Intent
Support serendipitous reuse across an architecture
Motivation
Producer P and consumer C want to transfer a document. P has a particular
abstract data schema in mind, and C has a compatible schema. Registry ensures
that P encodes and transmits the information in a form that C understands.
The methods for applying patterns of information transfer and
forms in which information are encoded may be defined either by local
convention or by a wider convention. Local protocols may be "jargonised"
versions of more widely-applicable protocols. In time, jargon may move from
a satellite architecture such as that of a particular enterprise and become
standardised in a wider setting such as the Web.
Applicability
Registry is applicable for REST architectures large and small. However,
architectures with only a small number of components may not need a formal
registry.
Structure
Participants
- Document Type
-
- Provides a defined way to encode a given information schema
- May have one or more parent types, whose parser is also appropriate
to read this derived document type
- Is defined in a Document Type Specification, including accommodation
of document type evolution through must-ignore semantics on information
that is not understood by processors.
- May be a target encoding for a number of a number of different
information schemas, including internal information schemas for various
kinds of applications. For example, HTML can be used to encode
information schemas as diverse as journal entry publication and
warnings about severe weather events.
- May act as a bridge between different Information Schema. For
example, ODF is able to bridge between OpenOffice and Gnome Office
internal document schemas.
- Is defined to be independent of how it is transported, and
orthogonal to Operations that are defined in the architecture.
- Is developed using the least power principle:
The least powerful language that is capable of expressing a set of
information is used.
- Minimises coupling by limiting reliance on local or domain-specific
conventions.
- Operation
-
- Provides a way to transport information around the architecture
- Corresponds to one or more Interaction Patterns that are
understood by applicable information end-points
- Is defined to be independent of the information it transports,
and the specific endpoints it transfers information between. Operations
are orthogonal to the set of URLs in the architecture.
- Management Body and Process
-
- Manages the evolution of Document Types and Operations
- Oversees the adoption and uptake of new or updated Document Types
and Operations in the architecture
- May depend heavily on external bodies, especially those that manage
the Web
- Typically do not have the authority to upgrade any given component
of the architecture. Instead, components are upgraded over time as
their respective owners see fit in line with the Registry
specifications.
- Typically do not have any control over the set of URLs in the
architecture
Collaboration
- The Management Body and Processes manages the independent evolution of
Document Types and Operations in the architecture. The set of URLs is not
controlled by this body.
Consequences
The orthogonal nature of document types, operations, and URLs throughout
the architecture allow for freedom of evolution. This separation and evolution
is supported through the major patterns of the architecture as old and new
components interact, including
GET
and
PUT.
The balance of control and freedom is capable of delivering
application-level forwards and backwards compatibility that lasts for
a relative eternity in our industry. For example, HTML and HTTP have
been able to continuously evolve over a time-frame spanning decades.
More domain-specific document types will likely have shorter lifespans
as they develop from local conventions to a point of widespread global
agreement. However, as critical mass is created a semantic web where machines
can communicate generally based on widely-understood conventions is likely
to emerge without specific coordination or intervention.
Implementation
Sample Code
Known Uses
The IETF defines a registry and change management process that includes
a list of protocol interactions for document transfer
(HTTP/1.1)
and legal document types
(the IANA).
Related Patterns
