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<tcl>hd_keywords *appformat {application file-format} \
     {Application File Format} {application file format}</tcl>
<title>SQLite As An Application File Format</title>

<h1 align="center">
SQLite As An Application File Format
</h1>

<h2>Executive Summary</h2>

<p>An SQLite database file with a defined schema
often makes an excellent application file format.
Here are a dozen reasons why this is so:

<ol>
<li> Simplified Application Development
<li> Single-File Documents
<li> High-Level Query Language
<li> Accessible Content
<li> Cross-Platform
<li> Atomic Transactions
<li> Incremental And Continuous Updates
<li> Easily Extensible
<li> Performance
<li> Concurrent Use By Multiple Processes
<li> Multiple Programming Languages
<li> Better Applications
</ol>

<p>Each of these points will be described in more detail below,
after first considering more closely the meaning of
"application file format".

<h2>What Is An Application File Format?</h2>

<p>
An "application file format" is the file format
used to persist application state to disk or to exchange
information between programs.
There are thousands of application file formats in use today.
Here are just a few examples:

<ul>
<li>DOC - Word Perfect and Microsoft Office documents
<li>DWG - AutoCAD drawings
<li>PDF - Portable Document Format from Adobe
<li>XLS - Microsoft Excel Spreadsheet
<li>GIT - Git source code repository
<li>EPUB - The Electronic Publication format used by non-Kindle eBooks
<li>ODT - The Open Document format used by OpenOffice and others
<li>PPT - Microsoft PowerPoint presentations
<li>ODP - The Open Document presentation format used by OpenOffice and others
</ul>

<p>We make a distinction between a "file format" and an "application format".
A file format is used to store a single object.  So, for example, a GIF or
JPEG file stores a single image, and an XHTML file stores text,
so those are "file formats" and not "application formats".  An EPUB file, 
in contrast, stores both text and images (as contained XHTML and GIF/JPEG
files) and so it is considered a "application format".  This article is
about "application formats".

<p>The boundary between a file format and an application format is fuzzy.
This article calls JPEG a file format, but for an image editor, JPEG 
might be considered the application format.  Much depends on context.
For this article, let us say that a file format stores a single object
and an application format stores many different objects and their relationships
to one another.

<p>Most application formats fit into one of these three categories:

<ol>
<li><p><b>Fully Custom Formats.</b>
Custom formats are specifically designed for a single application.
DOC, DWG, PDF, XLS, and PPT are examples of custom formats.  Custom
formats are usually contained within a single file, for ease of transport.
They are also usually binary, though the DWG format is a notable exception.
Custom file formats require specialized application code
to read and write and are not normally accessible from commonly
available tools such as unix command-line programs and text editors.
In other words, custom formats are usually "opaque blobs".
To access the content of a custom application file format, one needs
a tool specifically engineered to read and/or write that format.

<li><p><b>Pile-of-Files Formats.</b>
Sometimes the application state is stored as a hierarchy of
files.  Git is a prime example of this, though the phenomenon occurs
frequently in one-off and bespoke applications.  A pile-of-files format
essentially uses the filesystem as a key/value database, storing small
chunks of information into separate files.  This gives the
advantage of making the content more accessible to common utility
programs such as text editors or "awk" or "grep".  But even if many 
of the files in a pile-of-files format
are easily readable, there are usually some files that have their
own custom format (example: Git "Packfiles") and are hence
"opaque blobs" that are not readable
or writable without specialized tools.  It is also much less convenient
to move a pile-of-files from one place or machine to another, than
it is to move a single file.  And it is hard to make a pile-of-files
document into an email attachment, for example.  Finally, a pile-of-files
format breaks the "document metaphor":
there is no one file that a user can point to
that is "the document".

<li><p><b>Wrapped Pile-of-Files Formats.</b>
Some applications use a Pile-of-Files that is then encapsulated into
some kind of single-file container, usually a ZIP archive.  
EPUB, ODT,and ODP are examples of this approach.
An EPUB book is really just a ZIP archive that contains various
XHTML files for the text of book chapters, GIF and JPEG images for
the artwork, and a specialized catalog file that tells the eBook
reader how all the XML and image files fit together.  OpenOffice
documents (ODT and ODP) are also ZIP archives containing XML and
images that represent their content as well as "catalog" files that
show the interrelationships between the component parts.

