Meru Virtual World Architecture
Stanford: Daniel Horn,
Princeton: Jeff Terrace,
Introduction: We believe that one of the next major
application platforms will be 3-dimensional, online virtual worlds.
Virtual worlds are shared, interactive spaces. Objects inhabit the
space, have programmable behaviors, and can discover and communicate
with other objects. Users commonly experience the world through an
avatar and can interact with objects in the world, much as they would
in the real world. Many currently deployed applications are virtual worlds:
multiplayer online games such as World of Warcraft and social
environments such as Second Life are popular examples. Other examples
include environments for virtual collaboration, distance learning
applications, and augmented reality.
Unfortunately the early evolution of virtual worlds has been
ad hoc. They have completely independent constructions, share few
architectural aspects, and offer little or no interoperability.
Because these systems are designed for very specific applications,
each suffers from at least one of poor scalability, centralized
control, and a lack of extensibility.
The Meru Project is designing and implementing an architecture for the
virtual worlds of the future. By learning how to build applications
and services before they are subject to the short-term necessities of
commercial development, we hope to avoid many of the complexities
other application platforms, such as the Web, have encountered.
We're currently working on the following projects:
Meru addresses the issues of
scalability and federation by carefully separating the components of a
virtual world. The core components of any virtual world system are the
simulation of the world, the simulation of individual object
behaviors, and the storage and distribution of the content of the
world. The Meru architecture separates these concerns:
- Object Host: handles simulation of
individual objects: receiving messages, handling events, and
simulating behavior using user-created scripts.
Space: handles inter-object behavior: it gives objects
names in the world, helps objects discover the names of other objects
they might be interested in, enables communication between objects,
and might provide physical simulation such as rigid body physics.
Persistence Services: handle storing and serving the large,
read-mostly data a virtual world needs, such as textures and meshes.
Currently we are focusing on the architecture of space servers. They
provide four basic services:
We are designing our space service to handle all of these scalably.
Some specific challenges we face in designing and implementing the
components that provide these services are:
Naming - the space is a communication medium for objects, so it must
give them names they can use to refer to each other. In this sense,
the space can be though of as an address space. We use a simple flat
namespace and assign these identifiers randomly.
Discovery - in order to send messages, objects must have other
objects' identifiers. The space must provide a way for objects to
discover other object identifiers. Of course this mechanism should aim
to return the identifiers of the most relevant objects to the querier.
Communication - the space acts as a communication medium and mediates
all inter-object messages. It must provide routing of messages and
possibly apply rules to restrict communication.
Physical Simulation - much physical simulation depends on the state of
multiple objects, and is simplified by having a single authoritative
simulator. The space may provide some form of physical simulation,
ranging from simple collision detection and response to a complex
rigid body simulation, depending on what the particular world calls for.
Efficient and Scalable Discovery - most existing systems use a simple
distance cut-off approach to discovery, where all objects within some
radius will be returned. While it is known how to efficiently
implement this approach when the radius is relatively small, the
nature of the query requires the radius to be large to find the
majority of objects that are important. Further, these systems return
many objects that could be quickly discarded as irrelevant. We're
looking into different types of queries that more efficiently find
important objects, but can still be implemented efficiently and scalably.
Scalable Communication - without restrictions on communication,
object messaging can very quickly overload the system, leading to poor
service for everyone. We are investigating how we can control message
queuing to provide physically plausible quality of service under load
while making the best use of resources when not under load.
Load Balancing - interests naturally collect -- we know that interests
often follow a Zipf distribution. This implies that as the world
grows, the maximum load on a single space server using fixed size
regions will increase. The resulting problems are already evident in
most other systems that split their worlds into fixed sized regions: a
few central hubs are overloaded or the number of participants is
simply capped to avoid the problem. Instead, we are investigating
ways to balance the load by dynamically segmenting the world, allowing
loaded servers to split in order to double available bandwidth and compute
power, and underloaded servers to merge so unused regions do not waste
In a seamless, scalable, and federated virtual world scripted objects
are distributed across many hosts and users may generate and host
scripts. Communication between objects via asynchronous messages is
the norm, and objects may not trust each other. These challenges make
popular scripting languages poorly suited to this domain. Most
existing languages designed for virtual worlds are ad hoc and often
lack even basic features for event-driven programming, code reuse, and
We are developing Emerson, a scripting language based on
concepts: entity-based isolation and concurrency, an event driven
model with concise and expressive pattern matching to find handlers
for messages, and strong support for example-based programming within
the live virtual environment.
