair to attempt to re-establish - its connection/session indefinitely).</p> -</div> - - -<a name="ch_zkWatches"></a> -<h2 class="h3">ZooKeeper Watches</h2> -<div class="section"> -<p>All of the read operations in ZooKeeper - <strong>getData()</strong>, <strong>getChildren()</strong>, and <strong>exists()</strong> - have the option of setting a watch as a - side effect. Here is ZooKeeper's definition of a watch: a watch event is - one-time trigger, sent to the client that set the watch, which occurs when - the data for which the watch was set changes. There are three key points - to consider in this definition of a watch:</p> -<ul> - -<li> - -<p> -<strong>One-time trigger</strong> -</p> - - -<p>One watch event will be sent to the client when the data has changed. - For example, if a client does a getData("/znode1", true) and later the - data for /znode1 is changed or deleted, the client will get a watch - event for /znode1. If /znode1 changes again, no watch event will be - sent unless the client has done another read that sets a new - watch.</p> - -</li> - - -<li> - -<p> -<strong>Sent to the client</strong> -</p> - - -<p>This implies that an event is on the way to the client, but may - not reach the client before the successful return code to the change - operation reaches the client that initiated the change. Watches are - sent asynchronously to watchers. ZooKeeper provides an ordering - guarantee: a client will never see a change for which it has set a - watch until it first sees the watch event. Network delays or other - factors may cause different clients to see watches and return codes - from updates at different times. The key point is that everything seen - by the different clients will have a consistent order.</p> - -</li> - - -<li> - -<p> -<strong>The data for which the watch was - set</strong> -</p> - - -<p>This refers to the different ways a node can change. It - helps to think of ZooKeeper as maintaining two lists of - watches: data watches and child watches. getData() and - exists() set data watches. getChildren() sets child - watches. Alternatively, it may help to think of watches being - set according to the kind of data returned. getData() and - exists() return information about the data of the node, - whereas getChildren() returns a list of children. Thus, - setData() will trigger data watches for the znode being set - (assuming the set is successful). A successful create() will - trigger a data watch for the znode being created and a child - watch for the parent znode. A successful delete() will trigger - both a data watch and a child watch (since there can be no - more children) for a znode being deleted as well as a child - watch for the parent znode.</p> - -</li> - -</ul> -<p>Watches are maintained locally at the ZooKeeper server to which the - client is connected. This allows watches to be lightweight to set, - maintain, and dispatch. When a client connects to a new server, the watch - will be triggered for any session events. Watches will not be received - while disconnected from a server. When a client reconnects, any previously - registered watches will be reregistered and triggered if needed. In - general this all occurs transparently. There is one case where a watch - may be missed: a watch for the existence of a znode not yet created will - be missed if the znode is created and deleted while disconnected.</p> -<a name="sc_WatchSemantics"></a> -<h3 class="h4">Semantics of Watches</h3> -<p> We can set watches with the three calls that read the state of - ZooKeeper: exists, getData, and getChildren. The following list details - the events that a watch can trigger and the calls that enable them: - </p> -<ul> - -<li> - -<p> -<strong>Created event:</strong> -</p> - -<p>Enabled with a call to exists.</p> - -</li> - - -<li> - -<p> -<strong>Deleted event:</strong> -</p> - -<p>Enabled with a call to exists, getData, and getChildren.</p> - -</li> - - -<li> - -<p> -<strong>Changed event:</strong> -</p> - -<p>Enabled with a call to exists and getData.</p> - -</li> - - -<li> - -<p> -<strong>Child event:</strong> -</p> - -<p>Enabled with a call to getChildren.</p> - -</li> - -</ul> -<a name="sc_WatchGuarantees"></a> -<h3 class="h4">What ZooKeeper Guarantees about Watches</h3> -<p>With regard to watches, ZooKeeper maintains these - guarantees:</p> -<ul> - -<li> - -<p>Watches are ordered with respect to other events, other - watches, and asynchronous replies. The ZooKeeper client libraries - ensures that everything is dispatched in order.</p> - -</li> - -</ul> -<ul> - -<li> - -<p>A client will see a watch event for a znode it is watching - before seeing the new data that corresponds to that znode.</p> - -</li> - -</ul> -<ul> - -<li> - -<p>The order of watch events from ZooKeeper corresponds to the - order of the updates as seen by the ZooKeeper service.</p> - -</li> - -</ul> -<a name="sc_WatchRememberThese"></a> -<h3 class="h4">Things to Remember about Watches</h3> -<ul> - -<li> - -<p>Watches are one time triggers; if you get a watch event and - you want to get notified of future changes, you must set another - watch.