whentouse.tcl

sqlite-3.4.1,嵌入式数据库.是一个功能强大的开源数据库,给学习和研发以及小型公司的发展带来了全所未有的好处.

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# Run this TCL script to generate HTML for the goals.html file.
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set rcsid {$Id: whentouse.tcl,v 1.7 2007/04/14 12:04:39 drh Exp $}
source common.tcl
header {Appropriate Uses For SQLite}

puts {
<p>
SQLite is different from most other SQL database engines in that its
primary design goal is to be simple:
</p>

<ul>
<li>Simple to administer</li>
<li>Simple to operate</li>
<li>Simple to embed in a larger program</li>
<li>Simple to maintain and customize</li>
</ul>

<p>
Many people like SQLite because it is small and fast.  But those
qualities are just happy accidents.
Users also find that SQLite is very reliable.  Reliability is
a consequence of simplicity.  With less complication, there is
less to go wrong.  So, yes, SQLite is small, fast, and reliable,
but first and foremost, SQLite strives to be simple.
</p>

<p>
Simplicity in a database engine can be either a strength or a
weakness, depending on what you are trying to do.  In order to
achieve simplicity, SQLite has had to sacrifice other characteristics
that some people find useful, such as high concurrency, fine-grained
access control, a rich set of built-in functions, stored procedures,
esoteric SQL language features, XML and/or Java extensions,
tera- or peta-byte scalability, and so forth.  If you need some of these
features and do not mind the added complexity that they
bring, then SQLite is probably not the database for you.
SQLite is not intended to be an enterprise database engine.  It
not designed to compete with Oracle or PostgreSQL.
</p>

<p>
The basic rule of thumb for when it is appropriate to use SQLite is
this:  Use SQLite in situations where simplicity of administration,
implementation, and maintenance are more important than the countless
complex features that enterprise database engines provide.
As it turns out, situations where simplicity is the better choice
are more common than many people realize.
</p>

<h2>Situations Where SQLite Works Well</h2>

<ul>
<li><p><b>Websites</b></p>

<p>SQLite usually will work great as the database engine for low to
medium traffic websites (which is to say, 99.9% of all websites).
The amount of web traffic that SQLite can handle depends, of course,
on how heavily the website uses its database.  Generally
speaking, any site that gets fewer than a 100000 hits/day should work
fine with SQLite.
The 100000 hits/day figure is a conservative estimate, not a
hard upper bound.
SQLite has been demonstrated to work with 10 times that amount
of traffic.</p>
</li>

<li><p><b>Embedded devices and applications</b></p>

<p>Because an SQLite database requires little or no administration,
SQLite is a good choice for devices or services that must work
unattended and without human support.  SQLite is a good fit for
use in cellphones, PDAs, set-top boxes, and/or appliances.  It also
works well as an embedded database in downloadable consumer applications.
</p>
</li>

<li><p><b>Application File Format</b></p>

<p>
SQLite has been used with great success as the on-disk file format
for desktop applications such as financial analysis tools, CAD
packages, record keeping programs, and so forth.  The traditional
File/Open operation does an sqlite3_open() and executes a
BEGIN TRANSACTION to get exclusive access to the content.  File/Save
does a COMMIT followed by another BEGIN TRANSACTION.  The use
of transactions guarantees that updates to the application file are atomic,
durable, isolated, and consistent.
</p>

<p>
Temporary triggers can be added to the database to record all
changes into a (temporary) undo/redo log table.  These changes can then
be played back when the user presses the Undo and Redo buttons.  Using
this technique, a unlimited depth undo/redo implementation can be written
in surprising little code.
</p>
</li>

<li><p><b>Replacement for <i>ad hoc</i> disk files</b></p>

<p>Many programs use fopen(), fread(), and fwrite() to create and
manage files of data in home-grown formats.  SQLite works 
particularly well as a
replacement for these <i>ad hoc</i> data files.</p>
</li>

<li><p><b>Internal or temporary databases</b></p>

<p>
For programs that have a lot of data that must be sifted and sorted
in diverse ways, it is often easier and quicker to load the data into
an in-memory SQLite database and use queries with joins and ORDER BY
clauses to extract the data in the form and order needed rather than
to try to code the same operations manually.
Using an SQL database internally in this way also gives the program
greater flexibility since new columns and indices can be added without
having to recode every query.
</p>
</li>

