Open Stack

SUMMARY: 
----------------

We are changing the syntax for inlining YAQL expressions in Mistral YAML from {1+$.my.var} (or “{1+$.my.var}”) to <% 1+$.my.var %>

Below I explain the rationale and the criteria for the choice. Comments and suggestions welcome.

DETAILS: 
-------------

We faced a number of problems with using YAQL expressions in Mistral DSL: [1] must handle any YAQL, not only the ones started with $; [2] must preserve types and [3] must comply with YAML. We fixed these problems by applying Ansible style syntax, requiring quotes around delimiters (e.g. “{1+$.my.yaql.var}”). However, it lead to unbearable confusion in DSL readability, in regards to types:

    publish:
       intvalue1: "{1+1}” # Confusing: you expect quotes to be string.
       intvalue2: "{int(1+1)}” # Even this doestn’ clean the confusion
       whatisthis:"{$.x + $.y}” # What type would this return? 

We got a very strong push back from users in the filed on this syntax. 

The crux of the problem is using { } as delimiters YAML. It is plain wrong to use the reserved character. The clean solution is to find a delimiter that won’t conflict with YAML.

Criteria for selecting best alternative are: 
1) Consistently applies to to all cases of using YAML in DSL
2) Complies with YAML 
3) Familiar to target user audience - openstack and devops

We prefer using two-char delimiters to avoid requiring extra escaping within the expressions.

The current winner is <% %>. It fits YAML well. It is familiar to openstack/devops as this is used for embedding Ruby expressions in Puppet and Chef (for instance, [4]). It plays relatively well across all cases of using expressions in Mistral (see examples in [5]):

ALTERNATIVES considered:
--------------------------------------------------

1) Use Ansible-like syntax: http://docs.ansible.com/YAMLSyntax.html#gotchas
Rejected for confusion around types. See above.

2) Use functions, like Heat HOT or TOSCA:

HOT templates and TOSCA doesn’t seem to have a concept of typed variables to borrow from (please correct me if I missed it). But they have functions: function: { function_name: {foo: [parameter1, parameter 2], bar:"xxx”}}. Applied to Mistral, it would look like:

    publish:
     - bool_var: { yaql: “1+1+$.my.var < 100” } 

Not bad, but currently rejected as it reads worse than delimiter-based syntax, especially in simplified one-line action invocation.

3) < > paired with other symbols: php-styoe  <? ..?>


REFERENCES: 
----------------------

[1] Allow arbitrary YAQL expressions, not just ones started with $ : https://github.com/stackforge/mistral/commit/5c10fb4b773cd60d81ed93aec33345c0bf8f58fd
[2] Use Ansible-like syntax to make YAQL expressions YAML complient https://github.com/stackforge/mistral/commit/d9517333b1fc9697d4847df33d3b774f881a111b
[3] Preserving types in YAQL https://github.com/stackforge/mistral/blob/d9517333b1fc9697d4847df33d3b774f881a111b/mistral/tests/unit/test_expressions.py#L152-L184
[4]Using <% %> in Puppet https://docs.puppetlabs.com/guides/templating.html#erb-is-plain-text-with-embedded-ruby 
[5] Etherpad with discussion https://etherpad.openstack.org/p/mistral-YAQL-delimiters
[6] Blueprint https://blueprints.launchpad.net/mistral/+spec/yaql-delimiters

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openstack.org/pipermail/openstack-dev/attachments/20150216/b9b6a626/attachment.html>