Patterns and precedence

[Josh Smift]

http://docs.ansible.com/intro_patterns.html shows

webservers:dbservers:&staging:!phoenix

as an example of a combination of unions and intersections, and says that
this means

all machines in the groups `webservers' and `dbservers' are to be
managed if they are in the group `staging' also, but the machines are
not to be managed if they are in the group `phoenix'

To simplify slightly, because I don't actualy need the negation in my
example, it's saying that

webservers:dbservers:&staging

means

(websevers UNION dbservers) INTERSECTION staging

which is cool.

But, I have a use case where I want to say

webservers UNION (dbservers INTERSECTION staging)

i.e. I want to manage with all webservers and only staging dbservers. I
didn't see an obvious way to do that; all three of

webservers:dbservers:&staging
webservers:staging:&dbservers
dbservers:&staging:webservers
staging:&dbservers:webservers

seem to be equivalent, and adding parentheses didn't seem to be supported
(I got "ERROR: provided hosts list is empty").

I particularly want to use it in a --limit argument, if that matters.

Is there a way to do this, or does union always take precedence over
intersection?

-Josh (j...@care.com)



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[Giovanni Tirloni]

There magic happens in lib/ansible/inventory.get_hosts() [1].

# when applying the host selectors, run those without the "&" or "!"
# first, then the &s, then the !s.
patterns = pattern_regular + pattern_intersection + pattern_exclude

[...]

if p.startswith("!"):
hosts = [ h for h in hosts if h not in that ]
elif p.startswith("&"):
hosts = [ h for h in hosts if h in that ]
else:
to_append = [ h for h in that if h.name not in [ y.name for y in hosts
] ]
hosts.extend(to_append)

My brain can't find a way to define what you want using patterns alone.

Filled issue#10234 as a possible improvement [2].

[1] -
https://github.com/ansible/ansible/blob/devel/lib/ansible/inventory/__init__.py#L176
[2] - https://github.com/ansible/ansible/issues/10234

Giovanni