Building a CalDAV client

This document is a general howto on how to build a CalDAV client. The document is language-agnostic, and considering the massive scope of CalDAV, not complete.

General synchronization concerns

The primary formats for transferring information is iCalendar for calendar objects (events and tasks) and xml for most other data.

CalDAV is based on WebDAV, which itself is an extension to HTTP.

Some operations will be very familiar if you already have experience with HTTP services (GET, PUT and DELETE), but many are added too (PROPFIND, PROPPATCH, REPORT, MKCOL, MKCALENDAR, ACL).

Most HTTP clients should just support methods they don't know about, so it's wise to simply use a stock HTTP client (or better yet, a DAV or CalDAV client).

One thing in which CalDAV differs from some other synchronization models, is that the 'truth' is always on the server. There should in general never really be a situation where there are conflicts, as the server is always correct.

One implication is that ideally the user should not be bothered by a 'Synchronize Now!' interface element. In an ideal world changes are submitted to the server the very instant that the user has made them.

Every single event and task is encoded as an iCalendar object. I strongly recommend always retaining the iCalendar the server sent to you.

Clients may add non-standard properties to iCalendar objects. It is important that when you GET and later on PUT an updated iCalendar object, any non-standard properties you may not have built-in support for gets retained.

Syncing a calendar

Simple clients tend to just access CalDAV servers based on the follow 3 setting:

username
password
calendar url

An example of this is Thunderbird Lightning. So this is where we start.

Retrieving calendar information

This is the recommended way to do an initial sync with SabreDAV. Every calendar has a so-called ctag. This ctag works like a change id. Every time the ctag has changed, you know something in the calendar has changed too.

An example request to get the ctag:

PROPFIND /calendars/johndoe/home/ HTTP/1.1
Depth: 0
Prefer: return-minimal
Content-Type: application/xml; charset=utf-8

<d:propfind xmlns:d="DAV:" xmlns:cs="http://calendarserver.org/ns/">
  <d:prop>
     <d:displayname />
     <cs:getctag />
  </d:prop>
</d:propfind>

The PROPFIND request is a HTTP request, defined by WebDAV. PROPFIND allows the client to fetch properties from a url.

CalDAV uses many properties like this, but in this case we just fetch the 'displayname', which is the human-readable name the user gave the calendar, and the ctag. The ctag must be stored for subsequent requests.

The request will return something like:

HTTP/1.1 207 Multi-status
Content-Type: application/xml; charset=utf-8

<d:multistatus xmlns:d="DAV:" xmlns:cs="http://calendarserver.org/ns/">
    <d:response>
        <d:href>/calendars/johndoe/home/</d:href>
        <d:propstat>
            <d:prop>
                <d:displayname>Home calendar</d:displayname>
                <cs:getctag>3145</cs:getctag>
            </d:prop>
            <d:status>HTTP/1.1 200 OK</d:status>
        </d:propstat>
    </d:response>
</d:multistatus>

This multistatus response is very common for Cal and WebDAV. Many requests return an xml document in this exact format, so it is worthwhile writing a standard parser.

The response gives us back the user, the values for the 2 properties and the status.

If the user did not have access to these properties, it's also possible that you get a response like this back:

HTTP/1.1 207 Multi-status
Content-Type: application/xml; charset=utf-8

<d:multistatus xmlns:d="DAV:" xmlns:cs="http://calendarserver.org/ns/">
    <d:response>
        <d:href>/calendars/johndoe/home/</d:href>
        <d:propstat>
            <d:prop>
                <d:displayname />
                <cs:getctag />
            </d:prop>
            <d:status>HTTP/1.1 403 Forbidden</d:status>
        </d:propstat>
    </d:response>
</d:multistatus>

So it is important that when you parse the response, you make sure that the status for the properties was actually 200 OK.

Downloading objects

Now we download every single object in this calendar. To do this, we use a REPORT method.

REPORT /calendars/johndoe/home/ HTTP/1.1
Depth: 1
Prefer: return-minimal
Content-Type: application/xml; charset=utf-8

<c:calendar-query xmlns:d="DAV:" xmlns:c="urn:ietf:params:xml:ns:caldav">
    <d:prop>
        <d:getetag />
        <c:calendar-data />
    </d:prop>
    <c:filter>
        <c:comp-filter name="VCALENDAR" />
    </c:filter>
</c:calendar-query>

This request will give us every object that's a VCALENDAR object, and its etag.

