Complex selector expressions

5 minute read

A selector is an expression using Java Unified Expression Language (JUEL) syntax that enables API Gateway configuration settings to be evaluated and expanded at runtime, based on metadata values (for example, message attributes, Key Property Store (KPS), or environment variables).

For example, when a HTTP request is received, it is converted into a set of message attributes on the message whiteboard. Each message attribute represents a specific characteristic of the HTTP request, such as the HTTP headers, HTTP body, and so on.

Example selector expressions

For more information on selector syntax and some examples of selector expressions, see Select configuration values at runtime.

The following sections describe some more complex examples of selectors.

KPS with multiple read keys

${kps.matrix.row.column} ${kps.matrix["row"]["column"]} 

For a KPS with multiple read keys, the values for each key are provided in ascending order. The result of the expression is also indexable:

set property test = ${kps.matrix.row} 

${test["column"]} looks up the KPS for [row/column].

Parameter from FormURLEncodedBody formatted content body

${content.body.getParameters().get("grant_type")} 

Gets the HTTP form post field called grant_type.

Parameter from JSON formatted content body

${content.body.getJSON().findValue('access_token').textValue()} 

If a body is of type application/json, then it is automatically treated as a com.vordel.mime.JSONBody. A JSONBody object returns an com.fasterxml.jackson.databind.JsonNode object by way of a getJSON() call.

For more information, see the Javadoc for JsonNode class, and the Javadoc for API Gateway classes documentation.

For example, if the body contains the following JSON content:

{  "access_token":"2YotnFZFEj",  "token_type":"example",  "expires_in":3600 } 

This selector results in the value 2YotnFZFEj.

Test if an attribute is empty

${empty my.attribute} 

This evaluates to true if my.attribute is null, or an empty string and is false otherwise.

Ternary operator

${empty my.attribute ? "DEFAULT VALUE" : my.attribute} 

The ternary operator takes the following form:

expression ? value_if_true : value_if_false 

This evaluates to the first value when the expression is true, and the second value when the expression is false.

The example in this section shows how to create an expression that evaluates to a default value when no value is supplied for my.attribute and which otherwise leaves the value unchanged. Note that the evaluation of this selector does not change the value stored in the attribute. Use this type of selector expression as the value for the Set Attribute filter if you wish to overwrite the information stored in the attribute.

Convert JSON content body to string

${content.body.getJSON().toString()} 

If a body is of type com.vordel.mime.JSONBody, this selector converts the JSON contained in the body to a string value.

Environment variable

${environment.VINSTDIR} 

Accesses the environment variable VINSTDIR, which will resolve to the home directory of the current instance, e.g. /opt/Axway/apigateawy/groups/group-2/instance-1/.

HTTP path

${http.path[2]} 

If you have the filter Extract REST Request Attributes in your policy, this filter adds the incoming URI to the message whiteboard as a String array, so that you can index into the path. If the incoming path is /thisisa/test, using this type of selector results in the following attributes on the whiteboard:

${http.path[1]} = thisisa ${http.path[2]} = test 

Database query results

You can use the Retrieve from or write to database filter to retrieve user attributes from a database or write user attributes to a database. You can select whether to place database query results in message attributes on the Advanced tab of the filter. By default, the Place query results into user attribute list option is selected.

The query results are represented as a list of properties. Each element in the list represents a query result row returned from the database. These properties represent pairs of attribute names and values for each column in the row. The Prefix for message attribute field in the filter is required to name the list of returned properties (for example, user).

Results in user attribute list

The following table shows some example selectors when the option Place query results into user attribute list is selected.

You can also use standard Java function calls on the attributes. For example:

• ${user.size()}: Number of properties (number of rows) retrieved from the database
• ${user[0].NAME.equals("John")}: Returns true if the NAME attribute (value of column NAME in first row) is "John"

For more information, see the java.util.ArrayList and java.lang.String class interfaces.

Results not in user attribute list

The following table shows some example selectors when the option Place query results into user attribute list is not selected.

You can also use standard Java function calls on the attributes. For example:

• ${user.NAME.size()}: Number of NAME attributes (number of rows with column NAME) retrieved from the database.
• ${user.NAME[0].equals("John")}: Returns true if the first NAME attribute (value of column NAME in first row) is "John".

For more information, see the java.util.ArrayList and java.lang.String class interfaces.

LDAP directory server search results

You can use the Retrieve from directory server filter to retrieve user profile data.

The filter can look up a user and retrieve that user’s attributes represented as a list of search results. Each element of the list represents a list of multivalued attributes returned from the directory server. The Prefix for message attribute field in the filter is required to name the list of search results (for example, user).

The following table shows some example selectors:

You can also use standard Java function calls on the attributes. For example:

• ${user.size()}: Number of search results returned by the LDAP directory server.
• ${user[0].memberOf.size()}: Number of memberOf attribute values returned in the search result.
• ${user[0].memberOf.contains("CN=Operator,OU=Sales")}: Returns true if one of the returned memberOf attributes is "CN=Operators,OU=Sales".
• ${user[0].memberOf[0].equals("CN=Operator,OU=Sales")}: Returns true if the first memberOf attribute is "CN=Operators,OU=Sales".

For more information, see java.util.ArrayList and java.lang.String class interfaces.

More information on KPS

A Key Property Store (KPS) is an external data store of API Gateway policy properties, which is typically read frequently, and seldom written to. Using a KPS enables metadata-driven policies, whereby policy configuration is stored in an external data store, and looked up dynamically when policies are executed.

For more information on KPS, see the API Gateway Policy Developer Guide and also the API Gateway Key Property Store User Guide.