http://git-wip-us.apache.org/repos/asf/hbase/blob/e80b3092/src/main/docbkx/cp.xml ---------------------------------------------------------------------- diff --git a/src/main/docbkx/cp.xml b/src/main/docbkx/cp.xml deleted file mode 100644 index 8624309..0000000 --- a/src/main/docbkx/cp.xml +++ /dev/null @@ -1,431 +0,0 @@ -<?xml version="1.0" encoding="UTF-8"?> -<chapter - version="5.0" - xml:id="cp" - xmlns="http://docbook.org/ns/docbook"; - xmlns:xlink="http://www.w3.org/1999/xlink"; - xmlns:xi="http://www.w3.org/2001/XInclude"; - xmlns:svg="http://www.w3.org/2000/svg"; - xmlns:m="http://www.w3.org/1998/Math/MathML"; - xmlns:html="http://www.w3.org/1999/xhtml"; - xmlns:db="http://docbook.org/ns/docbook";> - <!-- -/** - * Licensed to the Apache Software Foundation (ASF) under one - * or more contributor license agreements. See the NOTICE file - * distributed with this work for additional information - * regarding copyright ownership. The ASF licenses this file - * to you 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. - */ ---> - <title>Apache HBase Coprocessors</title> - <para> HBase coprocessors are modeled after the coprocessors which are part of Google's BigTable - (<link xlink:href="http://www.scribd.com/doc/21631448/Dean-Keynote-Ladis2009"/>, pages - 66-67.). Coprocessors function in a similar way to Linux kernel modules. They provide a way to - run server-level code against locally-stored data. The functionality they provide is very - powerful, but also carries great risk and can have adverse effects on the system, at the level - of the operating system. The information in this chapter is primarily sourced and heavily reused - from Mingjie Lai's blog post at <link - xlink:href="https://blogs.apache.org/hbase/entry/coprocessor_introduction"/>. </para> - - <para> Coprocessors are not designed to be used by end users of HBase, but by HBase developers who - need to add specialized functionality to HBase. One example of the use of coprocessors is - pluggable compaction and scan policies, which are provided as coprocessors in <link - xlink:href="HBASE-6427">HBASE-6427</link>. </para> - - <section> - <title>Coprocessor Framework</title> - <para>The implementation of HBase coprocessors diverges from the BigTable implementation. The - HBase framework provides a library and runtime environment for executing user code within the - HBase region server and master processes. </para> - <para> The framework API is provided in the <link - xlink:href="https://hbase.apache.org/apidocs/org/apache/hadoop/hbase/coprocessor/package-summary.html";>coprocessor</link> - package.</para> - <para>Two different types of coprocessors are provided by the framework, based on their - scope.</para> - <variablelist> - <title>Types of Coprocessors</title> - <varlistentry> - <term>System Coprocessors</term> - <listitem> - <para>System coprocessors are loaded globally on all tables and regions hosted by a region - server.</para> - </listitem> - </varlistentry> - <varlistentry> - <term>Table Coprocessors</term> - <listitem> - <para>You can specify which coprocessors should be loaded on all regions for a table on a - per-table basis.</para> - </listitem> - </varlistentry> - </variablelist> - - <para>The framework provides two different aspects of extensions as well: - <firstterm>observers</firstterm> and <firstterm>endpoints</firstterm>.</para> - <variablelist> - <varlistentry> - <term>Observers</term> - <listitem> - <para>Observers are analogous to triggers in conventional databases. They allow you to - insert user code by overriding upcall methods provided by the coprocessor framework. - Callback functions are executed from core HBase code when events occur. Callbacks are - handled by the framework, and the coprocessor itself only needs to insert the extended - or alternate functionality.</para> - <variablelist> - <title>Provided Observer Interfaces</title> - <varlistentry> - <term><link - xlink:href="https://hbase.apache.org/apidocs/org/apache/hadoop/hbase/coprocessor/RegionObserver.html";>RegionObserver</link></term> - <listitem> - <para>A RegionObserver provides hooks for data manipulation events, such as Get, - Put, Delete, Scan. An instance of a RegionObserver coprocessor exists for each - table region. The scope of the observations a RegionObserver can make is - constrained to that region. </para> - </listitem> - </varlistentry> - <varlistentry> - <term><link - xlink:href="https://hbase.apache.org/apidocs/org/apache/hadoop/hbase/coprocessor/RegionServerObserver.html";>RegionServerObserver</link></term> - <listitem> - <para>A RegionServerObserver provides for operations related to the RegionServer, - such as stopping the RegionServer and performing operations before or after - merges, commits, or rollbacks.</para> - </listitem> - </varlistentry> - <varlistentry> - <term><link - xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/coprocessor/WALObserver.html";>WALObserver</link></term> - <listitem> - <para>A WALObserver provides hooks for operations related to the write-ahead log - (WAL). You can observe or intercept WAL writing and reconstruction events. A - WALObserver runs in the context of WAL processing. A single WALObserver exists on - a single region server.</para> - </listitem> - </varlistentry> - <varlistentry> - <term><link - xlink:href="https://hbase.apache.org/apidocs/org/apache/hadoop/hbase/coprocessor/MasterObserver.html";>MasterObserver</link></term> - <listitem> - <para>A MasterObserver provides hooks for DDL-type operation, such as create, - delete, modify table. The MasterObserver runs within the context of the HBase - master. </para> - </listitem> - </varlistentry> - </variablelist> - <para>More than one observer of a given type can be loaded at once. Multiple observers are - chained to execute sequentially by order of assigned priority. Nothing prevents a - coprocessor implementor from communicating internally among its installed - observers.</para> - <para>An observer of a higher priority can preempt lower-priority observers by throwing an - IOException or a subclass of IOException.</para> - </listitem> - </varlistentry> - <varlistentry> - <term>Endpoints (HBase 0.96.x and later)</term> - <listitem> - <para>The implementation for endpoints changed significantly in HBase 0.96.x due to the - introduction of protocol buffers (protobufs) (<link - xlink:href="https://issues.apache.org/jira/browse/HBASE-5448";>HBASE-5488</link>). If - you created endpoints before 0.96.x, you will need to rewrite them. Endpoints are now - defined and callable as protobuf services, rather than endpoint invocations passed - through as Writable blobs</para> - <para>Dynamic RPC endpoints resemble stored procedures. An endpoint can be invoked at any - time from the client. When it is invoked, it is executed remotely at the target region - or regions, and results of the executions are returned to the client.</para> - <para>The endpoint implementation is installed on the server and is invoked using HBase - RPC. The client library provides convenience methods for invoking these dynamic - interfaces. </para> - <para>An endpoint, like an observer, can communicate with any installed observers. This - allows you to plug new features into HBase without modifying or recompiling HBase - itself.