snmp_index

Module

snmp_index

Module Summary

Abstract Data Type for SNMP Indexing

Description

The module snmp_index implements an Abstract Data Type (ADT) for an SNMP index structure for SNMP tables. It is implemented as an ets table of the ordered_set data-type, which means that all operations are O(log n). In the table, the key is an ASN.1 OBJECT IDENTIFIER.

This index is used to separate the implementation of the SNMP ordering from the actual implementation of the table. The SNMP ordering, that is implementation of GET NEXT, is implemented in this module.

For example, suppose there is an SNMP table, which is best implemented in Erlang as one process per SNMP table row. Suppose further that the INDEX in the SNMP table is an OCTET STRING. The index structure would be created as follows:

snmp_index:new(string)

For each new process we create, we insert an item in an snmp_index structure:

new_process(Name, SnmpIndex) ->
  Pid = start_process(),
  NewSnmpIndex = 
    snmp_index:insert(SnmpIndex, Name, Pid),
  <...>

With this structure, we can now map an OBJECT IDENTIFIER in e.g. a GET NEXT request, to the correct process:

get_next_pid(Oid, SnmpIndex) ->
  {ok, {_, Pid}} = snmp_index:get_next(SnmpIndex, Oid),
  Pid.

Common data types

The following data types are used in the functions below:

• index()

• oid() = [byte()]

• key_types = type_spec() | {type_spec(), type_spec(), ...}

• type_spec() = fix_string | string | integer

• key() = key_spec() | {key_spec(), key_spec(), ...}

• key_spec() = string() | integer()

The index() type denotes an snmp index structure.

The oid() type is used to represent an ASN.1 OBJECT IDENTIFIER.

The key_types() type is used when creating the index structure, and the key() type is used when inserting and deleting items from the structure.

The key_types() type defines the types of the SNMP INDEX columns for the table. If the table has one single INDEX column, this type should be a single atom, but if the table has multiple INDEX columns, it should be a tuple with atoms.

If the INDEX column is of type INTEGER, or derived from INTEGER, the corresponding type should be integer. If it is a variable length type (e.g. OBJECT IDENTIFIER, OCTET STRING), the corresponding type should be string. Finally, if the type is of variable length, but with a fixed size restriction (e.g. IpAddress), the corresponding type should be fix_string.

For example, if the SNMP table has two INDEX columns, the first one an OCTET STRING with size 2, and the second one an OBJECT IDENTIFER, the corresponding key_types parameter would be {fix_string, string}.

The key() type correlates to the key_types() type. If the key_types() is a single atom, the corresponding key() is a single type as well, but if the key_types() is a tuple, key must be a tuple of the same size.

In the example above, valid keys could be {"hi", "mom"} and {"no", "thanks"}, whereas "hi", {"hi", 42} and {"hello", "there"} would be invalid.

Exports