Apart from the different type of use cases, what is the difference between an asset materialization and a observation materialization?

[sryza]

Disclaimer: existencial question

Apart from the different type of use cases, what is the difference between an asset materialization and a observation materialization? I mean, in previous version there used to be only asset materializations and I believe they covered the actual purposes of the current observation materialization, since we could yield an asset materialization to flag some kind of events/observations/job completions...

Is there a reason for the existence of these two materializations in the newest version?

Originally asked as a question on Slack: https://dagster.slack.com/archives/C01U954MEER/p1647531889027089

[sryza]

A materialization signifies that an asset changed, while an observation doesn't. Some situations where this difference can come into play:

• You might write a sensor that listens for materializations to an asset, but not observations. I.e. you want to kick off whenever the asset changes, but not every time that a job records metadata about it.
• In software-defined assets, we show an "Upstream Changed" indicator in Dagit when an asset has been materialized less recently than one of its parents. Asset observations don't affect that indicator.