This page describes the XML elements comprising the FreezeScript transformation descriptors.

On this page:

<transformdb> Descriptor Element

The top-level descriptor in a descriptor file. It requires at least one <database> descriptor, and supports any number of <transform> and <init> child descriptors. This descriptor has no attributes.

<database> Descriptor Element

The attributes of this descriptor define the old and new key and value types for the database to be transformed, and optionally the name of the database to which these types apply. It supports any number of child descriptors, but at most one <record> descriptor. The <database> descriptor also creates a global scope for user-defined symbols.

The attributes supported by the <database> descriptor are described in the following table:

As an example, consider the following <database> descriptor. In this case, the Freeze map to be transformed currently has key type int and value type ::Employee, and is migrating to a key type of string:

<database key="int,string" value="::Employee">

<record> Descriptor Element

Commences the transformation. Child descriptors are executed for each record in the database, providing the user with an opportunity to examine the record's old key and value, and optionally modify the new key and value. Default transformations, as well as <transform> and <init> descriptors, are executed before the child descriptors. The <record> descriptor introduces the following symbols into a local scope: oldkey, newkey, oldvalue, newvalue, facet. These symbols are accessible to child descriptors, but not to <transform> or <init> descriptors. The oldkey and oldvalue symbols are read-only. The facet symbol is a string indicating the facet name of the object in the current record, and is only relevant for Freeze evictor databases.

Use caution when modifying database keys to ensure that duplicate keys do not occur. If a duplicate database key is encountered, transformation fails immediately.

Note that database transformation only occurs if a <record> descriptor is present.

<transform> Descriptor Element

Customizes the transformation for all instances of a type in the new Slice definitions. The children of this descriptor are executed after the optional default transformation has been performed. Only one <transform> descriptor can be specified for a type, but a <transform> descriptor is not required for every type. The symbols old and new are introduced into a local scope and represent the old and new values, respectively. The old symbol is read-only. The attributes supported by this descriptor are described in the following table:

Below is an example of a <transform> descriptor that initializes a new data member:

<transform type="::Product">
    <set target="new.salePrice" value="old.listPrice * old.discount"/>
</transform>

For class types, transformdb first attempts to locate a <transform> descriptor for the object's most-derived type. If no descriptor is found, transformdb proceeds up the class hierarchy in an attempt to find a descriptor. The base object type, Object, is the root of every class hierarchy and is included in the search for descriptors. It is therefore possible to define a <transform> descriptor for type Object, which will be invoked for every class instance.

Note that <transform> descriptors are executed recursively. For example, consider the following Slice definitions:

struct Inner {
    int sum;
};
struct Outer {
    Inner i;
};

When transformdb is performing the default transformation on a value of type Outer, it recursively performs the default transformation on the Inner member, then executes the <transform> descriptor for Inner, and finally executes the <transform> descriptor for Outer. However, if default transformation is disabled for Outer, then no transformation is performed on the Inner member and therefore the <transform> descriptor for Inner is not executed.

<init> Descriptor Element

Defines custom initialization rules for all instances of a type in the new Slice definitions. Child descriptors are executed each time the type is instantiated. The typical use case for this descriptor is for types that have been introduced in the new Slice definitions and whose instances require default values different than what transformdb supplies. The symbol value is introduced into a local scope to represent the instance. The attributes supported by this descriptor are described in the following table:

Here is a simple example of an <init> descriptor:

<init type="::Player">
    <set target="value.currency" value="100"/>
</init>

Note that, like <transform>, <init> descriptors are executed recursively. For example, if an <init> descriptor is defined for a struct type, the <init> descriptors of the struct's members are executed before the struct's descriptor.

<iterate> Descriptor Element

Iterates over a dictionary or sequence, executing child descriptors for each element. The symbol names selected to represent the element information may conflict with existing symbols in the enclosing scope, in which case those outer symbols are not accessible to child descriptors. The attributes supported by this descriptor are described in the following table:

Shown below is an example of an <iterate> descriptor that sets the new data member reviewSalary to true if the employee's salary is greater than $3000:

<iterate target="new.employeeMap" key="id" value="emp">
    <if test="emp.salary > 3000">
        <set target="emp.reviewSalary" value="true"/>
    </if>
</iterate>

<if> Descriptor Element

Conditionally executes child descriptors. The attributes supported by this descriptor are described in the following table:

Child descriptors are executed if the expression in test evaluates to true.

<set> Descriptor Element

Modifies a value. The value and type attributes are mutually exclusive. If target denotes a dictionary element, that element must already exist (i.e., <set> cannot be used to add an element to a dictionary). The attributes supported by this descriptor are described in the following table:

The <set> descriptor below modifies a member of a dictionary element:

<set target="new.parts['P105J3'].cost" value="new.parts['P105J3'].cost * 1.05"/>

This <set> descriptor adds an element to a sequence and initializes its value:

<set target="new.partsList" length="new.partsList.length + 1" value="'P105J3'"/>

As another example, the following <set> descriptor changes the value of an enumeration:

<set target="new.ingredient" value="::New::Apple"/>

Notice in this example that the value refers to a symbol in the new Slice definitions.

<add> Descriptor Element

Adds a new element to a sequence or dictionary. It is legal to add an element while traversing the sequence or dictionary using <iterate>, however the traversal order after the addition is undefined. The key and index attributes are mutually exclusive, as are the value and type attributes. If neither value nor type is specified, the new element is initialized with a default value. The attributes supported by this descriptor are described in the following table:

Below is an example of an <add> descriptor that adds a new dictionary element and then initializes its member:

<add target="new.parts" key="'P105J4'"/>
<set target="new.parts['P105J4'].cost" value="3.15"/>

<define> Descriptor Element

Defines a new symbol in the current scope. The attributes supported by this descriptor are described in the following table:

Below are two examples of the <define> descriptor. The first example defines the symbol identity to have type Ice::Identity, and proceeds to initialize its members using <set>:

<define name="identity" type="::New::Ice::Identity"/>
<set target="identity.name" value="steve"/>
<set target="identity.category" value="Admin"/>

The second example uses the enumeration we first saw in our discussion of custom database migration to define the symbol manufacturer and assign it a default value:

<define name="manufacturer" type="::New::BigThree" value="::New::Daimler"/>

<remove> Descriptor Element

Removes an element from a sequence or dictionary. It is legal to remove an element while traversing a sequence or dictionary using <iterate>, however the traversal order after removal is undefined. The attributes supported by this descriptor are described in the following table:

<fail> Descriptor Element

Causes transformation to fail immediately. If test is specified, transformation fails only if the expression evaluates to true. The attributes supported by this descriptor are described in the following table:

The following <fail> descriptor terminates the transformation if a range error is detected:

<fail message="range error occurred in ticket count!" test="old.ticketCount > 32767"/>

<delete> Descriptor Element

Causes transformation of the current database record to cease, and removes the record from the transformed database. This descriptor has no attributes.

<echo> Descriptor Element

Displays values and informational messages. If no attributes are specified, only a newline is printed. The attributes supported by this descriptor are described in the following table:

Shown below is an <echo> descriptor that uses both message and value attributes:

<if test="old.ticketCount > 32767">
    <echo message="deleting record with invalid ticket count: " value="old.ticketCount"/>
    <delete/>
</if>

See Also

• Custom Database Migration
• Freeze Maps
• Freeze Evictors
• Automatic Database Migration
• FreezeScript Descriptor Expression Language