Hi,

[Note : I work here]

We are rolling out a new feature called "machine tags" that allows users to
be more precise in how they tag, and how they search, their photos.

Many of you may already be familiar with machine tags by another name
(triple tags) or because you are already using them, informally, in your
code (for example, "geo:long=123.456").

"Machine tags" is the technical term for the extra hamsters we've added to
the Flickr servers to formalize how these sorts of tags are treated. I've
included a "Ceci n'est pas un FAQ" below with all the details.

For the moment, machine tags are principally an API "thing". The photo pages
have been updated to display tags a little differently but otherwise all the
magic you can perform with machine tags happens here at the API layer. (This
includes the special wildcard syntax for searching photos with machine
tags.)

Enjoy!

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# What are machine tags?

Machine tags are tags that use a special syntax to define extra information
about a tag.

Machine tags have a namespace, a predicate and a value. The namespace
defines a class or a facet that a tag belongs to ('geo', 'flickr', etc.) The
predicate is name of the property for a namespace ('latitude', 'user', etc.)
The value is, well, the value.

Like tags, there are no rules for machine tags beyond the syntax to specify
the parts of a machine tag. For example, you could tag a photo with :

* flickr:user=straup

* flora:tree=coniferous

* medium:paint=oil

* geo:cartier="plateau mont royal"

* geo:neighbourhood=geo:cartier

Flickr has already used machine tags, informally, on a couple of occasions :

- When we launched Maps, we provided a way for people who had
"geotagged" their photos to import their location data. This was
done using the "geo:lat=..." and "geo:lon=..." tags.

- When a user tags an event with an upcoming ID (for example :
"upcoming:event=81334") we display a link back to the upcoming.org
site. A similar example is the excellent "Flickr Upcoming Event"
greasemonkey script :

http://userscripts.org/scripts/show/5464

Dan Catt wrote a very good piece about machine tags - he called them "triple
tags" - last year :

http://geobloggers.com/archives/2006/01/11/advanced-tagging-and-tripletags/

---

# What is the spec for machine tags?

Machine tags are divided in to three parts :

1) A "namespace" :

Namespaces MUST begin with any character between a - z; remaining
characters MAY be a - z, 0 - 9 and underbars. Namespaces are
case-insensitive.

2) A "predicate" :

Predicates MUST begin with any character between a - z; remaining
characters MAY be a - z, 0 - 9 and underbars. Namespaces are
case-insensitive.

3) A "value" :

Values MAY contain any characters that a "plain vanilla" tags
use. Values may also contain spaces but, like regular tags, they
need to wrapped in quotes.

Namespace and predicates are separated by a colon : ":"

Predicates and values are separated by an equals symbol : "="

For example :

* flickr:user=straup

* geo:locality="san francisco"

---

# Why can't I use non-ASCII characters for namespaces and predicates ?

Simple steps, first.

---

# Can I define another machine tag as the value of a machine tag?

Sure, but it will not be processed as a machine tag itself.

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# How do I add machine tags?

By adding tags! No, really.

Machine tags are added *exactly* the same as any other tag whether it is
done through the website or the API.

When the Flickr supercomputer processes your tags, we take a moment to check
whether it is a machine tag. If it is we leverage the powerful Do What I
Mean engine to, well, do what you mean.

---

# How do I query machine tags?

Via the API!

Specifically, using the "machinetags" parameter in the
'flickr.photos.search' method. Like tags, you can specify multiple machine
tags as a comma separated list.

---

# Can I query the various part of a machine tag?

Yes. Aside from passing in a fully formed machine tag, there is a special
syntax for searching on specific properties :

* Find photos using the 'dc' namespace :

{"machine_tags" => "dc:"}

* Find photos with a title in the 'dc' namespace :

{"machine_tags" => "dc:title="}

* Find photos titled "mr. camera" in the 'dc' namespace :

{"machine_tags" => "dc:title=\"mr. camera\"}

* Find photos whose value is "mr. camera" :

{"machine_tags" => "*:*=\"mr. camera\""}

* Find photos that have a title, in any namespace :

{"machine_tags" => "*:title="}

* Find photos that have a title, in any namespace, whose value is "mr.
camera" :

{"machine_tags" => "*:title=\"mr. camera\""}

* Find photos, in the 'dc' namespace whose value is "mr. camera" :

{"machine_tags" => "dc:*=\"mr. camera\""}

---

# Is there a limit to the number of machine tags I can query?

Yes. The limit depends on the tag mode (ALL or ANY) that you are querying
with. "ALL" (AND) queries are limited to (16) machine tags. "ANY" (OR)
queries are limited to (8).

---

# Can I do range queries on machine tags?

No. Not yet, anyway.

It is a hard problem for reasons far too dull to get in to here. It's on the
list.

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# Are machine tag namespaces reserved?

No. Anyone can use a namespace for anything they want.

If you are concerned about colliding namespaces you should consider adding
an additional machine tag to define your namespace. For example :

dc:subject=tags
xmlns:dc=http://purl.org/dc/elements/1.1/

Like tags, in general, we expect (hope?) that the community will develop its
own standards by consensus over time.

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# What about all the machine tags that are already in the Flickr database?

At the moment, they are still treated as plain old tags.

We have plans to go back and re-import them as machine tags but for now,
only new tags will be processed as machine tags.

In the meantime, if you re-save a machine from the 'edit this tag' it will
be re-imported as a machine tag.

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# Is the predicate *really* a predicate?

You are in a dark cave. In the corner is a fire and a man making bunny
shadows on the wall with his hands. Whether or not it's really a 'predicate'
depends on how much time you spend on the semantic-web mailing list. ;-)

It's close enough to being a predicate that it makes for a good short-hand.

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# Wait, aren't machine tags just RDF?

No, machine tags are not RDF; they could play RDF on television, though.

See also :

http://weblog.scifihifi.com/2005/08/05/meta-tags-the-poor-mans-rdf

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# Huh, what is RDF ?

RDF Describes Flickr. That's really all you need to know about RDF.

--

Aaron Straup Cope
Hackr