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    <copyright>
      <year>2000</year><year>2016</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
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    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
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    <title>Filters and the Constraint Language BNF</title>
    <prepared></prepared>
    <docno></docno>
    <date>2000-04-13</date>
    <rev></rev>
    <file>ch_BNF.xml</file>
  </header>

  <section>
    <title>Filters and the Constraint Language BNF</title>
    <p>This chapter describes, the grammar supported by 
      <seealso marker="CosNotifyFilter_Filter">CosNotifyFilter_Filter</seealso> and 
      <seealso marker="CosNotifyFilter_MappingFilter">CosNotifyFilter_MappingFilter</seealso>,
      and how to create and use filter objects. 
      </p>

    <section>
      <title>How to create filter objects</title>
      <p>To be able to filter events we must create a filter and associate
        it with one, or more, of the administrative or proxy objects. In the example
        below, we choose to associate the filter with a ConsumerAdmin object.</p>
      <code type="none">
FilterFactory = cosNotificationApp:start_filter_factory(),
Filter = 'CosNotifyFilter_FilterFactory':
                     create_filter(FilterFactory,"EXTENDED_TCL"),
ConstraintInfoSeq = 'CosNotifyFilter_Filter':
                     add_constraints(Filter, ConstraintExpSeq),
FilterID = 'CosNotifyChannelAdmin_ConsumerAdmin':
                     add_filter(AdminConsumer, Filter),
      </code>
      <p><c>"EXTENDED_TCL"</c> is the only grammar supported by Orber Notification
        Service.</p>
      <p>Depending on which operation type the Admin object uses, i.e., 
        <c>'AND_OP'</c> or <c>'OR_OP'</c>, events will be tested using the 
        associated filter. The operation properties are:</p>
      <p></p>
      <list type="bulleted">
        <item>
          <p>'AND_OP' - must be approved by the proxy's <em>and</em> its parent admin's 
            filters. If all filters associated with an object (Admin or Proxy) 
            return false the event will be discarded. In this situation it is pointless
            to try and verify with the other object's associated filters since the outcome
            still would be the same.</p>
        </item>
        <item>
          <p>'OR_OP' - if one of the object's (Admin or Proxy) filters return true, the event 
            will not be checked against any other filter associated with a proxy or its
            parent admin. If a object's associated filters all return false,
            the event will be forwarded to related proxies/admins, and 
            tested against any associated filters.</p>
        </item>
      </list>
      <p>Initially, filters are empty and will always return true. Hence, we must
        add constraints by using <c>'CosNotifyFilter_Filter':add_constraints/2</c>.
        As input, the second argument must be a sequence of:</p>
      <code type="none">
#'CosNotifyFilter_ConstraintExp'{
       event_types = [#'CosNotification_EventType'{
                             domain_name = string(),
                             type_name = string()}], 
       constraint_expr = string()}
      </code>
      <p>The <c>event_types</c> describes which types of events that should be matched using
        the associated <c>constraint_expr</c>.</p>
      <p>If a constraint expression is supposed to apply for all events, then the <c>type_name</c> can 
        be set to the special event type <c>%ALL</c> in a constraint's event type sequence. The 
        <c>domain_name</c> should be <c>""</c> or <c>"*"</c>.</p>
      <p>In the following sections we will take a closer look on how to write
        constraint expressions.</p>
    </section>

    <section>
      <title>The CosNotification Constraint Language</title>
      <p>The constraint language supported by the Notification Service is:</p>
      <code type="none"><![CDATA[
<constraint> := /* empty */ 
    | <bool> 

<bool> := <bool_or> 

<bool_or> := <bool_or> or <bool_and> 
    | <bool_and> 

<bool_and> := <bool_and> and <bool_compare> 
    | <bool_compare> 

<bool_compare> := <expr_in> == <expr_in> 
    | <expr_in> != <expr_in> 
    | <expr_in> < <expr_in> 
    | <expr_in> <= <expr_in>
    | <expr_in> > <expr_in> 
    | <expr_in> >= <expr_in> 
    | <expr_in> 

<expr_in> := <expr_twiddle> in <Ident> /* sequence only */
    | <expr_twiddle> 
    | <expr_twiddle> in $ <Component> /* sequence only */

<expr_twiddle> := <expr> ~ <expr> /* string data types only */
    | <expr> 

<expr> := <expr> + <term> 
    | <expr> - <term> 
    | <term> 

<term> := <term> * <factor_not> 
    | <term> / <factor_not> 
    | <factor_not> 

<factor_not> := not <factor> 
    | <factor> 

<factor> := ( <bool_or> ) 
    | exist <Ident> 
    | <Ident> 
    | <Number> 
    | - <Number> 
    | <String> 
    | TRUE 
    | FALSE
    | + <Number> 
    | exist $ <Component> 
    | $ <Component> 
    | default $ <Component> /* discriminated unions only */