Much of the content that makes up a virtual world is long-lived,
static content: meshes, textures, scripts, and audio. Content
distribution networks address the problem of efficiently serving these
assets to millions of clients.|
Virtual worlds have a few properties which suggest a different CDN
design might be warranted. First, most content can be split into
levels of detail and accessed incrementally. For instance, a texture
can be displayed at lower resolution before the entire texture has
been downloaded. Further, the client may not request all chunks: a
higher resolution version of the texture might not improve the
rendering because the mesh is too far away for it to be noticable.
Second, the relative importance of requests varies quickly over
time. Unlike the resources on a web page, which are all required to
display the page, the content required to display a virtual world may
vary quickly. If the user turns, the relative importance of different
meshes and textures in the scene change suddenly. Still, out-of-view
elements should still have some priority since the user may turn back
towards them soon.
Finally, virtual worlds have spatial locality that could be exploited
by the CDN: resources that are geometrically nearby in the world are
likely to be requested by the same client. For instance, the meshes
for objects that are next to each other will likely be requested by
all avatars in the region.
We are building a CDN which exploits these features, as well as a
client library to ease interaction with the CDN and high frequency
updates to requests.
A client displaying a virtual world must decide which assets to
download and display, taking into account the effect that asset (or
lack of the asset) will have on the fidelity of the world, the current
and possible viewpoint of the user, the size of the content (both for
download and as stored on the graphics card), and that the data must
be streamed from the CDN. |
We are building a flexible graphics asset manager which can account
for these challenges and allows us to experiment with different
algorithms for prioritizing and downloading assets and LODs of assets.
Built on top of our CDN client library, it will be able to quickly
update the priorities of assets. The algorithms could take into
account resource constraints of the graphics hardware, perceptual
metrics taking into account the objects the assets are associated
with, the available levels of detail, and the time to transfer the
data from the CDN.
 Ewen Cheslack-Postava, Tahir Azim, Behram F. T. Mistree, Daniel Reiter Horn, Jeff Terrace, Philip Levis, and Michael J. Freedman.
"A Scalable Server for 3D Metaverses",
To appear in Proceedings of the USENIX Annual Technical Conference (ATC '12).
 Jeff Terrace, Ewen Cheslack-Postava, Philip Levis, and Michael J. Freedman.
"Unsupervised Conversion of 3D Models for Interactive Metaverses",
Proceedings of the 2012 International Conference on Multimedia and Expo (ICME 2012)
 Behram F. T. Mistree, Bhupesh Chandra, Ewen Cheslack-Postava, Philip Levis, and David Gay.
"Emerson: Accessible Scripting for Applications in an Extensible Virtual World", Proceedings of the ACM international conference on Object oriented programming systems languages and applications (Onward 2011).
 Bhupesh Chandra, Ewen Cheslack-Postava, Behram F. T. Mistree, Philip Levis, and David Gay.
"Emerson: Scripting for Federated Virtual Worlds", Proceedings of the 15th International Conference on Computer Games: AI, Animation, Mobile, Interactive Multimedia, Educational & Serious Games (CGAMES 2010 USA).
 Daniel Horn, Ewen Cheslack-Postava, Tahir Azim, Michael J. Freedman,
Philip Levis, "Scaling Virtual Worlds with a Physical
Metaphor", IEEE Pervasive Computing, vol. 8, no. 3, pp. 50-54, July-Sept. 2009,
 Daniel Horn, Ewen Cheslack-Postava, Behram F.T. Mistree, Tahir Azim, Jeff Terrace, Michael J. Freedman, and Philip Levis
"To Infinity and Not Beyond: Scaling Communication in Virtual Worlds with Meru",
CSTR 2010-01 5/11/09