Multi-Process Client VAG -- draft
- This is a pre-draft of the draft
- This is an early draft so scope and focus are still fairly open. Please add comments to the Talk:Multi-Process Client VAG -- draft if you have slightly different concerns so that we can try to converge on a common viewpoint. This discussion could also expose other similar VAGs that are needed in this area.
- 1 Summary
- 2 Purpose
- 3 Multi-Process Client VAG Glossary
- 4 Source of Viewpoint
- 5 General concerns addressed by this viewpoint
- 6 Specific concerns within this viewpoint
- 7 Rationale for a Multi-Process Client
- 8 Use Cases
- 9 Multi-Process Client Architecture
- 10 Facility-Mediator Communications
- 11 Facility-Facility Communications
- 12 Mediator-VirtualWorld Communications
- 13 Initial Facilities in More Detail
- 14 Organization
- 15 Architectural Descriptions/Views used to express this viewpoint
- 16 Tools employed by this VAG
- 17 References and Resources
- 18 External Links
- 19 Members (Stakeholders)
The Multi-Process Client Viewpoint Advocacy Group (VAG) exists to provide input to the overall system archtecture design process in those areas where client-side functionality needs to evolve to track server-side changes.
More specifically, the Multi-Process Client VAG is concerned with identifying the key server-side changes that impact on the client, proposing at least one reference design for an "MPClient" that can address all the expected changes, and assisting with the design and implementation of such a reference client. This work is expected to generate feedback in the opposite direction as well: a range of client-based Use_Cases will be examined in this VAG and used to generate requirements for the system architecture, particularly in the areas of scalability and interoperability.
As is evident from its name, this Multi-Process Client VAG has predefined one specific aspect of the required client, namely the change from the monolithic process structure of the original Linden client to a multi-process one. This will be discussed and justified in detail below, but it is not exclusionary: other client-oriented viewpoint groups may proceed along different lines.
This is a technical VAG.
Areas addressed by this viewpoint
- Server-side APIs visible from the client.
- Scalability of the combined client and server systems from a client-side perspective.
- Interoperability of grids and worlds from a client-side perspective.
- Client diversity and its impact on client structure.
- Designing clients for extensibility.
- Use of clients as test tools.
- Harnessing the power of community for rapid client evolution.
Areas not addressed by this viewpoint
- Server-side details that are not visible from the client.
For brevity, the client developed here is referred to as the "MPClient".
The linked glossary defines the following terms:
Backend, Busy waiting, Client Script, Facility, Facility API, Facility Optimization, Facility Socket, Frontend, GUI, LCC (Limited Capability Client) , Mediator, Monolithic Client, Multi-Process Client (MPClient), Multicore, Plug-in, Process, Regression Testing, Renderer, TDD (Test-Driven Design), Test driver, Test harness, UI, Viewer (deprecated).
See Architecture Working Group Glossary for terms not defined here.
Source of Viewpoint
No existing sources for this viewpoint have (yet) been sought. This viewpoint is however highly reusable, and therefore existing sources are very likely to exist and should be investigated.
Translated into mortalspeak: other applications probably use the same structure, so we ought to check them out, even if they don't claim an IEEE_1471 viewpoint.
Applications with similar viewpoints
While not described using IEEE-1471 viewpoints, the following applications are also highly process-factored:
General concerns addressed by this viewpoint
- These are expressed in the form of a Rationale for a Multi-Process Client.
Specific concerns within this viewpoint
- These are presented in detail in the section Proposals and Analysis.
Rationale for a Multi-Process Client
Problems with the Monolithic Client
The original SL client performs its intended role as a fixed-function viewer for the self-contained Second Life service reasonably well, but it does not meet the requirements of a platform client, for the following reasons:
- The platform view of an extended, highly scalable, and widely interoperable Second Life embraces third-party worlds, grids, assets, and contributions, which implies client diversity and flexibility which are absent in the monolithic client.
