Tutorial – CalDAV/CardDAV

If you keep a digital calendar or address book, you want your data to be stored in one central location and accessible from any device, wherever and whenever you need it. You also want ownership of your data and metadata. By using free and open source software (FOSS), you can create calendars and address books in a private cloud that allows you to synchronize and share that data, without being locked into some corporate, data-harvesting walled garden.

In this tutorial, I will explain the open standards, CalDAV and CardDAV, that make independent storing and sharing of calendar and address book data possible. Then, I will show you how to automatically import or export calendars and contacts, from any source, to a Nextcloud instance, process that data, and migrate it to another server. Finally, I will outline how to set up your own standalone calendar and address book.

CalDAV/CardDAV

CalDAV and CardDAV, the open standards that allow centralized storage and management of personal data, are both supersets of the Web Distributed Authoring and Versioning (WebDAV) system. WebDAV's specification describes how software programs can edit remote content over the Internet, using the same protocol (HTTP) that browsers use to load pages from websites. CalDav is the WebDAV extension for calendars, and CardDAV is the extension for personal contacts.

Both CalDAV and CardDAV are client-server protocols: They let many users, each with their own interfaces, simultaneously access the same set of events or contacts stored on a common server.

At the lower level, single events or whole calendars are stored inside files with the iCalendar extension .ics (also .ical or .icalender) [1]. Files that contain address books, instead, have the Virtual Contact File extension .vcf [2].

Both .ics (Listing 1) and .vcf (Listing 2) files use plain text formats with a relatively simple syntax that could be written manually with any text editor. The calendar event in Listing 1 shows an annual all staff meeting that must occur from February 12 to 13 (2008 in this example), as described in the RRULE (recurrence rule) field. The DESCRIPTION variable describes the meeting's purpose (a project status checkup) and the GEO variable gives the meeting's location. The whole event is enclosed by BEGIN:VEVENT and END:VEVENT tags, inside a VCALENDAR that may contain other events both before or after this event.

BEGIN:VCALENDAR
BEGIN:VEVENT
SUMMARY:All Staff Meeting
STATUS:CONFIRMED
RRULE:FREQ=YEARLY;INTERVAL=1;BYMONTH=2;BYMONTHDAY=12
DTSTART:20080212
DTEND:20080213
CATEGORIES:teamwork, project management
GEO:87.5739497;-45.7399606
DESCRIPTION: project status check-up
END:VEVENT
END:VCALENDAR

BEGIN:VCARD
VERSION:4.0
FN:Bruce Wayne
ADR;TYPE=WORK:;;;Gotham City;USA;;Planet Earth
EMAIL;TYPE=HOME:batman@example.com
TEL;TYPE="HOME,VOICE":555-5555
URL:https://batman.example.com
ORG:Billionaire
TITLE:Dark Knight
END:VCARD

Besides VEVENTs, an .ics calendar may also store to-do items (VTODO), journal entries (VJOURNAL), and time zone information (VTIMEZONE). Rich text in HTML format can be specified by the parameter X-ALT-DESC.

Both .ics and .vcf files also contain unique identifiers (UID) that allow the servers to index them. On the host computers, the files are stored in folders with a defined structure called vdir. Each vdir corresponds to a different calendar or address book. While it is possible to create, delete, or edit a vdir's contents, I recommend using a proper CalDAV or CardDAV client to prevent corruption of vdir's indexing.

Nextcloud

In this tutorial, I use Nextcloud [3], which is currently the most promising solution for open source, self-hosted cloud services. To install Nextcloud and understand its importance, please see my blog [4].

Under the hood, both the calendars and contacts of all of a Nextcloud instance's users are managed by an embedded CalDAV/CardDAV server.

Figures 1 and 2 show details of the address book and calendar interfaces that I created for a test Nextcloud account. In Figure 1, you can see the data that Nextcloud saved in Listing 2.

Figure 1: Nextcloud's Contacts app is the interface to its own CardDAV server.

I describe the Nextcloud GUIs shown in Figures 1 and 2 in more detail in my blog [4]. However, there are at least two instances when there is a better way to process this personal data: The first is mass migration to or from another server or bulk export to some other database; the second is whenever you want to simultaneously modify contacts or calendars. Especially in the second instance, it can be much faster to make local copies of all the records, modify them with a script, and then re-upload everything to the server. The tools described in this tutorial can be used for all these tasks.

vdirsyncer

Theoretically, the simplest way to process automatically a CalDAV or CardDAV server's contents would be a command-line client that can connect directly to that server. In practice, I have found an alternate approach that seems easier to implement with standard tools and even more flexible: Copy the server data on your computer, change as needed those copies, and then sync the vdirs on the server with those on your local archive. This approach also has the advantage of keeping contacts and events accessible even when the server is unreachable. I use vdirsyncer [5] to perform the synchronization.

