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snmp_index Abstract Data Type for SNMP Indexing

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:

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.

All API functions that update the index return a NewIndex term. This is for backward compatibility with a previous implementation that used a B+ tree written purely in Erlang for the index. The NewIndex return value can now be ignored. The return value is now the unchanged table identifier for the ets table.

The implementation using ets tables introduces a semantic incompatibility with older implementations. In those older implementations, using pure Erlang terms, the index was garbage collected like any other Erlang term and did not have to be deleted when discarded. An ets table is deleted only when the process creating it explicitly deletes it or when the creating process terminates.

A new interface delete/1 is now added to handle the case when a process wants to discard an index table (i.e. to build a completely new). Any application using transient snmp indexes has to be modified to handle this.

As an snmp adaption usually keeps the index for the whole of the systems lifetime, this is rarely a problem.
delete(Index) -> true Delete an index table Index = NewIndex = index() Key = key()

Deletes a complete index structure (i.e. the ets table holding the index). The index can no longer be referenced after this call. See the warning note above.

 delete(Index, Key) -> NewIndex Delete an item from the index Index = NewIndex = index() Key = key()

Deletes a key and its value from the index structure. Returns a new structure.

 get(Index, KeyOid) -> {ok, {KeyOid, Value}} | undefined Get the item with KeyOid Index = index() KeyOid = oid() Value = term()

Gets the item with key KeyOid. Could be used from within an SNMP instrumentation function.

 get_last(Index) -> {ok, {KeyOid, Value}} | undefined Get the last item in the index structure Index = index() KeyOid = oid() Value = term()

Gets the last item in the index structure.

 get_next(Index, KeyOid) -> {ok, {NextKeyOid, Value}} | undefined Get the next item Index = index() KeyOid = NextKeyOid = oid() Value = term()

Gets the next item in the SNMP lexicographic ordering, after KeyOid in the index structure. KeyOid does not have to refer to an existing item in the index.

 insert(Index, Key, Value) -> NewIndex Insert an item into the index Index = NewIndex = index() Key = key() Value = term()

Inserts a new key value tuple into the index structure. If an item with the same key already exists, the new Value overwrites the old value.

 key_to_oid(Index, Key) -> KeyOid Convert a key to an OBJECT IDENTIFIER Index = index() Key = key() KeyOid = NextKeyOid = oid()

Converts Key to an OBJECT IDENTIFIER.

 new(KeyTypes) -> Index Create a new snmp index structure KeyTypes = key_types() Index = index()

Creates a new snmp index structure. The key_types() type is described above.