- The fixed functions of the original client represent only a small subset of the functions envisaged for that extended platform.
- Many Limited Capability Clients (LCCs) will have hardware constraints which will make them unable to run the original client, for example cellphones.
- Many LCCs will have user-interface constraints which require a radically different client structure, for example portable audio-only clients.
- The monolithic structure of the original client does not allow wholesale omission of large and inappropriate subsystems (at best, the capabilities can only be disabled).
- The original client does not support user choice of local facilities unless they have been pre-defined by Linden Lab. This is inappropriate in a platform client that needs to interoperate with third-party systems and assets.
- Any change of facilities outside of those envisioned by Linden Lab for the specific Second Life service would require a complete client rebuild and release. The manpower implications of this alone would would be immense, let alone the impact on client stability and interoperability.
- The monolithic client is a very large and complex application. Large and complex applications run counter to good software engineering practice, for numerous well proven reasons.
- Monolithic clients cannot make good use of the new generation of multicore CPUs unless they are extensively threaded internally. Such threading can be very successful, but in general this hardwires the client-side architecture to a fixed multithreading structure and very often leads to development restrictions. In addition, the lack of thread address space separation in a threaded application tends to reduce stability and harms resilience.
Benefits of a Multi-Process Client
The above concerns are addressed quite effectively in a multi-process client architecture, as follows:
- Plugging together alternative modules provides the needed diversity and flexibility that turns the client into a generic platform. It makes it extensible to the larger feature set of the distributed supergrid, and also allows it to be scaled down for Limited Capability Clients.
- With a multi-process architecture, 3rd parties are no longer dependent on the Linden design process to include their extensions, nor do they need to perform a complete client build and release. Extensions can be deployed incrementally.
- Facility modules or extensions are inherently small and of limited complexity, because they address a specific function. This benefits stability and ease of development.
- Facilities can be programmed in any language whatsoever as long as it can use network sockets. This eliminates computing language barriers between developers and allows the greatest possible number of them to contribute and collaborate.
- Networking sockets are license barriers and hence, like all client-server program pairs on the Internet, Facility programs can be covered by arbitrary licenses and still interoperate freely. This eliminates balkanization of the developer community by license.
- The multi-process client architecture harnesses multicore CPUs naturally and automatically, while still working on single-core CPUs through normal multitasking.
Concerns in a Multi-Process Client
- Because the modules of an MPClient reside in separate processes, and communication between those processes over a socket has much greater overheads than any normal inter-module communication within a single-image address space, this client architecture is viable only if the following conditions apply:
- communications over sockets are relatively infrequent
- communicating processes are not highly sensitive to socket latency
- serialization overhead does not add significantly to communication costs
- target platforms are suitably efficient at multitasking / context switching
- It is a central task of this project to produce a functional decomposition and overall design that meets the above criteria for viability. While some target platforms are not noted for their efficiency at multitasking, the work should seek to reduce the impact of such deficiencies as far as possible through target-specific optimizations.
QA tool: Test harness / Regression test driver
- Historically, this use case provided the initial motivation for the Multi-Process Client.
- The new architecture being designed under the auspices of the Architecture Working Group is expected to deliver reference implementations of key REST services fairly rapidly, whereas development of the monolithic SL client moves slowly because of conflicting requirements and insufficient manpower. Of particular concern here is that the official build repository is not open to community merging, which means that alternative community client-code repositories are continually chasing taillights and having to re-merge their changes. This is not conducive to rapid progress, and hence an alternative client platform that would be free of such constraints was sought.
- A Multi-Process Client as defined here would be even more free of constraints than a monolithic client built from an open community SVN repository. Because of the mix'n'match attachment of Facility modules to meet personal requirements, as well as the license barrier created by network sockets, and also the ability to write Facility programs in any language whatsoever that supports sockets, such a client would in practice have fewer developer constraints than almost any other application in existence. This then is a very powerful approach to the problem.
