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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 detailed information about creating + distributed applications that use ZooKeeper. It discusses the basic + operations ZooKeeper supports, and how these can be used to build + higher-level abstractions. It contains solutions to common tasks, a + troubleshooting guide, and links to other information.</para> + + <para>$Revision: 1.14 $ $Date: 2008/09/19 05:31:45 $</para> + </abstract> + </articleinfo> + + <section id="_introduction"> + <title>Introduction</title> + + <para>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.</para> + + <para>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:</para> + + <itemizedlist> + <listitem> + <para><xref linkend="ch_zkDataModel" /></para> + </listitem> + + <listitem> + <para><xref linkend="ch_zkSessions" /></para> + </listitem> + + <listitem> + <para><xref linkend="ch_zkWatches" /></para> + </listitem> + + <listitem> + <para><xref linkend="ch_zkGuarantees" /></para> + </listitem> + </itemizedlist> + + <para>The next four sections provide practical programming + information. These are:</para> + + <itemizedlist> + <listitem> + <para><xref linkend="ch_guideToZkOperations" /></para> + </listitem> + + <listitem> + <para><xref linkend="ch_bindings" /></para> + </listitem> + + <listitem> + <para><xref linkend="ch_programStructureWithExample" /> + <emphasis>[tbd]</emphasis></para> + </listitem> + + <listitem> + <para><xref linkend="ch_gotchas" /></para> + </listitem> + </itemizedlist> + + <para>The book concludes with an <ulink + url="#apx_linksToOtherInfo">appendix</ulink> containing links to other + useful, ZooKeeper-related information.</para> + + <para>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 <ulink url="#ch_zkDataModel">ZooKeeper Data Model</ulink> and <ulink + url="#ch_guideToZkOperations">ZooKeeper Basic Operations</ulink>. Also, + the <ulink url="#ch_programStructureWithExample">Simple Programmming + Example</ulink> <emphasis>[tbd]</emphasis> is helpful for understanding the basic + structure of a ZooKeeper client application.</para> + </section> + + <section id="ch_zkDataModel"> + <title>The ZooKeeper Data Model</title> + + <para>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:</para> + + <itemizedlist> + <listitem> + <para>The null character (\u0000) cannot be part of a path name. (This + causes problems with the C binding.)</para> + </listitem> + + <listitem> + <para>The following characters can't be used because they don't + display well, or render in confusing ways: \u0001 - \u0019 and \u007F + - \u009F.</para> + </listitem> + + <listitem> + <para>The following characters are not allowed: \ud800 -uF8FFF, + \uFFF0 - uFFFF.</para> + </listitem> + + <listitem> + <para>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".</para> + </listitem> + + <listitem> + <para>The token "zookeeper" is reserved.</para> + </listitem> + </itemizedlist> + + <section id="sc_zkDataModel_znodes"> + <title>ZNodes</title> + + <para>Every node in a ZooKeeper tree is referred to as a + <emphasis>znode</emphasis>. 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... )<emphasis>[tbd...]</emphasis></para> + + <note> + <para>In distributed application engineering, the word + <emphasis>node</emphasis> can refer to a generic host machine, a + server, a member of an ensemble, a client process, etc. In the ZooKeeper + documentation, <emphasis>znodes</emphasis> refer to the data nodes. + <emphasis>Servers</emphasis> refer to machines that make up the + ZooKeeper service; <emphasis>quorum peers</emphasis> refer to the + servers that make up an ensemble; client refers to any host or process + which uses a ZooKeeper service.</para> + </note> + + <para> A znode is the main abstraction a programmer needs to be aware of. Znodes have + several characteristics that are worth mentioning here.</para> + + <section id="sc_zkDataMode_watches"> + <title>Watches</title> + + <para>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 + <ulink url="#ch_zkWatches">ZooKeeper Watches</ulink>.</para> + </section> + + <section> + <title>Data Access</title> + + <para>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.</para> + + <para>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.</para> + </section> + + <section> + <title>Ephemeral Nodes</title> + + <para>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.</para> + </section> + + <section> + <title>Sequence Nodes -- Unique Naming</title> + + <para>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 + <ulink url="recipes.html#sc_recipes_Queues">Queue + Recipe</ulink> 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").</para> + </section> + </section> + + <section id="sc_timeInZk"> + <title>Time in ZooKeeper</title> + + <para>ZooKeeper tracks time multiple ways:</para> + + <itemizedlist> + <listitem> + <para><emphasis role="bold">Zxid</emphasis></para> + + <para>Every change to the ZooKeeper state receives a stamp in the + form of a <emphasis>zxid</emphasis> (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.