</para> - <itemizedlist> - <title>Steps to Implement an Endpoint</title> - <listitem><para>Define the coprocessor service and related messages in a <filename>.proto</filename> file</para></listitem> - <listitem><para>Run the <command>protoc</command> command to generate the code.</para></listitem> - <listitem><para>Write code to implement the following:</para> - <itemizedlist> - <listitem><para>the generated protobuf Service interface</para></listitem> - <listitem> - <para>the new <link - xlink:href="https://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/HTable.html#coprocessorService(byte[])">org.apache.hadoop.hbase.coprocessor.CoprocessorService</link> - interface (required for the <link - xlink:href="https://hbase.apache.org/apidocs/org/apache/hadoop/hbase/regionserver/RegionCoprocessorHost.html";>RegionCoprocessorHost</link> - to register the exposed service)</para></listitem> - </itemizedlist> - </listitem> - <listitem><para>The client calls the new HTable.coprocessorService() methods to perform the endpoint RPCs.</para></listitem> - </itemizedlist> - - <para>For more information and examples, refer to the API documentation for the <link - xlink:href="https://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/coprocessor/package-summary.html";>coprocessor</link> - package, as well as the included RowCount example in the - <filename>/hbase-examples/src/test/java/org/apache/hadoop/hbase/coprocessor/example/</filename> - of the HBase source.</para> - </listitem> - </varlistentry> - <varlistentry> - <term>Endpoints (HBase 0.94.x and earlier)</term> - <listitem> - <para>Dynamic RPC endpoints resemble stored procedures. An endpoint can be invoked at any - time from the client. When it is invoked, it is executed remotely at the target region - or regions, and results of the executions are returned to the client.</para> - <para>The endpoint implementation is installed on the server and is invoked using HBase - RPC. The client library provides convenience methods for invoking these dynamic - interfaces. </para> - <para>An endpoint, like an observer, can communicate with any installed observers. This - allows you to plug new features into HBase without modifying or recompiling HBase - itself.</para> - <itemizedlist> - <title>Steps to Implement an Endpoint</title> - <listitem> - <bridgehead>Server-Side</bridgehead> - <itemizedlist> - <listitem> - <para>Create new protocol interface which extends CoprocessorProtocol.</para> - </listitem> - <listitem> - <para>Implement the Endpoint interface. The implementation will be loaded into and - executed from the region context.</para> - </listitem> - <listitem> - <para>Extend the abstract class BaseEndpointCoprocessor. This convenience class - hides some internal details that the implementer does not need to be concerned - about, Ë such as coprocessor framework class loading.</para> - </listitem> - </itemizedlist> - </listitem> - <listitem> - <bridgehead>Client-Side</bridgehead> - <para>Endpoint can be invoked by two new HBase client APIs:</para> - <itemizedlist> - <listitem> - <para><code>HTableInterface.coprocessorProxy(Class<T> protocol, byte[] - row)</code> for executing against a single region</para> - </listitem> - <listitem> - <para><code>HTableInterface.coprocessorExec(Class<T> protocol, byte[] - startKey, byte[] endKey, Batch.Call<T,R> callable)</code> for executing - over a range of regions</para> - </listitem> - </itemizedlist> - </listitem> - </itemizedlist> - </listitem> - </varlistentry> - </variablelist> - </section> - - <section> - <title>Examples</title> - <para>An example of an observer is included in - <filename>hbase-examples/src/test/java/org/apache/hadoop/hbase/coprocessor/example/TestZooKeeperScanPolicyObserver.java</filename>. - Several endpoint examples are included in the same directory.</para> - </section> - - - - <section> - <title>Building A Coprocessor</title> - - <para>Before you can build a processor, it must be developed, compiled, and packaged in a JAR - file. The next step is to configure the coprocessor framework to use your coprocessor. You can - load the coprocessor from your HBase configuration, so that the coprocessor starts with HBase, - or you can configure the coprocessor from the HBase shell, as a table attribute, so that it is - loaded dynamically when the table is opened or reopened.</para> - <section> - <title>Load from Configuration</title> - <para> To configure a coprocessor to be loaded when HBase starts, modify the RegionServer's - <filename>hbase-site.xml</filename> and configure one of the following properties, based - on the type of observer you are configuring: </para> - <itemizedlist> - <listitem> - <para><code>hbase.coprocessor.region.classes</code>for RegionObservers and - Endpoints</para> - </listitem> - <listitem> - <para><code>hbase.coprocessor.wal.classes</code>for WALObservers</para> - </listitem> - <listitem> - <para><code>hbase.coprocessor.master.classes</code>for MasterObservers</para> - </listitem> - </itemizedlist> - <example> - <title>Example RegionObserver Configuration</title> - <para>In this example, one RegionObserver is configured for all the HBase tables.</para> - <screen language="xml"><![CDATA[ -<property> - <name>hbase.coprocessor.region.classes</name> - <value>org.apache.hadoop.hbase.coprocessor.AggregateImplementation</value> - </property> ]]> - </screen> - </example> - - <para> If multiple classes are specified for loading, the class names must be comma-separated. - The framework attempts to load all the configured classes using the default class loader. - Therefore, the jar file must reside on the server-side HBase classpath.</para> - - <para>Coprocessors which are loaded in this way will be active on all regions of - all tables. These are the system coprocessor introduced earlier. The first listed - coprocessors will be assigned the priority <literal>Coprocessor.Priority.SYSTEM</literal>. - Each subsequent coprocessor in the list will have its priority value incremented by one - (which reduces its priority, because priorities have the natural sort order of Integers). </para> - <para>When calling out to registered observers, the framework executes their callbacks methods - in the sorted order of their priority. Ties are broken arbitrarily.