science

Tag URI scheme

Tag URI scheme is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Tag URI scheme rather than just read about it. In short: The tag URI scheme is a uniform resource identifier (URI) scheme for unique identifiers called tags, defined by RFC 4151 in October 2005. The RFC identifies four requirements for tags: Identifiers are likely to be unique across space and time, and come from a practically inexhaustible supply.

Key takeaways

Tag URI scheme belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Tag URI scheme to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Tag URI scheme from memory before moving on to harder problems.

Reference excerpt

The tag URI scheme is a uniform resource identifier (URI) scheme for unique identifiers called tags, defined by RFC 4151 in October 2005. The RFC identifies four requirements for tags:

Identifiers are likely to be unique across space and time, and come from a practically inexhaustible supply. Identifiers are relatively convenient for humans to mint (create), read, type, remember etc. No central registration is necessary, at least for holders of domain names or email addresses; and there is negligible cost to mint each new identifier. The identifiers are independent of any particular resolution scheme. Tags are used extensively in YAML.

Format The general syntax of a tag URI is:

"tag:" authorityName "," YYYY-MM-DD-date ":" specific [ "#" fragment ] In this syntax, the authorityName is either a domain name or an email address, and the date is in the YYYY-MM-DD format, such as 2017-01-01. Thus, a specific tag is tied to a specific domain name or email address at a specific point of time. It is required that the "tagging entity" creating the tag be in control of the specified domain or email address as of 00:00 UTC on the specified date. This requirement makes each tag globally and persistently unique. The authority name alone would not suffice for global uniqueness, since the ownership of domains and email addresses is subject to change. The date used in a tag may be a past date, provided the tagging entity controlled the authority name on that past date. An entity which acquires control of an authority name immediately after a period when it was unassigned is allowed to mint tags as if it controlled the authority name during the unassigned period, provided the entity has evidence that the name was unassigned. The date used in a newly-minted tag may not be a date in the future. The date may be abbreviated; the month and day default to 01, but tags which have defaulted month and day values are reckoned as different from tags where the 01 value is specified. Thus,

tag:example.com,2004:1234 tag:example.com,2004-01:1234 tag:example.com,2004-01-01:1234 are all different tags and might have different referents. They all should have been minted by the same tagging entity (specifically, whoever controlled example.com on January 1, 2004 00:00 UTC). If a different entity gained control of example.com at 00:05 UTC on that date, it would not be allowed to mint tags until the following date, and it would not ever be able to use the abbreviated forms, 2004 or 2004-01. Examples of tag URIs are:

tag:timothy@hpl.hp.com,2001:web/externalHome tag:sandro@w3.org,2004-05:Sandro tag:my-ids.com,2001-09-15:TimKindberg:presentations:UBath2004-05-19 tag:blogger.com,1999:blog-555 tag:yaml.org,2002:int#section1

See also Universally unique identifier (UUID) Uniform resource name (URN) iSCSI Qualified Name (IQN)

References

Worked examples

Example 1 — a first encounter with Tag URI scheme

Start with the simplest possible case. Write down what Tag URI scheme claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Tag URI scheme before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Tag URI scheme ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Tag URI scheme

In research: Tag URI scheme appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Tag URI scheme in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Tag URI scheme is common in secondary-school and first-year university syllabi. It links to neighbouring topics Identifiers, Index (publishing), Internet Standards, so understanding it makes those chapters shorter.
In everyday life: Look for Tag URI scheme outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Tag URI scheme in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Tag URI scheme means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Tag URI scheme out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Tag URI scheme in simple terms?

The tag URI scheme is a uniform resource identifier (URI) scheme for unique identifiers called tags, defined by RFC 4151 in October 2005. The RFC identifies four requirements for tags: Identifiers are likely to be unique across space and time, and come from a practically inexhaustible supply.

Why does Tag URI scheme matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Tag URI scheme?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Tag URI scheme.