</p> - -</li> - -</ul> -<ul> - -<li> - -<p>Because watches are one time triggers and there is latency - between getting the event and sending a new request to get a watch - you cannot reliably see every change that happens to a node in - ZooKeeper. Be prepared to handle the case where the znode changes - multiple times between getting the event and setting the watch - again. (You may not care, but at least realize it may - happen.)</p> - -</li> - -</ul> -<ul> - -<li> - -<p>A watch object, or function/context pair, will only be - triggered once for a given notification. For example, if the same - watch object is registered for an exists and a getData call for the - same file and that file is then deleted, the watch object would - only be invoked once with the deletion notification for the file. - </p> - -</li> - -</ul> -<ul> - -<li> - -<p>When you disconnect from a server (for example, when the - server fails), you will not get any watches until the connection - is reestablished. For this reason session events are sent to all - outstanding watch handlers. Use session events to go into a safe - mode: you will not be receiving events while disconnected, so your - process should act conservatively in that mode.</p> - -</li> - -</ul> -</div> - - -<a name="sc_ZooKeeperAccessControl"></a> -<h2 class="h3">ZooKeeper access control using ACLs</h2> -<div class="section"> -<p>ZooKeeper uses ACLs to control access to its znodes (the - data nodes of a ZooKeeper data tree). The ACL implementation is - quite similar to UNIX file access permissions: it employs - permission bits to allow/disallow various operations against a - node and the scope to which the bits apply. Unlike standard UNIX - permissions, a ZooKeeper node is not limited by the three standard - scopes for user (owner of the file), group, and world - (other). ZooKeeper does not have a notion of an owner of a - znode. Instead, an ACL specifies sets of ids and permissions that - are associated with those ids.</p> -<p>Note also that an ACL pertains only to a specific znode. In - particular it does not apply to children. For example, if - <em>/app</em> is only readable by ip:172.16.16.1 and - <em>/app/status</em> is world readable, anyone will - be able to read <em>/app/status</em>; ACLs are not - recursive.</p> -<p>ZooKeeper supports pluggable authentication schemes. Ids are - specified using the form <em>scheme:id</em>, - where <em>scheme</em> is a the authentication scheme - that the id corresponds to. For - example, <em>ip:172.16.16.1</em> is an id for a - host with the address <em>172.16.16.1</em>.</p> -<p>When a client connects to ZooKeeper and authenticates - itself, ZooKeeper associates all the ids that correspond to a - client with the clients connection. These ids are checked against - the ACLs of znodes when a clients tries to access a node. ACLs are - made up of pairs of <em>(scheme:expression, - perms)</em>. The format of - the <em>expression</em> is specific to the scheme. For - example, the pair <em>(ip:19.22.0.0/16, READ)</em> - gives the <em>READ</em> permission to any clients with - an IP address that starts with 19.22.</p> -<a name="sc_ACLPermissions"></a> -<h3 class="h4">ACL Permissions</h3> -<p>ZooKeeper supports the following permissions:</p> -<ul> - -<li> -<p> -<strong>CREATE</strong>: you can create a child node</p> -</li> - -<li> -<p> -<strong>READ</strong>: you can get data from a node and list its children.</p> -</li> - -<li> -<p> -<strong>WRITE</strong>: you can set data for a node</p> -</li> - -<li> -<p> -<strong>DELETE</strong>: you can delete a child node</p> -</li> - -<li> -<p> -<strong>ADMIN</strong>: you can set permissions</p> -</li> - -</ul> -<p>The <em>CREATE</em> - and <em>DELETE</em> permissions have been broken out - of the <em>WRITE</em> permission for finer grained - access controls. The cases for <em>CREATE</em> - and <em>DELETE</em> are the following:</p> -<p>You want A to be able to do a set on a ZooKeeper node, but - not be able to <em>CREATE</em> - or <em>DELETE</em> children.</p> -<p> -<em>CREATE</em> - without <em>DELETE</em>: clients create requests by - creating ZooKeeper nodes in a parent directory. You want all - clients to be able to add, but only request processor can - delete. (This is kind of like the APPEND permission for - files.)</p> -<p>Also, the <em>ADMIN</em> permission is there - since ZooKeeper doesn’t have a notion of file owner. In some - sense the <em>ADMIN</em> permission designates the - entity as the owner. ZooKeeper doesn’t support the LOOKUP - permission (execute permission bit on directories to allow you - to LOOKUP even though you can't list the directory). Everyone - implicitly has LOOKUP permission. This allows you to stat a - node, but nothing more. (The problem is, if you want to call - zoo_exists() on a node that doesn't exist, there is no - permission to check.)</p> -<a name="sc_BuiltinACLSchemes"></a> -<h4>Builtin ACL Schemes</h4> -<p>ZooKeeeper has the following built in schemes:</p> -<ul> - -<li> -<p> -<strong>world</strong> has a - single id, <em>anyone</em>, that represents - anyone.</p> -</li> - - -<li> -<p> -<strong>auth</strong> doesn't - use any id, represents any authenticated - user.