ODL (Object Definition Language) — Elements You Can Define When authoring an object schema with ODL, which categories of elements can be defined directly?

Introduction:
ODL is a schema definition language for object databases that describes persistent types and their behavior. Understanding the breadth of elements you can define in ODL helps you design expressive object schemas that capture state (attributes and structures) and behavior (operations) where appropriate.

Given Data / Assumptions:

• ODL is associated with ODMG object databases.
• Object schemas typically comprise classes, attributes, relationships, and operations.
• The language supports composite type definitions (structures) and behavior declarations.

Concept / Approach:
Attributes capture scalar or complex values on objects. Structures allow composition of multiple fields into a single typed value. Operations define callable behaviors associated with classes or interfaces. Together, they form the core of an object schema's state and behavior specification in ODL.

Step-by-Step Solution:
1) Confirm that attributes are first-class schema elements in ODL.2) Recognize that structured types (records) can be declared for reuse and clarity.3) Note that operations (methods) can be declared in interfaces/classes to define behavior signatures.4) Therefore, all listed elements are definable in ODL.

Verification / Alternative check:
Review ODL examples that declare interfaces with attributes, relationships, struct types, and operation signatures; these constructs are standard in ODMG-style schemas.

Why Other Options Are Wrong:

• Only one category: Too narrow; ODL supports multiple element kinds.
• Only classes: Misstates ODL's scope; attributes and operations are integral.

Common Pitfalls:
Designing overly complex structures without considering normalization and navigability, or declaring operations in the schema that belong better in application-layer services.

Final Answer:
All of the above.