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# snmp_index (snmp v5.20.2)

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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:

    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.

## Warnings

#### Warning

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.

## Summary

### Types

index()

This type denotes an snmp index structure.

key()

This type correlates to the `key_types/0` type. If the `key_types/0` is a single atom, the corresponding `key/0` is a single type as well, but if the `key_types/0` is a tuple, `key/0` must be a tuple of the same size.

key_spec()
key_types()

This type is used when creating the index structure, and the `key/0` type is used when inserting and deleting items from the structure.

type_spec()

### Functions

delete(Index)

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)

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

get(Index, KeyOid)

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

get_last(Index)

Gets the last item in the index structure.

get_next(Index, KeyOid)

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)

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)

Converts `Key` to an OBJECT IDENTIFIER.

new(KeyTypes)

Create an new anonymous snmp index structure.

new(KeyTypes, Name)

Creates a new named snmp index structure.

## Types

# index()

    -opaque index()

This type denotes an snmp index structure.

# key()

    -type key() :: key_spec() | tuple().

This type correlates to the `key_types/0` type. If the `key_types/0` is a single atom, the corresponding `key/0` is a single type as well, but if the `key_types/0` is a tuple, `key/0` 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.

There is no way to propely describe this type in the erlang type language, which is why `tuple/0` was used above. The proper definition looks like:

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

# key_spec()

    -type key_spec() :: string() | integer().

# key_types()

    -type key_types() :: type_spec() | tuple().

This type is used when creating the index structure, and the `key/0` type is used when inserting and deleting items from the structure.

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`.

There is no way to propely describe this type in the erlang type language, which is why `tuple/0` was used above. The proper definition looks like:

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

# type_spec()

    -type type_spec() :: fix_string | string | integer.

## Functions

# delete(Index)

    -spec delete(Index) -> true when Index :: index().

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)

    -spec delete(Index, Key) -> NewIndex when Index :: index(), Key :: key(), NewIndex :: index().

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

# get(Index, KeyOid)

    -spec get(Index, KeyOid) -> {ok, {KeyOid, Value}} | undefined
                 when Index :: index(), KeyOid :: snmp:oid(), Value :: term().

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

# get_last(Index)

    -spec get_last(Index) -> {ok, {KeyOid, Value}} | undefined
                      when Index :: index(), KeyOid :: snmp:oid(), Value :: term().

Gets the last item in the index structure.

# get_next(Index, KeyOid)

    -spec get_next(Index, KeyOid) -> {ok, {NextKeyOid, Value}} | undefined
                      when Index :: index(), KeyOid :: snmp:oid(), NextKeyOid :: snmp: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)

    -spec insert(Index, Key, Value) -> NewIndex
                    when Index :: index(), Key :: key(), Value :: term(), NewIndex :: index().

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)

    -spec key_to_oid(Index, Key) -> KeyOid when Index :: index(), Key :: key(), KeyOid :: snmp:oid().

Converts `Key` to an OBJECT IDENTIFIER.

# new(KeyTypes)

    -spec new(KeyTypes) -> Index when KeyTypes :: key_types(), Index :: index().

Create an new anonymous snmp index structure.

# new(KeyTypes, Name)

(since OTP 27.0)

    -spec new(KeyTypes, Name) -> Index when KeyTypes :: key_types(), Name :: atom(), Index :: index().

Creates a new named snmp index structure.