<p>A wrapped pile-of-files format is a compromise between a full
custom file format and a pure pile-of-files format.
A wrapped pile-of-files format is not an opaque blob in the same sense
as a custom format, since the component parts can still be accessed
using any common ZIP archiver, but the format is not quite as accessible
as a pure pile-of-files format because one does still need the ZIP 
archiver, and one cannot normally use command-line tools like "find"
on the file hierarchy without first un-zipping it.  On the other
hand, a wrapped pile-of-files format does preserve the document
metaphor by putting all content into a single disk file.  And
because it is compressed, the wrapped pile-of-files format tends to
be more compact.

<p>As with custom file formats, and unlike pure pile-of-file formats,
a wrapped pile-of-files format is not as easy to edit, since
usually the entire file must be rewritten in order to change any
component part.
</ol>

<p>The purpose of this document is to argue in favor of a fourth
new catagory of application file format: An SQLite database file.

<h2>SQLite As The Application File Format</h2>

<p>
Any application state that can be recorded in a pile-of-files can
also be recorded in an SQLite database with a simple key/value schema
like this:
<blockquote><pre>
CREATE TABLE files(filename TEXT PRIMARY KEY, content BLOB);
</pre></blockquote>
If the content is compressed, then such an SQLite database is only
slightly larger, and [SQLAR smaller than ZIP|sometimes smaller] 
than an equivalent ZIP archive, and it has the advantage
of being able to update individual "files" without rewrite
the entire document.

<p>
But an SQLite database is not limited to a simple key/value structure
like a pile-of-files database.  An SQLite database can have dozens
or hundreds or thousands of different of tables, with dozens or
hundreds or thousands of fields per table, each with different datatypes
and constraints and particular meanings, all cross-referencing each other,
appropriately and automatically indexed for rapid retrieval,
and all stored efficiently and compactly in a single disk file.
And all of this structure is succinctly documented for humans
by the SQL schema.

<p>In other words, an SQLite database can do everything that a 
pile-of-files or wrapped pile-of-files format can do, plus much more,
and with greater lucidity.
An SQLite database is a more versatile container than key/value
filesystem or a ZIP archive.  (For a detailed example, see the
[OpenOffice case study] essay.)

<p>The power of an SQLite database could, in theory, be achieved using
a custom file format.  But any custom file format that is as expressive
as a relational database would likely require an enormous design specification 
and many tens or hundreds of thousands of lines of code to 
implement.  And the end result would be an "opaque blob" that is
inaccessible without specialized tools.

<p>
Hence, in comparison to other approaches, the use of
an SQLite database as an application file format has
compelling advantages.  Here are a few of these advantages,
enumerated and expounded:
</p>

<ol>
<li><p><b>Simplified Application Development.</b>
No new code is needed for reading or writing the application file.
One has merely to link against the SQLite library, or include the 
[amalgamation | single "sqlite3.c" source file] with the rest of the
application C code, and SQLite will take care of all of the application
file I/O.  This can reduce application code size by many thousands of
lines, with corresponding saving in development and maintenance costs.

<p>SQLite is widely used and stable.  There are literally billions of
SQLite database files in use daily, on smartphones and gadgets
and in desktop applications.
SQLite is [tested | carefully tested] and proven reliable.  It is not
a component that needs much tuning or debugging, allowing developers
to stay focused on application logic.

<li><p><b>Single-File Documents.</b>
An SQLite database is contained in a single file, which is easily
copied or moved or attached.  The "document" metaphor is preserved.

<p>SQLite does not have any file naming requirements
and so the application can use any custom file suffix that it wants
to help identify the file as "belonging" to the application.
SQLite database files contain a 4-byte [Application ID] in
their headers that can be set to an application-defined value
and then used to identify the "type" of the document for utility
programs such as [http://linux.die.net/man/1/file | file(1)], further
enhancing the document metaphor.


<li><p><b>High-Level Query Language.</b>
SQLite is a complete relational database engine, which means that the
application can access content using high-level queries.  Application
developers need not spend time thinking about "how" to retrieve the
information they need from a document.  Developers write SQL that
expresses "what" information they want and let the database engine
to figure out how to best retrieve that content.  This helps developers
operate "heads up" and remain focused on solving the user's problem,
and avoid time spent "heads down" fiddling with low-level file
formatting details.