If you're only interested in VTODO (because you're writing a todo app) you can also filter for just those:

REPORT /calendars/johndoe/home/ HTTP/1.1
Depth: 1
Prefer: return-minimal
Content-Type: application/xml; charset=utf-8

<c:calendar-query xmlns:d="DAV:" xmlns:c="urn:ietf:params:xml:ns:caldav">
    <d:prop>
        <d:getetag />
        <c:calendar-data />
    </d:prop>
    <c:filter>
        <c:comp-filter name="VCALENDAR">
            <c:comp-filter name="VTODO" />
        </c:comp-filter>
    </c:filter>
</c:calendar-query>

Similarly it's also possible to filter to just events, or only get events within a specific time-range.

This report will return a multi-status object again:

HTTP/1.1 207 Multi-status
Content-Type: application/xml; charset=utf-8

<d:multistatus xmlns:d="DAV:" xmlns:cs="http://calendarserver.org/ns/">
    <d:response>
        <d:href>/calendars/johndoe/home/132456762153245.ics</d:href>
        <d:propstat>
            <d:prop>
                <d:getetag>"2134-314"</d:getetag>
                <c:calendar-data>BEGIN:VCALENDAR
                    VERSION:2.0
                    CALSCALE:GREGORIAN
                    BEGIN:VTODO
                    UID:132456762153245
                    SUMMARY:Do the dishes
                    DUE:20121028T115600Z
                    END:VTODO
                    END:VCALENDAR
                </c:calendar-data>
            </d:prop>
            <d:status>HTTP/1.1 200 OK</d:status>
        </d:propstat>
    </d:response>
    <d:response>
        <d:href>/calendars/johndoe/home/132456-34365.ics</d:href>
        <d:propstat>
            <d:prop>
                <d:getetag>"5467-323"</d:getetag>
                <c:calendar-data>BEGIN:VCALENDAR
                    VERSION:2.0
                    CALSCALE:GREGORIAN
                    BEGIN:VEVENT
                    UID:132456-34365
                    SUMMARY:Weekly meeting
                    DTSTART:20120101T120000
                    DURATION:PT1H
                    RRULE:FREQ=WEEKLY
                    END:VEVENT
                    END:VCALENDAR
                </c:calendar-data>
            </d:prop>
            <d:status>HTTP/1.1 200 OK</d:status>
        </d:propstat>
    </d:response>
</d:multistatus>

This calendar only contained 2 objects. A todo and a weekly event.

So after you retrieved and processed these, for each object you must retain:

The calendar data itself
The url
The etag

In this case all urls ended with .ics. This is often the case, but you must not rely on this. In this case the UID in the calendar object was also identical to a part of the url. This too is often the case, but again not something you can rely on, so don't make any assumptions.

The url and the UID have no meaningful relationship, so treat both those items as separate unique identifiers.

Finding out if anything changed

To see if anything in a calendar changed, we simply request the ctag again on the calendar. If the ctag did not change, you still have the latest copy.

If it did change, you must request all the etags in the entire calendar again:

REPORT /calendars/johndoe/home/ HTTP/1.1
Depth: 1
Prefer: return-minimal
Content-Type: application/xml; charset=utf-8

<c:calendar-query xmlns:d="DAV:" xmlns:c="urn:ietf:params:xml:ns:caldav">
    <d:prop>
        <d:getetag />
    </d:prop>
    <c:filter>
        <c:comp-filter name="VCALENDAR">
            <c:comp-filter name="VTODO" />
        </c:comp-filter>
    </c:filter>
</c:calendar-query>

Note that this last request is extremely similar to a previous one, but we are only asking for the etag, not the calendar-data.

The reason for this, is that calendars can be rather huge. It will save a TON of bandwidth to only check the etag first.

HTTP/1.1 207 Multi-status
Content-Type: application/xml; charset=utf-8

<d:multistatus xmlns:d="DAV:" xmlns:cs="http://calendarserver.org/ns/">
    <d:response>
        <d:href>/calendars/johndoe/home/132456762153245.ics</d:href>
        <d:propstat>
            <d:prop>
                <d:getetag>"xxxx-xxx"</d:getetag>
            </d:prop>
            <d:status>HTTP/1.1 200 OK</d:status>
        </d:propstat>
    </d:response>
    <d:response>
        <d:href>/calendars/johndoe/home/fancy-caldav-client-1234253678.ics</d:href>
        <d:propstat>
            <d:prop>
                <d:getetag>"5-12"</d:getetag>
            </d:prop>
            <d:status>HTTP/1.1 200 OK</d:status>
        </d:propstat>
    </d:response>
</d:multistatus>

Judging from this last request, 3 things have changed:

The etag for the task has changed, so the contents must be different
There's a new url, some other client must have added an object
One object is missing, something must have deleted it.