</para> - </section> - - <section> - <title>Load from the HBase Shell</title> - <para> You can load a coprocessor on a specific table via a table attribute. The following - example will load the <systemitem>FooRegionObserver</systemitem> observer when table - <systemitem>t1</systemitem> is read or re-read. </para> - <example> - <title>Load a Coprocessor On a Table Using HBase Shell</title> - <screen> -hbase(main):005:0> <userinput>alter 't1', METHOD => 'table_att', - 'coprocessor'=>'hdfs:///foo.jar|com.foo.FooRegionObserver|1001|arg1=1,arg2=2'</userinput> -<computeroutput>Updating all regions with the new schema... -1/1 regions updated. -Done. -0 row(s) in 1.0730 seconds</computeroutput> - -hbase(main):006:0> <userinput>describe 't1'</userinput> -<computeroutput>DESCRIPTION ENABLED - {NAME => 't1', coprocessor$1 => 'hdfs:///foo.jar|com.foo.FooRegio false - nObserver|1001|arg1=1,arg2=2', FAMILIES => [{NAME => 'c1', DATA_B - LOCK_ENCODING => 'NONE', BLOOMFILTER => 'NONE', REPLICATION_SCOPE - => '0', VERSIONS => '3', COMPRESSION => 'NONE', MIN_VERSIONS => - '0', TTL => '2147483647', KEEP_DELETED_CELLS => 'false', BLOCKSIZ - E => '65536', IN_MEMORY => 'false', ENCODE_ON_DISK => 'true', BLO - CKCACHE => 'true'}, {NAME => 'f1', DATA_BLOCK_ENCODING => 'NONE', - BLOOMFILTER => 'NONE', REPLICATION_SCOPE => '0', VERSIONS => '3' - , COMPRESSION => 'NONE', MIN_VERSIONS => '0', TTL => '2147483647' - , KEEP_DELETED_CELLS => 'false', BLOCKSIZE => '65536', IN_MEMORY - => 'false', ENCODE_ON_DISK => 'true', BLOCKCACHE => 'true'}]} -1 row(s) in 0.0190 seconds</computeroutput> - </screen> - </example> - - <para>The coprocessor framework will try to read the class information from the coprocessor - table attribute value. The value contains four pieces of information which are separated by - the <literal>|</literal> character.</para> - - <itemizedlist> - <listitem> - <para>File path: The jar file containing the coprocessor implementation must be in a - location where all region servers can read it. You could copy the file onto the local - disk on each region server, but it is recommended to store it in HDFS.</para> - </listitem> - <listitem> - <para>Class name: The full class name of the coprocessor.</para> - </listitem> - <listitem> - <para>Priority: An integer. The framework will determine the execution sequence of all - configured observers registered at the same hook using priorities. This field can be - left blank. In that case the framework will assign a default priority value.</para> - </listitem> - <listitem> - <para>Arguments: This field is passed to the coprocessor implementation.</para> - </listitem> - </itemizedlist> - <example> - <title>Unload a Coprocessor From a Table Using HBase Shell</title> - <screen> -hbase(main):007:0> <userinput>alter 't1', METHOD => 'table_att_unset',</userinput> -hbase(main):008:0* <userinput>NAME => 'coprocessor$1'</userinput> -<computeroutput>Updating all regions with the new schema... -1/1 regions updated. -Done. -0 row(s) in 1.1130 seconds</computeroutput> - -hbase(main):009:0> <userinput>describe 't1'</userinput> -<computeroutput>DESCRIPTION ENABLED - {NAME => 't1', FAMILIES => [{NAME => 'c1', DATA_BLOCK_ENCODING => false - 'NONE', BLOOMFILTER => 'NONE', REPLICATION_SCOPE => '0', VERSION - S => '3', COMPRESSION => 'NONE', MIN_VERSIONS => '0', TTL => '214 - 7483647', KEEP_DELETED_CELLS => 'false', BLOCKSIZE => '65536', IN - _MEMORY => 'false', ENCODE_ON_DISK => 'true', BLOCKCACHE => 'true - '}, {NAME => 'f1', DATA_BLOCK_ENCODING => 'NONE', BLOOMFILTER => - 'NONE', REPLICATION_SCOPE => '0', VERSIONS => '3', COMPRESSION => - 'NONE', MIN_VERSIONS => '0', TTL => '2147483647', KEEP_DELETED_C - ELLS => 'false', BLOCKSIZE => '65536', IN_MEMORY => 'false', ENCO - DE_ON_DISK => 'true', BLOCKCACHE => 'true'}]} -1 row(s) in 0.0180 seconds </computeroutput> - </screen> - </example> - <warning> - <para>There is no guarantee that the framework will load a given coprocessor successfully. - For example, the shell command neither guarantees a jar file exists at a particular - location nor verifies whether the given class is actually contained in the jar file. - </para> - </warning> - </section> - </section> - <section> - <title>Check the Status of a Coprocessor</title> - <para>To check the status of a coprocessor after it has been configured, use the - <command>status</command> HBase Shell command.</para> - <screen> -hbase(main):020:0> <userinput>status 'detailed'</userinput> -<computeroutput>version 0.92-tm-6 -0 regionsInTransition -master coprocessors: [] -1 live servers - localhost:52761 1328082515520 - requestsPerSecond=3, numberOfOnlineRegions=3, usedHeapMB=32, maxHeapMB=995 - -ROOT-,,0 - numberOfStores=1, numberOfStorefiles=1, storefileUncompressedSizeMB=0, storefileSizeMB=0, memstoreSizeMB=0, -storefileIndexSizeMB=0, readRequestsCount=54, writeRequestsCount=1, rootIndexSizeKB=0, totalStaticIndexSizeKB=0, -totalStaticBloomSizeKB=0, totalCompactingKVs=0, currentCompactedKVs=0, compactionProgressPct=NaN, coprocessors=[] - .META.,,1 - numberOfStores=1, numberOfStorefiles=0, storefileUncompressedSizeMB=0, storefileSizeMB=0, memstoreSizeMB=0, -storefileIndexSizeMB=0, readRequestsCount=97, writeRequestsCount=4, rootIndexSizeKB=0, totalStaticIndexSizeKB=0, -totalStaticBloomSizeKB=0, totalCompactingKVs=0, currentCompactedKVs=0, compactionProgressPct=NaN, coprocessors=[] - t1,,1328082575190.c0491168a27620ffe653ec6c04c9b4d1. - numberOfStores=2, numberOfStorefiles=1, storefileUncompressedSizeMB=0, storefileSizeMB=0, memstoreSizeMB=0, -storefileIndexSizeMB=0, readRequestsCount=0, writeRequestsCount=0, rootIndexSizeKB=0, totalStaticIndexSizeKB=0, -totalStaticBloomSizeKB=0, totalCompactingKVs=0, currentCompactedKVs=0, compactionProgressPct=NaN, -coprocessors=[AggregateImplementation] -0 dead servers </computeroutput> - </screen> - </section> - <section> - <title>Monitor Time Spent in Coprocessors</title> - <para>HBase 0.98.5 introduced the ability to monitor some statistics relating to the amount of - time spent executing a given coprocessor. You can see these statistics via the HBase Metrics - framework (see <xref linkend="hbase_metrics"/> or the Web UI for a given Region Server, via - the <guilabel>Coprocessor Metrics</guilabel> tab. These statistics are valuable for debugging - and benchmarking the performance impact of a given coprocessor on your cluster. Tracked - statistics include min, max, average, and 90th, 95th, and 99th percentile. All times are shown - in milliseconds. The statistics are calculated over coprocessor - execution samples recorded during the reporting interval, which is 10 seconds by default. The - metrics sampling rate as described in <xref linkend="hbase_metrics" />.</para> - <figure> - <title>Coprocessor Metrics UI</title> - <mediaobject> - <imageobject> - <imagedata fileref="coprocessor_stats.png" width="100%"/> - </imageobject> - <caption> - <para>The Coprocessor Metrics UI shows statistics about time spent executing a given - coprocessor, including min, max, average, and 90th, 95th, and 99th percentile.</para> - </caption> - </mediaobject> - </figure> - </section> - <section> - <title>Status of Coprocessors in HBase</title> - <para> Coprocessors and the coprocessor framework are evolving rapidly and work is ongoing on - several different JIRAs. </para> - </section> -</chapter>
