Hi,
I'm considering of writing a plugin, but would need some pointers/tips
what's the best way to achieve what I'm after.
Basically I do a lot of firewall rules for host to host connections (based
on groups), so I've currently done something like:
- source: "{{ groups.mygroup | map('extract', hostvars, 'my_ip_variable') |
list }}"
The syntax is quite verbose, but there's a bigger problem that accessing
hostvars like that is extremely slow when working with 1000+ hosts. So I
though I could construct a dict that would contain my groups and the
required variables, something like this:
my_net_vars:
mygroup:
my_ip_variable: [10.0.0.1]
my_other_ip_variable: [192.168.0.1]
which would allow me to have simplified usage just with:
- source:"{{ my_net_vars.mygroup.my_ip_variable }}"
There seems to be a few alternatives I've been looking in to:
1) lookup plugin
since lookup plugins already have the variable information available, it
would be rather simple to construct this type of information from that.
Then I could just set
my_net_vars: "{ lookup('my plugin') }}"
in my group_vars/all. The issue here is still the slowness. The lookup
seems to happen for every host every time a variable is used which results
in a lot of calls. I managed to get it faster by manually generating a json
that would be exactly the same for each host, but in practice this would
mean I would need to implement my own caching that would live either "for
the current ansible run" or then I would need to look for ways to detect if
the inventory has changed. Would there be anything in ansible that could be
used for this kind of caching?
2) vars plugin
This would probably suit, but I'm struggling to get information about hosts
and their groups. What would be an easy way to have that information
available for a variable plugin, since although it gets groups and hosts as
entities, my play might be limited to a specific set?
3) inventory plugin
I'm already relying on the ini inventory plugin and host_group_vars plugin,
so would it then just to make sense to subclass this one and inject my own
variables here instead?
I can expect my variables be available in ansible base
dir/host_vars/hostname/host.yml in a non-templated format, so I could even
parse these files to make it possibly quicker, unless those are available
as cached somehow.
Also constructed plugin sounded nice, but doesn't seem to work if the
variables come from a variable plugin such as the built in host_group_vars,
as the variables are not visible at that point. So any suggested
approaches? Or other alternatives to these ones? I realize if I wrote an
inventory plugin, that could be extended later on to cover multiple
inventory sources I might have in the future, but for now I'm using
something that's readily available.
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