<li><p><b>Command-line dataset analysis tool</b></p>

<p>
Experienced SQL users can employ
the command-line <b>sqlite</b> program to analyze miscellaneous
datasets. Raw data can be imported from CSV files, then that
data can be sliced and diced to generate a myriad of summary
reports.  Possible uses include website log analysis, sports
statistics analysis, compilation of programming metrics, and
analysis of experimental results.
</p>

<p>
You can also do the same thing with a enterprise client/server
database, of course.  The advantages to using SQLite in this situation
are that SQLite is much easier to set up and the resulting database 
is a single file that you can store on a floppy disk or flash-memory stick
or email to a colleague.
</p>
</li>

<li><p><b>Stand-in for an enterprise database during demos or testing</b></p>

<p>
If you are writing a client application for an enterprise database engine,
it makes sense to use a generic database backend that allows you to connect
to many different kinds of SQL database engines.  It makes even better
sense to
go ahead and include SQLite in the mix of supported database and to statically
link the SQLite engine in with the client.  That way the client program
can be used standalone with an SQLite data file for testing or for
demonstrations.
</p>
</li>

<li><p><b>Database Pedagogy</b></p>

<p>
Because it is simple to setup and use (installation is trivial: just
copy the <b>sqlite</b> or <b>sqlite.exe</b> executable to the target machine
and run it) SQLite makes a good database engine for use in teaching SQL.
Students can easily create as many databases as they like and can
email databases to the instructor for comments or grading.  For more
advanced students who are interested in studying how an RDBMS is
implemented, the modular and well-commented and documented SQLite code
can serve as a good basis.  This is not to say that SQLite is an accurate
model of how other database engines are implemented, but rather a student who
understands how SQLite works can more quickly comprehend the operational
principles of other systems.
</p>
</li>

<li><p><b>Experimental SQL language extensions</b></p>

<p>The simple, modular design of SQLite makes it a good platform for
prototyping new, experimental database language features or ideas.
</p>
</li>


</ul>

<h2>Situations Where Another RDBMS May Work Better</h2>

<ul>
<li><p><b>Client/Server Applications</b><p>

<p>If you have many client programs accessing a common database
over a network, you should consider using a client/server database
engine instead of SQLite.  SQLite will work over a network filesystem,
but because of the latency associated with most network filesystems,
performance will not be great.  Also, the file locking logic of
many network filesystems implementation contains bugs (on both Unix
and windows).  If file locking does not work like it should,
it might be possible for two or more client programs to modify the
same part of the same database at the same time, resulting in 
database corruption.  Because this problem results from bugs in
the underlying filesystem implementation, there is nothing SQLite
can do to prevent it.</p>

<p>A good rule of thumb is that you should avoid using SQLite
in situations where the same database will be accessed simultaneously
from many computers over a network filesystem.</p>
</li>

<li><p><b>High-volume Websites</b></p>

<p>SQLite will normally work fine as the database backend to a website.
But if you website is so busy that your are thinking of splitting the
database component off onto a separate machine, then you should 
definitely consider using an enterprise-class client/server database
engine instead of SQLite.</p>
</li>

<li><p><b>Very large datasets</b></p>

<p>When you start a transaction in SQLite (which happens automatically
before any write operation that is not within an explicit BEGIN...COMMIT)
the engine has to allocate a bitmap of dirty pages in the disk file to
help it manage its rollback journal.  SQLite needs 256 bytes of RAM for
every 1MiB of database (assuming a 1024-byte page size: less memory is
used with larger page sizes, of course).  
For smaller databases, the amount of memory
required is not a problem, but when database begin to grow into the
multi-gigabyte range, the size of the bitmap can get quite large.  If
you need to store and modify more than a few dozen GB of data, you should
consider using a different database engine.
</p>
</li>

<li><p><b>High Concurrency</b></p>

<p>
SQLite uses reader/writer locks on the entire database file.  That means
if any process is reading from any part of the database, all other
processes are prevented from writing any other part of the database.
Similarly, if any one process is writing to the database,
all other processes are prevented from reading any other part of the
database.
For many situations, this is not a problem.  Each application
does its database work quickly and moves on, and no lock lasts for more
than a few dozen milliseconds.  But there are some applications that require
more concurrency, and those applications may need to seek a different
solution.
</p>
</li>

</ul>

}
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