http://git-wip-us.apache.org/repos/asf/hbase/blob/e80b3092/src/main/docbkx/customization-pdf.xsl ---------------------------------------------------------------------- diff --git a/src/main/docbkx/customization-pdf.xsl b/src/main/docbkx/customization-pdf.xsl deleted file mode 100644 index b21236f..0000000 --- a/src/main/docbkx/customization-pdf.xsl +++ /dev/null @@ -1,129 +0,0 @@ -<?xml version="1.0"?> -<xsl:stylesheet - xmlns:xsl="http://www.w3.org/1999/XSL/Transform"; - version="1.0"> -<!-- -/** - * Licensed to the Apache Software Foundation (ASF) under one - * or more contributor license agreements. See the NOTICE file - * distributed with this work for additional information - * regarding copyright ownership. The ASF licenses this file - * to you under the Apache License, Version 2.0 (the - * "License"); you may not use this file except in compliance - * with the License. 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See the NOTICE file - * distributed with this work for additional information - * regarding copyright ownership. The ASF licenses this file - * to you 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. - */ ---> - - <title>Data Model</title> - <para>In HBase, data is stored in tables, which have rows and columns. This is a terminology - overlap with relational databases (RDBMSs), but this is not a helpful analogy. Instead, it can - be helpful to think of an HBase table as a multi-dimensional map.</para> - <variablelist> - <title>HBase Data Model Terminology</title> - <varlistentry> - <term>Table</term> - <listitem> - <para>An HBase table consists of multiple rows.</para> - </listitem> - </varlistentry> - <varlistentry> - <term>Row</term> - <listitem> - <para>A row in HBase consists of a row key and one or more columns with values associated - with them. Rows are sorted alphabetically by the row key as they are stored. For this - reason, the design of the row key is very important. The goal is to store data in such a - way that related rows are near each other. A common row key pattern is a website domain. - If your row keys are domains, you should probably store them in reverse (org.apache.www, - org.apache.mail, org.apache.jira). This way, all of the Apache domains are near each - other in the table, rather than being spread out based on the first letter of the - subdomain.</para> - </listitem> - </varlistentry> - <varlistentry> - <term>Column</term> - <listitem> - <para>A column in HBase consists of a column family and a column qualifier, which are - delimited by a <literal>:</literal> (colon) character.</para> - </listitem> - </varlistentry> - <varlistentry> - <term>Column Family</term> - <listitem> - <para>Column families physically colocate a set of columns and their values, often for - performance reasons. Each column family has a set of storage properties, such as whether - its values should be cached in memory, how its data is compressed or its row keys are - encoded, and others. Each row in a table has the same column - families, though a given row might not store anything in a given column family.</para> - <para>Column families are specified when you create your table, and influence the way your - data is stored in the underlying filesystem. Therefore, the column families should be - considered carefully during schema design.</para> - </listitem> - </varlistentry> - <varlistentry> - <term>Column Qualifier</term> - <listitem> - <para>A column qualifier is added to a column family to provide the index for a given - piece of data. Given a column family <literal>content</literal>, a column qualifier - might be <literal>content:html</literal>, and another might be - <literal>content:pdf</literal>. Though column families are fixed at table creation, - column qualifiers are mutable and may differ greatly between rows.</para> - </listitem> - </varlistentry> - <varlistentry> - <term>Cell</term> - <listitem> - <para>A cell is a combination of row, column family, and column qualifier, and contains a - value and a timestamp, which represents the value's version.</para> - <para>A cell's value is an uninterpreted array of bytes.</para> - </listitem> - </varlistentry> - <varlistentry> - <term>Timestamp</term> - <listitem> - <para>A timestamp is written alongside each value, and is the identifier for a given - version of a value. By default, the timestamp represents the time on the RegionServer - when the data was written, but you can specify a different timestamp value when you put - data into the cell.</para> - <caution> - <para>Direct manipulation of timestamps is an advanced feature which is only exposed for - special cases that are deeply integrated with HBase, and is discouraged in general. - Encoding a timestamp at the application level is the preferred pattern.</para> - </caution> - <para>You can specify the maximum number of versions of a value that HBase retains, per column - family. When the maximum number of versions is reached, the oldest versions are - eventually deleted. By default, only the newest version is kept.</para> - </listitem> - </varlistentry> - </variablelist> - - <section - xml:id="conceptual.view"> - <title>Conceptual View</title> - <para>You can read a very understandable explanation of the HBase data model in the blog post <link - xlink:href="http://jimbojw.com/wiki/index.php?title=Understanding_Hbase_and_BigTable";>Understanding - HBase and BigTable</link> by Jim R. Wilson. Another good explanation is available in the - PDF <link - xlink:href="http://0b4af6cdc2f0c5998459-c0245c5c937c5dedcca3f1764ecc9b2f.r43.cf2.rackcdn.com/9353-login1210_khurana.pdf";>Introduction - to Basic Schema Design</link> by Amandeep Khurana. It may help to read different - perspectives to get a solid understanding of HBase schema design. The linked articles cover - the same ground as the information in this section.</para> - <para> The following example is a slightly modified form of the one on page 2 of the <link - xlink:href="http://research.google.com/archive/bigtable.html";>BigTable</link> paper. There - is a table called <varname>webtable</varname> that contains two rows - (<literal>com.cnn.www</literal> - and <literal>com.example.www</literal>), three column families named - <varname>contents</varname>, <varname>anchor</varname>, and <varname>people</varname>. In - this example, for the first row (<literal>com.cnn.www</literal>), - <varname>anchor</varname> contains two columns (<varname>anchor:cssnsi.com</varname>, - <varname>anchor:my.look.ca</varname>) and <varname>contents</varname> contains one column - (<varname>contents:html</varname>). This example contains 5 versions of the row with the - row key <literal>com.cnn.www</literal>, and one version of the row with the row key - <literal>com.example.www</literal>. The <varname>contents:html</varname> column qualifier contains the entire - HTML of a given website. Qualifiers of the <varname>anchor</varname> column family each - contain the external site which links to the site represented by the row, along with the - text it used in the anchor of its link. The <varname>people</varname> column family represents - people associated with the site. - </para> - <note> - <title>Column Names</title> - <para> By convention, a column name is made of its column family prefix and a - <emphasis>qualifier</emphasis>. For example, the column - <emphasis>contents:html</emphasis> is made up of the column family - <varname>contents</varname> and the <varname>html</varname> qualifier. The colon - character (<literal>:</literal>) delimits the column family from the column family - <emphasis>qualifier</emphasis>. </para> - </note> - <table - frame="all"> - <title>Table <varname>webtable</varname></title> - <tgroup - cols="5" - align="left" - colsep="1" - rowsep="1"> - <colspec - colname="c1" /> - <colspec - colname="c2" /> - <colspec - colname="c3" /> - <colspec - colname="c4" /> - <colspec - colname="c5" /> - <thead> - <row> - <entry>Row Key</entry> - <entry>Time Stamp</entry> - <entry>ColumnFamily <varname>contents</varname></entry> - <entry>ColumnFamily <varname>anchor</varname></entry> - <entry>ColumnFamily <varname>people</varname></entry> - </row> - </thead> - <tbody> - <row> - <entry>"com.cnn.www"</entry> - <entry>t9</entry> - <entry /> - <entry><varname>anchor:cnnsi.com</varname> = "CNN"</entry> - <entry /> - </row> - <row> - <entry>"com.cnn.www"</entry> - <entry>t8</entry> - <entry /> - <entry><varname>anchor:my.look.ca</varname> = "CNN.com"</entry> - <entry /> - </row> - <row> - <entry>"com.cnn.www"</entry> - <entry>t6</entry> - <entry><varname>contents:html</varname> = "<html>..."