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16.1. Coordinating Instances

Achieving a unified application experience often involves controlling what instance each user connects to and making users aware of what other people are doing in other instances. The broad categories of applications are (1) those in which each instance can pretty much manage itself—often with help from an application server and database—and (2) those in which instances must cooperate with one another in real time. An example of the first type of application is a video conference application in which one person manages a room instance and controls who is invited to visit it. A list of invitees held in a database is often all that is needed to invite users and control access.

An example of the second type of application is an online gaming system in which users enter one of a series of lobbies depending on their interests or how busy each lobby is. From there, they can see what game rooms are available and agree to move to one that contains a game of interest to them, such as chess. Another example in which real-time coordination is required is a help desk waiting room in which users wait in a queue to get into a help operator's room for assistance. Each operator may need to see who is in the queue and even select some users out of order based on information each user provides or information about ongoing problems.


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