[Freeipa-devel] [DESIGN] Text-based rules for CSR autogeneration using Jinja2

[Simo Sorce]

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:
> > > 
> > > Hi,
> > > 
> > > I have updated the design page
> > > http://www.freeipa.org/page/V4/Automatic_Certificate_Request_Gene
> > > rati
> > > on/Mapping_Rules
> > > with my plan for implementing user-configurable rules for mapping
> > > IPA
> > > data into certificate requests. In brief: we will use Jinja2 for
> > > templating. Data rules (which map individual data items) and
> > > syntax
> > > rules (which group them into certificate fields) will both be
> > > snippets
> > > of Jinja2 markup. The formatting process will be as follows:
> > > 1. Syntax rules will be rendered using Jinja2. Data rules (rule
> > > text,
> > > not rendered) will be passed as the datarules attribute.
> > > 2. Rendered syntax rules will be processed by the Formatter class
> > > for
> > > the selected CSR generation helper (e.g. openssl or certutil).
> > > The
> > > formatter combines these partial rules into a full template for
> > > the
> > > config.
> > > 3. The template will be rendered using Jinja2. Relevant data from
> > > the
> > > IPA database will be available in the context for this rendering.
> > > 4. The final rendered template will be returned to the caller,
> > > labeled
> > > with its function (e.g. a command line or a config file).
> > > 
> > > Are there any comments or objections to this approach? Here's an
> > > example
> > > to show what it might look like in practice.
> > > 
> > > Example data rules:
> > > email={{subject.email}}
> > > O={{config.ipacertificatesubjectbase}}\nCN={{subject.username}}
> > > 
> > > Example syntax rule:
> > > subjectAltName=@{% section %}{{datarules|join('\n')}}{%
> > > endsection %}
> > > 
> > > Example composed config template:
> > > [ req ]
> > > prompt = no
> > > encrypt_key = no
> > > 
> > > distinguished_name = {% section
> > > %}O={{config.ipacertificatesubjectbase}}
> > > 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
> > > http://blog.benjaminlipton.com/2016/07/19/csr-generation-templati
> > > ng.h
> > > tml
> > > if you're interested, as well.
> > Nice work Ben,
> > it's been really nice to be able to follow your notes on the blog
> > post,
> > one question remains lingering in my head, why jinja2 ?
> > I know that engine relatively well as I used it in ipsilon, so I am
> > not
> > questioning the choice just asking why specifically jinja2 and not
> > something else, potentially language agnostic.
> > 
> > Simo.
> 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
> where 
> the implementation is taking place (in the IPA RPC server). I
> thought 
> 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)
> seemed 
> 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 ?

Simo.