</para> + </listitem> + + <listitem> + <para><emphasis role="bold">Version numbers</emphasis></para> + + <para>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).</para> + </listitem> + + <listitem> + <para><emphasis role="bold">Ticks</emphasis></para> + + <para>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.</para> + </listitem> + + <listitem> + <para><emphasis role="bold">Real time</emphasis></para> + + <para>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.</para> + </listitem> + </itemizedlist> + </section> + + <section id="sc_zkStatStructure"> + <title>ZooKeeper Stat Structure</title> + + <para>The Stat structure for each znode in ZooKeeper is made up of the + following fields:</para> + + <itemizedlist> + <listitem> + <para><emphasis role="bold">czxid</emphasis></para> + + <para>The zxid of the change that caused this znode to be + created.</para> + </listitem> + + <listitem> + <para><emphasis role="bold">mzxid</emphasis></para> + + <para>The zxid of the change that last modified this znode.</para> + </listitem> + + <listitem> + <para><emphasis role="bold">pzxid</emphasis></para> + + <para>The zxid of the change that last modified children of this znode.</para> + </listitem> + + <listitem> + <para><emphasis role="bold">ctime</emphasis></para> + + <para>The time in milliseconds from epoch when this znode was + created.</para> + </listitem> + + <listitem> + <para><emphasis role="bold">mtime</emphasis></para> + + <para>The time in milliseconds from epoch when this znode was last + modified.</para> + </listitem> + + <listitem> + <para><emphasis role="bold">version</emphasis></para> + + <para>The number of changes to the data of this znode.</para> + </listitem> + + <listitem> + <para><emphasis role="bold">cversion</emphasis></para> + + <para>The number of changes to the children of this znode.</para> + </listitem> + + <listitem> + <para><emphasis role="bold">aversion</emphasis></para> + + <para>The number of changes to the ACL of this znode.</para> + </listitem> + + <listitem> + <para><emphasis role="bold">ephemeralOwner</emphasis></para> + + <para>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.</para> + </listitem> + + <listitem> + <para><emphasis role="bold">dataLength</emphasis></para> + + <para>The length of the data field of this znode.</para> + </listitem> + + <listitem> + <para><emphasis role="bold">numChildren</emphasis></para> + + <para>The number of children of this znode.</para> + </listitem> + + </itemizedlist> + </section> + </section> + + <section id="ch_zkSessions"> + <title>ZooKeeper Sessions</title> + + <para>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:</para> + + <mediaobject id="fg_states" > + <imageobject> + <imagedata fileref="images/state_dia.jpg"/> + </imageobject> + </mediaobject> + + <para>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.</para> + + <para> <emphasis role="bold">Added in 3.2.0</emphasis>: 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.</para> + + <para>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.</para> + + <para>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.</para> + + <para>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).</para> + + <para>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.</para> + + <para>Example state transitions for an expired session as seen by + the expired session's watcher:</para> + + <orderedlist> + <listitem><para>'connected' : session is established and client + is communicating with cluster (client/server communication is + operating properly)</para></listitem> + <listitem><para>.... client is partitioned from the + cluster</para></listitem> + <listitem><para>'disconnected' : client has lost connectivity + with the cluster</para></listitem> + <listitem><para>.... time elapses, after 'timeout' period the + cluster expires the session, nothing is seen by client as it is + disconnected from cluster</para></listitem> + <listitem><para>.... time elapses, the client regains network + level connectivity with the cluster</para></listitem> + <listitem><para>'expired' : eventually the client reconnects to + the cluster, it is then notified of the + expiration</para></listitem> + </orderedlist> + + <para>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.</para> + + <para>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.</para> + + <para> + 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: + </para> + + <orderedlist> + <listitem><para>The application calls an operation on a session that is no + longer alive/valid</para></listitem> + <listitem><para>The ZooKeeper client disconnects from a server when there + are pending operations to that server, i.e., there is a pending asynchronous call. + </para></listitem> + </orderedlist> + + <para> <emphasis role="bold">Added in 3.2.0 -- SessionMovedException</emphasis>. 