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<!DOCTYPE erlref SYSTEM "erlref.dtd">
<erlref>
<header>
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<year>1997</year><year>2016</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
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<title>snmp_index</title>
<prepared></prepared>
<responsible></responsible>
<docno></docno>
<approved></approved>
<checked></checked>
<date></date>
<rev></rev>
<file>snmp_index.xml</file>
</header>
<module since="">snmp_index</module>
<modulesummary>Abstract Data Type for SNMP Indexing</modulesummary>
<description>
<p>The module <c>snmp_index</c> 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.
</p>
<p>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.
</p>
<p>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:
</p>
<code type="none">
snmp_index:new(string)
</code>
<p>For each new process we create, we insert an item in an
<c>snmp_index</c> structure:
</p>
<code type="none"><![CDATA[
new_process(Name, SnmpIndex) ->
Pid = start_process(),
NewSnmpIndex =
snmp_index:insert(SnmpIndex, Name, Pid),
<...>
]]></code>
<p>With this structure, we can now map an OBJECT IDENTIFIER in
e.g. a GET NEXT request, to the correct process:
</p>
<code type="none">
get_next_pid(Oid, SnmpIndex) ->
{ok, {_, Pid}} = snmp_index:get_next(SnmpIndex, Oid),
Pid.
</code>
</description>
<section>
<title>Common data types</title>
<p>The following data types are used in the functions below:
</p>
<list type="bulleted">
<item>
<p><c>index()</c></p>
</item>
<item>
<p><c>oid() = [byte()]</c></p>
</item>
<item>
<p><c>key_types = type_spec() | {type_spec(), type_spec(), ...}</c></p>
</item>
<item>
<p><c>type_spec() = fix_string | string | integer</c></p>
</item>
<item>
<p><c>key() = key_spec() | {key_spec(), key_spec(), ...}</c></p>
</item>
<item>
<p><c>key_spec() = string() | integer()</c></p>
</item>
</list>
<p>The <c>index()</c> type denotes an snmp index structure.
</p>
<p>The <c>oid()</c> type is used to represent an ASN.1 OBJECT
IDENTIFIER.
</p>
<p>The <c>key_types()</c> type is used when creating the
index structure, and the <c>key()</c> type is used when inserting
and deleting items from the structure.
</p>
<p>The <c>key_types()</c> 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.
</p>
<p>If the INDEX column is of type INTEGER, or derived from
INTEGER, the corresponding type should be <c>integer</c>. If it
is a variable length type (e.g. OBJECT IDENTIFIER, OCTET STRING),
the corresponding type should be <c>string</c>. Finally, if the
type is of variable length, but with a fixed size restriction
(e.g. IpAddress), the corresponding type should be
<c>fix_string</c>.
</p>
<p>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 <c>key_types</c> parameter would be
<c>{fix_string, string}</c>.
</p>
<p>The <c>key()</c> type correlates to the <c>key_types()</c>
type. If the <c>key_types()</c> is a single atom, the
corresponding <c>key()</c> is a single type as well, but if the
<c>key_types()</c> is a tuple, <c>key</c> must be a tuple of the
same size.
</p>
<p>In the example above, valid <c>keys</c> could be <c>{"hi", "mom"}</c> and <c>{"no", "thanks"}</c>, whereas <c>"hi"</c>,
<c>{"hi", 42}</c> and <c>{"hello", "there"}</c> would be invalid.</p>
<warning>
<marker id="1"></marker>
<p>All API functions that update the index return a <c>NewIndex</c>
term. This is for backward compatibility with a previous
implementation that used a B+ tree written purely in Erlang for
the index. The <c>NewIndex</c> return value can now be ignored.
The return value is now the unchanged table identifier for the
ets table.</p>
<p>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.</p>
<p>A new interface <c>delete/1</c> 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.</p>
<p>As an snmp adaption usually keeps the index for the whole of the
systems lifetime, this is rarely a problem.</p>
</warning>
</section>
<funcs>
<func>
<name since="">delete(Index) -> true</name>
<fsummary>Delete an index table</fsummary>
<type>
<v>Index = NewIndex = index()</v>
<v>Key = key()</v>
</type>
<desc>
<p>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 <seealso marker="#1">warning note</seealso>
above.</p>
</desc>
</func>
<func>
<name since="">delete(Index, Key) -> NewIndex</name>
<fsummary>Delete an item from the index</fsummary>
<type>
<v>Index = NewIndex = index()</v>
<v>Key = key()</v>
</type>
<desc>
<p>Deletes a key and its value from the index structure.
Returns a new structure.</p>
</desc>
</func>
<func>
<name since="">get(Index, KeyOid) -> {ok, {KeyOid, Value}} | undefined</name>
<fsummary>Get the item with <c>KeyOid</c></fsummary>
<type>
<v>Index = index()</v>
<v>KeyOid = oid()</v>
<v>Value = term()</v>
</type>
<desc>
<p>Gets the item with key <c>KeyOid</c>. Could be used from
within an SNMP instrumentation function.</p>
</desc>
</func>
<func>
<name since="">get_last(Index) -> {ok, {KeyOid, Value}} | undefined</name>
<fsummary>Get the last item in the index structure</fsummary>
<type>
<v>Index = index()</v>
<v>KeyOid = oid()</v>
<v>Value = term()</v>
</type>
<desc>
<p>Gets the last item in the index structure.</p>
</desc>
</func>
<func>
<name since="">get_next(Index, KeyOid) -> {ok, {NextKeyOid, Value}} | undefined</name>
<fsummary>Get the next item</fsummary>
<type>
<v>Index = index()</v>
<v>KeyOid = NextKeyOid = oid()</v>
<v>Value = term()</v>
</type>
<desc>
<p>Gets the next item in the SNMP lexicographic ordering,
after <c>KeyOid</c> in the index structure. <c>KeyOid</c>
does not have to refer to an existing item in the index.</p>
</desc>
</func>
<func>
<name since="">insert(Index, Key, Value) -> NewIndex</name>
<fsummary>Insert an item into the index</fsummary>
<type>
<v>Index = NewIndex = index()</v>
<v>Key = key()</v>
<v>Value = term()</v>
</type>
<desc>
<p>Inserts a new key value tuple into the index structure. If
an item with the same key already exists, the new <c>Value</c>
overwrites the old value.</p>
</desc>
</func>
<func>
<name since="">key_to_oid(Index, Key) -> KeyOid</name>
<fsummary>Convert a key to an OBJECT IDENTIFIER</fsummary>
<type>
<v>Index = index()</v>
<v>Key = key()</v>
<v>KeyOid = NextKeyOid = oid()</v>
</type>
<desc>
<p>Converts <c>Key</c> to an OBJECT IDENTIFIER.</p>
</desc>
</func>
<func>
<name since="">new(KeyTypes) -> Index</name>
<fsummary>Create a new snmp index structure</fsummary>
<type>
<v>KeyTypes = key_types()</v>
<v>Index = index()</v>
</type>
<desc>
<p>Creates a new snmp index structure. The <c>key_types()</c>
type is described above.</p>
</desc>
</func>
</funcs>
</erlref>