</p> -</li> - - -<li> -<p> -<strong>digest</strong> uses - a <em>username:password</em> string to generate - MD5 hash which is then used as an ACL ID - identity. Authentication is done by sending - the <em>username:password</em> in clear text. When - used in the ACL the expression will be - the <em>username:base64</em> - encoded <em>SHA1</em> - password <em>digest</em>.</p> - -</li> - - -<li> -<p> -<strong>ip</strong> uses the - client host IP as an ACL ID identity. The ACL expression is of - the form <em>addr/bits</em> where the most - significant <em>bits</em> - of <em>addr</em> are matched against the most - significant <em>bits</em> of the client host - IP.</p> -</li> - - -</ul> -<a name="ZooKeeper+C+client+API"></a> -<h4>ZooKeeper C client API</h4> -<p>The following constants are provided by the ZooKeeper C - library:</p> -<ul> - -<li> -<p> -<em>const</em> <em>int</em> ZOO_PERM_READ; //can read node’s value and list its children</p> -</li> - -<li> -<p> -<em>const</em> <em>int</em> ZOO_PERM_WRITE;// can set the node’s value</p> -</li> - -<li> -<p> -<em>const</em> <em>int</em> ZOO_PERM_CREATE; //can create children</p> -</li> - -<li> -<p> -<em>const</em> <em>int</em> ZOO_PERM_DELETE;// can delete children</p> -</li> - -<li> -<p> -<em>const</em> <em>int</em> ZOO_PERM_ADMIN; //can execute set_acl()</p> -</li> - -<li> -<p> -<em>const</em> <em>int</em> ZOO_PERM_ALL;// all of the above flags OR’d together</p> -</li> - -</ul> -<p>The following are the standard ACL IDs:</p> -<ul> - -<li> -<p> -<em>struct</em> Id ZOO_ANYONE_ID_UNSAFE; //(‘world’,’anyone’)</p> -</li> - -<li> -<p> -<em>struct</em> Id ZOO_AUTH_IDS;// (‘auth’,’’)</p> -</li> - -</ul> -<p>ZOO_AUTH_IDS empty identity string should be interpreted as “the identity of the creator”.</p> -<p>ZooKeeper client comes with three standard ACLs:</p> -<ul> - -<li> -<p> -<em>struct</em> ACL_vector ZOO_OPEN_ACL_UNSAFE; //(ZOO_PERM_ALL,ZOO_ANYONE_ID_UNSAFE)</p> -</li> - -<li> -<p> -<em>struct</em> ACL_vector ZOO_READ_ACL_UNSAFE;// (ZOO_PERM_READ, ZOO_ANYONE_ID_UNSAFE)</p> -</li> - -<li> -<p> -<em>struct</em> ACL_vector ZOO_CREATOR_ALL_ACL; //(ZOO_PERM_ALL,ZOO_AUTH_IDS)</p> -</li> - -</ul> -<p>The ZOO_OPEN_ACL_UNSAFE is completely open free for all - ACL: any application can execute any operation on the node and - can create, list and delete its children. The - ZOO_READ_ACL_UNSAFE is read-only access for any - application. CREATE_ALL_ACL grants all permissions to the - creator of the node. The creator must have been authenticated by - the server (for example, using “<em>digest</em>” - scheme) before it can create nodes with this ACL.</p> -<p>The following ZooKeeper operations deal with ACLs:</p> -<ul> -<li> - -<p> -<em>int</em> <em>zoo_add_auth</em> - (zhandle_t *zh,<em>const</em> <em>char</em>* - scheme,<em>const</em> <em>char</em>* - cert, <em>int</em> certLen, void_completion_t - completion, <em>const</em> <em>void</em> - *data);</p> - -</li> -</ul> -<p>The application uses the zoo_add_auth function to - authenticate itself to the server. The function can be called - multiple times if the application wants to authenticate using - different schemes and/or identities.</p> -<ul> -<li> - -<p> -<em>int</em> <em>zoo_create</em> - (zhandle_t *zh, <em>const</em> <em>char</em> - *path, <em>const</em> <em>char</em> - *value,<em>int</em> - valuelen, <em>const</em> <em>struct</em> - ACL_vector *acl, <em>int</em> - flags,<em>char</em> - *realpath, <em>int</em> - max_realpath_len);</p> - -</li> -</ul> -<p>zoo_create(...) operation creates a new node. The acl - parameter is a list of ACLs associated with the node. The parent - node must have the CREATE permission bit set.</p> -<ul> -<li> - -<p> -<em>int</em> <em>zoo_get_acl</em> - (zhandle_t *zh, <em>const</em> <em>char</em> - *path,<em>struct</em> ACL_vector - *acl, <em>struct</em> Stat *stat);</p> - -</li> -</ul> -<p>This operation returns a node’s ACL info.</p> -<ul> -<li> - -<p> -<em>int</em> <em>zoo_set_acl</em> - (zhandle_t *zh, <em>const</em> <em>char</em> - *path, <em>int</em> - version,<em>const</em> <em>struct</em> - ACL_vector *acl);</p> - -</li> -</ul> -<p>This function replaces node’s ACL list with a new one. The - node must have the ADMIN permission set.</p> -<p>Here is a sample code that makes use of the above APIs to - authenticate itself using the “<em>foo</em>” scheme - and create an ephemeral node “/xyz” with create-only - permissions.</p> -<div class="note"> -<div class="label">Note</div> -<div class="content"> -<p>This is a very simple example which is intended to show - how to interact with ZooKeeper ACLs - specifically. See <span class="codefrag filename">.../trunk/src/c/src/cli.c</span> - for an example of a C client implementation</p> - -</div> -</div> -<pre class="code"> -#include <string.h> -#include <errno.h> - -#include "zookeeper.h" - -static zhandle_t *zh; - -/** - * In this example this method gets the cert for your - * environment -- you must provide - */ -char *foo_get_cert_once(char* id) { return 0; } - -/** Watcher function -- empty for this example, not something you should - * do in real code */ -void watcher(zhandle_t *zzh, int type, int state, const char *path, - void *watcherCtx) {} - -int main(int argc, char argv) { - char buffer[512]; - char p[2048]; - char *cert=0; - char appId[64]; - - strcpy(appId, "example.foo_test"); - cert = foo_get_cert_once(appId); - if(cert!