</entry> - <entry /> - <entry /> - </row> - <row> - <entry>"com.cnn.www"</entry> - <entry>t5</entry> - <entry><varname>contents:html</varname> = "<html>..."</entry> - <entry /> - <entry /> - </row> - <row> - <entry>"com.cnn.www"</entry> - <entry>t3</entry> - <entry><varname>contents:html</varname> = "<html>..."</entry> - <entry /> - <entry /> - </row> - <row> - <entry>"com.example.www"</entry> - <entry>t5</entry> - <entry><varname>contents:html</varname> = "<html>..."</entry> - <entry></entry> - <entry>people:author = "John Doe"</entry> - </row> - </tbody> - </tgroup> - </table> - <para>Cells in this table that appear to be empty do not take space, or in fact exist, in - HBase. This is what makes HBase "sparse." A tabular view is not the only possible way to - look at data in HBase, or even the most accurate. The following represents the same - information as a multi-dimensional map. This is only a mock-up for illustrative - purposes and may not be strictly accurate.</para> - <programlisting><![CDATA[ -{ - "com.cnn.www": { - contents: { - t6: contents:html: "<html>..." - t5: contents:html: "<html>..." - t3: contents:html: "<html>..." - } - anchor: { - t9: anchor:cnnsi.com = "CNN" - t8: anchor:my.look.ca = "CNN.com" - } - people: {} - } - "com.example.www": { - contents: { - t5: contents:html: "<html>..." - } - anchor: {} - people: { - t5: people:author: "John Doe" - } - } -} - ]]></programlisting> - - </section> - <section - xml:id="physical.view"> - <title>Physical View</title> - <para> Although at a conceptual level tables may be viewed as a sparse set of rows, they are - physically stored by column family. A new column qualifier (column_family:column_qualifier) - can be added to an existing column family at any time.</para> - <table - frame="all"> - <title>ColumnFamily <varname>anchor</varname></title> - <tgroup - cols="3" - align="left" - colsep="1" - rowsep="1"> - <colspec - colname="c1" /> - <colspec - colname="c2" /> - <colspec - colname="c3" /> - <thead> - <row> - <entry>Row Key</entry> - <entry>Time Stamp</entry> - <entry>Column Family <varname>anchor</varname></entry> - </row> - </thead> - <tbody> - <row> - <entry>"com.cnn.www"</entry> - <entry>t9</entry> - <entry><varname>anchor:cnnsi.com</varname> = "CNN"</entry> - </row> - <row> - <entry>"com.cnn.www"</entry> - <entry>t8</entry> - <entry><varname>anchor:my.look.ca</varname> = "CNN.com"</entry> - </row> - </tbody> - </tgroup> - </table> - <table - frame="all"> - <title>ColumnFamily <varname>contents</varname></title> - <tgroup - cols="3" - align="left" - colsep="1" - rowsep="1"> - <colspec - colname="c1" /> - <colspec - colname="c2" /> - <colspec - colname="c3" /> - <thead> - <row> - <entry>Row Key</entry> - <entry>Time Stamp</entry> - <entry>ColumnFamily "contents:"</entry> - </row> - </thead> - <tbody> - <row> - <entry>"com.cnn.www"</entry> - <entry>t6</entry> - <entry><varname>contents:html</varname> = "<html>..."</entry> - </row> - <row> - <entry>"com.cnn.www"</entry> - <entry>t5</entry> - <entry><varname>contents:html</varname> = "<html>..."</entry> - </row> - <row> - <entry>"com.cnn.www"</entry> - <entry>t3</entry> - <entry><varname>contents:html</varname> = "<html>..."</entry> - </row> - </tbody> - </tgroup> - </table> - <para>The empty cells shown in the - conceptual view are not stored at all. - Thus a request for the value of the <varname>contents:html</varname> column at time stamp - <literal>t8</literal> would return no value. Similarly, a request for an - <varname>anchor:my.look.ca</varname> value at time stamp <literal>t9</literal> would - return no value. However, if no timestamp is supplied, the most recent value for a - particular column would be returned. Given multiple versions, the most recent is also the - first one found, since timestamps - are stored in descending order. Thus a request for the values of all columns in the row - <varname>com.cnn.www</varname> if no timestamp is specified would be: the value of - <varname>contents:html</varname> from timestamp <literal>t6</literal>, the value of - <varname>anchor:cnnsi.com</varname> from timestamp <literal>t9</literal>, the value of - <varname>anchor:my.look.ca</varname> from timestamp <literal>t8</literal>. </para> - <para>For more information about the internals of how Apache HBase stores data, see <xref - linkend="regions.arch" />. </para> - </section> - - <section - xml:id="namespace"> - <title>Namespace</title> - <para> A namespace is a logical grouping of tables analogous to a database in relation - database systems. This abstraction lays the groundwork for upcoming multi-tenancy related - features: <itemizedlist> - <listitem> - <para>Quota Management (HBASE-8410) - Restrict the amount of resources (ie regions, - tables) a namespace can consume.</para> - </listitem> - <listitem> - <para>Namespace Security Administration (HBASE-9206) - provide another level of security - administration for tenants.</para> - </listitem> - <listitem> - <para>Region server groups (HBASE-6721) - A namespace/table can be pinned onto a subset - of regionservers thus guaranteeing a course level of isolation.</para> - </listitem> - </itemizedlist> - </para> - <section - xml:id="namespace_creation"> - <title>Namespace management</title> - <para> A namespace can be created, removed or altered. Namespace membership is determined - during table creation by specifying a fully-qualified table name of the form:</para> - - <programlisting language="xml"><![CDATA[<table namespace>:<table qualifier>]]></programlisting> - - - <example> - <title>Examples</title> - - <programlisting language="bourne"> -#Create a namespace -create_namespace 'my_ns' - </programlisting> - <programlisting language="bourne"> -#create my_table in my_ns namespace -create 'my_ns:my_table', 'fam' - </programlisting> - <programlisting language="bourne"> -#drop namespace -drop_namespace 'my_ns' - </programlisting> - <programlisting language="bourne"> -#alter namespace -alter_namespace 'my_ns', {METHOD => 'set', 'PROPERTY_NAME' => 'PROPERTY_VALUE'} - </programlisting> - </example> - </section> - <section - xml:id="namespace_special"> - <title>Predefined namespaces</title> - <para> There are two predefined special namespaces: </para> - <itemizedlist> - <listitem> - <para>hbase - system namespace, used to contain hbase internal tables</para> - </listitem> - <listitem> - <para>default - tables with no explicit specified namespace will automatically fall into - this namespace.