Hypergrid Implementation

= Hyperlinks and Agent Transfers =

When you establish a link between your opensim and another, a message is sent out to that other opensim requesting information about it; the required information includes the network information of that opensim host, and the coordinates of its first region on its local grid in the form of a region handle. For example, suppose you are linking node X.com:9000, placing it in your local map at 900, 900. That opensim runs one or more regions that likely are not in 900, 900 on their own map; suppose the first region of that opensim is at 1100, 1100. From your point of view, it doesn't matter what those other coordinates are, and you don't need to know -- that's the key to being able to decentralize the "world" as given by a 2D map; you want to place it in your map at 900, 900. The "true" position of that simulator only matters for the LL viewer, when there are teleports between your world and that other opensim. This mapping between coordinate systems is the essence of hyperlinks for opensim; it's one simple but critical thing that the hypergrid implementation does. The mapping happens on the TeleportFinish event; instead of sending the local coordinates to the viewer, the hypergrid teleport wrapper sends the remote coordinates.

When an agent teleports through that hyperlink the following happens. First, before InformRegionOfChildAgent, the local opensim notifies the remote opensim of this foreign user via the "expect_hg_user" method. That message sends along the addresses of all the servers that this user uses, i.e. user, inventory and asset servers. The remote opensim places an entry for that user in its local user profile cache but not in its user database; the foreign user information is non-persistent. After that, the teleport process is exactly the same as the normal teleport process; the only difference is that the region handles are switched between the remote region's hyperlink position on the local grid and its actual position on its grid.

In summary, the two new concepts introduced by the hypergrid are the concept of an hyperlink and the concept of a "local user" vs. "foreign user".

= Inventory Access =

Inventory access from abroad is done by wrapping the existing scene-inventory interactions with additional code that gets or posts inventory assets from/to the user's asset server. When inventory is accessed, the hypergrid wrapper checks if the user is foreign and, if she is, the wrapper simply brings the necessary assets from the user's asset server to the local asset cache and server; from then on, the wrapper passes the control to the existing inventory access functions. When something is added to inventory, the hypergrid wrapper is notified via an event, and posts the assets to the user's asset server. A cache of the exchanged item identifiers is maintained so that they aren't brought back over and over again.

The result is that hypergrided opensim instances end up interacting with several asset servers, instead of just one. That interaction is implemented in a straightforward manner by instantiating several GridAssetClient objects, instead of just one.