Pattern statement

A pattern is a construct that maps string tuples to tabular expressions. Each pattern must declare a pattern name and optionally define a pattern mapping. Patterns that define a mapping return a tabular expression when invoked. Any two statements must be separated by a semicolon.

Empty patterns are patterns that are declared but don't define a mapping. When invoked, they return error SEM0036 along with the details of the missing pattern definitions in the HTTP header. Middle-tier applications that provide a Kusto Query Language (KQL) experience can use the returned details as part of their process to enrich KQL query results. For more information, see Working with middle-tier applications.

Syntax

• Declare an empty pattern:

  declare pattern PatternName ;

• Declare and define a pattern:

  declare pattern PatternName = (ArgName : ArgType [, ... ]) [[ PathName : PathArgType ]]

  {

        ( ArgValue1_1 [, ArgValue2_1, ... ] ) [ .[ PathValue_1 ] ] = { expression1 } ;

      [ ( ArgValue1_2 [, ArgValue2_2, ... ] ) [ .[ PathValue_2 ] ] = { expression2 } ; ... ]

  } ;

• Invoke a pattern:

  • PatternName ( ArgValue1 [, ArgValue2 ...] ).PathValue
  • PatternName ( ArgValue1 [, ArgValue2 ...] ).["PathValue"]

Learn more about syntax conventions.

Parameters

Name	Type	Required	Description
PatternName	string	✔️	The name of the pattern.
ArgName	string	✔️	The name of the argument. Patterns can have one or more arguments.
ArgType	string	✔️	The scalar data type of the ArgName argument. Possible values: string
PathName	string		The name of the path argument. Patterns can have no path or one path.
PathArgType	string		The type of the PathArgType argument. Possible values: string
ArgValue	string	✔️	The ArgName and optional PathName tuple values to be mapped to an expression.
PathValue	string		The value to map for PathName.
expression	string	✔️	A tabular or lambda expression that references a function returning tabular data. For example: Logs | where Timestamp > ago(1h)

Examples

In each of the following examples, a pattern is declared, defined, and then invoked.

Define simple patterns

The following example defines a pattern that maps states to an expression that returns its capital/major city.

declare pattern country = (name:string)[state:string]
{
  ("USA").["New York"] = { print Capital = "Albany" };
  ("USA").["Washington"] = { print Capital = "Olympia" };
  ("Canada").["Alberta"] = { print Capital = "Edmonton" };
};
country("Canada").Alberta

Output

Capital
Edmonton

The following example defines a pattern that defines some scoped application data.

declare pattern App = (applicationId:string)[scope:string]  
{
    ('a1').['Data']    = { range x from 1 to 5 step 1 | project App = "App #1", Data    = x };
    ('a1').['Metrics'] = { range x from 1 to 5 step 1 | project App = "App #1", Metrics = rand() };
    ('a2').['Data']    = { range x from 1 to 5 step 1 | project App = "App #2", Data    = 10 - x };
    ('a3').['Metrics'] = { range x from 1 to 5 step 1 | project App = "App #3", Metrics = rand() };
};
union App('a2').Data, App('a1').Metrics

Output

App	Data	Metrics
App #2	9
App #2	8
App #2	7
App #2	6
App #2	5
App #1		0.53674122855537532
App #1		0.78304713305654439
App #1		0.20168860732346555
App #1		0.13249123867679469
App #1		0.19388305330563443

Normalization

There are syntax variations for invoking patterns. For example, the following union returns a single pattern expression since all the invocations are of the same pattern.

declare pattern app = (applicationId:string)[eventType:string]
{
    ("ApplicationX").["StopEvents"] = { database("AppX").Events | where EventType == "StopEvent" };
    ("ApplicationX").["StartEvents"] = { database("AppX").Events | where EventType == "StartEvent" };
};
union
  app("ApplicationX").StartEvents,
  app('ApplicationX').StartEvents,
  app("ApplicationX").['StartEvents'],
  app("ApplicationX").["StartEvents"]

No wildcards

There's no special treatment given to wildcards in a pattern. For example, the following query returns a single missing pattern invocation.

declare pattern app = (applicationId:string)[eventType:string]
{
    ("ApplicationX").["StopEvents"] = { database("AppX").Events | where EventType == "StopEvent" };
    ("ApplicationX").["StartEvents"] = { database("AppX").Events | where EventType == "StartEvent" };
};
union app("ApplicationX").["*"]
| count

Returns semantic error

One or more pattern references were not declared. Detected pattern references: ["app('ApplicationX').['*']"]

Work with middle-tier applications

A middle-tier application provides its users with the ability to use KQL and wants to enhance the experience by enriching the query results with augmented data from its internal service.

To this end, the application provides users with a pattern statement that returns tabular data that their users can use in their queries. The pattern's arguments are the keys the application will use to retrieve the enrichment data. When the user runs the query, the application does not parse the query itself but instead plans to leverage the error returned by an empty pattern to retrieve the keys it requires. So it prepends the query with the empty pattern declaration, sends it to the cluster for processing, and then parses the returned HTTP header to retrieve the values of missing pattern arguments. The application uses these values to look up the enrichment data and builds a new declaration that defines the appropriate enrichment data mapping. Finally, the application prepends the new definition to the user's query, resends it for processing, and returns the result it receives to the user.

Example

In the following example, a middle-tier application provides the ability to enrich queries with longitude/latitude locations. The application uses an internal service to map IP addresses to longitude/latitude locations, and provides a pattern called map_ip_to_longlat for this purpose. Let's suppose the application gets the following query from the user:

map_ip_to_longlat("10.10.10.10")

The application does not parse this query and hence does not know which IP address (10.10.10.10) was passed to the pattern. So it prepends the user query with an empty map_ip_to_longlat pattern declaration and sends it for processing:

declare pattern map_ip_to_longlat;
map_ip_to_longlat("10.10.10.10")

The application receives the following error in response.

One or more pattern references were not declared. Detected pattern references: ["map_ip_to_longlat('10.10.10.10')"]

The application inspects the error, determines that the error indicates a missing pattern reference, and retrieves the missing IP address (10.10.10.10). It uses the IP address to look up the enrichment data in its internal service and builds a new pattern defining the mapping of the IP address to the corresponding longitude and latitude data. The new pattern is prepended to the user's query and run again. This time the query succeeds because the enrichment data is now declared in the query, and the result is sent to the user.

declare pattern map_ip_to_longlat = (address:string)
{
  ("10.10.10.10") = { print Lat=37.405992, Long=-122.078515 }
};
map_ip_to_longlat("10.10.10.10")

Output

Lat	Long
37.405992	-122.078515

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