Documentation for variables

[mithun]

As a new custodian user, I'm trying to understand the usage of variables in policies. There seems to be multiple types of variables.

A non-exhaustive list for a beginner can be:

• vars in a policy yaml
• standard runtime variables for interpolation

Neither of the above ☝️ seem to have documentation around (1) how they are intended to be used, and (2) Examples.

It would be ideal to have a dedicated section within the documentation that describes the different types of variables, their use-cases and examples.

[kapilt]

agreed we should have some better docs here, quick recap fwiw

vars in a policy yml is at the moment just for yaml anchor and reference usage within a file. its mostly to help achieve DRY on policies with common configuration sections.

runtime variable interpolation is to enable better reuse across policies by enabling {region} {account_id}, {now} and a few others to be determined at execution time. alot of the values you see in that file are compatibility with filters/actions that were previously doing some degree of variable interpolation.

c7n-org also includes a notion of variables associated to an account, which allows variable interpolation using user defined variables scoped to an account on policy files.

[nitrocode]

I hit this recently when trying to merge filters. If examples are still undocumented, could someone add an example here of vars usage so I can DRY up my yaml ?

Edit: I found this on gitter by @jtroberts83 https://gitter.im/cloud-custodian/cloud-custodian?at=59fa4b4432e080696e530931

vars:
  common-filters: &age-description-filters
        - type: age
          days: 7
          op: ge
        - type: value
          key: Description
          op: regex
          value: ^(Automated,Backup,-?)\w+

policies:
- name: delete-old-snapshots-under-20-gb
  resource: ebs-snapshot
  filters:
   - type: value
     key: VolumeSize
     op: lt
     value: 20
   - and: *age-description-filters
  actions:
   - delete

Test:

cat policies/iam-attach-default-policy.yml.mod | \
  python -c "import sys, yaml; y=yaml.safe_load(sys.stdin.read()); print(y['policies'][0]['filters'][0]['and']);"