Data Retrieval From Large Simple Tables

Data retrieval from a simple table requires you to use the single, integer index subidentifier to index into an existing data structure.

With some modules, this underlying table might be relatively large, or only accessible through a cumbersome interface. Data retrieval might be very slow, particularly if performing a walk of a MIB tree requires the table to be reloaded for each variable requested. In these circumstances, a useful technique is to cache the table on the first read and use that cache for subsequent requests.

To cache the table, you must have a separate routine to read in the table. The routine uses two static variables. One variable is a structure or array for the data. The other variable is an additional timestamp to indicate when the table was last loaded. When a call is made to the routine to read the table, the routine can first determine whether the cached table is sufficiently new. If the data is recent enough, the routine can return immediately. The system then uses the cached data. If the cached version is old enough to be considered out of date, the routine can retrieve the current data. The routine updates the cached version of the data and the timestamp value. This approach is particularly useful if the data is relatively static, such as a list of mounted file systems.