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of this are to centralise the conflict code to allow further change, as well as to allow passing through the full reason for the conflict through to the conflicting backends. Backend state alters how we can handle different types of conflict so this is now required. As originally suggested by Heikki, no longer optional.
$PostgreSQL: pgsql/src/backend/storage/ipc/README,v 1.5 2008/03/20 17:55:15 momjian Exp $ Cache Invalidation Synchronization Routines =========================================== Mon Jul 18 11:09:22 PDT 1988 W.KLAS The cache synchronization is done using a message queue. Every backend can register a message which then has to be read by all backends. A message read by all backends is removed from the queue automatically. If a message has been lost because the buffer was full, all backends that haven't read this message will be told that they have to reset their cache state. This is done at the time when they try to read the message queue. The message queue is implemented as a shared buffer segment. Actually, the queue is a circle to allow fast inserting, reading (invalidate data) and maintaining the buffer.