Second Life Grid Glossary

A glossary of terms with specific meanings within the project, defined so that discussions can be concise and misunderstandings fewer. We also encourage you to take a look at the IEEE 1471 article on designing an effective system architecture which addresses all of the project goals.

Please note that this glossary is specific to the SL Architecture Working Group and is intended for use only within the context of Second Life. The definitions are not expressed in a generic manner, and should not be interpreted that way.

Agent

An Agent is a software resource which represents some portion of a user in the virtual world. Agents are hosted in agent servers. An agent may cause an avatar to be instantiated on behalf of a user.

Agents:

• Mediate the user's message traffic
• Mediate the user's asset inventory
• Act as a focal point for the user's access to services in other domains
• Act as a focal point for the user's access to utilties

Agents do not:

• Store persistent assets (They are stored in asset servers)

Architecture

An abstraction (or mental model) of the SL system which corresponds to its system design. It is revealed through architectural descriptions and multiple views (ADV).

Alternatively, the end product of a set of processes yielding a set of normative documents which define compliance to the architecture. This is not, inherently in opposition to the first definition as it captures the architecture at different points in its evolution.

Architectural descriptions and views (ADV)

Documents which describe (textually and/or graphically) how the various pieces of the SecondLife system fulfil their expected goals. More precisely, ADVs express in detail how the system architecture is conformant with the concerns expressed in viewpoints. Some examples:

For a message flow viewpoint, ADVs describe how system components interact with one another by describing their relations, protocols and message formats.

For a scalability-oriented viewpoint, ADVs describe the relevant system resource pools and their capacities, the flow paths and choke points, the bandwidth of links and access mechanisms, the means of expanding capacity, and the analysed scalability of key elements.

For a client capability viewpoint, ADVs ennumerate and detail the client-related capabilities, client-server capability negociation mechanisms and protocols, means of activating capability negociation, and client-side capability state transitions.

Asset

• An entity which can be transferred from agent to agent or from agent to region or from region to agent. It can be something like an object, texture, sound, link, landmark, and so forth.

Architecturally, an asset consists of asset properties and asset data (also called meta data).

• (temp note): We need some clarity here, and elsewhere in the glossary between a Web Services description of resources such as an asset, and an in use description. If we view all assets as having URLs and a singular place in an asset server where the definitive copy of the asset exists, then we need to separate that from the potentially many places where copies of that set of information may be cached and used. -- Zha

• Ah.. Actually, if we wish to make a distinction between the asset's data, and its meta-data, then, we have the asset, which would be what we "get" from the URL, and the meta-data, which we seperate from the core item, in one of several ways. There was a nice bit of discussion of this in Zeros office hours on October 18 transcript -- Zha October 18, 2007, 5:42 PDT
• asset is currently under discussion on sldev -- Dr Scofield 00:19, 18 October 2007 (PDT)

Avatar

The representation of an agent in a region (or somewhere else, like on the web)

Component

A logical composition that is proportional to a whole. In the domain architecture of the AWG, these compositions include viewpoints and views. In software and hardware, these include modules, units, and devices.

Domain

A partition of architectural resources, and services. A domain can be characterized by its membership and the set of properties that the members share. In Internet usage, a domain is commonly tied to DNS namespace, with the shared property being name resolution. In terms of our architectural structure, domains share a set of properties, and gain some benefit from this distinction.

The architecture may expose domain membership, and the properties which define the domain through a standard web service.

Examples of domains might include

• All the region servers supporting Linden Lab's mainland
• All the corporate hosted servers supporting XYZ Co's private islands. (Which might include Region, Asset, and Utility serviced specific to XYZ co.)
• All the servers and services within HostingCorps trusted firewall.

In each case, the partition permits the members to act differently with respect to each other, than members outside the partition.

To understand the rationale for this definition, see Domain rationale discussion.

Meta data

Meta data is data about data: it describes the data and its properties. In the architecture discussions we use meta data for assets.

Permissions

A kind of license digest that specifies what rights the creator of an asset grants to a user of that asset (or what rights she doesn't want to grant) [see also Protecting content in an open grid]

Refactored Composition

Refactored Composition refers to a technique to arrange the factors of a composition such that the composition is more orthogonal to a new model. This term, or like terms, is often mistakenly used in expression to just rewrite, remake, or recompose the factors, or lesser constituents, within the same model the composition already exists.

Region

Some space. It can have any form. It can be grouped together with other regions. It is part of a region domain.

Region Domain

Set of regions sharing one or more properties, permitting them to be grouped into a domain.

Examples of region domains might include:

• All of the regions forming a continent run by a service provider
• All of the regions hosted by the XYZ corporation
• All of the regions which support a new release of a physics engine

Resource

• [REST]: software entity, or stored value which can be addressed via a URL One basic design point of the architecture is that it follows RESTful web practices.

• [Scalability]: Any finite architectural component or property or behaviour that is subject to exhaustion or which can become a bottleneck to system performance. Resources are the primary focus in designing for scalability. Examples: client-server bandwidth, agent pool depth, database access mechanism, locks, transaction times, utilities, services.

Scalability

The ability of a system to grow effectively in a given dimension in proportion to the amount of resource or capacity provided. The given dimension is often associated with a related dimension. A specific system may attempt to be scaled by adding resources, but this is effective only if the system is scalable. Example: scalability in the number of avatars at an event, viewed against the size of the user population, in proportion to the hardware allocated.

Service

A Web Services invocable resource which performs some task on behalf of a region

Simulation (sim)

A computation over time that mimics real world events within a part of the virtual world.

Stakeholder

Anyone who has a technical viewpoint that impacts on AWG work on system architecture.

Non-technical viewpoints exist and have validity, but do not fall within the current scope.

Use case

A technique used to capture the requirements of a system or architecture. Use cases avoid technical jagon although this often depends on the scope and actors of the use case. Generally, the archtectural terms in this glosary are valid (i.e. Identity, Viewer, Service) while specific technical terms are not (C++, REST, XML). Detail of a AWG use case can vary:

• A brief use case consists of a few sentences summarizing the use case.
• A casual use case consists of a few paragraphs of text or a minimal template, summarizing the use case.
• A fully dressed use case is a formal document based on a detailed template with fields for various sections; and it is the most common understanding of the meaning of a use case.

User

An person in control of an agent.

Utility

A Service, or collection of services which provides a utility which does not manifest as a region, agent or avatar within the virtual world. Examples: Currency, Identity, Asset Storage, Messaging, Presence, Topology Management.

Viewer

Currently, a monolithic client-side program which establishes communication with system servers and displays a visual image of the virtual world. It usually controls an agent represented by an avatar, eventually inside a region. As client programming evolves, the viewer per se may become only a client subprogram which deals with 3D rendering and UI handling exclusively, assisted by other subprograms.

Viewpoint

A set of related concerns about the architecture, and the representations or views used to describe the architecture to address those concerns. Examples: Client viewpoint, Client UI viewpoint, Functional viewpoint, REST Services viewpoint, Scalability viewpoint, Region Scalability viewpoint, Network viewpoint, Manpower viewpoint.

See Also

• Glossary
• Software and Systems Engineering Vocabulary