Modeling for cloud part2
This is the Wiki version of the article published in IEEE Internet Computing July/August 2010 Issue. External Reference
Semantic Modeling
Part 1 of this two-part article discussed
challenges related to cloud computing,
cloud interoperability, and multidimensional analysis of cloud-modeling requirements
(see the May/June issue). Here, we look more
specifically at areas in which semantic models
can support cloud computing. tics, however, are similar. Metadata added
through annotations pointing to generic operational models would play a key role in consolidating these APIs and enable interoperability
among the heterogeneous cloud environments. | ||
to the availability of extensible interpreted programming languages such as Ruby and Python. Unlike UML, a DSL is applicable only in a given domain but enables a light-weight model in that domain, often without requiring proprietary tools. For example, you can use IBM’s Sharable Code DSL (http://services.alphaworks.ibm.com/isc), which is a mashup generator, with a basic text editor. (However, providing graphical abstractions and specialized tooling would be more convenient for users.) “Lightweight” signifies that these models don’t use rich knowledge representation languages and so have limited reasoning capabilities. Our Cirrocumulus project for cloud interoperability (Cirrocumulus) uses DSLs to bridge the gap between executable artifacts and high-level semantic models. A DSL, although domain specific, can provide a more programmer-oriented representation of functional, non-functional, or even data descriptions. A best-of-both-worlds approach is to use annotations to link models, which provides the convenience of lightweight models while supporting high-level operations when required. Figure 2 shows an annotation referring to an ontology from a fictitious DSL script for configuration. The script is more program- mer-oriented (in fact, it’s derived from Ruby) but lacks an ontology’s richness. However, the annotation links the relevant components between the different levels, providing a way to facilitate high-level operations while maintaining a simpler representation. From the perspective based on the type of semantics and software lifecycle stage—that is, looking at the cube in Figure 1 from the front — you can see the modeling coverage for software deployment and management. Elastra’s Elastic Computing Modeling Language (ECML),
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exploiting these data stores requires substantial redesign of many data-driven applications and often makes porting data to a traditional relational database extremely difficult. The current practice is to address such transitions case-by-case. A better approach is to model the data in RDF and generate the specific target representations, and in some cases even the code for the application’s data access layer. This method can formulate transformations from one representation to another using the lifting-lowering mechanism. Semantic Annotations for WSDL and XML Schema (SAWSDL) demonstrated this mechanism’s use for data mediation. 2 Lightweight modeling in terms of DSLs also applies here. For example, the Web services community has long used XML Schema definitions as platform-agnostic data definitions. Schema definitions serve as inputs to code generation tools that generate platform-specific data definitions. From the perspective of the type of semantics and software lifecycle stage, most of this data modeling applies during application development. Concrete artifacts generated from these high-level models would be used mostly during subsequent lifecycle stages.
Service Enrichment microformat is part of the “lowercase semantic web” movement, which emphasizes lightweight models. Embedding rich semantic meta- data in cloud service descriptions has three main benefits that go beyond customized search capabilities. The first benefit deals with Representational State Transfer (REST) style services. Many cloud service providers adopt REST-style Web services that don’t advocate a formal service description. These services are described using HTML pages. WSDL 2.0, the latest specification, explicitly supports formal description of “RESTful” services but hasn’t seen quick adoption. Alternative approaches such as SA-REST3 (SA stands for semantic annotation), a generic annotation scheme that follows microformat design principles, are becoming more applicable in this space. These annotations enable the seamless, flexible integration of formalizations into RESTful service descriptions. This opens the door to many exciting avenues such as faceted search to identify relevant reusable services and semiautomated service compositions. The second benefit deals with handling change. The cloud space is still evolving. If the history of software or component interoperability |
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is any guide, achieving consensus in
the cloud space will be difficult and
won’t likely happen soon. Attaching
formalizations via annotations, however, is flexible enough to accommodate an evolving model. This is
especially attractive to vendors who
aren’t willing to invest heavily in
interim standards.
The third benefit is that the formalizations apply not only to service descriptions but also to many
other aspects such as service-level
agreements (SLAs) and software
licenses. You can use annotations
to embed formalizations even for
these documents, facilitating more
automation in the cloud space. For
example, Web Service- Level Agreement (WSLA) specification provides a
way to formalize SLAs, but creating
and maintaining these formalizations is time-consuming.
Figure 3 illustrates using SA-REST annotations on the Amazon
Elastic Compute Cloud (EC2) SLA
document. It shows how a capable
processor could use these annotations to extract a WSLA equivalent
of the human-readable SLA.
These benefits’ importance comes
into perspective when you consider
the enormous body of research on
standard-driven service compositions and agreement matching.
The informal, non-standard-driven
nature of many cloud services made
most of the previous research inapplicable. However, being able to
glean formalizations from existing
documents opens the doors to apply
many well-researched techniques.
models excel in providing solutions.
However, learning from the past,
we advocate a multilevel modeling
strategy to provide smooth transitions into different granularity
levels. We also think that DSLs can
play an important role in the cloud
space to provide lightweight modeling in an appealing manner to the
software engineering community.
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