</para> - </listitem> - </itemizedlist> - <example> - <title>Examples</title> - - <programlisting language="bourne"> -#namespace=foo and table qualifier=bar -create 'foo:bar', 'fam' - -#namespace=default and table qualifier=bar -create 'bar', 'fam' -</programlisting> - </example> - </section> - </section> - - <section - xml:id="table"> - <title>Table</title> - <para> Tables are declared up front at schema definition time. </para> - </section> - - <section - xml:id="row"> - <title>Row</title> - <para>Row keys are uninterrpreted bytes. Rows are lexicographically sorted with the lowest - order appearing first in a table. The empty byte array is used to denote both the start and - end of a tables' namespace.</para> - </section> - - <section - xml:id="columnfamily"> - <title>Column Family<indexterm><primary>Column Family</primary></indexterm></title> - <para> Columns in Apache HBase are grouped into <emphasis>column families</emphasis>. All - column members of a column family have the same prefix. For example, the columns - <emphasis>courses:history</emphasis> and <emphasis>courses:math</emphasis> are both - members of the <emphasis>courses</emphasis> column family. The colon character - (<literal>:</literal>) delimits the column family from the <indexterm><primary>column - family qualifier</primary><secondary>Column Family Qualifier</secondary></indexterm>. - The column family prefix must be composed of <emphasis>printable</emphasis> characters. The - qualifying tail, the column family <emphasis>qualifier</emphasis>, can be made of any - arbitrary bytes. Column families must be declared up front at schema definition time whereas - columns do not need to be defined at schema time but can be conjured on the fly while the - table is up an running.</para> - <para>Physically, all column family members are stored together on the filesystem. Because - tunings and storage specifications are done at the column family level, it is advised that - all column family members have the same general access pattern and size - characteristics.</para> - - </section> - <section - xml:id="cells"> - <title>Cells<indexterm><primary>Cells</primary></indexterm></title> - <para>A <emphasis>{row, column, version} </emphasis>tuple exactly specifies a - <literal>cell</literal> in HBase. Cell content is uninterrpreted bytes</para> - </section> - <section - xml:id="data_model_operations"> - <title>Data Model Operations</title> - <para>The four primary data model operations are Get, Put, Scan, and Delete. Operations are - applied via <link - xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/Table.html";>Table</link> - instances. - </para> - <section - xml:id="get"> - <title>Get</title> - <para><link - xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/Get.html";>Get</link> - returns attributes for a specified row. Gets are executed via <link - xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/Table.html#get(org.apache.hadoop.hbase.client.Get)"> - Table.get</link>. </para> - </section> - <section - xml:id="put"> - <title>Put</title> - <para><link - xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/Put.html";>Put</link> - either adds new rows to a table (if the key is new) or can update existing rows (if the - key already exists). Puts are executed via <link - xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/Table.html#put(org.apache.hadoop.hbase.client.Put)"> - Table.put</link> (writeBuffer) or <link - xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/Table.html#batch(java.util.List, java.lang.Object[])"> - Table.batch</link> (non-writeBuffer). </para> - </section> - <section - xml:id="scan"> - <title>Scans</title> - <para><link - xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/Scan.html";>Scan</link> - allow iteration over multiple rows for specified attributes. </para> - <para>The following is an example of a Scan on a Table instance. Assume that a table is - populated with rows with keys "row1", "row2", "row3", and then another set of rows with - the keys "abc1", "abc2", and "abc3". The following example shows how to set a Scan - instance to return the rows beginning with "row".</para> -<programlisting language="java"> -public static final byte[] CF = "cf".getBytes(); -public static final byte[] ATTR = "attr".getBytes(); -... - -Table table = ... // instantiate a Table instance - -Scan scan = new Scan(); -scan.addColumn(CF, ATTR); -scan.setRowPrefixFilter(Bytes.toBytes("row")); -ResultScanner rs = table.getScanner(scan); -try { - for (Result r = rs.next(); r != null; r = rs.next()) { - // process result... -} finally { - rs.close(); // always close the ResultScanner! -</programlisting> - <para>Note that generally the easiest way to specify a specific stop point for a scan is by - using the <link - xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/filter/InclusiveStopFilter.html";>InclusiveStopFilter</link> - class. </para> - </section> - <section - xml:id="delete"> - <title>Delete</title> - <para><link - xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/Delete.html";>Delete</link> - removes a row from a table. Deletes are executed via <link - xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/Table.html#delete(org.apache.hadoop.hbase.client.Delete)"> - HTable.delete</link>. </para> - <para>HBase does not modify data in place, and so deletes are handled by creating new - markers called <emphasis>tombstones</emphasis>. These tombstones, along with the dead - values, are cleaned up on major compactions. </para> - <para>See <xref - linkend="version.delete" /> for more information on deleting versions of columns, and - see <xref - linkend="compaction" /> for more information on compactions. </para> - - </section> - - </section> - - - <section - xml:id="versions"> - <title>Versions<indexterm><primary>Versions</primary></indexterm></title> - - <para>A <emphasis>{row, column, version} </emphasis>tuple exactly specifies a - <literal>cell</literal> in HBase. It's possible to have an unbounded number of cells where - the row and column are the same but the cell address differs only in its version - dimension.</para> - - <para>While rows and column keys are expressed as bytes, the version is specified using a long - integer. Typically this long contains time instances such as those returned by - <code>java.util.Date.getTime()</code> or <code>System.currentTimeMillis()</code>, that is: - <quote>the difference, measured in milliseconds, between the current time and midnight, - January 1, 1970 UTC</quote>.</para> - - <para>The HBase version dimension is stored in decreasing order, so that when reading from a - store file, the most recent values are found first.</para> - - <para>There is a lot of confusion over the semantics of <literal>cell</literal> versions, in - HBase. In particular:</para> - <itemizedlist> - <listitem> - <para>If multiple writes to a cell have the same version, only the last written is - fetchable.