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.
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.
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.
Functions
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.
Index = NewIndex = index()
Key = key()
Deletes a key and its value from the index structure. Returns a new structure.
Index = index()
KeyOid = oid()
Value = term()
Gets the item with key KeyOid
. Could be used from
within an SNMP instrumentation function.
Index = index()
KeyOid = oid()
Value = term()
Gets the last item in the index structure.
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.
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.
Index = index()
Key = key()
KeyOid = NextKeyOid = oid()
Converts Key
to an OBJECT IDENTIFIER.
KeyTypes = key_types()
Index = index()
Creates a new snmp index structure. The key_types()
type is described above.