Use case: QA tool
- Only those Facilities required in a QA tool would need to be developed. These include:
- Scripting system
- State machine engine
- Test logger
- Regression test manager
- Performance instrumentation
- Test documentation generator
- An ever-expanding suite of individual unit tests
- Dynamic workload generators
- Scaling trend visualizers
- The early candidates for testing are expected to be Login and REST Services. Unit tests would therefore be created for those first.
- Since a Facility can trivially create a network-accessible endpoint, the QA tool could in principle be network-accessible and driven by the community wherever it is located.
- The most common uses of the "Mult-Process Client as QA tool" are expected to include:
- Individual unit testing of the new unified Login service
- Individual unit testing of REST services, including early experimental designs
- Test-driven development framework for reference or third-party implementations
- Workload generation for testing the scalability trends of REST services
- Regression testing of all client-visible subsystems after rebuilding
- Pressure testing of service robustness when under maximum load.
Limited Capability Client (LCC)
- stub only, please fill in
Use case: LCC
- stub only, please fill in
- The first rule of "Full client" is not to talk about "Full client".
- Joking aside, since the Multi-Process Client is completely open-ended even without developer involvement as a result of its user-level attachment of Facilities, a "Full client" is merely one with a lot of Facilities attached, and even that would vary according to need.
- A "Normal client" can easily be defined for any given local grid, but in a global supergrid of interoperating grids such a concept is somewhat limiting as well.
- A more appropriate model might be one in which a "Minimal normal client" is packaged with commonly used Facilities, and the user downloads additional Facilities from a suitable repository as needed ... in other words, the model employed by modern web browsers.
Use case: Minimal normal client
- Taking the capabilities of the current monolithic client as the basis for required functionality, a "Minimal normal client" would include the following Facility modules:
- Login session facility
- Events subsystem facility
- Suite of 2D UI facilities
- Suite of world-rendering facilities
- Suite of media handling facilities
- Suite of common I/O facilities
- Suite of resident communication facilities
- Usage: as per current monolithic client.
Multi-Process Client Architecture
The Multi-Process Client architecture detailed below represents only one possible factoring of the client into a suite of communicating processes. This initial factoring provides a concrete basis for analysis and discussion, but is very likely to change if a better factoring is found.
It is proposed that the client be structured as follows at the level of operating system processes and communications flows:
- A frontend Mediator process should handle all communications with virtual world servers.
- This Mediator process should provide a dynamic number of network Facility Sockets.
- Several backend Facility processes should attach to these Facility Sockets to implement Facilities.
- The functions of the client are performed through concurrent operation of the Mediator and the set of attached Facility processes.
Processes and Communication Flows. This diagram is representative of the general client architecture, rather than describing a specific design. The Facilities illustrated are a subset of those possible, and the list of candidates is inherently open-ended by the extensibility of the model. Also, no Facility Optimization is shown in the above diagram, and this may introduce additional high-bandwidth channels between Facilities and/or merging.
NOTE. One specific Facility Optimization is so simple and generic that it should be the default for this architecture: after attachment through the Mediator, Facility processes should talk to each other directly, rather than through the Mediator. This optimization requires little change of design, but conceptually adds a sparse mesh of connections between Facilities (connected only where there is a functional dependency), and is clearly more efficient.
Multi-Process Client Mediator
The Mediator is the communications hub of the client. As such, it is expected to benefit from being as fast as possible and as small as possible (for CPU cache locality). To this end, and in keeping with the client design architecture, all non-communication functions should be placed in attached processes, even if those functions relate to the Mediator alone. For example, Mediator configuration and subsequent control should be carried out by a Facility, not in the Mediator itself.
The Mediator is expected to comprise the following elements:
- Dynamic backend socket interface able to accept an arbitrary number of connections from Facility processes. Note that any processes that might be considered logically "frontend" in the sense of lying between VW servers and the Mediator are actually implemented as Facilities as well --- at the level of process abstraction, there is no distinction.