So based on those 3 items we know that we need to delete an object from our local list, and fetch the contents for the new item, and the updated one.

To fetch the data for these, you can simply issue GET requests:

GET /calendars/johndoe/home/132456762153245.ics HTTP/1.1

But, because in a worst-case scenario this could result in a LOT of GET requests we can do a 'multiget'.

REPORT /calendars/johndoe/home/ HTTP/1.1
Depth: 1
Prefer: return-minimal
Content-Type: application/xml; charset=utf-8

<c:calendar-multiget xmlns:d="DAV:" xmlns:c="urn:ietf:params:xml:ns:caldav">
    <d:prop>
        <d:getetag />
        <c:calendar-data />
    </d:prop>
    <d:href>/calendars/johndoe/home/132456762153245.ics</d:href>
    <d:href>/calendars/johndoe/home/fancy-caldav-client-1234253678.ics</d:href>
</c:calendar-multiget>

This request will simply return a multi-status again with the calendar-data and etag.

A small note about application design

If you read this far and understood what's been said, you may have realized that it's a bit cumbersome to have a separate step for the initial sync, and subsequent updates.

It would totally be possible to skip the 'initial sync', and just use calendar-query and calendar-multiget REPORTS for the initial sync as well.

Updating a calendar object

Updating a calendar object is rather simple:

PUT /calendars/johndoe/home/132456762153245.ics HTTP/1.1
Content-Type: text/calendar; charset=utf-8
If-Match: "2134-314"

BEGIN:VCALENDAR
....
END:VCALENDAR

A response to this will be something like this:

HTTP/1.1 204 No Content
ETag: "2134-315"

The update gave us back the new ETag. SabreDAV gives this ETag on updates back most of the time, but not always.

There are cases where the caldav server must modify the iCalendar object right after storage. In those cases an ETag will not be returned, and you should issue a GET request immediately to get the correct object.

A few notes:

You must not change the UID of the original object
Every object should hold only 1 event or task.
You cannot change an VEVENT into a VTODO.

Creating a calendar object

Creating a calendar object is almost identical, except that you don't have a url yet to a calendar object.

Instead, it is up to you to determine the new url.

PUT /calendars/johndoe/home/somerandomstring.ics HTTP/1.1
Content-Type: text/calendar; charset=utf-8

BEGIN:VCALENDAR
....
END:VCALENDAR

A response to this will be something like this:

HTTP/1.1 201 Created
ETag: "21345-324"

Similar to updating, an ETag is often returned, but there are cases where this is not true.

Deleting a calendar object

Deleting is simple enough:

DELETE /calendars/johndoe/home/132456762153245.ics HTTP/1.1
If-Match: "2134-314"

Speeding up Sync with WebDAV-Sync

WebDAV-Sync is a protocol extension that is defined in rfc6578. Because this extension was defined later, some servers may not support this yet.

SabreDAV supports this since 2.0.

WebDAV-Sync allows a client to ask just for calendars that have changed. The process on a high-level is as follows:

Client requests sync-token from server.
Server reports token 15.
Some time passes.
Client does a Sync REPORT on an calendar, and supplied token 15.
Server returns calendar object urls that have changed or have been deleted and returns token 17.

As you can see, after the initial sync, only items that have been created, modified or deleted will ever be sent.

This has a lot of advantages. The transmitted xml bodies can generally be a lot shorter, and is also easier on both client and server in terms of memory and CPU usage, because only a limited set of items will have to be compared.

It's important to note, that a client should only do Sync operations, if the server reports that it has support for it. The quickest way to do so, is to request {DAV}sync-token on the calendar you wish to sync.

Technically, a server may support 'sync' on one calendar, and it may not support it on another, although this is probably rare.

