Caching of Fieldbus Data in the Gateway

There are situations where many clients request the same datapoint value from the Internet/fieldbus gateway. Traditionally, the gateway would acquire this value from the fieldbus, which may lead to high loads on the fieldbus.

To circumvent this problem, the gateway may implement data caching, so that a request does not necessarily trigger fieldbus access. Instead of reading the requested data from the datapoint, it sends the cached data from the gateway.

Although this technique may greatly reduce the load on the fieldbus, the delivered data may be outdated - the ``real'' data in the fieldbus may have changed some time ago. This problem may be circumvented by the following means:

1. Expiration:

The cached data is accompanied by a timestamp, called ``Expiration Date''. When a client requests this data from the gateway, it checks if the expiration date is older than a certain amount of time, in which case the cache is refreshed by retrieving the new value from the datapoint.

2. Subscription:

Many fieldbus systems support a technique called ``System Notifications''. This way, the gateway observes certain datapoints for value changes. In case of a value change, the fieldbus node sends the updated value to the gateway, which refreshes the cache entry.