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).</para> + + </section> + + <section id="ch_zkWatches"> + <title>ZooKeeper Watches</title> + + <para>All of the read operations in ZooKeeper - <emphasis + role="bold">getData()</emphasis>, <emphasis + role="bold">getChildren()</emphasis>, and <emphasis + role="bold">exists()</emphasis> - 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:</para> + + <itemizedlist> + <listitem> + <para><emphasis role="bold">One-time trigger</emphasis></para> + + <para>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.</para> + </listitem> + + <listitem> + <para><emphasis role="bold">Sent to the client</emphasis></para> + + <para>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.</para> + </listitem> + + <listitem> + <para><emphasis role="bold">The data for which the watch was + set</emphasis></para> + + <para>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.</para> + </listitem> + </itemizedlist> + + <para>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.</para> + + <section id="sc_WatchSemantics"> + <title>Semantics of Watches</title> + + <para> 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: + </para> + + <itemizedlist> + <listitem> + <para><emphasis role="bold">Created event:</emphasis></para> + <para>Enabled with a call to exists.</para> + </listitem> + + <listitem> + <para><emphasis role="bold">Deleted event:</emphasis></para> + <para>Enabled with a call to exists, getData, and getChildren.</para> + </listitem> + + <listitem> + <para><emphasis role="bold">Changed event:</emphasis></para> + <para>Enabled with a call to exists and getData.</para> + </listitem> + + <listitem> + <para><emphasis role="bold">Child event:</emphasis></para> + <para>Enabled with a call to getChildren.</para> + </listitem> + </itemizedlist> + </section> + + <section id="sc_WatchGuarantees"> + <title>What ZooKeeper Guarantees about Watches</title> + + <para>With regard to watches, ZooKeeper maintains these + guarantees:</para> + + <itemizedlist> + <listitem> + <para>Watches are ordered with respect to other events, other + watches, and asynchronous replies. The ZooKeeper client libraries + ensures that everything is dispatched in order.</para> + </listitem> + </itemizedlist> + + <itemizedlist> + <listitem> + <para>A client will see a watch event for a znode it is watching + before seeing the new data that corresponds to that znode.</para> + </listitem> + </itemizedlist> + + <itemizedlist> + <listitem> + <para>The order of watch events from ZooKeeper corresponds to the + order of the updates as seen by the ZooKeeper service.</para> + </listitem> + </itemizedlist> + </section> + + <section id="sc_WatchRememberThese"> + <title>Things to Remember about Watches</title> + + <itemizedlist> + <listitem> + <para>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.</para> + </listitem> + </itemizedlist> + + <itemizedlist> + <listitem> + <para>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.)</para> + </listitem> + </itemizedlist> + + <itemizedlist> + <listitem> + <para>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. + </para> + </listitem> + </itemizedlist> + + <itemizedlist> + <listitem> + <para>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.</para> + </listitem> + </itemizedlist> + </section> + </section> + + <section id="sc_ZooKeeperAccessControl"> + <title>ZooKeeper access control using ACLs</title> + + <para>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.</para> + + <para>Note also that an ACL pertains only to a specific znode. In + particular it does not apply to children. For example, if + <emphasis>/app</emphasis> is only readable by ip:172.16.16.1 and + <emphasis>/app/status</emphasis> is world readable, anyone will + be able to read <emphasis>/app/status</emphasis>; ACLs are not + recursive.</para> + + <para>ZooKeeper supports pluggable authentication schemes. Ids are + specified using the form <emphasis>scheme:id</emphasis>, + where <emphasis>scheme</emphasis> is a the authentication scheme + that the id corresponds to. For + example, <emphasis>ip:172.16.16.1</emphasis> is an id for a + host with the address <emphasis>172.16.16.1</emphasis>.</para> + + <para>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 <emphasis>(scheme:expression, + perms)</emphasis>. The format of + the <emphasis>expression</emphasis> is specific to the scheme. For + example, the pair <emphasis>(ip:19.22.0.0/16, READ)</emphasis> + gives the <emphasis>READ</emphasis> permission to any clients with + an IP address that starts with 19.22.</para> + + <section id="sc_ACLPermissions"> + <title>ACL Permissions</title> + + <para>ZooKeeper supports the following permissions:</para> + + <itemizedlist> + <listitem><para><emphasis role="bold">CREATE</emphasis>: you can create a child node</para></listitem> + <listitem><para><emphasis role="bold">READ</emphasis>: you can get data from a node and list its children.