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SQLite Archive Files

1. Introduction

An "SQLite Archive" is a file container similar to a ZIP archive or Tarball but based on an SQLite database.

An SQLite Archive is an ordinary SQLite database file that contains the following table as part of its schema:

CREATE TABLE sqlar(
  name TEXT PRIMARY KEY,  -- name of the file
  mode INT,               -- access permissions
  mtime INT,              -- last modification time
  sz INT,                 -- original file size
  data BLOB               -- compressed content
);

Each row of the SQLAR table holds the content of a single file. The filename (the full pathname relative to the root of the archive) is in the "name" field. The "mode" field is an integer which is the unix-style access permissions for the file. "mtime" is the modification time of the file in seconds since 1970. "sz" is the original uncompressed size of the file. The "data" field contains the file content. The content is usually compressed using Deflate, though not always. If the "sz" field is equal to the size of the "data" field, then the content is stored uncompressed.

1.1. Database As Container Object

An SQLite Archive is one example of a more general idea that an SQLite database can behave as a container object holding lots of smaller data components.

With client/server databases like PostgreSQL or Oracle, users and developers tend to think of the database as a service or a "node", not as an object. This is because the database content is spread out across multiple files on the server, or possibly across multiple servers in a service cluster. One cannot point to a single file or even a single directory and say "this is the database".

SQLite, in contrast, stores all content in a single file on disk. That single file is something you can point to and say "this is the database". It behaves as an object. An SQLite database file can be copied, renamed, sent as an email attachment, passed as the argument a POST HTTP request, or otherwise treated as other data object such as an image, document, or media file.

Studies show that many applications already use SQLite as a container object. For example, Kennedy (no relation to the SQLite developer) reports that 14% of Android applications never write to their SQLite databases. It is believed that these applications are downloading entire databases from the cloud and then using the information locally as needed. In other words, the applications are using SQLite not so much as a database but as a queryable wire-transfer format.

1.2. Applications Using SQLite Archives

The Fossil Distributed Version Control system provides users with the option to download check-ins as either Tarballs, ZIP Archives, or SQLite Archives.

2. Advantages Of SQLite Archives

  1. An SQLite Archive is flexible. ZIP Archives and Tarballs are limited to storing only files. An SQLite Archive stores files plus whatever other tabular and/or relational data seems useful to the application.

  2. An SQLite Archive is transactional. Updates are atomic and durable, even if there are crashes or power losses in the middle of the update. Readers see a consistent and unchanging version of the content even is some other process is simultaneously updating the archive.

  3. An SQLite Archive can be updated incrementally. Individual files can be added or removed or replaced without having to rewrite the entire archive.

  4. An SQLite Archive can be queried using a high-level query language (SQL). Some examples:

    • What is the total size of all files in the archive whose names end in ".h" or ".cpp"?
    • What percentage of the files are compressed by less than 25%?
    • How many executable files are in the archive?
    Questions like these (and countless others) can be answered without having to uncompress or extract any content.
  5. Applications that already use SQLite for other purposes can easily add support for SQLite Archives using a small extension (https://sqlite.org/src/file/ext/misc/sqlar.c) to handle the compression and decompression of content. Even this tiny extension can be omitted if the files in the archive are uncompressed. In contrast, supporting ZIP Archives and/or Tarballs requires either separate libraries or lots of extra custom code, or sometimes both.

  6. An SQLite Archive can work around firewall-imposed censorship. For example, certain file types that are considered "dangerous" (examples: DLLs) will be blocked by Gmail and probably many other email services and firewall, even if those files are wrapped inside a ZIP Archive or Tarball. But these firewall usually do not care about SQLite Archives and so content can be put inside an SQLite Archive to evade censorship.

3. Disadvantages Of SQLite Archives

  1. The SQLite Archive is a relatively new format. It was first described in in 2014. ZIP Archives and Tarballs, on the other hand, have been around for decades and are well-entrenched as standard formats. Most programmers know what a ZIP Archive or Tarball is, but if you say "SQLite Archive" you are more likely to get a reply of "What?" Tooling to process ZIP Archives and Tarballs is more likely to be installed on stock computers.

