HBASE-11476 Expand 'Conceptual View' section of Data Model chapter (Misty Stanley-Jones)
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Jonathan M Hsieh
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<chapter
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xml:id="datamodel">
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<title>Data Model</title>
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<para>In short, applications store data into an HBase table. Tables are made of rows and
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columns. All columns in HBase belong to a particular column family. Table cells -- the
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intersection of row and column coordinates -- are versioned. A cell’s content is an
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uninterpreted array of bytes. </para>
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<para>Table row keys are also byte arrays so almost anything can serve as a row key from strings
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to binary representations of longs or even serialized data structures. Rows in HBase tables
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are sorted by row key. The sort is byte-ordered. All table accesses are via the table row key
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-- its primary key. </para>
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<para>In HBase, data is stored in tables, which have rows and columns. This is a terminology
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overlap with relational databases (RDBMSs), but this is not a helpful analogy. Instead, it can
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be helpful to think of an HBase table as a multi-dimensional map.</para>
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<variablelist>
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<title>HBase Data Model Terminology</title>
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<varlistentry>
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<term>Table</term>
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<listitem>
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<para>An HBase table consists of multiple rows.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>Row</term>
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<listitem>
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<para>A row in HBase consists of a row key and one or more columns with values associated
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with them. Rows are sorted alphabetically by the row key as they are stored. For this
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reason, the design of the row key is very important. The goal is to store data in such a
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way that related rows are near each other. A common row key pattern is a website domain.
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If your row keys are domains, you should probably store them in reverse (org.apache.www,
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org.apache.mail, org.apache.jira). This way, all of the Apache domains are near each
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other in the table, rather than being spread out based on the first letter of the
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subdomain.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>Column</term>
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<listitem>
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<para>A column in HBase consists of a column family and a column qualifier, which are
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delimited by a <literal>:</literal> (colon) character.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>Column Family</term>
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<listitem>
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<para>Column families physically colocate a set of columns and their values, often for
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performance reasons. Each column family has a set of storage properties, such as whether
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its values should be cached in memory, how its data is compressed or its row keys are
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encoded, and others. Each row in a table has the same column
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families, though a given row might not store anything in a given column family.</para>
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<para>Column families are specified when you create your table, and influence the way your
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data is stored in the underlying filesystem. Therefore, the column families should be
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considered carefully during schema design.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>Column Qualifier</term>
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<listitem>
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<para>A column qualifier is added to a column family to provide the index for a given
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piece of data. Given a column family <literal>content</literal>, a column qualifier
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might be <literal>content:html</literal>, and another might be
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<literal>content:pdf</literal>. Though column families are fixed at table creation,
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column qualifiers are mutable and may differ greatly between rows.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>Cell</term>
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<listitem>
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<para>A cell is a combination of row, column family, and column qualifier, and contains a
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value and a timestamp, which represents the value's version.</para>
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<para>A cell's value is an uninterpreted array of bytes.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>Timestamp</term>
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<listitem>
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<para>A timestamp is written alongside each value, and is the identifier for a given
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version of a value. By default, the timestamp represents the time on the RegionServer
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when the data was written, but you can specify a different timestamp value when you put
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data into the cell.</para>
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<caution>
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<para>Direct manipulation of timestamps is an advanced feature which is only exposed for
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special cases that are deeply integrated with HBase, and is discouraged in general.
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Encoding a timestamp at the application level is the preferred pattern.</para>
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</caution>
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<para>You can specify the maximum number of versions of a value that HBase retains, per column
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family. When the maximum number of versions is reached, the oldest versions are
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eventually deleted. By default, only the newest version is kept.</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<section
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xml:id="conceptual.view">
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<title>Conceptual View</title>
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<para>You can read a very understandable explanation of the HBase data model in the blog post <link
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xlink:href="http://jimbojw.com/wiki/index.php?title=Understanding_Hbase_and_BigTable">Understanding
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HBase and BigTable</link> by Jim R. Wilson. Another good explanation is available in the
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PDF <link
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xlink:href="http://0b4af6cdc2f0c5998459-c0245c5c937c5dedcca3f1764ecc9b2f.r43.cf2.rackcdn.com/9353-login1210_khurana.pdf">Introduction
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to Basic Schema Design</link> by Amandeep Khurana. It may help to read different
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perspectives to get a solid understanding of HBase schema design. The linked articles cover
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the same ground as the information in this section.</para>
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<para> The following example is a slightly modified form of the one on page 2 of the <link
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xlink:href="http://research.google.com/archive/bigtable.html">BigTable</link> paper. There
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is a table called <varname>webtable</varname> that contains two column families named
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<varname>contents</varname> and <varname>anchor</varname>. In this example,
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is a table called <varname>webtable</varname> that contains two rows
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(<literal>com.cnn.www</literal>
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and <literal>com.example.www</literal>), three column families named
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<varname>contents</varname>, <varname>anchor</varname>, and <varname>people</varname>. In
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this example, for the first row (<literal>com.cnn.www</literal>),
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<varname>anchor</varname> contains two columns (<varname>anchor:cssnsi.com</varname>,
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<varname>anchor:my.look.ca</varname>) and <varname>contents</varname> contains one column
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(<varname>contents:html</varname>). <note>
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(<varname>contents:html</varname>). This example contains 5 versions of the row with the
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row key <literal>com.cnn.www</literal>, and one version of the row with the row key
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<literal>com.example.www</literal>. The <varname>contents:html</varname> column qualifier contains the entire
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HTML of a given website. Qualifiers of the <varname>anchor</varname> column family each
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contain the external site which links to the site represented by the row, along with the
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text it used in the anchor of its link. The <varname>people</varname> column family represents
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people associated with the site.