</para></listitem> + <listitem><para><emphasis role="bold">WRITE</emphasis>: you can set data for a node</para></listitem> + <listitem><para><emphasis role="bold">DELETE</emphasis>: you can delete a child node</para></listitem> + <listitem><para><emphasis role="bold">ADMIN</emphasis>: you can set permissions</para></listitem> + </itemizedlist> + + <para>The <emphasis>CREATE</emphasis> + and <emphasis>DELETE</emphasis> permissions have been broken out + of the <emphasis>WRITE</emphasis> permission for finer grained + access controls. The cases for <emphasis>CREATE</emphasis> + and <emphasis>DELETE</emphasis> are the following:</para> + + <para>You want A to be able to do a set on a ZooKeeper node, but + not be able to <emphasis>CREATE</emphasis> + or <emphasis>DELETE</emphasis> children.</para> + + <para><emphasis>CREATE</emphasis> + without <emphasis>DELETE</emphasis>: 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.)</para> + + <para>Also, the <emphasis>ADMIN</emphasis> permission is there + since ZooKeeper doesnât have a notion of file owner. In some + sense the <emphasis>ADMIN</emphasis> permission designates the + entity as the owner. ZooKeeper doesnât 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.)</para> + + <section id="sc_BuiltinACLSchemes"> + <title>Builtin ACL Schemes</title> + + <para>ZooKeeeper has the following built in schemes:</para> + + <itemizedlist> + <listitem><para><emphasis role="bold">world</emphasis> has a + single id, <emphasis>anyone</emphasis>, that represents + anyone.</para></listitem> + + <listitem><para><emphasis role="bold">auth</emphasis> doesn't + use any id, represents any authenticated + user.</para></listitem> + + <listitem><para><emphasis role="bold">digest</emphasis> uses + a <emphasis>username:password</emphasis> string to generate + MD5 hash which is then used as an ACL ID + identity. Authentication is done by sending + the <emphasis>username:password</emphasis> in clear text. When + used in the ACL the expression will be + the <emphasis>username:base64</emphasis> + encoded <emphasis>SHA1</emphasis> + password <emphasis>digest</emphasis>.</para> + </listitem> + + <listitem><para><emphasis role="bold">ip</emphasis> uses the + client host IP as an ACL ID identity. The ACL expression is of + the form <emphasis>addr/bits</emphasis> where the most + significant <emphasis>bits</emphasis> + of <emphasis>addr</emphasis> are matched against the most + significant <emphasis>bits</emphasis> of the client host + IP.</para></listitem> + + </itemizedlist> + </section> + + <section> + <title>ZooKeeper C client API</title> + + <para>The following constants are provided by the ZooKeeper C + library:</para> + + <itemizedlist> + <listitem><para><emphasis>const</emphasis> <emphasis>int</emphasis> ZOO_PERM_READ; //can read nodeâs value and list its children</para></listitem> + <listitem><para><emphasis>const</emphasis> <emphasis>int</emphasis> ZOO_PERM_WRITE;// can set the nodeâs value</para></listitem> + <listitem><para><emphasis>const</emphasis> <emphasis>int</emphasis> ZOO_PERM_CREATE; //can create children</para></listitem> + <listitem><para><emphasis>const</emphasis> <emphasis>int</emphasis> ZOO_PERM_DELETE;// can delete children</para></listitem> + <listitem><para><emphasis>const</emphasis> <emphasis>int</emphasis> ZOO_PERM_ADMIN; //can execute set_acl()</para></listitem> + <listitem><para><emphasis>const</emphasis> <emphasis>int</emphasis> ZOO_PERM_ALL;// all of the above flags ORâd together</para></listitem> + </itemizedlist> + + <para>The following are the standard ACL IDs:</para> + + <itemizedlist> + <listitem><para><emphasis>struct</emphasis> Id ZOO_ANYONE_ID_UNSAFE; //(âworldâ,âanyoneâ)</para></listitem> + <listitem><para><emphasis>struct</emphasis> Id ZOO_AUTH_IDS;// (âauthâ,ââ)</para></listitem> + </itemizedlist> + + <para>ZOO_AUTH_IDS empty identity string should be interpreted as âthe identity of the creatorâ.</para> + + <para>ZooKeeper client comes with three standard ACLs:</para> + + <itemizedlist> + <listitem><para><emphasis>struct</emphasis> ACL_vector ZOO_OPEN_ACL_UNSAFE; //(ZOO_PERM_ALL,ZOO_ANYONE_ID_UNSAFE)</para></listitem> + <listitem><para><emphasis>struct</emphasis> ACL_vector ZOO_READ_ACL_UNSAFE;// (ZOO_PERM_READ, ZOO_ANYONE_ID_UNSAFE)</para></listitem> + <listitem><para><emphasis>struct</emphasis> ACL_vector ZOO_CREATOR_ALL_ACL; //(ZOO_PERM_ALL,ZOO_AUTH_IDS)</para></listitem> + </itemizedlist> + + <para>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 â<emphasis>digest</emphasis>â + scheme) before it can create nodes with this ACL.</para> + + <para>The following ZooKeeper operations deal with ACLs:</para> + + <itemizedlist><listitem> + <para><emphasis>int</emphasis> <emphasis>zoo_add_auth</emphasis> + (zhandle_t *zh,<emphasis>const</emphasis> <emphasis>char</emphasis>* + scheme,<emphasis>const</emphasis> <emphasis>char</emphasis>* + cert, <emphasis>int</emphasis> certLen, void_completion_t + completion, <emphasis>const</emphasis> <emphasis>void</emphasis> + *data);</para> + </listitem></itemizedlist> + + <para>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.