- Identity information for each Facility to allow message and event routing.
- Extensive runtime instrumentation, with all captured data passed asynchronously to Facilities for processing.
- A minimalist facility message handler/dispatcher optimized for Mediator functions only.
The Mediator is a centralized message router for the client, and as such it is a single point of failure. For advanced fault-tolerant clients such as might be running in a broadcast studio, either hot standby or load balancing techniques could be employed to reduce the impact of failure, using at least two Mediators in a resilient or high-availability configuration.
NOTE. After Facilities attach through the Mediator, the default Facility Optimization is for Facility processes to talk to each other directly, rather than through the Mediator. This is highly beneficial for reducing overheads and latency in inter-facility communications, but the Mediator still needs to be efficient in its normal duties as proxy to the VW servers.
Multi-Process Client Facilities
Each of the following Facilities is implemented as a process which attaches to a Facility Socket on the Mediator process. Two or more performance-critical Facilities may be combined if the engineering tradeoff is deemed acceptable. The list includes some Facilities that are large subsystems which have not yet been refactored into components (eg. Complete 3D Engine).
As noted earlier, any processes that are considered logically "frontend" in the sense of lying between VW servers and the Mediator are actually implemented as Facilities as well, despite these being primarily "backend". At the level of process abstraction and setup, there is no distinction. The only difference is that actual frontend processes also employ direct connections to remote servers. An example of this kind of Facility could be the libsecondlife-based LibSecondlife Gateway (if used). Direct external connections are considered a Facility Optimization within this client design model, since communication with remote parties could have been routed through the Mediator instead.
- Identity Manager
- Login Manager
- Configuration Manager
- Object cache
- 3D Scenegraph
- Standard 3D Renderer
- Extended 3D Renderers
- Vertex and Pixel Shaders
- Complete 3D Engine
- Event Subsystem
- Logging Subsystem
- 2D UI Controller
- Input Handlers
- Output Handlers
- Audio player
- Audio DSPs
- MIDI Interface
- VoIP interface
- Encryption Modules
- Local Scripting VMs
- Generic State Machine
- Unit Test Framework
- Fuzzing Tester
- Regression Test Manager
- Private Gateways
- LibSecondlife Gateway
- Private Asset Database
- Proxy for Monolithic Client
Note that Facilities are implemented as full processes, not as DLL/.so plugins. As a result, they can be run standalone for testing in a reduced capacity, or connected to an alternative fuzzing Mediator to check their robustness. They are not dependent on the Mediator process's internal symbols nor on those of any other Facility.
Because Facilities run as independent processes, the overall client can be made extremely robust and resilient to failure of individual facilities.
NOTE. Depending on the function implemented, some Facilities may elect to open higher-bandwidth channels to other Facilities, mediated through the Mediator Facility API. One such Facility Optimization is a default: Facilities obtain from the Mediator the service endpoint socket for other Facilities on which they have a direct dependency, and they then connect to those sockets directly. Consequently, normal inter-Facility communication is point-to-point. Some Facilities may even merge with others, when performance constraints leave no alternative (rare).
Where appropriate, multiple Facilities of the same type may be attached concurrently: for example, both a normal workstation renderer and an HDTV renderer could operate simultaneously, or dual Wiimotes could control a pair of katanas, or multiple MIDI instruments might be employed during a musical performance.
Facility Attachment Protocol
Please note that this is a preliminary and tentative specification.
- Facility and Mediator processes are normal TCP clients and server, respectively. All ports mentioned here are TCP server/listener ports. Normal TCP dynamically-assigned originator ports are present at the other end of each connection and are not mentioned here.
- The Mediator listens permanently on a predefined/configurable range of Facility Ports (TCP ports), waiting for attachment requests.
- A Facility process opens a TCP connection to the Mediator on the predefined Facility Port of its choosing, and writes a standard set of details over the connection stream.
- If the details are acceptable, the Mediator adds the new Facility to its facilities table and sets up appropriate message routing to it.
