The Message Definition Record (MDR) describes the layout of a Sabre Data Source (SDS) message. Like the SDS message, delimiters separate fields from one another. While the SDS message transmits actual data, the MDR describes the type of data being sent.

The following is an example an MDR:

The MDR uses much the same delimiters as the SDS message, except MDR does not use the "end of repeat data" delimiter.

The MDR Header is identical to the SDS message header. This fact allows an API to use the SDS message ID and version as a key to retrieve the MDR corresponding to that message. The MDR Header consists of two fields, a fourteen-byte MDR ID, and a two-digit MDR version number.

The MDR segment record describes an SDS segment that may (or must) appear in an SDS message. The MDR segment record has several fields. The first two fields of an MDR segment record and SDS message are identical. This allows an API to use the SDS fields as a key to find the corresponding segment in the MDR. The next two fields, mandatory/optional flag and repeat factor, indicate properties of the segment. The MDR segment record contains:

Segment Delimiter

The segment delimiter signals the start of a segment record.

Segment ID

The segment ID is a six-character segment identifier that follows the segment delimiter and combines with the nesting level to identify an MDR segment.

Nesting Level

The nesting level is a single digit number ranging from zero-to-nine that combines with the segment ID to identify an MDR segment. For more information, see Nesting_Rules. An element delimiter separates the nesting level from the segment ID.

Mandatory/Optional Flag

The mandatory/optional flag is a one-character field with the value of either "M" or "O" (mandatory or optional). A mandatory segment must appear at least once in a message. Nested mandatory segments must appear at least once under their parent segment. An optional segment may appear within an SDS message, but it is not required. An element delimiter separates the nesting level from the mandatory/optional flag.

Repeat Factor

The repeat factor is a one-to-three-digit field that indicates the number of times a segment may appear in a message. The minimum value for this field is one and the maximum is 999. The value 999 in this field acts as an infinite repeat factor.

Segment Delimiter

The segment delimiter terminates the segment record. This allows insertion of the new fields in the segment record and still allows processing by older software.

Element Record

The element record describes the properties of an element within a segment.

Element ID	(four-digit, base-36 number)
Data Type	(one-character)
Minimum Size	(one-to-four digits)
Maximum Size	(one-to-four digits)
Mandatory/Optional Flag	(one-character)
Repeat Factor	(one-to-three digits)

Element ID

The element ID is a four-digit, base-36 number. A base-36 number is a number whose digits range from 0-9, A-Z. Each element identifier uniquely defines a certain element such as flight number, number in party, etc. The data elements and their definitions are available from the application representative. An element delimiter follows the element ID field.

MDR Data Type

The data type is a one-character field that describes the type of data that transmits. An element delimiter follows the data type. The current data types are

MDR Minimum Size

The minimum size field defines the minimum number of characters (or digits) a data element must be if it is not omitted. An omitted element appears as a zero length data value within an SDS message. The minimum size field must be in the range 1-to-4096 inclusively. An element delimiter follows the minimum size field.

The mandatory/optional flag is a one-character value code with a value of M (mandatory) or O (optional). A mandatory data element is an element that must appear in the message segment. An optional data element can be omitted. An omitted optional element appears as a zero length element value in the SDS message. An element delimiter follows the mandatory/optional flag.

MDR Repeat Factor

The repeat factor indicates how many times this element occurs within the segment. This is a one-to-three-digit field with a value from 1-to-999. When this field has a value of more than one, the element is a repeating element.