</para> + + <itemizedlist><listitem> + <para><emphasis>int</emphasis> <emphasis>zoo_create</emphasis> + (zhandle_t *zh, <emphasis>const</emphasis> <emphasis>char</emphasis> + *path, <emphasis>const</emphasis> <emphasis>char</emphasis> + *value,<emphasis>int</emphasis> + valuelen, <emphasis>const</emphasis> <emphasis>struct</emphasis> + ACL_vector *acl, <emphasis>int</emphasis> + flags,<emphasis>char</emphasis> + *realpath, <emphasis>int</emphasis> + max_realpath_len);</para> + </listitem></itemizedlist> + + <para>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.</para> + + <itemizedlist><listitem> + <para><emphasis>int</emphasis> <emphasis>zoo_get_acl</emphasis> + (zhandle_t *zh, <emphasis>const</emphasis> <emphasis>char</emphasis> + *path,<emphasis>struct</emphasis> ACL_vector + *acl, <emphasis>struct</emphasis> Stat *stat);</para> + </listitem></itemizedlist> + + <para>This operation returns a nodeâs ACL info.</para> + + <itemizedlist><listitem> + <para><emphasis>int</emphasis> <emphasis>zoo_set_acl</emphasis> + (zhandle_t *zh, <emphasis>const</emphasis> <emphasis>char</emphasis> + *path, <emphasis>int</emphasis> + version,<emphasis>const</emphasis> <emphasis>struct</emphasis> + ACL_vector *acl);</para> + </listitem></itemizedlist> + + <para>This function replaces nodeâs ACL list with a new one. The + node must have the ADMIN permission set.</para> + + <para>Here is a sample code that makes use of the above APIs to + authenticate itself using the â<emphasis>foo</emphasis>â scheme + and create an ephemeral node â/xyzâ with create-only + permissions.</para> + + <note><para>This is a very simple example which is intended to show + how to interact with ZooKeeper ACLs + specifically. See <filename>.../trunk/src/c/src/cli.c</filename> + for an example of a C client implementation</para> + </note> + + <programlisting> +#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; +} + </programlisting> + </section> + </section> + </section> + + <section id="sc_ZooKeeperPluggableAuthentication"> + <title>Pluggable ZooKeeper authentication</title> + + <para>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.</para> + + <para>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 <<emphasis>idspec, + permissions</emphasis>> pairs. The <emphasis>idspec</emphasis> 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:</para> + + <programlisting> +public interface AuthenticationProvider { + String getScheme(); + KeeperException.Code handleAuthentication(ServerCnxn cnxn, byte authData[]); + boolean isValid(String id); + boolean matches(String id, String aclExpr); + boolean isAuthenticated(); +} + </programlisting> + + <para>The first method <emphasis>getScheme</emphasis> returns the string + that identifies the plugin. Because we support multiple methods of authentication, + an authentication credential or an <emphasis>idspec</emphasis> will always be + prefixed with <emphasis>scheme:</emphasis>. The ZooKeeper server uses the scheme + returned by the authentication plugin to determine which ids the scheme + applies to.</para> + + <para><emphasis>handleAuthentication</emphasis> 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 + <emphasis>getScheme</emphasis> matches the scheme passed by the client. The + implementor of <emphasis>handleAuthentication</emphasis> will usually return + an error if it determines that the information is bad, or it will associate information + with the connection using <emphasis>cnxn.getAuthInfo().add(new Id(getScheme(), data))</emphasis>. + </para> + + <para>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 <emphasis>isValid(String id)</emphasis> method. It is up to the plugin to verify + that the id has a correct form. For example, <emphasis>ip:172.16.0.0/16</emphasis> + is a valid id, but <emphasis>ip:host.com</emphasis> is not. If the new ACL includes + an "auth" entry, <emphasis>isAuthenticated</emphasis> 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.</para> + + <para>ZooKeeper invokes + <emphasis>matches(String id, String aclExpr)</emphasis> 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, <emphasis>matches(String id, String aclExpr)</emphasis> + will be called with <emphasis>id</emphasis> set to the authentication information + that was previously added to the connection by <emphasis>handleAuthentication</emphasis> and + <emphasis>aclExpr</emphasis> set to the id of the ACL entry. The authentication plugin + uses its own logic and matching scheme to determine if <emphasis>id</emphasis> is included + in <emphasis>aclExpr</emphasis>. + </para> + + <para>There are two built in authentication plugins: <emphasis>ip</emphasis> and + <emphasis>digest</emphasis>. 