=0) { - fprintf(stderr, - "Certificate for appid [%s] is [%s]\n",appId,cert); - strncpy(p,cert, sizeof(p)-1); - free(cert); - } else { - fprintf(stderr, "Certificate for appid [%s] not found\n",appId); - strcpy(p, "dummy"); - } - - zoo_set_debug_level(ZOO_LOG_LEVEL_DEBUG); - - zh = zookeeper_init("localhost:3181", watcher, 10000, 0, 0, 0); - if (!zh) { - return errno; - } - if(zoo_add_auth(zh,"foo",p,strlen(p),0,0)!=ZOK) - return 2; - - struct ACL CREATE_ONLY_ACL[] = {{ZOO_PERM_CREATE, ZOO_AUTH_IDS}}; - struct ACL_vector CREATE_ONLY = {1, CREATE_ONLY_ACL}; - int rc = zoo_create(zh,"/xyz","value", 5, &CREATE_ONLY, ZOO_EPHEMERAL, - buffer, sizeof(buffer)-1); - - /** this operation will fail with a ZNOAUTH error */ - int buflen= sizeof(buffer); - struct Stat stat; - rc = zoo_get(zh, "/xyz", 0, buffer, &buflen, &stat); - if (rc) { - fprintf(stderr, "Error %d for %s\n", rc, __LINE__); - } - - zookeeper_close(zh); - return 0; -} - </pre> -</div> - - -<a name="sc_ZooKeeperPluggableAuthentication"></a> -<h2 class="h3">Pluggable ZooKeeper authentication</h2> -<div class="section"> -<p>ZooKeeper runs in a variety of different environments with - various different authentication schemes, so it has a completely - pluggable authentication framework. Even the builtin authentication - schemes use the pluggable authentication framework.</p> -<p>To understand how the authentication framework works, first you must - understand the two main authentication operations. The framework - first must authenticate the client. This is usually done as soon as - the client connects to a server and consists of validating information - sent from or gathered about a client and associating it with the connection. - The second operation handled by the framework is finding the entries in an - ACL that correspond to client. ACL entries are <<em>idspec, - permissions</em>> pairs. The <em>idspec</em> may be - a simple string match against the authentication information associated - with the connection or it may be a expression that is evaluated against that - information. It is up to the implementation of the authentication plugin - to do the match. Here is the interface that an authentication plugin must - implement:</p> -<pre class="code"> -public interface AuthenticationProvider { - String getScheme(); - KeeperException.Code handleAuthentication(ServerCnxn cnxn, byte authData[]); - boolean isValid(String id); - boolean matches(String id, String aclExpr); - boolean isAuthenticated(); -} - </pre> -<p>The first method <em>getScheme</em> returns the string - that identifies the plugin. Because we support multiple methods of authentication, - an authentication credential or an <em>idspec</em> will always be - prefixed with <em>scheme:</em>. The ZooKeeper server uses the scheme - returned by the authentication plugin to determine which ids the scheme - applies to.</p> -<p> -<em>handleAuthentication</em> is called when a client - sends authentication information to be associated with a connection. The - client specifies the scheme to which the information corresponds. The - ZooKeeper server passes the information to the authentication plugin whose - <em>getScheme</em> matches the scheme passed by the client. The - implementor of <em>handleAuthentication</em> will usually return - an error if it determines that the information is bad, or it will associate information - with the connection using <em>cnxn.getAuthInfo().add(new Id(getScheme(), data))</em>. - </p> -<p>The authentication plugin is involved in both setting and using ACLs. When an - ACL is set for a znode, the ZooKeeper server will pass the id part of the entry to - the <em>isValid(String id)</em> method. It is up to the plugin to verify - that the id has a correct form. For example, <em>ip:172.16.0.0/16</em> - is a valid id, but <em>ip:host.com</em> is not. If the new ACL includes - an "auth" entry, <em>isAuthenticated</em> is used to see if the - authentication information for this scheme that is assocatied with the connection - should be added to the ACL. Some schemes - should not be included in auth. For example, the IP address of the client is not - considered as an id that should be added to the ACL if auth is specified.</p> -<p>ZooKeeper invokes - <em>matches(String id, String aclExpr)</em> when checking an ACL. It - needs to match authentication information of the client against the relevant ACL - entries. To find the entries which apply to the client, the ZooKeeper server will - find the scheme of each entry and if there is authentication information - from that client for that scheme, <em>matches(String id, String aclExpr)</em> - will be called with <em>id</em> set to the authentication information - that was previously added to the connection by <em>handleAuthentication</em> and - <em>aclExpr</em> set to the id of the ACL entry. The authentication plugin - uses its own logic and matching scheme to determine if <em>id</em> is included - in <em>aclExpr</em>. - </p> -<p>There are two built in authentication plugins: <em>ip</em> and - <em>digest</em>. Additional plugins can