- Facilities are detached through simple closing of the TCP connection by either party. Detaching a Facility removes its entry from the Mediator's facilities table.
- Attaching the same Facility multiple times with suitable parameters creates a Facility Pool for multi-input or multi-output fanout, or for load balanced computation.
- STCP may be considered as a replacement for TCP in this application later, as it has benefits for messaging.
- The main reason for keeping request ports and attachment ports separate is for clarity and ease of monitoring/diagnosis during development. It does not preclude attachment ports being configured to use the same TCP port as used for requests.
- The above can easily be seen as a TCP-oriented description of services setup similar to that in standard pub/sub practice. Subsequent to the setup stage, the Multi-Process Client becomes a classic event-driven application.
- An architecture in which Facilities can be developed in isolation by third parties and attached in user-defined numbers and combinations is subject to a variety of potential dangers to its integrity, not the least resulting from variable quality. It is important that the Mediator be designed with some resilience against common faults, for instance to avoid passing on rapid reconnections to world servers, and to throttle repeated connection requests from Facilities. Provision of simple network access protection may also be relevant.
Facility Definition Protocol
- aka. How does a Facility tell the Mediator what services it has to offer?
- IMPORTANT: What is the absolute minimum that the Mediator needs to know about the services offered by a Facility? Can we reduce it down to nothing, other than a service name/number? The less the Mediator knows, the faster it can be (at least on a simplistic level), although whether this translates into a faster client overall is debatable.
Facility Operation Protocol
- aka. 1) How does one Facility use the services of another Facility?
- 2) How does the Mediator manage the communication between Facilities?
- See External Links for some background on event-driven architecture.
Facility Data Transfer Format
- The options available and the advantages/disadvantages of each in this application are currently being examined.
A number of well-known options exist:
- XML is an obvious candidate since it already lies at the core of AWG's REST services.
- JSON has several benefits, including clarity.
- YAML is a superset of JSON, with added benefits.
- Lua is a lightweight programming language often used for structured data transfer.
- Use a plug-in (DLL/.so) approach so that the data structuring technology is not hardwired into the design. [Rejected: this would balkanize Facilities badly.]
- Use an even more lightweight ad hoc textual encapsulation for data transfer.
- Dispense with textual formats altogether and use direct binary communications instead.
- Use the "best" of the above without regard to overheads, and once usage has stablized create a binary equivalent form to handle the most common cases more efficiently. Eg. XML+binary. This approach could be used at the framework level (Facility -> Mediator -> Facility), or only in direct Facility -> Facility communications as a Facility Optimization, or in both. This has good evolutionary properties.
A choice needs to be made but there is no single obvious candidate. All have advantages and disadvantages, and these are not the same ones as apply in AWG server-side. Additional input (lots of it!) is essential here. Trialing alternatives may be required as well.
Initial Facilities in More Detail
This section focuses on those Facilities which individual stakeholders consider relevant to their particular viewpoint, at this time. This is a very limited set at present, as it derives from the expected use of the client as a QA and measurement tool. The list of Facilities described here will expand as the Multi-Process Client evolves towards a full virtual-worlds client.
The descriptions in this section are brief enough to be read together as a set. Once full design and implementation begins on a given Facility, the relevant section below should link to a page dedicated to that Facility.
- The Login Manager is heavily capability-based, and the many capabilities present in the system present a considerable complexity issue. The capabilities need to be documented concisely but effectively. A separate Capability Manager may be appropriate in this area.
- The Multi-Process Client is a typical event-driven application: input received from user-controlled input devices generates events, as does data received from all network connections, and additional events are generated by internal timers. All these events trigger actions (event handlers) within the Facility processes, routed appropriately by the Mediator, and these Facilities in turn generate more events for other Facilities to handle.
- The other side of the coin is worth emphasizing: when no events are being received and/or handled, the client is entirely quiescent. There are no busy-waiting loops to waste CPU unnecessarily in the Multi-Process Client.