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 + <emphasis>-Dzookeeeper.authProvider.X=com.f.MyAuth</emphasis> or adding entries such as + the following in the server configuration file:</para> + + <programlisting> +authProvider.1=com.f.MyAuth +authProvider.2=com.f.MyAuth2 + </programlisting> + + <para>Care should be taking to ensure that the suffix on the property is unique. If there are + duplicates such as <emphasis>-Dzookeeeper.authProvider.X=com.f.MyAuth -Dzookeeper.authProvider.X=com.f.MyAuth2</emphasis>, + 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. + </para> + </section> + + <section id="ch_zkGuarantees"> + <title>Consistency Guarantees</title> + + <para>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:</para> + + <variablelist> + <varlistentry> + <term>Sequential Consistency</term> + + <listitem> + <para>Updates from a client will be applied in the order that they + were sent.</para> + </listitem> + </varlistentry> + + <varlistentry> + <term>Atomicity</term> + + <listitem> + <para>Updates either succeed or fail -- there are no partial + results.</para> + </listitem> + </varlistentry> + + <varlistentry> + <term>Single System Image</term> + + <listitem> + <para>A client will see the same view of the service regardless of + the server that it connects to.</para> + </listitem> + </varlistentry> + + <varlistentry> + <term>Reliability</term> + + <listitem> + <para>Once an update has been applied, it will persist from that + time forward until a client overwrites the update. This guarantee + has two corollaries:</para> + + <orderedlist> + <listitem> + <para>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 <emphasis>monotonicity condition</emphasis> in Paxos.)</para> + </listitem> + + <listitem> + <para>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.</para> + </listitem> + </orderedlist> + </listitem> + </varlistentry> + + <varlistentry> + <term>Timeliness</term> + + <listitem> + <para>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.</para> + </listitem> + </varlistentry> + </variablelist> + + <para>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 <ulink url="recipes.html">Recipes and Solutions</ulink> + for more details.</para> + + <note> + <para>Sometimes developers mistakenly assume one other guarantee that + ZooKeeper does <emphasis>not</emphasis> in fact make. This is:</para> + + <variablelist> + <varlistentry> + <term>Simultaneously Consistent Cross-Client Views</term> + + <listitem> + <para>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 <emphasis + role="bold">sync()</emphasis> method from the ZooKeeper API + method before it performs its read.</para> + + <para>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 <ulink + url="recipes.html">ZooKeeper Recipes</ulink>. + <emphasis>[tbd:..]</emphasis>).</para> + </listitem> + </varlistentry> + </variablelist> + </note> + </section> + + <section id="ch_bindings"> + <title>Bindings</title> + + <para>The ZooKeeper client libraries come in two languages: Java and C. + The following sections describe these.</para> + + <section> + <title>Java Binding</title> + + <para>There are two packages that make up the ZooKeeper Java binding: + <emphasis role="bold">org.apache.zookeeper</emphasis> and <emphasis + role="bold">org.apache.zookeeper.data</emphasis>. The rest of the + packages that make up ZooKeeper are used internally or are part of the + server implementation. The <emphasis + role="bold">org.apache.zookeeper.data</emphasis> package is made up of + generated classes that are used simply as containers.</para> + + <para>The main class used by a ZooKeeper Java client is the <emphasis + role="bold">ZooKeeper</emphasis> 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.</para> + + <para>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:</para> + + <itemizedlist> + <listitem> + <para>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.</para> + </listitem> + + <listitem> + <para>Callbacks do not block the processing of the IO thread or the + processing of the synchronous calls.</para> + </listitem> + + <listitem> + <para>Synchronous calls may not return in the correct order. For + example, assume a client does the following processing: issues an + asynchronous read of node <emphasis role="bold">/a</emphasis> with + <emphasis>watch</emphasis> set to true, and then in the completion + callback of the read it does a synchronous read of <emphasis + role="bold">/a</emphasis>. (Maybe not good practice, but not illegal + either, and it makes for a simple example.)</para> + + <para>Note that if there is a change to <emphasis + role="bold">/a</emphasis> between the asynchronous read and the + synchronous read, the client library will receive the watch event + saying <emphasis role="bold">/a</emphasis> 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 <emphasis role="bold">/a</emphasis> + before the watch event is processed.