In addition to installing manually, you can find vdirsyncer binary packages for several distributions, but not in the usual places. On my Ubuntu 19.10 desktop, for instance, I had to install from a custom repository with the sequence of commands shown in Listing 3, which I found in vdirsyncer's very complete manual [6].

01 curl -s https://packagecloud.io/install/repositories/pimutils/vdirsyncer/script.deb.sh | sudo bash
02 sudo apt-get update
03 sudo apt-get install vdirsyncer-latest
04 export PATH="$PATH:/opt/venvs/vdirsyncer-latest/bin"

The command in line 1 of Listing 3 downloads and executes a shell script that fetches data for the custom repository and adds them to the Ubuntu package database. Line 3 is what actually installs vdirsyncer on your system. Line 4 tells Ubuntu to add the vdirsyncer executable's folder (/opt/...) to the list of folders ($PATH) where programs are stored.

Once vdirsyncer is installed, it needs to know which vdirs it should sync and their location. You do this with configuration files like vdirsyncer-nextcloud.conf (Listing 4).

01 [general]
02 status_path = "~/.vdirsyncer/status/"
03 [pair contacts_nextcloud_to_local]
04 a = "my_contacts_local"
05 b = "my_contacts_nextcloud"
06 collections = ["from a", "from b"]
07 [storage my_contacts_local]
08 type = "filesystem"
09 path = "~/.contacts/"
10 fileext = ".vcf"
11 [storage my_contacts_nextcloud]
12 type = "carddav"
13 url = https://full-url-of-my-private-nextcloud-instance/"
14 username = "nextcloud_test_user"
15 password = "nextcloud_test_password"
16
17 [pair cal_nextcloud_to_local]
18 a = "my_cal_local"
19 b = "my_cal_nextcloud"
20 collections = ["from a", "from b"]
21 [storage my_cal_local]
22 type = "filesystem"
23 path = "~/.calendars/"
24 fileext = ".ics"
25 [storage my_cal_nextcloud]
26 type = "caldav"
27 url = https://full-url-of-my-private-nextcloud-instance/"
28 username = "nextcloud_test_user"
29 password = "nextcloud_test_password"

Lines 1 and 2 of Listing 4 tell vdirsyncer where to store the synchronization status of all the vdirs it manages.

Lines 3 to 15 define a pair of address books to sync. The address book on your local computer, my_contacts_local (aliased as a), is stored in normal folders located inside the ~/.contacts directory and contains .vcf files. The Nextcloud remote address book (lines 11 to 15) is accessible with the CardDAV protocol at the URL shown on line 13, with the credentials shown in lines 14 and 15. Line 6 means that vdirsyncer must sync all the collections in each pair of vdirs.

The only difference in lines 17 to 29, visible in the variable names and file extensions, is that this section tells vdirsyncer to sync two sets of calendars instead of address books.

After the initial configuration, but before the actual synchronization, you need to make vdirsyncer "discover" all the data on the remote server (unless you configured to only sync one calendar, as explained in the documentation). Listing 5 shows how to do this plus includes an excerpt of the output.

01 #> vdirsyncer -c vdirsyncer-nextcloud.conf discover
02 Discovering collections for pair contacts_nextcloud_to_local
03 my_contacts_local:
04   - "contacts"
05 my_contacts_nextcloud:
06   - "contacts" ("Contacts")
07 Saved for contacts_nextcloud_to_local: collections = ["contacts"]
08 Discovering collections for pair cal_nextcloud_to_local
09 my_cal_local:
10 my_cal_nextcloud:
11   - "work" ("Work")
12 .....
13 warning: No collection "work" found for storage my_cal_local.
14 Should vdirsyncer attempt to create it? [y/N]: y
15 Saved for cal_nextcloud_to_local: collections = ["work"]

The output means that everything went well, and vdirsyncer discovered everything it was supposed to find. Passing the configuration file with the -c option is necessary when you want to use vdirsyncer with different servers. The actual synchronization is done with the following single command (which you could make a regular cron job):

#> vdirsyncer -c vdirsyncer-nextcloud.conf sync

Vdirsyncer has many other options and uses. Due to space constraints, I will only discuss how to handle conflicts. If two calendars contain two different records for the same event, you must decide which calendar is the master that should win such conflicts and explicitly tell vdirsyncer. (Listing 5), if the Nextcloud calendar is the master, you should add a line like this:

conflict_resolution = "b wins"

right after the lines 6 and 20 of Listing 5. In addition to the vdirsyncer manual [6], see [7] for more conflict resolution examples and vdirsyncer configuration tips.