adding using system properties. At - startup the ZooKeeper server will look for system properties that start with - "zookeeper.authProvider." and interpret the value of those properties as the class name - of an authentication plugin. These properties can be set using the - <em>-Dzookeeeper.authProvider.X=com.f.MyAuth</em> or adding entries such as - the following in the server configuration file:</p> -<pre class="code"> -authProvider.1=com.f.MyAuth -authProvider.2=com.f.MyAuth2 - </pre> -<p>Care should be taking to ensure that the suffix on the property is unique. If there are - duplicates such as <em>-Dzookeeeper.authProvider.X=com.f.MyAuth -Dzookeeper.authProvider.X=com.f.MyAuth2</em>, - only one will be used. Also all servers must have the same plugins defined, otherwise clients using - the authentication schemes provided by the plugins will have problems connecting to some servers. - </p> -</div> - - -<a name="ch_zkGuarantees"></a> -<h2 class="h3">Consistency Guarantees</h2> -<div class="section"> -<p>ZooKeeper is a high performance, scalable service. Both reads and - write operations are designed to be fast, though reads are faster than - writes. The reason for this is that in the case of reads, ZooKeeper can - serve older data, which in turn is due to ZooKeeper's consistency - guarantees:</p> -<dl> - -<dt> -<term>Sequential Consistency</term> -</dt> -<dd> -<p>Updates from a client will be applied in the order that they - were sent.</p> -</dd> - - -<dt> -<term>Atomicity</term> -</dt> -<dd> -<p>Updates either succeed or fail -- there are no partial - results.</p> -</dd> - - -<dt> -<term>Single System Image</term> -</dt> -<dd> -<p>A client will see the same view of the service regardless of - the server that it connects to.</p> -</dd> - - -<dt> -<term>Reliability</term> -</dt> -<dd> -<p>Once an update has been applied, it will persist from that - time forward until a client overwrites the update. This guarantee - has two corollaries:</p> -<ol> - -<li> - -<p>If a client gets a successful return code, the update will - have been applied. On some failures (communication errors, - timeouts, etc) the client will not know if the update has - applied or not. We take steps to minimize the failures, but the - guarantee is only present with successful return codes. - (This is called the <em>monotonicity condition</em> in Paxos.)</p> - -</li> - - -<li> - -<p>Any updates that are seen by the client, through a read - request or successful update, will never be rolled back when - recovering from server failures.</p> - -</li> - -</ol> -</dd> - - -<dt> -<term>Timeliness</term> -</dt> -<dd> -<p>The clients view of the system is guaranteed to be up-to-date - within a certain time bound (on the order of tens of seconds). - Either system changes will be seen by a client within this bound, or - the client will detect a service outage.</p> -</dd> - -</dl> -<p>Using these consistency guarantees it is easy to build higher level - functions such as leader election, barriers, queues, and read/write - revocable locks solely at the ZooKeeper client (no additions needed to - ZooKeeper). See <a href="recipes.html">Recipes and Solutions</a> - for more details.</p> -<div class="note"> -<div class="label">Note</div> -<div class="content"> - -<p>Sometimes developers mistakenly assume one other guarantee that - ZooKeeper does <em>not</em> in fact make. This is:</p> - - -<dl> - -<dt> -<term>Simultaneously Consistent Cross-Client Views</term> -</dt> -<dd> -<p>ZooKeeper does not guarantee that at every instance in - time, two different clients will have identical views of - ZooKeeper data. Due to factors like network delays, one client - may perform an update before another client gets notified of the - change. Consider the scenario of two clients, A and B. If client - A sets the value of a znode /a from 0 to 1, then tells client B - to read /a, client B may read the old value of 0, depending on - which server it is connected to. If it - is important that Client A and Client B read the same value, - Client B should should call the <strong>sync()</strong> method from the ZooKeeper API - method before it performs its read.</p> -<p>So, ZooKeeper by itself doesn't guarantee that changes occur - synchronously across all servers, but ZooKeeper - primitives can be used to construct higher level functions that - provide useful client synchronization. (For more information, - see the <a href="recipes.html">ZooKeeper Recipes</a>. - <em>[tbd:..]</em>).</p> -</dd> - -</dl> - -</div> -</div> -</div> - - -<a name="ch_bindings"></a> -<h2 class="h3">Bindings</h2> -<div class="section"> -<p>The ZooKeeper client libraries come in two languages: Java and C. - The following sections describe these.</p> -<a name="Java+Binding"></a> -<h3 class="h4">Java Binding</h3> -<p>There are two packages that make up the ZooKeeper Java binding: - <strong>org.apache.zookeeper</strong> and <strong>org.apache.zookeeper.data</strong>. The rest of the - packages that make up ZooKeeper are used internally or are part of the - server implementation. The <strong>org.apache.zookeeper.data</strong> package is made up of - generated classes that are used simply as containers.