array-user Frequently Asked Questions Page

Container indexes vs MIB indexes

SNMP tables have indexes. A table index may have multiple components which, taken together, uniquely identify a row.

Now, when the two are used together, it easy to get confused. All of the MIB indexes, taken together, are the primary container index. Even if a MIB table has a dozen indexes, the container only has one.

Dave Shield put it this way:

The (primary) container index is effectively equivalent to the sequence of OID instance subidentifiers, i.e. everything following the object OID itself. So using the Host Resources hrPartitionTable as an example, and taking the two index values hrDeviceIndex = 99 hrPartitionIndex = 42 then this instance of hrPartitionLabel would have the OID .1.3.6.1.2.1.25.3.7.1.2.99.42 where ".1.3.6.1.2.1.25.3.7.1.2" is the OID of the hrPartitionLabel object, and "99.42" are the instance subidentifiers. The primary container index would then also be "99.42"

An example

guestTable guestEntry INDEX { building, room } building INTEGER room INTEGER name STRING

So, the MIB has two indexes, the building and room numbers. The primary MIB index is the building, and the secondary MIB index is the room. However, the primary index ofr the container will be the combined index OID (building.room). The default compare routine for containers assumes that the data record's first component is a netsnmp_index (and the array-user code generates a structure that starts with a netsnmp_index), so you don't need to provide a comparison routine. If you wanted to provide your own compare routine, it might look something like this:

int netsnmp_compare_netsnmp_index(guestTable *lh_guest, guestTable *rh_guests) { /* compare building, then room */ if(lh_guest->building == rh_guest->building) { if(lh_guest->room == rh_guest->room) return 0; else { if(lh_guest->room < rh_guest->room) return -1; else return 1; } } else { if(lh_guest->building < rh_guest->building) return -1; else return 1; } }

Remember, for the array-user, as provided, the data for a table is internal to the agent. So we now have a container with the guest data, and we can look up data by building and room number. What if our application now needs to generate a guest report, but sorted by name? We have the data, but in the wrong sort order. This is where you would use a secondary index to the container. The new compare might look like this:

int netsnmp_compare_netsnmp_index(guestTable *lh_guest, guestTable *rh_guests) { /* compare name, then building and room */ int rc = strcmp(lh_guest->name,rh_guest->name); if(rc != 0) return rc; if(lh_guest->building == rh_guest->building) { if(lh_guest->room == rh_guest->room) return 0; else { if(lh_guest->room < rh_guest->room) return -1; else return 1; } } else { if(lh_guest->building < rh_guest->building) return -1; else return 1; } }

Creating new rows

    xxTable_context *context;
    netsnmp_index   index;
    oid             tmp_oid[MAXLEN];

    /* set temp oid for new row */
    tmp_oid[ 0 ] = 42; 

    /* set temp index */
    index.oids = tmp_oid;
    index.len = 1

    /* create row */
    context = xxTable_create_row(&index);

    /* fill in data */
    context->abc = some_value;

    /* insert row in container */
    CONTAINER_INSERT(cb.container, context);

    xxTable_context *context;
    netsnmp_index   index;
    oid             tmp_oid[MAXLEN];

    /* set temp oid for new row with string index 'hi' */
    tmp_oid[ 0 ] = 2;  /* length *
    tmp_oid[ 1 ] = 'h'; 
    tmp_oid[ 2 ] = 'i'; 

    /* set temp index */
    index.oids = tmp_oid;
    index.len = 3;

    /* create row */
    context = xxTable_create_row(&index);

    /* fill in data */
    context->abc = "bob";

    /* insert row in container */
    CONTAINER_INSERT(cb.container, context);