</para> + </listitem> + </itemizedlist> + + <para>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. </para> + </section> + + <section> + <title>C Binding</title> + + <para>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.</para> + + <para>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 + <emphasis>pthread</emphasis> 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.</para> + + <section> + <title>Installation</title> + + <para>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 <emphasis + role="bold">3</emphasis>.</para> + + <orderedlist> + <listitem> + <para>Run <command>ant compile_jute</command> from the ZooKeeper + top level directory (<filename>.../trunk</filename>). + This will create a directory named "generated" under + <filename>.../trunk/src/c</filename>.</para> + </listitem> + + <listitem> + <para>Change directory to the<filename>.../trunk/src/c</filename> + and run <command>autoreconf -if</command> to bootstrap <emphasis + role="bold">autoconf</emphasis>, <emphasis + role="bold">automake</emphasis> and <emphasis + role="bold">libtool</emphasis>. Make sure you have <emphasis + role="bold">autoconf version 2.59</emphasis> or greater installed. + Skip to step<emphasis role="bold"> 4</emphasis>.</para> + </listitem> + + <listitem> + <para>If you are building from a project source package, + unzip/untar the source tarball and cd to the<filename> + zookeeper-x.x.x/src/c</filename> directory.</para> + </listitem> + + <listitem> + <para>Run <command>./configure <your-options></command> to + generate the makefile. Here are some of options the <emphasis + role="bold">configure</emphasis> utility supports that can be + useful in this step:</para> + + <itemizedlist> + <listitem> + <para><command>--enable-debug</command></para> + + <para>Enables optimization and enables debug info compiler + options. (Disabled by default.)</para> + </listitem> + + <listitem> + <para><command>--without-syncapi </command></para> + + <para>Disables Sync API support; zookeeper_mt library won't be + built. (Enabled by default.)</para> + </listitem> + + <listitem> + <para><command>--disable-static </command></para> + + <para>Do not build static libraries. (Enabled by + default.)</para> + </listitem> + + <listitem> + <para><command>--disable-shared</command></para> + + <para>Do not build shared libraries. (Enabled by + default.)</para> + </listitem> + </itemizedlist> + + <note> + <para>See INSTALL for general information about running + <emphasis role="bold">configure</emphasis>.</para> + </note> + </listitem> + + <listitem> + <para>Run <command>make</command> or <command>make + install</command> to build the libraries and install them.</para> + </listitem> + + <listitem> + <para>To generate doxygen documentation for the ZooKeeper API, run + <command>make doxygen-doc</command>. 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 <command>./configure --help</command></para> + </listitem> + </orderedlist> + </section> + + <section> + <title>Building Your Own C Client</title> + + <para>In order to be able to use the ZooKeeper API in your application + you have to remember to</para> + + <orderedlist> + <listitem> + <para>Include ZooKeeper header: #include + <zookeeper/zookeeper.h></para> + </listitem> + + <listitem> + <para>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 + <emphasis>zookeeper_mt</emphasis> library. If you are building a + single-threaded client, do not compile with -DTHREADED, and be + sure to link against the<emphasis> zookeeper_st + </emphasis>library.</para> + </listitem> + </orderedlist> + + <note><para> + See <filename>.../trunk/src/c/src/cli.c</filename> + for an example of a C client implementation</para> + </note> + </section> + </section> + </section> + + <section id="ch_guideToZkOperations"> + <title>Building Blocks: A Guide to ZooKeeper Operations</title> + + <para>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:</para> + + <itemizedlist> + <listitem> + <para><xref linkend="sc_connectingToZk" /></para> + </listitem> + </itemizedlist> + + <section id="sc_errorsZk"> + <title>Handling Errors</title> + + <para>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.</para> + </section> + + <section id="sc_connectingToZk"> + <title>Connecting to ZooKeeper</title> + + <para></para> + </section> + + <section id="sc_readOps"> + <title>Read Operations</title> + + <para></para> + </section> + + <section id="sc_writeOps"> + <title>Write Operations</title> + + <para></para> + </section> + + <section id="sc_handlingWatches"> + <title>Handling Watches</title> + + <para></para> + </section> + + <section id="sc_miscOps"> + <title>Miscelleaneous ZooKeeper Operations</title> + <para></para> + </section> + + + </section> + + <section id="ch_programStructureWithExample"> + <title>Program Structure, with Simple Example</title> + + <para><emphasis>[tbd]</emphasis></para> + </section> + + <section id="ch_gotchas"> + <title>Gotchas: Common Problems and Troubleshooting</title> + + <para>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:</para> + + <orderedlist> + <listitem> + <para>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.