What is the problem with how it is currently?

There are four related things:

• Outputs - sometimes used to mean trigger:message pairs, but sometimes used to refer to triggers or messages.
• Triggers - the things you can use in the graph or satisfy with cylc set. These are often referred to as "labels".
• Messages - the strings you can send through cylc message.
• Events - kinda like messages that come from within the system like submission failed or retry, but confusingly, also started, succeeded and failed which are also task outputs.

Whilst working on #5658 it was discovered that triggers/messages were sometimes mislabelled as their opposite and outputs was sometimes used ambiguously to refer to one or the other. No functional problems, it all works fine, just confusing when you need to work on this code.

Cheat Sheet:

• DB - messages
• Data Store - messages & triggers
• TaskOutputs - messages & triggers
• cylc set - triggers
• cylc message - messages
• cylc trigger - trick question
• Scheduler.message_queue - messages & events
• TaskEventsManager.process_message - messages & events

I've been diving into this code over the past few days whilst working on the optional outputs and struggling with this a bit (completion expressions need triggers, most of the rest of the code needs messages, trying to avoid storing surplus maps whilst keeping it efficient). I'm coming to the conclusion that messages should be mapped onto triggers in the task events code, from there on it would be simpler to stick to triggers.

Triggers are the more fundamental and user facing concept. They are used in the graph, the completion expression, cylc show, universal ID, etc. Messages can be used to provide supplementary detail and may be patterns, great for sending supplementary information back to the scheduler (not that anyone does sadly, I've checked!), but not so good for internal interfaces where it's not clear whether we're dealing with raw messages or matching message patterns. Note raw messages may not be unique and the mapping between raw messages and patterns may be broken by reload/restart.