Getting the first sync-token

Initially, we just request a sync token when asking for calendar information:

PROPFIND /calendars/johndoe/home/ HTTP/1.1
Depth: 0
Content-Type: application/xml; charset=utf-8

<d:propfind xmlns:d="DAV:" xmlns:cs="http://calendarserver.org/ns/">
  <d:prop>
     <d:displayname />
     <cs:getctag />
     <d:sync-token />
  </d:prop>
</d:propfind>

This would return something as follows:

<d:multistatus xmlns:d="DAV:" xmlns:cs="http://calendarserver.org/ns/">
    <d:response>
        <d:href>/calendars/johndoe/home/</d:href>
        <d:propstat>
            <d:prop>
                <d:displayname>My calendar</d:displayname>
                <cs:getctag>3145</cs:getctag>
                <d:sync-token>http://sabredav.org/ns/sync-token/3145</d:sync-token>
            </d:prop>
            <d:status>HTTP/1.1 200 OK</d:status>
        </d:propstat>
    </d:response>
</d:multistatus>

As you can see, the sync-token is a url. It always should be a url. Even though a number appears in the url, you are not allowed to attach any meaning to that url. Some servers may have use an increasing number, another server may use a completely random string.

Receiving changes

After a sync token has been obtained, and the client already has the initial copy of the calendar, the client is able to request all changes since the token was issued.

This is done with a REPORT request that may look like this:

REPORT /calendars/johndoe/home/ HTTP/1.1
Host: dav.example.org
Content-Type: application/xml; charset="utf-8"

<?xml version="1.0" encoding="utf-8" ?>
<d:sync-collection xmlns:d="DAV:">
  <d:sync-token>http://sabredav.org/ns/sync/3145</d:sync-token>
  <d:sync-level>1</d:sync-level>
  <d:prop>
    <d:getetag/>
  </d:prop>
</d:sync-collection>

This requests all the changes since sync-token identified by http://sabredav.org/ns/sync/3145, and for the calendar objects that have been added or modified, we're requesting the etag.

The response to a query like this is another multistatus xml body. Example:

HTTP/1.1 207 Multi-Status
Content-Type: application/xml; charset="utf-8"

<?xml version="1.0" encoding="utf-8" ?>
<d:multistatus xmlns:d="DAV:">
    <d:response>
        <d:href>/calendars/johndoe/home/newevent.ics</d:href>
        <d:propstat>
            <d:prop>
                <d:getetag>"33441-34321"</d:getetag>
            </d:prop>
            <d:status>HTTP/1.1 200 OK</d:status>
        </d:propstat>
    </d:response>
    <d:response>
        <d:href>/calendars/johndoe/home/updatedevent.ics</d:href>
        <d:propstat>
            <d:prop>
                <d:getetag>"33541-34696"</d:getetag>
            </d:prop>
            <d:status>HTTP/1.1 200 OK</d:status>
        </d:propstat>
    </d:response>
    <d:response>
        <d:href>/calendars/johndoe/home/deletedevent.ics</d:href>
        <d:status>HTTP/1.1 404 Not Found</d:status>
    </d:response>
    <d:sync-token>http://sabredav.org/ns/sync/5001</d:sync-token>
 </d:multistatus>

The last response reported two changes: newevent.ics and updatedevent.ics. There's no way to tell from the response whether those calendar objects got created or updated, you, as a client, can only infer this based on the objects you are already aware of.

The entry with name deletedevent.ics got deleted as indicated by the 404 status. Note that the status element is here a child of d:response when in all previous examples it has been a child of d:propstat.

The other difference with the other multi-status examples, is that this one has a sync-token element with the latest sync-token.

Caveats

Note that a server is free to 'forget' any sync-tokens that have been previously issued. In this case it may be needed to do a full-sync again.

In case the supplied sync-token is not recognized by the server, a HTTP error is emitted. SabreDAV emits a 403.

Discovery

Ideally you will want to make sure that all the calendars in an account are automatically discovered. The best user interface would be to just have to ask for three items:

Username
Password
Server

And the server should be as short as possible. This is possible with most servers.

If, for example a user specified 'dav.example.org' for the server, the first thing you should do is attempt to send a PROPFIND request to https://dav.example.org/. Note that you SHOULD try the https url before the http url.

This PROPFIND request looks as follows:

PROPFIND / HTTP/1.1
Depth: 0
Prefer: return-minimal
Content-Type: application/xml; charset=utf-8

<d:propfind xmlns:d="DAV:">
  <d:prop>
     <d:current-user-principal />
  </d:prop>
</d:propfind>

This will return a response such as the following:

HTTP/1.1 207 Multi-status
Content-Type: application/xml; charset=utf-8

<d:multistatus xmlns:d="DAV:" xmlns:cs="http://calendarserver.org/ns/">
    <d:response>
        <d:href>/</d:href>
        <d:propstat>
            <d:prop>
                <d:current-user-principal>
                    <d:href>/principals/users/johndoe/</d:href>
                </d:current-user-principal>
            </d:prop>
            <d:status>HTTP/1.1 200 OK</d:status>
        </d:propstat>
    </d:response>
</d:multistatus>

A 'principal' is a user. The url that's being returned, is a url that refers to the current user. On this url you can request additional information about the user.

