Meru Virtual World Architecture


Stanford: Daniel Horn, Ewen Cheslack-Postava, Tahir Azim, Behram Mistree, Bhupesh Chandra, Lily Guo, Philip Levis
Princeton: Jeff Terrace, Xiaozhou Li, Mike Freedman
Win32 client work by: Matteo Borri


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:

Virtual World Components:

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.

Space Architecture:

Currently we are focusing on the architecture of space servers. They provide four basic services:
  • 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.
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:
  • 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 resources.


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 interactive scripting.

We are developing Emerson, a scripting language based on JavaScript. Emerson addresses these challenges with three core design 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.

Content Distribution for Virtual Worlds:

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.


[1] 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).

[2] 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)

[3] 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).

[4] 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).

[5] 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, doi:10.1109/MPRV.2009.54

Technical Reports:

[6] 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