When a SNMP client, such as an SNMP management console, requests information that is located within one of the supported MIBs, the values that are returned to the client are determined by the MBeans that are deployed in the JMX agent that supports the MIB. The MBeans interact directly with the underlying managed resource to obtain the values. For example, the J2EE MIB provides details associated with EAServer. This information is extracted from EAServer by the EAServer proxy MBean, using EAServer-specific APIs—see “EAServer proxy MBean”.
Because an SNMP client can generate many requests in rapid succession, it is not reasonable to generate a corresponding request against EAServer for each request that is received from the client. Instead, the JMX agent MBeans cache information retrieved from EAServer, and re-query the server only when the cached data becomes stale. Cached data includes information about the existence of rows in a table (there may be a row for each deployed EJB), and the values in each row. Cached data becomes stale at different rates; by default, 60 seconds for the existence of rows, and 10 seconds for the values in a row. JMX agent MBeans recalculate the existence or rows within a table when 60 seconds have elapsed since the last time the rows were calculated. The MBeans refresh the contents of a row when 10 seconds have elapsed since the last time the row was refreshed. Typically, the structure of a table changes infrequently, if at all; the content of rows is more dynamic.
