User-Defined Functions (StreamInsight)
The examples in this topic demonstrate how to extend the possible expressions in StreamInsight LINQ operators by using method calls to existing .NET functions or user-defined functions (UDFs).
By using UDFs, expressions of any complexity are possible. They can be used wherever ordinary expressions occur: filter predicates, projections, join predicates, and so on. However, be aware that such a method call is evaluated for each event. A user-defined function must be compiled into an assembly that is accessible by the StreamInsight server at run time.
The parameters and the return value of a UDF must be of one of the StreamInsight primitive types. Additionally, culture-related parameters are accepted for UDFs.
Use Cases
The following use cases illustrate ways that user-defined functions can be used to extend the functionality of StreamInsight queries.
User-defined Filters
You can create a user-defined filter operator that reasons about the incoming event and passes the event to the output or filters the event out. Such a user-defined filter may reason about user-defined data and may include complex or specialized logic in the process of event filtration.
User-defined Projects
Similar to user-defined filters, a user-defined project expression can produce a new payload value, whose computation is beyond the means of the provided expression functionality in StreamInsight.
Examples
The following example uses a user-defined calculation in a filter expression by applying the function MyFunc on the payload fields.
// Assuming the following input event type:
public class MyPayload
{
public int id;
public int a;
public int b;
public double value;
}
var filteredStream = from e in stream
where e.value > MyFunc(e.a * e.b)
select e;
The following example specifies the user-defined function MyFunctions.valThreshold in the filter predicate.
var filteredStream = from e in stream
where e.value < MyFunctions.valThreshold(e.id)
select e;
The following example uses the user-defined function valThreshold, which takes a single parameter and returns a value.
public class MyFunctions
{
public static int valThreshold(int id)
{
switch (id)
{
case 1:
return 15;
case 2:
return 11;
case 3:
return 18;
default:
return 0;
}
}
}
A user-defined project function can be used like any other expression in a projection, as shown in the following example.
var result = from e in stream
select new { location = MyFunctions.Lookup(e.id) };
In addition to taking built-in primitive types as parameters, user-defined functions can take culture-related information as parameters. The accepted types are:
CultureInfo
StringComparison
CompareOptions
The following example shows how to provide culture-specific information to a UDF.
// Assuming the following input event type:
public class MyPayload
{
public int id;
public string name;
}
var result = from e in stream
select new { location = MyFunctions.Lookup2(
e.name,
new CultureInfo("en-US"),
CompareOptions.IgnoreCase)
};
The culture information can also be taken from an event field in the input stream, using the helper function CepUtility.GetEventFieldCulture as shown in the following example.
var result = from e in stream
select new { location = MyFunctions.Lookup2(
e.name,
CepUtility.GetEventFieldCulture(e.name),
CompareOptions.IgnoreCase)
};