<p>A pile-of-files format can be viewed as a key/value database.  
A key/value database is better than no database at all.
But without transactions or indices or a high-level query language or
a proper schema,
it much harder and more error prone to use a key/value database than
a relational database.

<li><p><b>Accessible Content.</b>
Information held in an SQLite database file is accessible using 
commonly available open-source command-line tools - tools that 
are installed by default on Mac and Linux systems and that are 
freely available as a self-contained EXE file on Windows.
Unlike custom file formats, application-specific programs are
not required to read or write content in an SQLite database.
An SQLite database file is not an opaque blob.  It is true
that command-line tools such as text editors or "grep" or "awk" are
not useful on an SQLite database, but the SQL query language is a much
more powerful and convenient way for examining the content, so the
inability to use "grep" and "awk" and the like is not seen as a loss.

<p>An SQLite database is a [file format | well-defined and well-documented]
file format that is in widespread use by literally hundreds of 
thousands of applications and
is backwards compatible to its inception in 2004 and which promises
to continue to be compatible in years to come.  The longevity of
SQLite database files is particularly important to bespoke applications,
since it allows the document content to be accessed years or decades in the
future, long after all traces of the original application have been lost.
Data lives longer than code.

<li><p><b>Cross-Platform.</b>
SQLite database files are portable between 32-bit and 64-bit machines and
between big-endian and little-endian architectures and between any of the
various flavors of Windows and Unix-like operating systems.
The application using an SQLite application file format can store
binary numeric data without having to worry about the byte-order of
integers or floating point numbers.
Text content can be read or written as UTF-8, UTF-16LE, or UTF-16BE and 
SQLite will automatically perform any necessary translations on-the-fly.

<li><p><b>Atomic Transactions.</b>
Writes to an SQLite database are [atomic commit | atomic].  
They either happen completely
or not at all, even during system crashes or power failures.  So
there is no danger of corrupting a document just because the power happened
to go out at the same instant that a change was being written to disk.

<p>SQLite is transactional, meaning that multiple changes can be grouped
together such that either all or none of them occur, and so that the
changes can be rolled back if a problem is found prior to commit.
This allows an application to make a change incrementally, then run
various sanity and consistency checks on the resulting data prior to
committing the changes to disk.  The
[http://www.fossil-scm.org/ | Fossil] DVCS 
[http://www.fossil-scm.org/fossil/doc/tip/www/selfcheck.wiki|uses this technique]
to verify that no repository history has been lost prior to each change.

<li><p><b>Incremental And Continuous Updates.</b>
When writing to an SQLite database file, only those parts of the file that
actually change are written out to disk.  This makes the writing happen faster
and saves wear on SSDs.  This is an enormous advantage over custom
and wrapped pile-of-files formats, both of which usually require a
rewrite of the entire document in order to change a single byte.  
Pure pile-of-files formats can also
do incremental updates to some extent, though the granularity of writes is 
usually larger with pile-of-file formats (a single file) than with SQLite
(a single page).

<p>SQLite also supports continuous update.
Instead of collecting changes in memory and then writing
them to disk only on a File/Save action, changes can be written back to
the disk as they occur.  This avoids loss of work on a system crash or
power failure.  The undo/redo stack can also be kept in the on-disk 
database, meaning that undo/redo can occur across session boundaries.
Maintenance of the undo/redo stack can often be automated using SQL
triggers.

<li><p><b>Easily Extensible.</b>
As an application grows, new features can be added to an
SQLite application file format simply by adding new tables to the schema
or by adding new columns to existing tables.  Adding columns or tables
does not change the meaning of prior queries, so with a 
modicum of care to ensuring that the meaning of legacy columns and
tables are preserved, backwards compatibility is maintained.

<p>It is possible to extend custom or pile-of-files formats too, of course,
but doing is often much harder.  If indices are added, then all application
code that changes the corresponding tables must be located and modified to
keep those indices up-to-date.  If columns are added, then all application
code that accesses the corresponding table must be located and modified to 
take into account the new columns.

<li><p><b>Performance.</b>
In many cases, an SQLite application file format will be faster than a
custom or pile-of-files format.  In the case of a custom format, SQLite
often dramatically improves start-up times because instead of having to
read and parse the entire document into memory, the application can
do queries to extract only the information needed for the initial screen.
As the application progresses, it only needs to load as much material as
is needed to draw the next screen, and can discard information from
prior screens that is no longer in use.  This helps keep the memory
footprint of the application under control.

