Generic Functions for Implementing SNMP Objects in a Database

The module snmp_generic contains generic functions for implementing tables (and variables) using the SNMP built-in database or Mnesia. These default functions are used if no instrumentation function is provided for a managed object in a MIB. Sometimes, it might be necessary to customize the behaviour of the default functions. For example, in some situations a trap should be sent if a row is deleted or modified, or some hardware is to be informed, when information is changed.

The overall structure is shown in the following figure:

         +---------------+
         |   SNMP Agent  |
         +- - - - - - - -+
         |      MIB      |
         +---------------+
                 |
         Association file       (associates a MIB object with
                 |               snmp_generic:table_funct
                 |               snmp_generic:variable_func)
+--------------------------------------+
|           snmp_generic               |  Support for get-next,
|                                      |  RowStatus operations
+----------------------+---------------+
|    snmpa_local_db    |    Mnesia     |  Database
+--------------+-------+---------------+
|     dets     |  ets  |
| (persistent) |       |
+--------------+-------+

Each function takes the argument NameDb, which is a tuple {Name, Db}, to identify which database the functions should use. Name is the symbolic name of the managed object as defined in the MIB, and Db is either volatile, persistent, or mnesia. If it is mnesia, all variables are stored in the Mnesia table snmp_variables which must be a table with two attributes (not a Mnesia SNMP table). The SNMP tables are stored in Mnesia tables with the same names as the SNMP tables. All functions assume that a Mnesia table exists with the correct name and attributes. It is the programmer's responsibility to ensure this. Specifically, if variables are stored in Mnesia, the table snmp_variables must be created by the programmer. The record definition for this table is defined in the file snmp/include/snmp_types.hrl.

If an instrumentation function in the association file for a variable myVar does not have a name when compiling an MIB, the compiler generates an entry.

{myVar, {snmp_generic, variable_func, [{myVar, Db]}}.

And for a table:

{myTable, {snmp_generic, table_func, [{myTable, Db]}}.

Example

The following example shows an implementation of a table which is stored in Mnesia, but with some checks performed at set-request operations.

myTable_func(new, NameDb) ->   % pass unchanged
  snmp_generic:table_func(new, NameDb).

myTable_func(delete, NameDb) ->   % pass unchanged
  snmp_generic:table_func(delete, NameDb).

%% change row
myTable_func(is_set_ok, RowIndex, Cols, NameDb) ->
  case snmp_generic:table_func(is_set_ok, RowIndex,
                               Cols, NameDb) of
    {noError, 0} ->
      myApplication:is_set_ok(RowIndex, Cols);
    Err ->
      Err
  end;

myTable_func(set, RowIndex, Cols, NameDb) ->
  case snmp_generic:table_func(set, RowIndex, Cols,
                               NameDb),
    {noError, 0} ->
      % Now the row is updated, tell the application
      myApplication:update(RowIndex, Cols);
    Err ->
      Err
  end;

myTable_func(Op, RowIndex, Cols, NameDb) ->   % pass unchanged
  snmp_generic:table_func(Op, RowIndex, Cols, NameDb).

The .funcs file would look like:

{myTable, {myModule, myTable_func, [{myTable, mnesia}]}}.