[Pulp-dev] Proposal and feedback request: un-nest urls
Jeff Ortel
jortel at redhat.com
Wed Nov 29 19:21:58 UTC 2017
Austin makes a compelling argument.
On 11/28/2017 02:16 PM, Austin Macdonald wrote:
> When I look at this, the most important point is that we have a hyperlinked REST API, which means that the
> urls are specifically not going to be built by users.
>
> For a user to retrieve an importer, they would first GET the importers for a repository. The next call would
> be the exact href returned by pulp. This workflow is exactly the same whether we nest or not. The only
> difference is that we no longer convey the information in the href, which seems fine to me since they aren't
> particularly readable anyway.
>
> It has already been discussed that filtering can make up for the use cases that use nesting, and that filters
> would be more flexible.
>
> So for me, nesting costs in (1) extra code to carry (2) extra dependency (3) complexity to use.
>
> To elaborate on the complexity, the problem is in declaring fields on the serializer. The serializer is
> responsible for building the urls, which requires all of the uuids for the entire nested structure. This is
> further complicated by master/detail, which is an entirely Pulp concept.
>
> Because of this, anyone working on the API (likely including plugin writers) will need to understand
> parent_lookup_kwargs and how to use then with:
> DetailNestedHyperlinkedRelatedField
> DetailNestedHyperlinkedidentityField
> DetailwritableNestedUrlRelatedField
> DetailRelatedField
> DetailIdentityField
> NestedHyperlinkedRelatedField
> HyperlinkedRelatedField.
>
> The complexity seems inherrent, so I doubt we will be able to simplify this much. So, is all this code and
> complexity worth the implied relationship in non-human-friendly urls? As someone who has spent a lot of time
> on this code, I don't think so.
>
>
>
> On Nov 28, 2017 06:12, "Patrick Creech" <pcreech at redhat.com <mailto:pcreech at redhat.com>> wrote:
>
> On Mon, 2017-11-27 at 16:10 -0600, Jeff Ortel wrote:
> > On 11/27/2017 12:19 PM, Jeff Ortel wrote:
> > >
> > >
> > > 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.
> >
> > This is not an argument to flatten the URLs but meant to clarify the relationships.
>
> I'm in agreement here. I was possibly a little hasty in lumping all things that have a Repositoy fk
> as being 'dependent' in that paragraph during the formation of my argument.
>
> > >
> > > >
> > > > --------------------------------------------------------------
> > > > 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
> <https://martinfowler.com/bliki/DDD_Aggregate.html>
> > > >
> > > >
> > > >
> > > > _______________________________________________
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> > > >
> >
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