<Component> := /* empty */ 
    | . <CompDot> 
    | <CompArray> 
    | <CompAssoc> 
    | <Ident> <CompExt> /* run-time variable */ 

<CompExt> := /* empty */ 
    | . <CompDot> 
    | <CompArray> 
    | <CompAssoc> 

<CompDot> := <Ident> <CompExt> 
    | <CompPos> 
    | <UnionPos> 
    | _length   /* only valid for arrays or sequences */
    | _d        /* discriminated unions only */
    | _type_id  /* only valid if possible to obtain */
    | _repos_id /* only valid if possible to obtain */

<CompArray> := [ <Digits> ] <CompExt> 

<CompAssoc> := ( <Ident> ) <CompExt> 

<CompPos> := <Digits> <CompExt>

<UnionPos> := ( <UnionVal> ) <CompExt> 

<UnionVal> := /* empty */ 
    | <Digits> 
    | - <Digits> 
    | + <Digits> 
    | <String>

/* Character set issues */
<Ident> :=<Leader> <FollowSeq> 
    | \ < Leader> <FollowSeq>

<FollowSeq> := /* <empty> */ 
    | <FollowSeq> <Follow> 

<Number> := <Mantissa> 
    | <Mantissa> <Exponent>

<Mantissa> := <Digits> 
    | <Digits> . 
    | . <Digits> 
    | <Digits> . <Digits> 

<Exponent> := <Exp> <Sign> <Digits> 

<Sign> := + 
    | -

<Exp> := E 
    | e 

<Digits> := <Digits> <Digit> 
    | <Digit> 

<String> := ' <TextChars> '   

<TextChars> := /* <empty> */ 
    | <TextChars> <TextChar> 

<TextChar> := <Alpha> 
    | <Digit> 
    | <Other> 
    | <Special> 

<Special> := \\ 
    | \' 

<Leader> := <Alpha>

<Follow> := <Alpha> 
    | <Digit> 
    | _

<Alpha> is the set of alphabetic characters [A-Za-z] 
<Digit> is the set of digits [0-9] 
<Other> is the set of ASCII characters that are not <Alpha>, <Digit>, or <Special>
      ]]></code>
      <p>In the absence of parentheses, the following precedence relations hold :</p>
      <list type="ordered">
        <item><c>()</c>, <c>exist</c>, <c>default</c>, <c>unary-sign</c></item>
        <item><c>not</c></item>
        <item><c>*</c>, <c>/</c></item>
        <item><c>+</c>, <c>-</c></item>
        <item><c>~</c></item>
        <item><c>in</c></item>
        <item><c>==</c>, <c>!=</c>, <c><![CDATA[<]]></c>, <c><![CDATA[<=]]></c>, <c>></c>, <c>>=</c></item>
        <item><c>and</c></item>
        <item><c>or</c></item>
      </list>
    </section>

