This is an introduction to the various objects which make up a
message model.
- Message
- A message is a set of data passed between applications. Messages
must have a structure and format which is agreed by the sending and receiving
applications.
- Message definition
- A message definition is a logical description of a message.
A message definition is a collection of simple elements organized in a tree-like
structure.
- Simple element
- A simple element describes one or more fields in a message.
It is based on a simple type (for example, string, integer or
float). It can repeat, and it can define a default or a fixed value.
- Simple type
- A simple type describes a class of data within a message.
It describes the type of data (for example, string, integer or float) and
it can have value constraints which place limits on the values of any simple
elements based on that simple type.
- Complex element
- A complex element is a named structure containing simple
elements within the message. Complex elements can contain other complex elements,
and can also contain groups. The content of a complex element
is defined by a complex type.
- Complex type
- A complex type describes a structure within a message. It
contains elements, attributes and groups organised
into a hierarchy.
- Group
- A group is a list of elements with information about how
those elements can appear in a message. Groups can be ordered (sequence) unordered
(all) or selective (choice).
- Attribute
- An attribute represents an XML Schema attribute. Attributes
are very similar to simple elements, but they require special treatment when
used with XML messages. In messages which are not XML, they are treated exactly
like a simple element based on the same simple type.
Global
and local objects
Many of the objects in the message model can be
either global or local. A global object must have a unique name, which is
used to refer to the object from one or more places in the message model.
Local objects are defined and used in only one place in the message model.
You are recommended to make objects local unless they need to be
used in more than one place. This reduces the probability of name clashes
among the global objects in the message model, and will make the message set
easier to work with.
Properties of message model objects
The properties
of all message model objects are listed on the 'properties' pane of the message
editor. The properties fall into three categories:
- Logical
- The logical properties of an object relate to the format-independent description
of the object called the 'logical model'. Logical properties describe what data
the object contains without saying anything about how it is written
down.
- Physical
- The physical properties of an object describe how the object is written
down. These properties control the parsing and writing of the object. There
is one set of physical properties for each physical format in your message
set.
- Documentation
- This field is present for all message model objects. It provides a standard
place for any description of the object which you might require. Text entered
here does not affect the processing of messages in any way.