Interceptor Configuration Expressions

The BAM interceptor configuration file uses filter expressions to identify an activity and uses data expressions to construct a data element for storage, use as a correlation ID or continuation token, or similar purpose. Regardless of the purpose, individual expressions are identified in the interceptor configuration file by the expression element and contain one or more operations using Reverse Polish Notation, also known as postfix notation.

About Reverse Polish Notation

In Reverse Polish Notation (RPN), operands precede the operator, thereby removing the need to use parentheses as precedence operators. A stack is used to hold values and all operations either push values onto the stack, pop (remove) values from the stack, or perform a combination of pushes and pops to complete an operation.

For example, if you wanted to evaluate the expression

5 + (10 - 2)

Converting this to the equivalent RPN results in

5 10 2 - +

This would be evaluated as follows:

Assuming the RPN system supported the string concatenate operation, you could also evaluate the expression

"The quick brown " + "fox " + "jumped over the lazy " + "dog."

Converting this to the equivalent RPN notation yields:

"The quick brown " "fox " "jumped over the lazy " "dog" + + +

This would be evaluated as follows:

As you can see, an arbitrary number of operations can be supported, including comparison, Boolean operations, and custom operations that retrieve values appropriate for the operations in which they participate. Values accumulate on the stack and are pushed and popped according to individual operations.

Reverse Polish Notation in the Interceptor Configuration File

You will write two kinds of expressions in the interceptor configuration file: filter expressions and data expressions. Filter expressions expect the result of the RPN expression to be a Boolean true or false while data expressions expect a single value on the stack.

Filter Expressions

Filter expressions evaluate to Boolean true or false and are used to identify a specific event to track in the WF or WFC application. In WF applications, it is common to filter based on activity name and event. For example, you may want to select the FoodAndDrinksPolicy activity when it is Closed. Using WF operations, you could express the filter as:

(GetActivityName = "FoodAndDrinksPolicy") && (GetActivityEvent = "Closed")

Converting this to RPN yields:

GetActivityName "FoodAndDrinksPolicy" == GetActivityEvent "Closed" == &&

Converting this expression to the equivalent expression for the interceptor configuration file results in the following XML:

<ic:Filter>  
  <ic:Expression>  
    <wf:Operation Name="GetActivityName"/>  
    <ic:Operation Name="Constant">  
      <ic:Argument>FoodAndDrinksPolicy</ic:Argument>  
    </ic:Operation>  
    <ic:Operation Name="Equals"/>  
    <wf:Operation Name="GetActivityEvent"/>  
    <ic:Operation Name="Constant">  
      <ic:Argument>Closed</ic:Argument>  
    </ic:Operation>  
    <ic:Operation Name="Equals"/>  
    <ic:Operation Name="And"/>  
  </ic:Expression>  
</ic:Filter>  

Finally, this expression would be evaluated as follows assuming GetActivityName returned "DessertPolicy" and GetActivityEvent returned "Closed":

The value left on the stack is Boolean False. This will cause the corresponding event to not fire. If the activity name were "FoodAndDrinksPolicy" then it would have evaluated to Boolean True.

You can construct a similar expression (with a similar evaluation) by using the WCF operations GetEndpointName and GetOperationName.

Data Expressions

Data expressions are used to define a single string data value. A data expression is any expression that is not enclosed by a Filter element. Data expressions are used by the OnEvent elements CorrelationID, ContinuationToken, Reference, and Update.

It is a common requirement to update the BAM activity database with a labeled time stamp. For example, you might want to capture the time an event starts with a string formatted as "Start: <EventTime>". To do this, you need to use an expression similar to the following (where + represents concatenation):

"Start: " + GetContextProperty(EventTime)

Converting this to RPN yields:

"Start: " GetContextProperty(EventTime) +

Converting this expression to the equivalent expression for an Update element in the interceptor configuration file results in the following XML:

<ic:Update DataItemName="NewOrderCreateTime" Type="NVARCHAR">  
  <ic:Expression>  
    <ic:Operation Name="Constant">  
      <ic:Argument>Start:</ic:Argument>  
    </ic:Operation>  
    <wf:Operation Name="GetContextProperty">  
      <wf:Argument>EventTime</wf:Argument>  
    </wf:Operation>  
    <ic:Operation Name="Concatenate" />  
  </ic:Expression>  
</ic:Update>  

The following table shows how this expression would be interpreted if the event time was "12:00 PM".

The value used by the update command would be "Start: 2006-09-27T12:00:34.000Z".

In This Section

Interceptor Operations

See Also

Structure of an Interceptor Configuration File