    <section>
      <title>The Constraint Language Data Types</title>
      <p>The Notification Service Constraint Language, defines how to write
        constraint expressions, which can be used to filter events. The 
        representation does, however, differ slightly from ordinary Erlang terms.</p>
      <p>When creating a <c>ConstraintExp</c>, the field <c>constraint_expr</c> must be
        set to contain a string, e.g., <c><![CDATA["1 < 2"]]></c>. The Notification Service Constraint 
        Language, is designed to be able to filter structured and unstructured events
        using the same constraint expression. The Constraint Language Types and Operations
        can be divided into two sub-groups:</p>
      <list type="bulleted">
        <item>
          <p>Basic - arithmetics, strings, constants, numbers etc.</p>
        </item>
        <item>
          <p>Complex - accessing members of complex data types, such as unions.</p>
        </item>
      </list>
      <p>Some of the basic types, e.g., integer, are self explanatory. Hence, they are not described further.</p>
      <table>
        <row>
          <cell align="center" valign="middle"><em>Type/Operation</em></cell>
          <cell align="center" valign="middle"><em>Examples</em></cell>
          <cell align="center" valign="middle"><em>Description</em></cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>string</c></cell>
          <cell align="left" valign="middle"><c>"'MyString'"</c></cell>
          <cell align="left" valign="middle">Strings are represented as a sequence of zero or more <c><![CDATA[<TextChar>]]></c>s enclosed in single quotes, e.g., <c>'string'</c>.</cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>~</c></cell>
          <cell align="left" valign="middle"><c>"'Sring1' ~ 'String2'"</c></cell>
          <cell align="left" valign="middle">The operator <c>~</c>is called the substring operator and mean "String1  is contained within String2".</cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>boolean</c></cell>
          <cell align="left" valign="middle"><c>"TRUE == (('lang' ~ 'Erlang' + 'fun' ~ 'functional') >= 2)"</c></cell>
          <cell align="left" valign="middle">Booleans may only be TRUE or FALSE, i.e., only capital letters. Expressions which evaluate to TRUE or FALSE can be summed up and matched, where TRUE equals 1 and FALSE 0.</cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>sequence</c></cell>
          <cell align="left" valign="middle"><c>"myIntegerSequence[2]"</c></cell>
          <cell align="left" valign="middle">The BNF use C/C++ notation, i.e., the example will return the <em>third</em>element.</cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>_length</c></cell>
          <cell align="left" valign="middle"><c>"myIntegerSequence._length"</c></cell>
          <cell align="left" valign="middle">Returns the length of an sequence or array.</cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>in</c></cell>
          <cell align="left" valign="middle"><c>"'Erlang' in $.FunctionalLanguagesStringSeq"</c></cell>
          <cell align="left" valign="middle">Returns <c>TRUE</c>if a given element is found in the given sequence. The element must be of a simple type and the same as the sequence is defined to contain.</cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>$</c></cell>
          <cell align="left" valign="middle"><c>"$ == 40"</c></cell>
          <cell align="left" valign="middle">Denote the current event as well as any run-time variables. If the event is unstructured and its contained value 40, the example will return <c>TRUE</c>.</cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>.</c></cell>
          <cell align="left" valign="middle"><c>"$.MyStructMember == 40"</c></cell>
          <cell align="left" valign="middle">The structure member operator <c>.</c>may be used to reference its members when the data  refers to a named structure, discriminated union, or CORBA::Any data structure.</cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>_type_id</c></cell>
          <cell align="left" valign="middle"><c>"$._type_id == 'MyStruct'"</c></cell>
          <cell align="left" valign="middle">Returns the unscoped IDL type name of the component. This operation is only valid if said information can be obtained.</cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>_repos_id</c></cell>
          <cell align="left" valign="middle"><c>"$._repos_id == 'IDL:MyModule/MyStruct:1.0'"</c></cell>
          <cell align="left" valign="middle">Returns the RepositoryId of the component. This operation is only valid if said information can be obtained.</cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>_d</c></cell>
          <cell align="left" valign="middle"><c>"$.eventUnion._d"</c></cell>
          <cell align="left" valign="middle">May only be used when accessing discriminated unions and refers to the discriminator.</cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>exist</c></cell>
          <cell align="left" valign="middle"><c>"exist $.eventUnion._d and $.eventUnion._d == 10"</c></cell>
          <cell align="left" valign="middle">To avoid that a filtering of an event fails due to that, for example, we try to compare a union discriminator which does not exist, we can use this operator.</cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>default</c></cell>
          <cell align="left" valign="middle"><c>"default $.eventUnion._d"</c></cell>
          <cell align="left" valign="middle">If the <c>_d</c>operation is in conjunction with the <c>default</c>operation, TRUE will be  returned if the union has a default member that is active.</cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>union</c></cell>
          <cell align="left" valign="middle"><c>"$.(0) ==  5"</c>eq. <c>"$.('zero') ==  5"</c></cell>
          <cell align="left" valign="middle">When the component refers to a union, with one of the cases defined as <c>case 0: short zero;</c>, we use <c>0</c>or <c>'zero'</c>. The result of the example is <c>TRUE</c>if the union has a discriminator set to <c>0</c>and the value <c>5</c>. If more than one case is defined to be<c>'zero'</c>, <c>$.('zero')</c>accepts both; <c>$.(0)</c>only returns <c>TRUE</c>if the discriminator is set to <c>0</c>. Leaving out the identifier, i.e., <c>$.()</c>, refers to the default value.</cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>name-value pairs</c></cell>
          <cell align="left" valign="middle"><c>"$.NameValueSeq('myID') == 5"</c>eq.<c>"$.NameValueSeq[1].name == 'myID' and $.NameValueSeq[1].value == 5"</c></cell>
          <cell align="left" valign="middle">The Notification service makes extensive use of <c>name-value pairs</c>sequences within  structured events, which allow us to via the identifier <c>name</c>access its <c>value</c>, as shown in the example.</cell>
        </row>
        <tcaption>Table 1: Type and Operator Examples</tcaption>
      </table>
      <p>In the next section we will take a closer look at how it is possible to write constraints using 
        different types of notation etc.</p>
    </section>