<p>A pile-of-files format can be read incrementally just like SQLite.
But many developers are surprised to learn that SQLite can read and 
write smaller BLOBs (less than about 100KB in size) from its database 
faster than those same blobs can be read or written as separate files 
from the filesystem.  (See [Internal Versus External BLOBs] for further 
information.)  There is overhead associated with operating a relational
database engine, however one should not assume that direct file I/O
is faster than SQLite database I/O, as often it is not.

<p>In either case, if performance problems do arise in an SQLite application 
those problems can often be resolved by adding one or two [CREATE INDEX]
statements to the schema or perhaps running [ANALYZE] one time
and without having to touch a single line of
application code.  But if a performance problem comes up in a custom or 
pile-of-files format, the fix will often require extensive changes
to application code to add and maintain new indices or to extract 
information using different algorithms.

<li><p><b>Concurrent Use By Multiple Processes.</b>
SQLite automatically coordinates concurrent access to the same
document from multiple threads and/or processes.  Two or more
applications can connect and read from the same document at the
same time.  Writes are serialized, but as writes normally only
take milliseconds, applications simply take turns writing.
SQLite automatically ensures that the low-level format of the
document is uncorrupted.  Accomplishing the same with a custom
or pile-of-files format, in contrast, requires extensive support
in the application.  And the application logic needed to support
concurrency is a notorious bug-magnet.

<li><p><b>Multiple Programming Languages.</b>
Though SQLite is itself written in ANSI-C, interfaces exist for
just about every other programming language you can think of:
C++, C#, Objective-C, Java, Tcl, Perl, Python, Ruby, Erlang,
JavaScript, and so forth.  So programmers can develop in whatever
language they are most comfortable with and which best matches
the needs of the project.

<p>An SQLite application file format is a great 
choice in cases where there is a collection or "federation" of
separate programs, often written in different languages and by
different development teams.
This comes up commonly in research or laboratory
environments where one team is responsible for data acquisition
and other teams are responsible for various stages of analysis.
Each team can use whatever hardware, operating system,
programming language and development methodology that they are 
most comfortable with, and as long as all programs use an SQLite 
database with a common schema, they can all interoperate.


<li><p><b>Better Applications.</b>
If the application file format is an SQLite database, the complete
documentation for that file format consists of the database schema,
with perhaps a few extra words about what each table and column
represents.  The description of a custom file format,
on the other hand, typically runs on for hundreds of 
pages.  A pile-of-files format, while much simpler and easier to
describe than a fully custom format, still tends to be much larger
and more complex than an SQL schema dump, since the names and format
for the individual files must still be described.

<p>This is not a trivial point.  A clear, concise, and easy to understand
file format is a crucial part of any application design.
Fred Brooks, in his all-time best-selling computer science text,
<i>The Mythical Man-Month</i> says:
<blockquote><i>Representation is the
essence of computer programming.<br />...<br />
Show me your flowcharts and conceal your tables, and I shall 
continue to be mystified.  Show me your tables, and I won't usually
need your flowcharts; they'll be obvious.</i></blockquote>
<p>Rob Pike, in his
<i>Rules of Programming</i> expresses the same idea this way:
<blockquote>
<i>Data dominates.  If you've chosen the right data structures
and organized things well, the algorithms will almost always
be self-evident.  Data structures, not algorithms, are central
to programming.</i></blockquote>
<p> Linus Torvalds used different words to say
much the same thing on the Git mailing list on 2006-06-27: 
<blockquote>
<i>Bad programmers worry about the code.  Good programmers worry
about data structures and their relationships.</i>
</blockquote>

<p>The point is this: an SQL database schema almost always does 
a far better job of defining and organizing the tables and 
data structures and their relationships.
And having clear, concise, and well-defined representation
almost always results in an application that performs better,
has fewer problems, and is easier to develop and maintain.
</ol>

<h2>Conclusion</h2>

<p>
SQLite is not the perfect application file format for every situation.
But in many cases, SQLite is a far better choice than either a custom
file format, a pile-of-files, or a wrapped pile-of-files.
SQLite is a high-level, stable, reliable, cross-platform, widely-deployed,
extensible, performant, accessible, concurrent file format.  It deserves
your consideration as the standard file format on your next application
design.