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hbase-5753.xml - adding schema design case study in Case Studies chapter 

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Doug Meil 2012-04-09 20:17:04 +00:00
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@ -34,6 +34,149 @@
<para>For more information on Performance and Troubleshooting, see <xref linkend="performance"/> and <xref linkend="trouble"/>.
</para>
</section>
<section xml:id="casestudies.schema">
<title>Schema Design</title>
<section xml:id="casestudies.schema.listdata">
<title>List Data</title>
<para>The following is an exchange from the user dist-list regarding a fairly common question:
how to handle per-user list data in HBase.
</para>
<para>*** QUESTION ***</para>
<para>
We're looking at how to store a large amount of (per-user) list data in
HBase, and we were trying to figure out what kind of access pattern made
the most sense. One option is store the majority of the data in a key, so
we could have something like:
</para>
<programlisting>
&lt;FixedWidthUserName&gt;&lt;FixedWidthValueId1&gt;:"" (no value)
&lt;FixedWidthUserName&gt;&lt;FixedWidthValueId2&gt;:"" (no value)
&lt;FixedWidthUserName&gt;&lt;FixedWidthValueId3&gt;:"" (no value)
</programlisting>
The other option we had was to do this entirely using:
<programlisting>
&lt;FixedWidthUserName&gt;&lt;FixedWidthPageNum0&gt;:&lt;FixedWidthLength&gt;&lt;FixedIdNextPageNum&gt;&lt;ValueId1&gt;&lt;ValueId2&gt;&lt;ValueId3&gt;...
&lt;FixedWidthUserName&gt;&lt;FixedWidthPageNum1&gt;:&lt;FixedWidthLength&gt;&lt;FixedIdNextPageNum&gt;&lt;ValueId1&gt;&lt;ValueId2&gt;&lt;ValueId3&gt;...
</programlisting>
<para>
where each row would contain multiple values.
So in one case reading the first thirty values would be:
</para>
<programlisting>
scan { STARTROW =&gt; 'FixedWidthUsername' LIMIT =&gt; 30}
</programlisting>
And in the second case it would be
<programlisting>
get 'FixedWidthUserName\x00\x00\x00\x00'
</programlisting>
<para>
The general usage pattern would be to read only the first 30 values of
these lists, with infrequent access reading deeper into the lists. Some
users would have &lt;= 30 total values in these lists, and some users would
have millions (i.e. power-law distribution)
</para>
<para>
The single-value format seems like it would take up more space on HBase,
but would offer some improved retrieval / pagination flexibility. Would
there be any significant performance advantages to be able to paginate via
gets vs paginating with scans?
</para>
<para>
My initial understanding was that doing a scan should be faster if our
paging size is unknown (and caching is set appropriately), but that gets
should be faster if we'll always need the same page size. I've ended up
hearing different people tell me opposite things about performance. I
assume the page sizes would be relatively consistent, so for most use cases
we could guarantee that we only wanted one page of data in the
fixed-page-length case. I would also assume that we would have infrequent
updates, but may have inserts into the middle of these lists (meaning we'd
need to update all subsequent rows).
</para>
<para>
Thanks for help / suggestions / follow-up questions.
</para>
<para>*** ANSWER ***</para>
<para>
If I understand you correctly, you're ultimately trying to store
triples in the form "user, valueid, value", right? E.g., something
like:
</para>
<programlisting>
"user123, firstname, Paul",
"user234, lastname, Smith"
</programlisting>
<para>
(But the usernames are fixed width, and the valueids are fixed width).
</para>
<para>
And, your access pattern is along the lines of: "for user X, list the
next 30 values, starting with valueid Y". Is that right? And these
values should be returned sorted by valueid?
</para>
<para>
The tl;dr version is that you should probably go with one row per
user+value, and not build a complicated intra-row pagination scheme on
your own unless you're really sure it is needed.
</para>
<para>
Your two options mirror a common question people have when designing
HBase schemas: should I go "tall" or "wide"? Your first schema is
"tall": each row represents one value for one user, and so there are
many rows in the table for each user; the row key is user + valueid,
and there would be (presumably) a single column qualifier that means
"the value". This is great if you want to scan over rows in sorted
order by row key (thus my question above, about whether these ids are
sorted correctly). You can start a scan at any user+valueid, read the
next 30, and be done. What you're giving up is the ability to have
transactional guarantees around all the rows for one user, but it
doesn't sound like you need that. Doing it this way is generally
recommended (see
here <link xlink:href="http://hbase.apache.org/book.html#schema.smackdown">http://hbase.apache.org/book.html#schema.smackdown</link>).
</para>
<para>
Your second option is "wide": you store a bunch of values in one row,
using different qualifiers (where the qualifier is the valueid). The
simple way to do that would be to just store ALL values for one user
in a single row. I'm guessing you jumped to the "paginated" version
because you're assuming that storing millions of columns in a single
row would be bad for performance, which may or may not be true; as
long as you're not trying to do too much in a single request, or do
things like scanning over and returning all of the cells in the row,
it shouldn't be fundamentally worse. The client has methods that allow
you to get specific slices of columns.
</para>
<para>
Note that neither case fundamentally uses more disk space than the
other; you're just "shifting" part of the identifying information for
a value either to the left (into the row key, in option one) or to the
right (into the column qualifiers in option 2). Under the covers,
every key/value still stores the whole row key, and column family
name. (If this is a bit confusing, take an hour and watch Lars
George's excellent video about understanding HBase schema design:
<link xlink:href="http://www.youtube.com/watch?v=_HLoH_PgrLk)">http://www.youtube.com/watch?v=_HLoH_PgrLk)</link>.
</para>
<para>
A manually paginated version has lots more complexities, as you note,
like having to keep track of how many things are in each page,
re-shuffling if new values are inserted, etc. That seems significantly
more complex. It might have some slight speed advantages (or
disadvantages!) at extremely high throughput, and the only way to
really know that would be to try it out. If you don't have time to
build it both ways and compare, my advice would be to start with the
simplest option (one row per user+value). Start simple and iterate! :)
</para>
</section> <!-- listdata -->
</section> <!-- schema design -->
<section xml:id="casestudies.perftroub">
<title>Performance/Troubleshooting</title>
<section xml:id="casestudies.slownode">
<title>Case Study #1 (Performance Issue On A Single Node)</title>
@ -175,5 +318,7 @@ Link detected: yes
<para>See also <xref linkend="dfs.datanode.max.xcievers"/>.
</para>
</section>
</section> <!-- performance/troubleshooting -->
</chapter>