Separating yaml files and passing in variables

Is there any way to separate a YAML file and pass in variables to sub files?

For Example:

Parent yaml file:

Fn::Merge:
- !Include /templates/first.yaml, variables: env: staging
- !Include /templates/second.yaml, variables: env: production


first.yaml file:

first.yaml

First:
Properties:
env: $env


1 Answer
1

This cannot be done in YAML itself, i.e. there is nothing in the
specification that speaks about including sub files or variable
expansion.

It is possible that a program that loads the YAML does something like
this, but there are some problems with the syntax that you indicate.

The $env looks like a template what you expect to be replaced by
staging during loading. However it is unclear from your example whether
you always expect a complete scalar node to be replaced, or if this
can be done mid-scalar and if so what the escape mechanism is
(i.e. how to specify something in an included file that results in a
string $env in your program).

$env

staging

$env

You should IMO explicitly tag scalars that
needs template expansion. You could make your first.yaml example into

first.yaml

env: !v env


where interpretation of the !v tag takes the full scalar. And if you
want expansion withing scalars you use a different tag and the more
verbose template option you have been using.

!v

xyz: !t This is a more verbose $env specficication


You might think you no longer need to worry about escaping, because if
there is no tag, the $env is not interpreted, but that is not the
case: you can still can have scalars where some $... patterns need
interpreting and other don't.

$env

$...

Your !Include tag has some problems as well. It is good that you
make things explicit by using a tag. Some programs like Symfony do
this kind of inclusion by magically interpreting special keys and scalar
syntax (but this could be a result of having to work around the rather
incomplete PHP YAML parser the use).

!Include

But your parameter passing is going to provide some problems as this:

!Include /templates/first.yaml, variables: env: staging

is going to be interpreted as if you wrote a single huge key with an embedded comma.
Your YAML is equivalent to

And I don't think that is what you want.
You should probably make the parameter for !Include an explicit sequence:

!Include

in which case order of parameters is important. Or use mappings for every parameter:

How this all should be actually implemented depends on your programming language and the parser you are using.

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