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<?xml version="1.0" encoding="iso-8859-1" ?>
<!DOCTYPE chapter SYSTEM "chapter.dtd">
<chapter>
<header>
<title>Mnesia Database Questions</title>
<prepared>Matthias Lang</prepared>
<docno></docno>
<date>2007-09-12</date>
<rev>1.0</rev>
<file>mnesia.xml</file>
</header>
<p>
Mnesia is the somewhat odd name for the real-time, distributed database which
comes with Erlang.
</p>
<section><title>What is Mnesia good at?</title>
<section> <title>Locking and Transactions</title>
<p>
If you need to keep a database that will be used by multiple
processes and/or nodes, using Mnesia means you don't have
to write your own access controls.
</p></section>
<section> <title>Distribution</title>
<p>
Tables can be replicated at many nodes, both for efficiency
(database lookup is a local activity) and robustness
(redundancy means if one node goes down, other nodes still
have copies of the data.)
</p></section>
<section> <title>Non-fully-normalised data</title>
<p>
Unlike most database systems, records can contain data
of arbitrary size and structure.
</p></section>
<section><title>Monitoring</title>
<p>
Monitoring - processes can subscribe to events which are
sent when various operations on the data take place
(update, delete, etc) The RDBMS package allows even
more fine-grained control.
</p></section>
</section>
<section><title>What is Mnesia not so good at?</title>
<p>
Mnesia is primarily intended to be a memory-resident database.
Some of its design tradeoffs reflect this.
</p><p>
Really large tables must be stored in a fragmented manner.
</p></section>
<section><title>Is Mnesia good for storing blobs?</title>
<p>
It depends. Erlang has no problem storing Erlang binary data
types of arbritary size, however due to the in-memory-database
design emphasis of mnesia, storing lots of binary
data will eventually hit one of a number of limitations.
These are driven by:
</p>
<list>
<item><p>
Storage type - Both <c>ram_copies</c> and
<c>disc_copies</c> tables
rely on storing a full copy of the whole table and data
in main memory. This will limit total blob storage to
the size of available memory.
</p><p>
On the other hand <c>disc_only_copies</c>
tables do not suffer
from this limitation but they are slow (from disk) and
the data is stored in dets tables which if not closed
properly (e.g. after system crash) can take a long time
to repair (this was improved in recent versions but is
still not quick).
</p></item>
<item><p>
Replication - if the table has a replica then updating
an entry and rebuilding after a restart will copy the
data over the network between the two machines. Depending
on available bandwidth and the uptime requirements
this may or may not be acceptable.
</p></item>
</list>
<p>
As always, measurement of the different mechanisms for your
specific application is recommended.
</p><p>
A more colourful discussion of the these topics can be found
in this <url href="http://erlang.org/pipermail/erlang-questions/1999-October/000618.html">
post to the mailing list.</url>
</p></section>
<section><title>Is there an SQL interface for mnesia?</title>
<p>
A partial one was built as a masters project, it's
of limited use and not widely used.
</p><p>
QLC is the query engine for Mnesia and ETS. It is
a much better fit to Erlang than SQL.
</p></section>
<section><title>How much data can be stored in Mnesia?</title>
<p>
Dets uses 32 bit integers for file offsets, so the
largest possible mnesia table (for now) is 4Gb.
</p><p>
In practice your machine will slow to a crawl way before
you reach this limit.
</p></section>
<section><title>Contributors</title>
<p> Thanks to Chris Pressey, Ulf Wiger and Sean Hinde for writing
the entries in this section.
</p></section></chapter>
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