</p> -<p>The main class used by a ZooKeeper Java client is the <strong>ZooKeeper</strong> class. Its two constructors differ only - by an optional session id and password. ZooKeeper supports session - recovery accross instances of a process. A Java program may save its - session id and password to stable storage, restart, and recover the - session that was used by the earlier instance of the program.</p> -<p>When a ZooKeeper object is created, two threads are created as - well: an IO thread and an event thread. All IO happens on the IO thread - (using Java NIO). All event callbacks happen on the event thread. - Session maintenance such as reconnecting to ZooKeeper servers and - maintaining heartbeat is done on the IO thread. Responses for - synchronous methods are also processed in the IO thread. All responses - to asynchronous methods and watch events are processed on the event - thread. There are a few things to notice that result from this - design:</p> -<ul> - -<li> - -<p>All completions for asynchronous calls and watcher callbacks - will be made in order, one at a time. The caller can do any - processing they wish, but no other callbacks will be processed - during that time.</p> - -</li> - - -<li> - -<p>Callbacks do not block the processing of the IO thread or the - processing of the synchronous calls.</p> - -</li> - - -<li> - -<p>Synchronous calls may not return in the correct order. For - example, assume a client does the following processing: issues an - asynchronous read of node <strong>/a</strong> with - <em>watch</em> set to true, and then in the completion - callback of the read it does a synchronous read of <strong>/a</strong>. (Maybe not good practice, but not illegal - either, and it makes for a simple example.)</p> - - -<p>Note that if there is a change to <strong>/a</strong> between the asynchronous read and the - synchronous read, the client library will receive the watch event - saying <strong>/a</strong> changed before the - response for the synchronous read, but because the completion - callback is blocking the event queue, the synchronous read will - return with the new value of <strong>/a</strong> - before the watch event is processed.</p> - -</li> - -</ul> -<p>Finally, the rules associated with shutdown are straightforward: - once a ZooKeeper object is closed or receives a fatal event - (SESSION_EXPIRED and AUTH_FAILED), the ZooKeeper object becomes invalid. - On a close, the two threads shut down and any further access on zookeeper - handle is undefined behavior and should be avoided. </p> -<a name="C+Binding"></a> -<h3 class="h4">C Binding</h3> -<p>The C binding has a single-threaded and multi-threaded library. - The multi-threaded library is easiest to use and is most similar to the - Java API. This library will create an IO thread and an event dispatch - thread for handling connection maintenance and callbacks. The - single-threaded library allows ZooKeeper to be used in event driven - applications by exposing the event loop used in the multi-threaded - library.</p> -<p>The package includes two shared libraries: zookeeper_st and - zookeeper_mt. The former only provides the asynchronous APIs and - callbacks for integrating into the application's event loop. The only - reason this library exists is to support the platforms were a - <em>pthread</em> library is not available or is unstable - (i.e. FreeBSD 4.x). In all other cases, application developers should - link with zookeeper_mt, as it includes support for both Sync and Async - API.</p> -<a name="Installation"></a> -<h4>Installation</h4> -<p>If you're building the client from a check-out from the Apache - repository, follow the steps outlined below. If you're building from a - project source package downloaded from apache, skip to step <strong>3</strong>.</p> -<ol> - -<li> - -<p>Run <span class="codefrag command">ant compile_jute</span> from the ZooKeeper - top level directory (<span class="codefrag filename">.../trunk</span>). - This will create a directory named "generated" under - <span class="codefrag filename">.../trunk/src/c</span>.</p> - -</li> - - -<li> - -<p>Change directory to the<span class="codefrag filename">.../trunk/src/c</span> - and run <span class="codefrag command">autoreconf -if</span> to bootstrap <strong>autoconf</strong>, <strong>automake</strong> and <strong>libtool</strong>. Make sure you have <strong>autoconf version 2.59</strong> or greater installed. - Skip to step<strong> 4</strong>.</p> - -</li> - - -<li> - -<p>If you are building from a project source package, - unzip/untar the source tarball and cd to the<span class="codefrag filename"> - zookeeper-x.x.x/src/c</span> directory.</p> - -</li> - - -<li> - -<p>Run <span class="codefrag command">./configure <your-options></span> to - generate the makefile. Here are some of options the <strong>configure</strong> utility supports that can be - useful in this step:</p> - - -<ul> - -<li> - -<p> -<span class="codefrag command">--enable-debug</span> -</p> - - -<p>Enables optimization and enables debug info compiler - options. (Disabled by default.)</p> - -</li> - - -<li> - -<p> -<span class="codefrag command">--without-syncapi </span> -</p> - - -<p>Disables Sync API support; zookeeper_mt library won't be - built. (Enabled by default.)