Overview

Purpose: The MQMD structure contains the control information that accompanies the application data when a message travels between the sending and receiving applications. The structure is an input/output parameter on the MQGET, MQPUT, and MQPUT1 calls.

Version: The current version of MQMD is MDVER2. Fields that exist only in the more-recent versions of the structure are identified as such in the descriptions that follow.

The COPY file provided contains the most recent version of MQMD that is supported by the environment, but with the initial value of the MDVER field set to MDVER1. To use fields that are not present in the version-1 structure, the application must set the MDVER field to the version number of the version required.

A declaration for the version-1 structure is available with the name MQMD1.

Character set and encoding: Data in MQMD must be in the character set and encoding of the local queue manager; these are given by the CodedCharSetId queue manager attribute and ENNAT, respectively. However, if the application is running as an MQ client, the structure must be in the character set and encoding of the client.

If the sending and receiving queue managers use different character sets or encodings, the data in MQMD is converted automatically. It is not necessary for the application to convert the MQMD.

Using different versions of MQMD: A version-2 MQMD is generally equivalent to using a version-1 MQMD and prefixing the message data with an MQMDE structure. However, if all of the fields in the MQMDE structure have their default values, the MQMDE can be omitted. A version-1 MQMD plus MQMDE are used as described below.

• On the MQPUT and MQPUT1 calls, if the application provides a version-1 MQMD, the application can optionally prefix the message data with an MQMDE, setting the MDFMT field in MQMD to FMMDE to indicate that an MQMDE is present. If the application does not provide an MQMDE, the queue manager assumes default values for the fields in the MQMDE.
  Note:

  Several of the fields that exist in the version-2 MQMD but not the version-1 MQMD are input/output fields on the MQPUT and MQPUT1 calls. However, the queue manager does not return any values in the equivalent fields in the MQMDE on output from the MQPUT and MQPUT1 calls; if the application requires those output values, it must use a version-2 MQMD.

• On the MQGET call, if the application provides a version-1 MQMD, the queue manager prefixes the message returned with an MQMDE, but only if one or more of the fields in the MQMDE has a non-default value. The MDFMT field in MQMD will have the value FMMDE to indicate that an MQMDE is present.

The default values that the queue manager used for the fields in the MQMDE are the same as the initial values of those fields, shown in Table 25.

When a message is on a transmission queue, some of the fields in MQMD are set to particular values; see MQXQH - Transmission-queue header for details.

Message context: Certain fields in MQMD contain the message context. There are two types of message context: identity context and origin context. Usually:

• Identity context relates to the application that originally put the message

• Origin context relates to the application that most recently put the message.

These two applications can be the same application, but they can also be different applications (for example, when a message is forwarded from one application to another).

Although identity and origin context usually have the meanings described above, the content of both types of context fields in MQMD actually depends on the PM* options that are specified when the message is put. As a result, identity context does not necessarily relate to the application that originally put the message, and origin context does not necessarily relate to the application that most recently put the message - it depends on the design of the application suite.

There is one class of application that never alters message context, namely the message channel agent (MCA). MCAs that receive messages from remote queue managers use the context option PMSETA on the MQPUT or MQPUT1 call. This allows the receiving MCA to preserve exactly the message context that travelled with the message from the sending MCA. However, the result is that the origin context does not relate to the application that most recently put the message (the receiving MCA), but instead relates to an earlier application that put the message (possibly the originating application itself).

In the descriptions below, the context fields are described as though they are used as described above. For more information about message context, see the WebSphere MQ Application Programming Guide.

Message expiry: Messages that have expired on a loaded queue (a queue that has been opened) are automatically removed from the queue within a reasonable period of time after their expiry. Some other new features of this release of WebSphere MQ can lead to loaded queues being scanned less frequently than in the previous product version, however expired messages on loaded queues are always removed within a reasonable period of their expiry.