Intent

Isolate database changes for index and/or client/server synchronization.

Also called

Index Synchronization, Choke Point, Ethereal Void

Problem

It is critical that all Spatial Indices remain synchronized with their associated Model Database. Similarly, a client must remain synchronized with a server. Since several things can cause a change to a Model Database, it is best to isolate all changes to two calls: push and pop (a.k.a. insert / remove). This creates two states: "in world" and "out of world" which is also known as "in ethereal void."
This pattern gets its name from the way that all insertions and deletions are limited to one place like a physical gateway where all people must move through it to get in or out which allows you to control flow.

Solution

A pop() method brings a model object out of the "void" and into the Model Database and its associated spatial index. A push() method removes the object from the database and index and into the void.
There are three basic operations:
When an object moves, it pushes itself into the void, updates its position, then pops itself back into the world.
When an object is created, it is created in the void then popped into the world.
When an object is destroyed, it is pushed into the void first, then destroyed.
The push / pop choke point may also be used to synchronize a client to a server by transmitting all database changes from server to client.
It should also be noted that having one gateway of change allows multiple changes to be made simultaneously with minimal synchronization effect. For example, push(); move(); changeSize(); pop(); prevents an unnecessary update from happening around the changeSize() call.

Structure:
Not available at this time.

Examples

Some games have different world or index states. For example:

An object may be allowed to be placed inside of another object (a container).
An object may be attached to another object (a weapon, armor, etc.)

In these cases, there may be more than one implementation of pop(). For example:

popIntoWorld( int x, int y); // pop into world
popIntoContainer( Model &container ); // pop into container
popOnto( Model &parent, Matrix4X4 &orient ); // pop onto another model.

Issues and Risks

none

Related Patterns

A Gateway simplifies the synchronization of a Spatial Index with a Model Database.
Gateways are often implemented inside of Model Database code.

Uses and References

Thanks to Herman Miller and Tony Zurovec.

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Date this article was posted to GameDev.net: 6/19/2001
(Note that this date does not necessarily correspond to the date the article was written)

See Also:
Design Patterns

Intent

Also called

Problem

Solution

Structure: Not available at this time.

Examples

Issues and Risks

Related Patterns

Uses and References

Structure:
Not available at this time.