  2. Since an SQLite database is a more general format (it is designed to do much more than simply store a bunch of files) it is not as compact as either the ZIP Archive or Tarball formats. An SQLite Archive is usually about 1% larger than the equivalent ZIP Archive. Tarballs are compressed as a single unit rather than compressing each file separately as is done by both SQLite and ZIP Archives. For these reason, Tarballs tend to be smaller than either ZIP or SQLite Archives.

    As an example, the following table show the relative sizes for an SQLite Archive, a ZIP Archive, and a Tarball of the 1,743 files in the SQLite 3.22.0 source tree:

    SQLite Archive10,754,048
    ZIP Archive (using Info-ZIP 3.0)10,662,365
    ZIP Archive (using zipfile)10,390,215
    Tarball 9,781,109
  3. An SQLite Archive supports only the Deflate compression method. Tarballs and ZIP Archive support a wider assortment of compression methods.

4. Managing An SQLite Archive From The Command-Line

The recommended way of creating, updating, listing, and extracting an SQLite Archive is to use the sqlite3.exe command-line shell for SQLite version 3.23.0 (2018-04-02) or later. This CLI supports the -A command-line option that allows easy management of SQLite Archives. The CLI for SQLite version 3.22.0 (2018-01-22) has the .archive command for managing SQLite Archives, but that requires interacting with the shell.

To list all of the files in an SQLite Archive named "example.sqlar" using one of these commands:

sqlite3 example.sqlar -At
sqlite3 example.sqlar -Atv

To extract all files from an SQLite Archive named "example.sqlar":

sqlite3 example.sqlar -Ax

To create a new SQLite Archive named "alltxt.sqlar" containing all *.txt files in the current directory:

sqlite3 alltxt.sqlar -Ac *.txt

For usage hints and a summary of all options, simply give the CLI the -A option with no additional arguments:

sqlite3 -A

4.1. Other command-line tools

Just as there is the "zip" program to manage ZIP Archives, and the "tar" program to manage Tarballs, the "sqlar" program exists to manage SQL Archives. The "sqlar" program is able to create a new SQLite Archive, list the content of an existing archive, add or remove files from the archive, and/or extract files from the archive. A separate "sqlarfs" program is able to mount the SQLite Archive as a Fuse Filesystem.

5. Managing SQLite Archives From Application Code

Applications can easily read or write SQLite Archives by linking against SQLite and including the ext/misc/sqlar.c extension to handle the compression and decompression. The sqlar.c extension creates two new SQL functions.

sqlar_compress(X)

The sqlar_compress(X) function attempts to compress a copy of the string or blob X using the Default algorithm and returns the result as a blob. If the input X is incompressible, then a copy of X is returned. This routine is used when inserting content into an SQLite Archive.

sqlar_uncompress(Y,SZ)

The sqlar_uncompress(Y,SZ) function will undo the compression accomplished by sqlar_compress(X). The Y parameter is the compressed content (the output from a prior call to sqlar_compress()) and SZ is the original uncompressed size of the input X that generated Y. If SZ is less than or equal to the size of Y, that indicates that no compression occurred, and so sqlar_uncompress(Y,SZ) returns a copy of Y. Otherwise, sqlar_uncompress(Y,SZ) runs the Inflate algorithm on Y to uncompress it and restore it to its original form and returns the uncompressed content. This routine is used when extracting content from an SQLite Archive.

Using the two routines above, it is simple for applications to insert new records into or extract existing records from an SQLite Archive. Insert a new into an SQLite Archive using code like this:

INSERT INTO sqlar(name,mode,mtime,sz,data)
 VALUES ($name,$mode,strftime('%s',$mtime),
         length($content),sqlar_compress($content));

Extract an entry from the SQLite Archive using code like this:

SELECT name, mode, datetime(mtime,'unixepoch'), sqlar_uncompress(data,sz)
  FROM sqlar
 WHERE ...;

The code above is for the general case. For the special case of an SQLite Archive that only stores uncompressed or uncompressible content (this might come up, for example, in an SQLite Archive that stores only JPEG, GIF, and/or PNG images) then the content can be inserted into and extracted from the database without using the sqlar_compress() and sqlar_uncompress() functions, and the sqlar.c extension is not required.