</p> - -</li> - - -<li> - -<p> -<span class="codefrag command">--disable-static </span> -</p> - - -<p>Do not build static libraries. (Enabled by - default.)</p> - -</li> - - -<li> - -<p> -<span class="codefrag command">--disable-shared</span> -</p> - - -<p>Do not build shared libraries. (Enabled by - default.)</p> - -</li> - -</ul> - - -<div class="note"> -<div class="label">Note</div> -<div class="content"> - -<p>See INSTALL for general information about running - <strong>configure</strong>.</p> - -</div> -</div> - -</li> - - -<li> - -<p>Run <span class="codefrag command">make</span> or <span class="codefrag command">make - install</span> to build the libraries and install them.</p> - -</li> - - -<li> - -<p>To generate doxygen documentation for the ZooKeeper API, run - <span class="codefrag command">make doxygen-doc</span>. All documentation will be - placed in a new subfolder named docs. By default, this command - only generates HTML. For information on other document formats, - run <span class="codefrag command">./configure --help</span> -</p> - -</li> - -</ol> -<a name="Building+Your+Own+C+Client"></a> -<h4>Building Your Own C Client</h4> -<p>In order to be able to use the ZooKeeper API in your application - you have to remember to</p> -<ol> - -<li> - -<p>Include ZooKeeper header: #include - <zookeeper/zookeeper.h></p> - -</li> - - -<li> - -<p>If you are building a multithreaded client, compile with - -DTHREADED compiler flag to enable the multi-threaded version of - the library, and then link against against the - <em>zookeeper_mt</em> library. If you are building a - single-threaded client, do not compile with -DTHREADED, and be - sure to link against the<em> zookeeper_st - </em>library.</p> - -</li> - -</ol> -<div class="note"> -<div class="label">Note</div> -<div class="content"> -<p> - See <span class="codefrag filename">.../trunk/src/c/src/cli.c</span> - for an example of a C client implementation</p> - -</div> -</div> -</div> - - -<a name="ch_guideToZkOperations"></a> -<h2 class="h3">Building Blocks: A Guide to ZooKeeper Operations</h2> -<div class="section"> -<p>This section surveys all the operations a developer can perform - against a ZooKeeper server. It is lower level information than the earlier - concepts chapters in this manual, but higher level than the ZooKeeper API - Reference. It covers these topics:</p> -<ul> - -<li> - -<p> -<a href="#sc_connectingToZk">Connecting to ZooKeeper</a> -</p> - -</li> - -</ul> -<a name="sc_errorsZk"></a> -<h3 class="h4">Handling Errors</h3> -<p>Both the Java and C client bindings may report errors. The Java client binding does so by throwing KeeperException, calling code() on the exception will return the specific error code. The C client binding returns an error code as defined in the enum ZOO_ERRORS. API callbacks indicate result code for both language bindings. See the API documentation (javadoc for Java, doxygen for C) for full details on the possible errors and their meaning.</p> -<a name="sc_connectingToZk"></a> -<h3 class="h4">Connecting to ZooKeeper</h3> -<p></p> -<a name="sc_readOps"></a> -<h3 class="h4">Read Operations</h3> -<p></p> -<a name="sc_writeOps"></a> -<h3 class="h4">Write Operations</h3> -<p></p> -<a name="sc_handlingWatches"></a> -<h3 class="h4">Handling Watches</h3> -<p></p> -<a name="sc_miscOps"></a> -<h3 class="h4">Miscelleaneous ZooKeeper Operations</h3> -<p></p> -</div> - - -<a name="ch_programStructureWithExample"></a> -<h2 class="h3">Program Structure, with Simple Example</h2> -<div class="section"> -<p> -<em>[tbd]</em> -</p> -</div> - - -<a name="ch_gotchas"></a> -<h2 class="h3">Gotchas: Common Problems and Troubleshooting</h2> -<div class="section"> -<p>So now you know ZooKeeper. It's fast, simple, your application - works, but wait ... something's wrong. Here are some pitfalls that - ZooKeeper users fall into:</p> -<ol> - -<li> - -<p>If you are using watches, you must look for the connected watch - event. When a ZooKeeper client disconnects from a server, you will - not receive notification of changes until reconnected. If you are - watching for a znode to come into existence, you will miss the event - if the znode is created and deleted while you are disconnected.</p> - -</li> - - -<li> - -<p>You must test ZooKeeper server failures. The ZooKeeper service - can survive failures as long as a majority of servers are active. The - question to ask is: can your application handle it? In the real world - a client's connection to ZooKeeper can break. (ZooKeeper server - failures and network partitions are common reasons for connection - loss.) The ZooKeeper client library takes care of recovering your - connection and letting you know what happened, but you must make sure - that you recover your state and any outstanding requests that failed. - Find out if you got it right in the test lab, not in production - test - with a ZooKeeper service made up of a several of servers and subject - them to reboots.