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The Dynamic Store

In a running Mac OS X system, the dynamic store contains a snapshot of the current network state. It also holds a copy of the preferences that define the currently active configuration. Because it contains the current configuration, as opposed to all configurations the user has defined, this part of the dynamic store is a subset of the persistent store. As such, the dynamic store’s hierarchical structure and combinations of key-value pairs are also determined by the System Configuration schema.

Various configuration agents (introduced in “Configuration Agents”) keep the dynamic store up to date. To do this, they use the information in some keys to decide what to do and they publish the results of their actions in other keys. This interaction between the configuration agents and the dynamic store is an ongoing process and is not limited to the initial configuration of network services that occurs when you turn on your computer. Instead, the configuration agents update the dynamic store whenever system or network events affect the current system state. The ability of the dynamic store to reflect the current system state is at the heart of Mac OS X network mobility.

In addition to holding the current network state, the dynamic store provides a level of abstraction between low-level networking and system events and the applications that need to know about them. For example, when you unplug your Ethernet cable a configuration agent gets this information from a low-level kernel event and updates the value of the appropriate dynamic store key. Through the dynamic store, an application can get this information without having to monitor low-level system events. This is because the System Configuration framework provides notification services that allow an application to register interest in specific keys. When a key’s value changes, an interested application is notified that a change occurred. It is then the application’s responsibility to inspect the value of the key.

Although you can register interest in any key, it’s best to be very selective about the notifications you choose to receive. For one thing, to be sure you’re watching the correct key and to interpret the information you get from it, you need detailed knowledge of the complex System Configuration schema. In many cases, you can use System Configuration APIs to receive notifications of specific events without resorting to watching individual keys. For information on some of these APIs, see “System Configuration APIs” and “Determining Reachability and Getting Connected.”

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