On 07/20/2016 10:37 AM, Simo Sorce wrote:
On Wed, 2016-07-20 at 10:17 -0400, Ben Lipton wrote:
On 07/20/2016 06:27 AM, Simo Sorce wrote:
On Tue, 2016-07-19 at 16:20 -0400, Ben Lipton wrote:

I have updated the design page
with my plan for implementing user-configurable rules for mapping
data into certificate requests. In brief: we will use Jinja2 for
templating. Data rules (which map individual data items) and
rules (which group them into certificate fields) will both be
of Jinja2 markup. The formatting process will be as follows:
1. Syntax rules will be rendered using Jinja2. Data rules (rule
not rendered) will be passed as the datarules attribute.
2. Rendered syntax rules will be processed by the Formatter class
the selected CSR generation helper (e.g. openssl or certutil).
formatter combines these partial rules into a full template for
3. The template will be rendered using Jinja2. Relevant data from
IPA database will be available in the context for this rendering.
4. The final rendered template will be returned to the caller,
with its function (e.g. a command line or a config file).

Are there any comments or objections to this approach? Here's an
to show what it might look like in practice.

Example data rules:

Example syntax rule:
subjectAltName=@{% section %}{{datarules|join('\n')}}{%
endsection %}

Example composed config template:
[ req ]
prompt = no
encrypt_key = no

distinguished_name = {% section
CN={{subject.username}}{% endsection %}

req_extensions = exts

[ exts ]
subjectAltName=@{% section %}email={{subject.email}}{% endsection

There's a lot more information about the thinking behind this at
if you're interested, as well.
Nice work Ben,
it's been really nice to be able to follow your notes on the blog
one question remains lingering in my head, why jinja2 ?
I know that engine relatively well as I used it in ipsilon, so I am
questioning the choice just asking why specifically jinja2 and not
something else, potentially language agnostic.

Honestly, my reasoning didn't go very far beyond that it seems to be
widely used and is compatible with python, which is the language
the implementation is taking place (in the IPA RPC server). I
about using the built-in python format strings or creating a simple
domain-specific language, but the likelihood of wanting the built-in
text processing features (join, replace, maybe even for loops)
high, and I didn't want to reimplement those features.

Will the additional package dependency be a problem?
I am more concerned a out the ability to process the data (which I
guess is stored in LDAP) by another client, or in the CLI.
Other than that the dependency does not concern me too much provided
jinja2 templating is stable and has some guarantee that it will be
supportable long term.

If that is not guaranteed it is a problem, we cannot easily swap out
one language for another once data is stored and used by the server.
So the most important consideration for me is whether we are locking
ourselves into something that will be hard to deal with later or not.

Should the jinja2 project fail by the wayside next year would we be
able to easily replace it with another engine without changing the
templates as stored ?


Ah, ok, I understand the concern. For now, the plan is that the server will do all the text processing, so I don't really forsee a need for any other client to read the mapping rules from LDAP. However, it's true that templates written in jinja2 would probably need at least minor changes to be compatible with another templating engine. (Same goes for any other choice - a lot of these engines seem to have very similar, but not exactly compatible, syntax). I don't really know how to judge the long-term viability of the jinja2 project, though it seems to be recognized by lots of projects (ansible[1], openstack[2], flask[3], even django[4] which has its own templating engine).

In any case, if the team prefers it, I'd be comfortable going with a more minimal DSL that only has the features we know we need. It might slightly limit the types of certs that can be generated, but that can be iterated on. But it would be another thing to design, build and maintain. Let me know what you think.


[1] http://docs.ansible.com/ansible/playbooks_variables.html#using-variables-about-jinja2
[2] http://lists.openstack.org/pipermail/openstack-dev/2013-July/012016.html
[3] http://flask.pocoo.org/docs/0.11/templating/
[4] https://docs.djangoproject.com/en/1.9/topics/templates/#django.template.backends.jinja2.Jinja2

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