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</para>
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<note>
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<title>Column Names</title>
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<para> By convention, a column name is made of its column family prefix and a
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<emphasis>qualifier</emphasis>. For example, the column
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<emphasis>contents:html</emphasis> is made up of the column family
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<varname>contents</varname> and <varname>html</varname> qualifier. The colon character
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(<literal>:</literal>) delimits the column family from the column family
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<varname>contents</varname> and the <varname>html</varname> qualifier. The colon
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character (<literal>:</literal>) delimits the column family from the column family
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<emphasis>qualifier</emphasis>. </para>
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</note>
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<table
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frame="all">
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<title>Table <varname>webtable</varname></title>
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<tgroup
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cols="4"
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cols="5"
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align="left"
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colsep="1"
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rowsep="1">
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@ -134,12 +225,15 @@
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colname="c3" />
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<colspec
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colname="c4" />
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<colspec
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colname="c5" />
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<thead>
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<row>
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<entry>Row Key</entry>
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<entry>Time Stamp</entry>
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<entry>ColumnFamily <varname>contents</varname></entry>
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<entry>ColumnFamily <varname>anchor</varname></entry>
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<entry>ColumnFamily <varname>people</varname></entry>
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</row>
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</thead>
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<tbody>
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@ -148,43 +242,85 @@
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<entry>t9</entry>
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<entry />
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<entry><varname>anchor:cnnsi.com</varname> = "CNN"</entry>
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<entry />
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</row>
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<row>
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<entry>"com.cnn.www"</entry>
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<entry>t8</entry>
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<entry />
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<entry><varname>anchor:my.look.ca</varname> = "CNN.com"</entry>
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<entry />
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</row>
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<row>
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<entry>"com.cnn.www"</entry>
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<entry>t6</entry>
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<entry><varname>contents:html</varname> = "<html>..."</entry>
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<entry />
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<entry />
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</row>
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<row>
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<entry>"com.cnn.www"</entry>
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<entry>t5</entry>
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<entry><varname>contents:html</varname> = "<html>..."</entry>
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<entry />
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<entry />
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</row>
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<row>
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<entry>"com.cnn.www"</entry>
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<entry>t3</entry>
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<entry><varname>contents:html</varname> = "<html>..."</entry>
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<entry />
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<entry />
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</row>
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<row>
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<entry>"com.example.www"</entry>
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<entry>t5</entry>
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<entry><varname>contents:html</varname> = "<html>..."</entry>
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<entry></entry>
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<entry>people:author = "John Doe"</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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</para>
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<para>Cells in this table that appear to be empty do not take space, or in fact exist, in
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HBase. This is what makes HBase "sparse." A tabular view is not the only possible way to
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look at data in HBase, or even the most accurate. The following represents the same
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information as a multi-dimensional map. This is only a mock-up for illustrative
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purposes and may not be strictly accurate.</para>
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<programlisting><![CDATA[
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{
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"com.cnn.www": {
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contents: {
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t6: contents:html: "<html>..."
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t5: contents:html: "<html>..."
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t3: contents:html: "<html>..."
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}
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anchor: {
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t9: anchor:cnnsi.com = "CNN"
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t8: anchor:my.look.ca = "CNN.com"
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}
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people: {}
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}
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"com.example.www": {
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contents: {
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t5: contents:html: "<html>..."
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}
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anchor: {}
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people: {
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t5: people:author: "John Doe"
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}
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}
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}
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]]></programlisting>
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</section>
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<section
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xml:id="physical.view">
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<title>Physical View</title>
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<para> Although at a conceptual level tables may be viewed as a sparse set of rows. Physically
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they are stored on a per-column family basis. New columns (i.e.,
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<varname>columnfamily:column</varname>) can be added to any column family without
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pre-announcing them. <table
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<para> Although at a conceptual level tables may be viewed as a sparse set of rows, they are
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physically stored by column family. A new column qualifier (column_family:column_qualifier)
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can be added to an existing column family at any time.</para>
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<table
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frame="all">
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<title>ColumnFamily <varname>anchor</varname></title>
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<tgroup
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@ -258,18 +394,20 @@
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</row>
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</tbody>
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</tgroup>
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</table> It is important to note in the diagram above that the empty cells shown in the
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conceptual view are not stored since they need not be in a column-oriented storage format.
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</table>
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<para>The empty cells shown in the
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conceptual view are not stored at all.
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Thus a request for the value of the <varname>contents:html</varname> column at time stamp
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<literal>t8</literal> would return no value. Similarly, a request for an
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<varname>anchor:my.look.ca</varname> value at time stamp <literal>t9</literal> would
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return no value. However, if no timestamp is supplied, the most recent value for a
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particular column would be returned and would also be the first one found since timestamps
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particular column would be returned. Given multiple versions, the most recent is also the
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first one found, since timestamps
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are stored in descending order. Thus a request for the values of all columns in the row
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<varname>com.cnn.www</varname> if no timestamp is specified would be: the value of
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<varname>contents:html</varname> from time stamp <literal>t6</literal>, the value of
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<varname>anchor:cnnsi.com</varname> from time stamp <literal>t9</literal>, the value of
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<varname>anchor:my.look.ca</varname> from time stamp <literal>t8</literal>. </para>
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<varname>contents:html</varname> from timestamp <literal>t6</literal>, the value of
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<varname>anchor:cnnsi.com</varname> from timestamp <literal>t9</literal>, the value of
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<varname>anchor:my.look.ca</varname> from timestamp <literal>t8</literal>. </para>
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<para>For more information about the internals of how Apache HBase stores data, see <xref
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linkend="regions.arch" />. </para>
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</section>
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