# snmp_generic

### MODULE

snmp_generic

### MODULE SUMMARY

Generic Functions for Implementing SNMP Objects in a Database

### DESCRIPTION

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]}}.

In the functions defined below, the following types are used:

`NameDb = {Name, Db}, Name = atom(), Db = volatile | persistent | mnesia`

`RowIndex = [int()]`

`Cols = [Col] | [{Col, Value}], Col = int(), Value = term()`

`RowIndex` denotes the last part of the OID which specifies the index of the row in the table (see RFC1212, 4.1.6 for more information about INDEX). `Cols` is a list of column numbers in the case of a `get` operation, and a list of column numbers and values in the case of a `set` operation. `Cols` is a list of column numbers in case of a get operation, and a list of column numbers and values in case of a set operation.

### EXPORTS

`get_status_col(Name,Cols)`
`get_status_col(NameDb,Cols) -> {ok, StatusVal} | false`

Gets the value of the status column from `Cols`.

This function can be used in instrumentation functions for `is_set_ok`, `undo` or `set` to check if the status column of a table is modified.

`get_index_types(Name)`

Gets the index types of `Name`

This function can be used in instrumentation functions to retrieve the index types part of the table info.

`table_func(Op1,NameDb)`
`table_func(Op2,RowIndex,Cols,NameDb) -> Ret`

Types:

`Op1 = new | delete`
`Op2 = get | next | is_set_ok | set | undo`

This is the default instrumentation function for tables.

- The `new` function creates the table if it does not exist, but only if the database is the SNMP internal db.
- The `delete` function does not delete the table from the database since unloading an MIB does not necessarily mean that the table should be destroyed.
- The `is_set_ok` function checks that a row which is to be modified or deleted exists, and that a row which is to be created does not exist.
- The `undo` function does nothing.
- The `set` function checks if it has enough information to make the row change its status from `notReady` to `notInService` (when a row has been been set to `createAndWait`). If a row is set to `createAndWait`, columns without a value are set to `noinit`. If Mnesia is used, the set functionality is handled within a transaction.

If it is possible for a manager to create or delete rows in the table, there must be a `RowStatus` column for `is_set_ok`, `set` and `undo` to work properly.

The function returns according to the specification of an instrumentation function.

`table_get_elements(NameDb,RowIndex,Cols) -> Values`

Types:

`Values = [term() | noinit]`

Returns a list with values for all columns in `Cols`. If a column is undefined, its value is `noinit`.

`table_next(NameDb,RestOid) -> RowIndex | endOfTable`

Types:

`RestOid = [int()]`

Finds the indices of the next row in the table. `RestOid` does not have to specify an existing row.

`table_row_exists(NameDb,RowIndex) -> bool()`

Checks if a row in a table exists.

`table_set_elements(NameDb,RowIndex,Cols) -> bool()`

Sets the elements in `Cols` to the row specified by `RowIndex`. No checks are performed on the new values.

If the Mnesia database is used, this function calls `mnesia:write` to store the values. This means that this function must be called from within a transaction (`mnesia:transaction/1` or `mnesia:dirty/1`).

`variable_func(Op1,NameDb)`
`variable_func(Op2,Val,NameDb) -> Ret`

Types:

`Op1 = new | delete | get`
`Op2 = is_set_ok | set | undo`

This is the default instrumentation function for variables.

The `new` function creates a new variable in the database with a default value as defined in the MIB, or a zero value (depending on the type). The `delete` function does not delete the variable from the database. The function returns according to the specification of an instrumentation function.

`variable_get(NameDb) -> {value, Value} | undefined`

Types:

`Value = term()`

Gets the value of a variable.

`variable_set(NameDb,NewVal) -> true | false`

Types:

`NewVal = term()`

Sets a new value to a variable. The variable is created if it does not exist. No checks are made on the type of the new value. Returns `false` if the `NameDb` argument is incorrectly specified, otherwise `true`.

### 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}]}}.

### AUTHORS

Martin Björklund - support@erlang.ericsson.se
Klas Eriksson - support@erlang.ericsson.se

snmp 4.8.2
Copyright © 1991-2006 Ericsson AB