[Pulp-dev] Proposal and feedback request: un-nest urls

[Jeff Ortel]

On 11/17/2017 08:55 AM, Patrick Creech wrote:
> One of the things I like to think about in these types of situations is, "what is good rest api
> design".  Nesting resources under other resources is a necessary part of good api design, and has
> its place.  To borrow some terms from domain driven development:
> 
> Collections of objects are called aggregates.  Think 'an order and its line items'.  Line items make
> no sense without having the order context, so they are an aggregate that is accessed under an
> Order.  This is called the aggregate root.  The rest api design for such an object, using order as
> the aggregate root, would look like:
> 
> '/orders/' -- all orders
> '/orders/{order_key}/' -- a specific order with key.
> '/orders/{order_key}/items/' -- All of the order's items.
> '/orders/{order_key}/items/{item_key}/' -- a specific line item of the order
> 
> When it comes to order items themselves, it isn't helpful to start with them as their own aggregate
> root in one large collection:
> 
> '/items/'   -- all order items in the system

The order/items is a good example of aggregation (or composition) and I agree it makes a strong case for
nesting.  In pulp, a repository is easily thought of as a collection or aggregation of content.

> 
> Because you lose the order context. Based on api design, this endpoint will need to respond with all
> order items across all orders and resort to parameter filtering to provide the context you need.
> 
> A quote borrowed from Martin Fowler [0]
> 
> "An aggregate will have one of its component objects be the aggregate root. Any references from
> outside the aggregate should only go to the aggregate root. The root can thus ensure the integrity
> of the aggregate as a whole."
> 
> Publishers, importers, and publications are all aggregates that don't make much sense outside of
> their aggregate root of Repository.  They are dependent on the Repository context, and from a domain
> view, should be accessed starting with their specific Repository endpoint.

I don't think the aggregation relationship exists between repository and importer/publisher.  There is a
strong association between repository and importer/publisher which /could/ even be characterized as
"ownership".  However, I don't think there is an aggregation (or composition) relationship.  The same for
publisher & publication.  A publication is associated to its creating publisher but the publisher isn't an
aggregation of publications.  The relationship mainly provides linkage to the repository.

> 
> --------------------------------------------------------------
> Specific items rebuttals:
>     
>     Yes, using the primary key uuid's as the immutable key adds some human readable challenges to
> the API.  That sounds more like a point to discuss in the human readable vs. not human readable
> immutable key debate.

Agreed.

Also, I don't think nesting impacts URL readability.

>     
>     One of the challenges in software engineering is ensuring the tools you are using don't limit
> your choices.  DRF limited the choices for pulp's rest API design, and drf-nested-routers was
> introduced to help remove that limit.  If working around these limitations is complex, take
> advantage of open source here and help improve the upstream dependencies for your workflow.
>     
>     As far as making things simpler for plugin writers, perhaps there are ways you can simplify it
> for them by providing some encapsulation in pulp's core instead.  Abstract away the nasty bits
> behind the scenes, and provide them with a simpler interface to do what they need.
>     
>     With respect to the invested time already in making this work, I agree with jeremy that it
> should be considered part of the sunken cost fallacy.  What does need to be evaluated though is how
> much time re-architecting at this point will cost you (discussion, planning, and development) vs the
> amount of time it will save, and weigh that against any planned milestones for pulp to see if it
> will push them out as well.
>     
>     I'm also in agreement that it is moot if pulp3 has a different api structure than pulp2.  Major
> version boundaries are the perfect time for evaluating and moving such things around.
> 
> [0] https://martinfowler.com/bliki/DDD_Aggregate.html
> 
> 
> 
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