The Vdir Storage Format
***********************

This document describes a standard for storing calendars and contacts
on a filesystem, with the main goal of being easy to implement.

Vdirsyncer synchronizes to vdirs via "filesystem". Each vdir
(basically just a directory with some files in it) represents a
calendar or addressbook.


Basic Structure
===============

The main folder (root) contains an arbitrary number of subfolders
(collections), which contain only files (items). Synonyms for
"collection" may be "addressbook" or "calendar".

An item is:

* A vCard file, in which case the file extension *must* be *.vcf*,
  *or*

* An iCalendar file, in which case the file extension *must* be
  *.ics*.

An item *should* contain a "UID" property as described by the vCard
and iCalendar standards. If it contains more than one "UID" property,
the values of those *must* not differ.

The file *must* contain exactly one event, task or contact. In most
cases this also implies only one "VEVENT"/"VTODO"/"VCARD" component
per file, but e.g.  recurrence exceptions would require multiple
"VEVENT" components per event.

The filename should have similar properties as the "UID" of the file
content. However, there is no requirement for these two to be the
same. Programs may choose to store additional metadata in that
filename, however, at the same time they *must not* assume that the
metadata they included will be preserved by other programs.


Metadata
========

Any of the below metadata files may be absent. None of the files
listed below have any file extensions.

* A file called "color" inside the vdir indicates the vdir's color, a
  property that is only relevant in UI design.

  Its content is an ASCII-encoded hex-RGB value of the form "#RRGGBB".
  For example, a file content of "#FF0000" indicates that the vdir has
  a red (user-visible) color. No short forms or informal values such
  as "red" (as known from CSS, for example) are allowed. The prefixing
  "#" must be present.

* Files called "displayname" and "description" contain a UTF-8 encoded
  label/ description, that may be used to represent the vdir in UIs.

* A file called "order" inside the vdir includes the relative order of
  the calendar, a property that is only relevant in UI design.


Writing to vdirs
================

Creating and modifying items or metadata files *should* happen
atomically.

Writing to a temporary file on the same physical device, and then
moving it to the appropriate location is usually a very effective
solution. For this purpose, files with the extension ".tmp" may be
created inside collections.

When changing an item, the original filename *must* be used.


Reading from vdirs
==================

* Any file ending with the ".tmp" or no file extension *must not* be
  treated as an item.

* The "ident" part of the filename *should not* be parsed to improve
  the speed of item lookup.


Considerations
==============

The primary reason this format was chosen is due to its compatibility
with the CardDAV and CalDAV standards.


Performance
-----------

Currently, vdirs suffer from a rather major performance problem, one
which current implementations try to mitigate by building up indices
of the collections for faster search and lookup.

The reason items' filenames don't contain any extra information is
simple: The solutions presented induced duplication of data, where one
duplicate might become out of date because of bad implementations. As
it stands right now, an index format could be formalized separately
though.

vdirsyncer doesn't really have to bother about efficient item lookup,
because its synchronization algorithm needs to fetch the whole list of
items anyway. Detecting changes is easily implemented by checking the
files' modification time.