</para> + </listitem> + + <listitem> + <para>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.</para> + </listitem> + + <listitem> + <para>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.</para> + </listitem> + + <listitem> + <para>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.</para> + </listitem> + + <listitem> + <para>Set your Java max heap size correctly. It is very important to + <emphasis>avoid swapping.</emphasis> 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.</para> + + <para>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 <emphasis>run load tests</emphasis>. 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.</para> + </listitem> + </orderedlist> + </section> + + <appendix id="apx_linksToOtherInfo"> + <title>Links to Other Information</title> + + <para>Outside the formal documentation, there're several other sources of + information for ZooKeeper developers.</para> + + <variablelist> + <varlistentry> + <term>ZooKeeper Whitepaper <emphasis>[tbd: find url]</emphasis></term> + + <listitem> + <para>The definitive discussion of ZooKeeper design and performance, + by Yahoo! Research</para> + </listitem> + </varlistentry> + + <varlistentry> + <term>API Reference <emphasis>[tbd: find url]</emphasis></term> + + <listitem> + <para>The complete reference to the ZooKeeper API</para> + </listitem> + </varlistentry> + + <varlistentry> + <term><ulink + url="http://us.dl1.yimg.com/download.yahoo.com/dl/ydn/zookeeper.m4v";>ZooKeeper + Talk at the Hadoup Summit 2008</ulink></term> + + <listitem> + <para>A video introduction to ZooKeeper, by Benjamin Reed of Yahoo! + Research</para> + </listitem> + </varlistentry> + + <varlistentry> + <term><ulink + url="https://cwiki.apache.org/confluence/display/ZOOKEEPER/Tutorial";>Barrier and + Queue Tutorial</ulink></term> + + <listitem> + <para>The excellent Java tutorial by Flavio Junqueira, implementing + simple barriers and producer-consumer queues using ZooKeeper.</para> + </listitem> + </varlistentry> + + <varlistentry> + <term><ulink + url="https://cwiki.apache.org/confluence/display/ZOOKEEPER/ZooKeeperArticles";>ZooKeeper + - A Reliable, Scalable Distributed Coordination System</ulink></term> + + <listitem> + <para>An article by Todd Hoff (07/15/2008)</para> + </listitem> + </varlistentry> + + <varlistentry> + <term><ulink url="recipes.html">ZooKeeper Recipes</ulink></term> + + <listitem> + <para>Pseudo-level discussion of the implementation of various + synchronization solutions with ZooKeeper: Event Handles, Queues, + Locks, and Two-phase Commits.</para> + </listitem> + </varlistentry> + + <varlistentry> + <term><emphasis>[tbd]</emphasis></term> + + <listitem> + <para>Any other good sources anyone can think of...</para> + </listitem> + </varlistentry> + </variablelist> + </appendix> +</article>
http://git-wip-us.apache.org/repos/asf/zookeeper/blob/c1efa954/zookeeper-docs/src/documentation/content/xdocs/zookeeperQuotas.xml ---------------------------------------------------------------------- diff --git a/zookeeper-docs/src/documentation/content/xdocs/zookeeperQuotas.xml b/zookeeper-docs/src/documentation/content/xdocs/zookeeperQuotas.xml new file mode 100644 index 0000000..7668e6a --- /dev/null +++ b/zookeeper-docs/src/documentation/content/xdocs/zookeeperQuotas.xml @@ -0,0 +1,71 @@ +<?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="bk_Quota"> + <title>ZooKeeper Quota's Guide</title> + <subtitle>A Guide to Deployment and Administration</subtitle> + <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 document contains information about deploying, + administering and mantaining ZooKeeper. It also discusses best + practices and common problems.</para> + </abstract> + </articleinfo> + <section id="zookeeper_quotas"> + <title>Quotas</title> + <para> ZooKeeper has both namespace and bytes quotas. You can use the ZooKeeperMain class to setup quotas. + ZooKeeper prints <emphasis>WARN</emphasis> messages if users exceed the quota assigned to them. The messages + are printed in the log of the ZooKeeper. + </para> + <para><computeroutput>$ bin/zkCli.sh -server host:port</computeroutput></para> + <para> The above command gives you a command line option of using quotas.</para> + <section> + <title>Setting Quotas</title> + <para>You can use + <emphasis>setquota</emphasis> to set a quota on a ZooKeeper node. It has an option of setting quota with + -n (for namespace) + and -b (for bytes). </para> + <para> 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. + </para> + </section> + <section> + <title>Listing Quotas</title> + <para> You can use + <emphasis>listquota</emphasis> to list a quota on a ZooKeeper node. + </para> + </section> + <section> + <title> Deleting Quotas</title> + <para> You can use + <emphasis>delquota</emphasis> to delete quota on a ZooKeeper node. + </para> + </section> + </section> + </article>
