Documentation Source Text

<tcl>hd_keywords *appformat {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

<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 (less than 2% larger)
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
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.

<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 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)], futher
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

<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 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 

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 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>A desktop application built on SQLite can also do continuous update.
In other words, 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.  The 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 

<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 as their application file format, 
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