    <section>
      <title>Accessing Data In Events</title>
      <p>To filter events, the supplied constraints must describe the contents of
        the events and desired values. We can, for example, state that we are only
        interested in receiving events which are of type <em>CommunicationsAlarm</em>.
        To be able to achieve this, the constraint must contain information
        that points out which fields to compare with. Figure one illustrates a conceptual overview of a
        structured event. The exact definition is found in the <c>CosNotification.idl</c> file.</p>
      <marker id="eventstructure"></marker>
      <image file="eventstructure.gif">
        <icaption>
Figure 1: The structure of a structured event.</icaption>
      </image>
      <p>The Notification Service supports different constraint expressions 
        notation:</p>
      <list type="bulleted">
        <item>
          <p>Fully scoped, e.g., "$.header.fixed_header.event_type.type_name == 'CommunicationsAlarm'"</p>
        </item>
        <item>
          <p>Short hand, e.g., "$type_name == 'CommunicationsAlarm'"</p>
        </item>
        <item>
          <p>Positional Notation, e.g., "$.0.0.0.1 == 'CommunicationsAlarm'"</p>
        </item>
      </list>
      <note>
        <p>Which notation to use is up to the user, however, the fully scoped may
          be easier to understand, but in some cases, if received from an ORB that do not populate ID:s of
          named parts, the positional notation is the only option.</p>
      </note>
      <note>
        <p>If a constraint, which access fields in a structured event structure,
          is supposed to handle unstructured events as well, the CORBA::Any must contain
          the same type of members.</p>
      </note>
      <p>How to filter against the fixed header fields, is described in the 
        table below.</p>
      <table>
        <row>
          <cell align="center" valign="middle">Field</cell>
          <cell align="center" valign="middle">Fully Scoped Constraint</cell>
          <cell align="center" valign="middle">Short Hand Constraint</cell>
        </row>
        <row>
          <cell align="left" valign="middle">type_name</cell>
          <cell align="left" valign="middle">"$.header.fixed_header.event_type.type_name == 'Type'"</cell>
          <cell align="left" valign="middle">"$type_name == 'Type'"</cell>
        </row>
        <row>
          <cell align="left" valign="middle">domain_name</cell>
          <cell align="left" valign="middle">"$.header.fixed_header.event_type.domain_name == 'Domain'"</cell>
          <cell align="left" valign="middle">"$domain_name == 'Domain'"</cell>
        </row>
        <row>
          <cell align="left" valign="middle">event_name</cell>
          <cell align="left" valign="middle">"$.header.fixed_header.event_name == 'Event'"</cell>
          <cell align="left" valign="middle">"$event_name == 'Event'"</cell>
        </row>
        <tcaption>Table 2: Fixed Header Constraint Examples</tcaption>
      </table>
      <p>If we are only interested in receiving events regarding 'Domain', 'Event'
        and 'Type', the constraint can look like 
        <c>"$domain_name == 'Domain' and $event_name == 'Event' and $type_name == 'Type'"</c>.</p>
      <p>The variable event header consists of a sequence of <em>name-value  pairs</em>. One way to filter on these are to use a constraint that looks
        like <c>"($.header.variable_header[1].name == 'priority' and  $.header.variable_header[1].value > 0)"</c>. An easier way to
        accomplish the same result is to use a constraint that treats the name-value
        pair as an associative array, i.e., when given a name the corresponding 
        value is returned. Hence, instead we can use 
        <c>"$.header.variable_header(priority) > 0"</c>.</p>
      <p>Accessing the event body is done in the same way as for the event header
        fields. The user must, however, be aware of, that if a run-time variable
        (<c>$variable</c>) is used data in the event header may take precedence.
        The order of precedence is:</p>
      <list type="ordered">
        <item>Reserved, e.g., <c>$curtime</c></item>
        <item>A simple-typed member of <c>$.header.fixed_header</c>.</item>
        <item>Properties in <c>$.header.variable_header</c>.</item>
        <item>Properties in <c>$.filterable_data</c>.</item>
        <item>If no match is found it is translated to <c>$.variable</c>.</item>
      </list>
    </section>

    <section>
      <title>Mapping Filters</title>
      <p>Mapping Filters may only be associated with Consumer Administrators or Proxy
        Suppliers. The purpose of a Mapping Filter is to override Quality of Service
        settings.</p>
      <p>Initially, Mapping Filters are empty and will always return true. Hence, we must
        add constraints by using <c>'CosNotifyFilter_MappingFilter':add_mapping_constraints/2</c>.
        If a constraint matches, the associated value will be used instead of the 
        related Quality of Service system settings.</p>
      <p>As input, the second argument must be a sequence of:</p>
      <code type="none">
#'CosNotifyFilter_MappingConstraintPair'{
    constraint_expression = #'CosNotifyFilter_ConstraintExp'{
       event_types = [#'CosNotification_EventType'{
                             domain_name = string(),
                             type_name = string()}], 
       constraint_expr = string()},
    result_to_set = any()}   
      </code>
    </section>
  </section>
</chapter>