- A good implementation of events is vital to the success of an event-driven application. It is proposed that existing well-reputed implementations be employed for this work, such as liboop, libevent, libev, or Fastevents for SDL. The above are being evaluated, and the shortlist may grow.
2D UI Controller
- There is no need for the 2D UI to be programmed as part of the 3D rendering process. Most of the creation and control of 2D windows and other graphic 2D elements is performed at very slow rates, and is best carried out by an independent Facility, or several. These can then communicate the occasional 2D UI changes to a separate 3D rendering Facility if 2D/3D integration is required, or they can display the 2D separately if desired, or both. Such separation reduces bloat in the 3D renderer, a subsystem which traditionally grows rapidly in complexity and so can benefit greatly from such refactoring.
- Input handlers map low-level inputs received from devices to input events, which are then sent to other Facilities for processing. When an input device is specific to one single Facility, the low-level handling may be performed directly in that process. However, in the general case this input handling is best placed in a separate Input Handler Facility process, so that the corresponding input events can be made available to as many other Facilities as required.
- Note that existing 3D subsystems can make refactoring input harder than it need be, by implicitly tying inputs from pointers and keyboards to the 3D graphics window. Because of this, it may be necessary to capture these inputs within the 3D rendering facility and pass the input data to the appropriate external Input Handler Facility for issuing events. Since responsiveness is a key property of successful UIs, efficiency may dictate that the entire Input Handler Facility may have to reside physically within the 3D renderer process in such cases. This would be a case of Facility Optimization.
- An Output Handler Facility is one which generates output that is not normally fed back into the client as events.
- A Logging Facility is a typical example of an Output Handler: it receives loggable information and stores it where specified, and it reacts to requests to add or redirect logging destinations.
- A view replicator is another example: one type might communicate with an existing 3D rendering facility to generate a second visual, potentially different to the first (eg. a visualization of scripting activity). Another type might simply replicate the normal visual and send it to a remote display using a standard mechanism such as VNC.
Local Scripting VMs
- Local scripting is central to the implementation of any powerful, flexible, extensible, and user-empowering client. This stems from the simple observation that users are all different, so no predefined set of functions can meet the needs of all users, and hence many will be disempowered by lack of user-level extensibility. Local scripting, if simple and flexible, can address this problem quite effectively, even for non-programmers since scripts can be generated by simple UIs.
- The socket-oriented structure of the Multi-Process Client offers the huge benefit of programming language independence, allowing scripters to use the scripting language of their choice, or the one best suited to the task in hand, or several. Consequently, no preference is expressed here for any particular programming language for scripting, and even the term "scripting" itself is not prescriptive: the important property is ease of change, rather than immediate execution. (Most modern scripting languages involve compilation anyway, so "scripting language" actually denotes a general area of the language implementation spectrum rather than a separate category.)
- Simple yet fast and powerful scripting languages are very likely to be the most successful ones, and hence to predominate in terms of popularity with users. One such language is Lua, which is used for scripting in a huge number of games and other applications owing to its high speed, small size, simplicity, and ease of embedding/extending. Another language much used in this role is Python, which is far less sleek but also far more endowed with sophisticated extensions and huge community support. Many other languages are also excellent in the role of embedding and extending, but have not achieved the popularity of these two. Both of the above languages are expected to be used where scripted code needs to be embedded within C/C++, or extended with C/C++, for speed.
- The Multi-Process Client is of course not constrained to only those languages that are good for embedding and extending. Each Facility is a separate process, and threfore a non-embeddable or non-extensible language can be used as well.
State Machine Facility (SMF)
- Numerous operations in a complex client require event-stepped sequencing through defined states. Rather than spreading ad hoc implementations of state sequencing throughout the client where sequencing is needed, we propose to implement a generic state machine framework just once, in its own dedicated Facility process which manages as many state machine instances as needed. Other Facilities can then employ this as a service.