What we need from this url, is their 'calendar home'. The calendar home is a collection that contains all of the users' calendars.

To request that, issue the following request:

PROPFIND /principals/users/johndoe/ HTTP/1.1
Depth: 0
Prefer: return-minimal
Content-Type: application/xml; charset=utf-8

<d:propfind xmlns:d="DAV:" xmlns:c="urn:ietf:params:xml:ns:caldav">
  <d:prop>
     <c:calendar-home-set />
  </d:prop>
</d:propfind>

This will return a response such as the following:

HTTP/1.1 207 Multi-status
Content-Type: application/xml; charset=utf-8

<d:multistatus xmlns:d="DAV:" xmlns:c="urn:ietf:params:xml:ns:caldav">
    <d:response>
        <d:href>/principals/users/johndoe/</d:href>
        <d:propstat>
            <d:prop>
                <c:calendar-home-set>
                    <d:href>/calendars/johndoe/</d:href>
                </c:calendar-home-set>
            </d:prop>
            <d:status>HTTP/1.1 200 OK</d:status>
        </d:propstat>
    </d:response>
</d:multistatus>

Lastly, to list all the calendars for the user, issue a PROPFIND request with Depth: 1.

PROPFIND /calendars/johndoe/ HTTP/1.1
Depth: 1
Prefer: return-minimal
Content-Type: application/xml; charset=utf-8

<d:propfind xmlns:d="DAV:" xmlns:cs="http://calendarserver.org/ns/" xmlns:c="urn:ietf:params:xml:ns:caldav">
  <d:prop>
     <d:resourcetype />
     <d:displayname />
     <cs:getctag />
     <c:supported-calendar-component-set />
  </d:prop>
</d:propfind>

In that last request, we asked for 4 properties.

The resourcetype tells us what type of object we're getting back. You must read out the resourcetype and ensure that it contains at least a calendar element in the CalDAV namespace. Other items may be returned, including non- calendar, which your application should ignore.

The displayname is a human-readable string for the calendarname, the ctag was already covered in an earlier section.

Lastly, supported-calendar-component-set. This gives us a list of components that the calendar accepts. This could be just VTODO, VEVENT, VJOURNAL or a combination of these three.

If you are just creating a todo-list application, this means you should only list the calendars that support the VTODO component.

HTTP/1.1 207 Multi-status
Content-Type: application/xml; charset=utf-8

<d:multistatus xmlns:d="DAV:" xmlns:cs="http://calendarserver.org/ns/" xmlns:c="urn:ietf:params:xml:ns:caldav">
    <d:response>
        <d:href>/calendars/johndoe/</d:href>
        <d:propstat>
            <d:prop>
                <d:resourcetype>
                    <d:collection/>
                </d:resourcetype>
            </d:prop>
            <d:status>HTTP/1.1 200 OK</d:status>
        </d:propstat>
    </d:response>
    <d:response>
        <d:href>/calendars/johndoe/home/</d:href>
        <d:propstat>
            <d:prop>
                <d:resourcetype>
                    <d:collection/>
                    <c:calendar/>
                </d:resourcetype>
                <d:displayname>Home calendar</d:displayname>
                <cs:getctag>3145</cs:getctag>
                <c:supported-calendar-component-set>
                    <c:comp name="VEVENT" />
                </c:supported-calendar-component-set>
            </d:prop>
            <d:status>HTTP/1.1 200 OK</d:status>
        </d:propstat>
    </d:response>
    <d:response>
        <d:href>/calendars/johndoe/tasks/</d:href>
        <d:propstat>
            <d:prop>
                <d:resourcetype>
                    <d:collection/>
                    <c:calendar/>
                </d:resourcetype>
                <d:displayname>My TODO list</d:displayname>
                <cs:getctag>3345</cs:getctag>
                <c:supported-calendar-component-set>
                    <c:comp name="VTODO" />
                </c:supported-calendar-component-set>
            </d:prop>
            <d:status>HTTP/1.1 200 OK</d:status>
        </d:propstat>
    </d:response>
</d:multistatus>

Advanced discovery topics

Read the Service Discovery documentation