MEAD Messages as Secondary Resources in an ERN Feed
One rule of the ERN Choreography is that when one primary Resource file (e.g. the resource files that make up the “essence” of the Release) is to be sent as part of an ERN message, all primary Resources are to be sent. Similarly, when one secondary Resource file (such as a cover art image) is to be sent as part of an ERN message, all secondary Resources are to be sent. This rule was agreed to simplify the logic for ingesting resource files.
MEAD files are communicated, when accompanying an ERN file, as if they were secondary resource files. However, the above rule does not apply to MEAD messages:
When a MEAD message is to be sent together with an ERN message there is no compulsion to also send the files containing the cover art and any other secondary Resources;
When the cover art or any other secondary Resource needs updating, there is no compulsion to also update any MEAD message(s);
When updating one MEAD message that was sent with an ERN message, there is no compulsion to also update any other MEAD messages that might have been sent with a previous version of the ERN message; and
When updating an ERN message (e.g. to provide a new Deal), there is no need to also update all associated MEAD messages. The MEAD information previously sent with an ERN message about the same Release(s) remains, thus, valid unless the sender of the ERN message choses to accompany its ERN message with an updated MEAD message. In that case the rules indicated in the article on ‘MEAD as a “Statement of Truth”’ apply.
Note: this rule will be added at some stage to the ERN Choreography standard.
 As there is no direct link from a MEAD message to an ERN message, it would be impossible to know which MEAD messages would need to be re-sent anyhow.