- In the envisaged usage scenario, a Facility (such as the Login manager) which requires event-driven sequencing will request the creation of a state machine, populate it with states, define per-state data, specify the trigger events which cause state transitions, and add pre- and/or post-transition callbacks for each possible transition. When enabled, relevant events (from any source) would be routed by the Mediator to the state machine, which would then perform the appropriate state transitions and dispatch actions anywhere within the client.
- Although state machines would normally be implemented within the SMF process and therefore callback activation would involve inter-Facility communication, Facility Optimization would be possible in the usual manner for high-speed event switching, namely by relocating key sections to their point of use.
Unit Test Framework
- See Unit testing for background.
Proxy for Monolithic Client
- A relatively simple way of analysing existing SL protocols in the absence of appropriate documentation might be to connect the existing monolithic client to the standard grid through a Facility which acts as a proxy. This would also have the benefit of providing early 3D visuals for the Multi-Process Client, and of reducing the impact of monolithic client crashes since the rest of the MPClient is not affected and login would not need to be repeated.
Anyone with an interest in this Viewpoint is welcome to join the roster of VAG members below. You should also join the AW_Groupies group in Second Life.
Non-stakeholders are also welcome to improve this VAG page --- this is after all a wiki. However, modifications that do not assist the stakeholders with the goals of this group will simply be reverted --- this wiki page is after all a workgroup tool of the group.
The above does not preclude modifications which identify and detail specific problems and incorrect designs so that they can be corrected; such contributions are very valuable. It does preclude wholesale negativity. If you're not interested in the project, just go away.
In world meetings
This group is currently active on a continuous basis, communicating over group and friend/calling-card IM and via wiki. We will also endeavor to meet at least once a week in-world, or more frequently if desired.
Members are active on the wiki and in the SLDEV mailing list.
Architectural Descriptions/Views used to express this viewpoint
This VAG expresses a client-oriented viewpoint, and hence is concerned only with those Architectural Descriptions/Views that impact on the client. This section identifies:
- the general form or representation of ADVs required to express the viewpoint of this VAG
- the elements within such ADVs which will be used to express the viewpoint
- how these elements within such ADVs map to the concerns of this viewpoint
- the traceability to viewpoint concerns required for conformance with the viewpoint.
Tools employed by this VAG
- Normal wiki textual and graphic representations are expected to be sufficient for group communications in this VAG. Proprietary formats should be avoided.
Testing / QA
- It is the main expected task of this VAG to develop the key elements of a Multi-Process Client, which will be a general QA tool for assisting the Quality Assurance VAG and Scalability VAG with their respective concerns. The same tool can be used for self-testing and test-driven development of the client itself.
- Interface testing and performance measurement for the above VAGs is likely to require visualization-oriented output. This same output is expected to provide visualization for client operations as well, which will be of particular interest in multi-grid applications.
- Since each component part of the Multi-Process Client will have a message-oriented API for socket-based communications, it would be very helpful to extend this interface with self-documentation and possibly also reflexion/introspection features. Even at its most minimal, built-in API usage information can be expected to be valuable for subsequent developers engaged in creating extended Facilities.
References and Resources
- SVG Overview Graphic of Multi-Process Client -- source file
- IEEE_1471 -- recommended practice for architecture description
- Monolithic client architecture
- Hegemons Login Analysis
- Client-server protocol and messaging
- lslsimcaps -- Accessing login and capabilities with LSL
- SLDev Mailing list and archives -- High traffic list spanning all SL developer topics
- libsecondlife -- useful library and protocol reference
- OpenSim sim project, wiki, and OSGrid -- a likely seed for 3rd party grids
- Event loop, event-driven programming, liboop -- some event-oriented resources.
- realxtend -- an open source platform for interconnected virtual worlds
Please note that the Multi-Process Client VAG is a non-hierarchical VAG in every respect, without exception. Any tags supplied with names are purely informational. Stakeholder ordering is alphabetical.