</para> - </listitem> - - <listitem> - <para>It is OK to write cells in a non-increasing version order.</para> - </listitem> - </itemizedlist> - - <para>Below we describe how the version dimension in HBase currently works. See <link - xlink:href="https://issues.apache.org/jira/browse/HBASE-2406";>HBASE-2406</link> for - discussion of HBase versions. <link - xlink:href="http://outerthought.org/blog/417-ot.html";>Bending time in HBase</link> - makes for a good read on the version, or time, dimension in HBase. It has more detail on - versioning than is provided here. As of this writing, the limiitation - <emphasis>Overwriting values at existing timestamps</emphasis> mentioned in the - article no longer holds in HBase. This section is basically a synopsis of this article - by Bruno Dumon.</para> - - <section xml:id="specify.number.of.versions"> - <title>Specifying the Number of Versions to Store</title> - <para>The maximum number of versions to store for a given column is part of the column - schema and is specified at table creation, or via an <command>alter</command> command, via - <code>HColumnDescriptor.DEFAULT_VERSIONS</code>. Prior to HBase 0.96, the default number - of versions kept was <literal>3</literal>, but in 0.96 and newer has been changed to - <literal>1</literal>.</para> - <example> - <title>Modify the Maximum Number of Versions for a Column</title> - <para>This example uses HBase Shell to keep a maximum of 5 versions of column - <code>f1</code>. You could also use <link - xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/HColumnDescriptor.html"; - >HColumnDescriptor</link>.</para> - <screen><![CDATA[hbase> alter ât1â², NAME => âf1â², VERSIONS => 5]]></screen> - </example> - <example> - <title>Modify the Minimum Number of Versions for a Column</title> - <para>You can also specify the minimum number of versions to store. By default, this is - set to 0, which means the feature is disabled. The following example sets the minimum - number of versions on field <code>f1</code> to <literal>2</literal>, via HBase Shell. - You could also use <link - xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/HColumnDescriptor.html"; - >HColumnDescriptor</link>.</para> - <screen><![CDATA[hbase> alter ât1â², NAME => âf1â², MIN_VERSIONS => 2]]></screen> - </example> - <para>Starting with HBase 0.98.2, you can specify a global default for the maximum number of - versions kept for all newly-created columns, by setting - <option>hbase.column.max.version</option> in <filename>hbase-site.xml</filename>. See - <xref linkend="hbase.column.max.version"/>.</para> - </section> - - <section - xml:id="versions.ops"> - <title>Versions and HBase Operations</title> - - <para>In this section we look at the behavior of the version dimension for each of the core - HBase operations.</para> - - <section> - <title>Get/Scan</title> - - <para>Gets are implemented on top of Scans. The below discussion of <link - xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/Get.html";>Get</link> - applies equally to <link - xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/Scan.html";>Scans</link>.</para> - - <para>By default, i.e. if you specify no explicit version, when doing a - <literal>get</literal>, the cell whose version has the largest value is returned - (which may or may not be the latest one written, see later). The default behavior can be - modified in the following ways:</para> - - <itemizedlist> - <listitem> - <para>to return more than one version, see <link - xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/Get.html#setMaxVersions()">Get.setMaxVersions()</link></para> - </listitem> - - <listitem> - <para>to return versions other than the latest, see <link - xlink:href="???">Get.setTimeRange()</link></para> - - <para>To retrieve the latest version that is less than or equal to a given value, thus - giving the 'latest' state of the record at a certain point in time, just use a range - from 0 to the desired version and set the max versions to 1.</para> - </listitem> - </itemizedlist> - - </section> - <section - xml:id="default_get_example"> - <title>Default Get Example</title> - <para>The following Get will only retrieve the current version of the row</para> - <programlisting language="java"> -public static final byte[] CF = "cf".getBytes(); -public static final byte[] ATTR = "attr".getBytes(); -... -Get get = new Get(Bytes.toBytes("row1")); -Result r = table.get(get); -byte[] b = r.getValue(CF, ATTR); // returns current version of value -</programlisting> - </section> - <section - xml:id="versioned_get_example"> - <title>Versioned Get Example</title> - <para>The following Get will return the last 3 versions of the row.</para> - <programlisting language="java"> -public static final byte[] CF = "cf".getBytes(); -public static final byte[] ATTR = "attr".getBytes(); -... -Get get = new Get(Bytes.toBytes("row1")); -get.setMaxVersions(3); // will return last 3 versions of row -Result r = table.get(get); -byte[] b = r.getValue(CF, ATTR); // returns current version of value -List<KeyValue> kv = r.getColumn(CF, ATTR); // returns all versions of this column -</programlisting> - </section> - - <section> - <title>Put</title> - - <para>Doing a put always creates a new version of a <literal>cell</literal>, at a certain - timestamp. By default the system uses the server's <literal>currentTimeMillis</literal>, - but you can specify the version (= the long integer) yourself, on a per-column level. - This means you could assign a time in the past or the future, or use the long value for - non-time purposes.</para> - - <para>To overwrite an existing value, do a put at exactly the same row, column, and - version as that of the cell you would overshadow.</para> - <section - xml:id="implicit_version_example"> - <title>Implicit Version Example</title> - <para>The following Put will be implicitly versioned by HBase with the current - time.</para> - <programlisting language="java"> -public static final byte[] CF = "cf".getBytes(); -public static final byte[] ATTR = "attr".getBytes(); -... -Put put = new Put(Bytes.toBytes(row)); -put.add(CF, ATTR, Bytes.toBytes( data)); -table.put(put); -</programlisting> - </section> - <section - xml:id="explicit_version_example"> - <title>Explicit Version Example</title> - <para>The following Put has the version timestamp explicitly set.</para> - <programlisting language="java"> -public static final byte[] CF = "cf".getBytes(); -public static final byte[] ATTR = "attr".getBytes(); -... -Put put = new Put( Bytes.toBytes(row)); -long explicitTimeInMs = 555; // just an example -put.add(CF, ATTR, explicitTimeInMs, Bytes.toBytes(data)); -table.put(put); -</programlisting> - <para>Caution: the version timestamp is internally by HBase for things like time-to-live - calculations. It's usually best to avoid setting this timestamp yourself. Prefer using - a separate timestamp attribute of the row, or have the timestamp a part of the rowkey, - or both. </para> - </section> - - </section> - - <section - xml:id="version.delete"> - <title>Delete</title> - - <para>There are three different types of internal delete markers. See Lars Hofhansl's blog - for discussion of his attempt adding another, <link - xlink:href="http://hadoop-hbase.blogspot.com/2012/01/scanning-in-hbase.html";>Scanning - in HBase: Prefix Delete Marker</link>. </para> - <itemizedlist> - <listitem> - <para>Delete: for a specific version of a column.