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Hi all,

I promised John to dump my thoughts on traits to the ML, so here we go :)

I see two roles of traits (or kinds of traits) for bare metal:
1. traits that say what the node can do already (e.g. "the node is
doing UEFI boot")
2. traits that say what the node can be *configured* to do (e.g. "the node can
boot in UEFI mode")

This seems confusing, but it's actually very useful. Say, I have a flavor that
requests UEFI boot via a trait. It will match both the nodes that are already in
UEFI mode, as well as nodes that can be put in UEFI mode.

This idea goes further with deploy templates (new concept we've been thinking
about). A flavor can request something like CUSTOM_RAID_5, and it will match the
nodes that already have RAID 5, or, more interestingly, the nodes on which we
can build RAID 5 before deployment. The UEFI example above can be treated in a
similar way.

This ends up with two sources of knowledge about traits in ironic:
1. Operators setting something they know about hardware ("this node is in UEFI
mode"),
2. Ironic drivers reporting something they
  2.1. know about hardware ("this node is in UEFI mode" - again)
  2.2. can do about hardware ("I can put this node in UEFI mode")

For case #1 we are planning on a new CRUD API to set/unset traits for a node.
Case #2 is more interesting. We have two options, I think:

a) Operators still set traits on nodes, drivers are simply validating them. E.g.
an operators sets CUSTOM_RAID_5, and the node's RAID interface checks if it is
possible to do. The downside is obvious - with a lot of deploy templates
available it can be a lot of manual work.

b) Drivers report the traits, and they get somehow added to the traits provided
by an operator. Technically, there are sub-cases again:
  b.1) The new traits API returns a union of operator-provided and
driver-provided traits
  b.2) The new traits API returns only operator-provided traits; driver-provided
traits are returned e.g. via a new field (node.driver_traits). Then nova will
have to merge the lists itself.

My personal favorite is the last option: I'd like a clear distinction between
different "sources" of traits, but I'd also like to reduce manual work for
operators.

A valid counter-argument is: what if an operator wants to override a
driver-provided trait? E.g. a node can do RAID 5, but I don't want this
particular node to do it for any reason. I'm not sure if it's a valid case, and
what to do about it.

Let me know what you think.

Dmitry