</p> - -</li> - - -<li> - -<p>The list of ZooKeeper servers used by the client must match the - list of ZooKeeper servers that each ZooKeeper server has. Things can - work, although not optimally, if the client list is a subset of the - real list of ZooKeeper servers, but not if the client lists ZooKeeper - servers not in the ZooKeeper cluster.</p> - -</li> - - -<li> - -<p>Be careful where you put that transaction log. The most - performance-critical part of ZooKeeper is the transaction log. - ZooKeeper must sync transactions to media before it returns a - response. A dedicated transaction log device is key to consistent good - performance. Putting the log on a busy device will adversely effect - performance. If you only have one storage device, put trace files on - NFS and increase the snapshotCount; it doesn't eliminate the problem, - but it can mitigate it.</p> - -</li> - - -<li> - -<p>Set your Java max heap size correctly. It is very important to - <em>avoid swapping.</em> Going to disk unnecessarily will - almost certainly degrade your performance unacceptably. Remember, in - ZooKeeper, everything is ordered, so if one request hits the disk, all - other queued requests hit the disk.</p> - - -<p>To avoid swapping, try to set the heapsize to the amount of - physical memory you have, minus the amount needed by the OS and cache. - The best way to determine an optimal heap size for your configurations - is to <em>run load tests</em>. If for some reason you - can't, be conservative in your estimates and choose a number well - below the limit that would cause your machine to swap. For example, on - a 4G machine, a 3G heap is a conservative estimate to start - with.</p> - -</li> - -</ol> -</div> - - -<a name="apx_linksToOtherInfo"></a> -<appendix id="apx_linksToOtherInfo"> - -<title>Links to Other Information</title> - - -<p>Outside the formal documentation, there're several other sources of - information for ZooKeeper developers.</p> - - -<dl> - -<dt> -<term>ZooKeeper Whitepaper <em>[tbd: find url]</em> -</term> -</dt> -<dd> -<p>The definitive discussion of ZooKeeper design and performance, - by Yahoo! Research</p> -</dd> - - -<dt> -<term>API Reference <em>[tbd: find url]</em> -</term> -</dt> -<dd> -<p>The complete reference to the ZooKeeper API</p> -</dd> - - -<dt> -<term> -<a href="http://us.dl1.yimg.com/download.yahoo.com/dl/ydn/zookeeper.m4v";>ZooKeeper - Talk at the Hadoup Summit 2008</a> -</term> -</dt> -<dd> -<p>A video introduction to ZooKeeper, by Benjamin Reed of Yahoo! - Research</p> -</dd> - - -<dt> -<term> -<a href="https://cwiki.apache.org/confluence/display/ZOOKEEPER/Tutorial";>Barrier and - Queue Tutorial</a> -</term> -</dt> -<dd> -<p>The excellent Java tutorial by Flavio Junqueira, implementing - simple barriers and producer-consumer queues using ZooKeeper.</p> -</dd> - - -<dt> -<term> -<a href="https://cwiki.apache.org/confluence/display/ZOOKEEPER/ZooKeeperArticles";>ZooKeeper - - A Reliable, Scalable Distributed Coordination System</a> -</term> -</dt> -<dd> -<p>An article by Todd Hoff (07/15/2008)</p> -</dd> - - -<dt> -<term> -<a href="recipes.html">ZooKeeper Recipes</a> -</term> -</dt> -<dd> -<p>Pseudo-level discussion of the implementation of various - synchronization solutions with ZooKeeper: Event Handles, Queues, - Locks, and Two-phase Commits.</p> -</dd> - - -<dt> -<term> -<em>[tbd]</em> -</term> -</dt> -<dd> -<p>Any other good sources anyone can think of...</p> -</dd> - -</dl> - -</appendix> - -<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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You can use the ZooKeeperMain class to setup quotas. - ZooKeeper prints <em>WARN</em> messages if users exceed the quota assigned to them. The messages - are printed in the log of the ZooKeeper. - </p> -<p> -<span class="codefrag computeroutput">$ bin/zkCli.sh -server host:port</span> -</p> -<p> The above command gives you a command line option of using quotas.</p> -<a name="Setting+Quotas"></a> -<h3 class="h4">Setting Quotas</h3> -<p>You can use - <em>setquota</em> to set a quota on a ZooKeeper node. It has an option of setting quota with - -n (for namespace) - and -b (for bytes). </p> -<p> The ZooKeeper quota are stored in ZooKeeper itself in /zookeeper/quota. To disable other people from - changing the quota's set the ACL for /zookeeper/quota such that only admins are able to read and write to it. - </p> -<a name="Listing+Quotas"></a> -<h3 class="h4">Listing Quotas</h3> -<p> You can use - <em>listquota</em> to list a quota on a ZooKeeper node. - </p> -<a name="Deleting+Quotas"></a> -<h3 class="h4"> Deleting Quotas</h3> -<p> You can use - <em>delquota</em> to delete quota on a ZooKeeper node. - </p> -</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/zookeeperQuotas.pdf ---------------------------------------------------------------------- diff --git a/docs/zookeeperQuotas.pdf b/docs/zookeeperQuotas.pdf deleted file mode 100644 index 1455fb9..0000000 Binary files a/docs/zookeeperQuotas.pdf and /dev/null differ http://git-wip-us.apache.org/repos/asf/zookeeper/blob/cf24deb2/docs/zookeeperReconfig.pdf ---------------------------------------------------------------------- diff --git a/docs/zookeeperReconfig.pdf b/docs/zookeeperReconfig.pdf deleted file mode 100644 index fbf2237..0000000 Binary files a/docs/zookeeperReconfig.pdf and /dev/null differ