</para> - </listitem> - <listitem> - <para>Delete column: for all versions of a column.</para> - </listitem> - <listitem> - <para>Delete family: for all columns of a particular ColumnFamily</para> - </listitem> - </itemizedlist> - <para>When deleting an entire row, HBase will internally create a tombstone for each - ColumnFamily (i.e., not each individual column). </para> - <para>Deletes work by creating <emphasis>tombstone</emphasis> markers. For example, let's - suppose we want to delete a row. For this you can specify a version, or else by default - the <literal>currentTimeMillis</literal> is used. What this means is <quote>delete all - cells where the version is less than or equal to this version</quote>. HBase never - modifies data in place, so for example a delete will not immediately delete (or mark as - deleted) the entries in the storage file that correspond to the delete condition. - Rather, a so-called <emphasis>tombstone</emphasis> is written, which will mask the - deleted values. When HBase does a major compaction, the tombstones are processed to - actually remove the dead values, together with the tombstones themselves. If the version - you specified when deleting a row is larger than the version of any value in the row, - then you can consider the complete row to be deleted.</para> - <para>For an informative discussion on how deletes and versioning interact, see the thread <link - xlink:href="http://comments.gmane.org/gmane.comp.java.hadoop.hbase.user/28421";>Put w/ - timestamp -> Deleteall -> Put w/ timestamp fails</link> up on the user mailing - list.</para> - <para>Also see <xref - linkend="keyvalue" /> for more information on the internal KeyValue format. </para> - <para>Delete markers are purged during the next major compaction of the store, unless the - <option>KEEP_DELETED_CELLS</option> option is set in the column family. To keep the - deletes for a configurable amount of time, you can set the delete TTL via the - <option>hbase.hstore.time.to.purge.deletes</option> property in - <filename>hbase-site.xml</filename>. If - <option>hbase.hstore.time.to.purge.deletes</option> is not set, or set to 0, all - delete markers, including those with timestamps in the future, are purged during the - next major compaction. Otherwise, a delete marker with a timestamp in the future is kept - until the major compaction which occurs after the time represented by the marker's - timestamp plus the value of <option>hbase.hstore.time.to.purge.deletes</option>, in - milliseconds. </para> - <note> - <para>This behavior represents a fix for an unexpected change that was introduced in - HBase 0.94, and was fixed in <link - xlink:href="https://issues.apache.org/jira/browse/HBASE-10118";>HBASE-10118</link>. - The change has been backported to HBase 0.94 and newer branches.</para> - </note> - </section> - </section> - - <section> - <title>Current Limitations</title> - - <section> - <title>Deletes mask Puts</title> - - <para>Deletes mask puts, even puts that happened after the delete - was entered. See <link xlink:href="https://issues.apache.org/jira/browse/HBASE-2256"; - >HBASE-2256</link>. Remember that a delete writes a tombstone, which only - disappears after then next major compaction has run. Suppose you do - a delete of everything <= T. After this you do a new put with a - timestamp <= T. This put, even if it happened after the delete, - will be masked by the delete tombstone. Performing the put will not - fail, but when you do a get you will notice the put did have no - effect. It will start working again after the major compaction has - run. These issues should not be a problem if you use - always-increasing versions for new puts to a row. But they can occur - even if you do not care about time: just do delete and put - immediately after each other, and there is some chance they happen - within the same millisecond.</para> - </section> - - <section - xml:id="major.compactions.change.query.results"> - <title>Major compactions change query results</title> - - <para><quote>...create three cell versions at t1, t2 and t3, with a maximum-versions - setting of 2. So when getting all versions, only the values at t2 and t3 will be - returned. But if you delete the version at t2 or t3, the one at t1 will appear again. - Obviously, once a major compaction has run, such behavior will not be the case - anymore...</quote> (See <emphasis>Garbage Collection</emphasis> in <link - xlink:href="http://outerthought.org/blog/417-ot.html";>Bending time in - HBase</link>.)</para> - </section> - </section> - </section> - <section xml:id="dm.sort"> - <title>Sort Order</title> - <para>All data model operations HBase return data in sorted order. First by row, - then by ColumnFamily, followed by column qualifier, and finally timestamp (sorted - in reverse, so newest records are returned first). - </para> - </section> - <section xml:id="dm.column.metadata"> - <title>Column Metadata</title> - <para>There is no store of column metadata outside of the internal KeyValue instances for a ColumnFamily. - Thus, while HBase can support not only a wide number of columns per row, but a heterogenous set of columns - between rows as well, it is your responsibility to keep track of the column names. - </para> - <para>The only way to get a complete set of columns that exist for a ColumnFamily is to process all the rows. - For more information about how HBase stores data internally, see <xref linkend="keyvalue" />. - </para> - </section> - <section xml:id="joins"><title>Joins</title> - <para>Whether HBase supports joins is a common question on the dist-list, and there is a simple answer: it doesn't, - at not least in the way that RDBMS' support them (e.g., with equi-joins or outer-joins in SQL). As has been illustrated - in this chapter, the read data model operations in HBase are Get and Scan. - </para> - <para>However, that doesn't mean that equivalent join functionality can't be supported in your application, but - you have to do it yourself. The two primary strategies are either denormalizing the data upon writing to HBase, - or to have lookup tables and do the join between HBase tables in your application or MapReduce code (and as RDBMS' - demonstrate, there are several strategies for this depending on the size of the tables, e.g., nested loops vs. - hash-joins). So which is the best approach? It depends on what you are trying to do, and as such there isn't a single - answer that works for every use case. - </para> - </section> - <section xml:id="acid"><title>ACID</title> - <para>See <link xlink:href="http://hbase.apache.org/acid-semantics.html";>ACID Semantics</link>. - Lars Hofhansl has also written a note on - <link xlink:href="http://hadoop-hbase.blogspot.com/2012/03/acid-in-hbase.html";>ACID in HBase</link>.</para> - </section> - </chapter>
