[Pulp-dev] Proposal and feedback request: un-nest urls
Jeff Ortel
jortel at redhat.com
Thu Nov 30 21:06:33 UTC 2017
On 11/29/2017 04:32 PM, Brian Bouterse wrote:
> For deletes, the db relationships are all there, so I expect deletes to cascade to other objects with any url
> structure. I believe closer to the release, we'll have to look at the cascading delete relationships to see if
> the behaviors that we have are correct.
>
> Overall, I'm +1 on un-nesting. I think it would result in a good user experience. I know it goes against the
> logical composition arguments, which have been well laid out. We want Pulp to be really simple, and the nested
> URL in the top of this thread is anything but simple. Consider another project like Ansible Galaxy (who also
> uses Django and DRF). Their API is very flat and as an outsider I find it very approachable:
> https://galaxy.ansible.com/api/v1/ Pulp could be that simple.
Clicking through the Galaxy API, there seems to be a good bit of nesting.
>
> My main concern in keeping the nesting is that this is going to be difficult for plugin writers. Making plugin
> writing easy is a primary goal if not the primary goal of Pulp3. If core devs are spending lots of time on it,
> a person doing this in their free time may not bother.
>
> I also see practical reasons motivating us to un-nest. We have been adding custom code regularly in this area,
> and it's been highly complexity and slow going. I think Austin described it well. Getting the viewsets working
> and to be simpler would allow us to move forward in many areas.
>
> So overall, un-nesting would give a better user experience (I think), a simpler plugin writer experience, and
> it would unblock a lot of work.
>
>
>
> On Wed, Nov 29, 2017 at 3:29 PM, Bihan Zhang <bizhang at redhat.com <mailto:bizhang at redhat.com>> wrote:
>
> I have a question about repository delete with the un-nested model.
> When a repository is deleted does the DELETE cascade to the importers/publishers that are linked to the
> repo? In an un-nested world I don't think they would. It would be odd for an object with its own endpoint
> to vanish without the user calling DELETE on the model.
>
> When nested it makes sense to cascade the delete so if /repo/1/ is deleted, everything thereafter
> (/repo/1/importer/2) should also be removed.
>
> Austin, I do see you point about it being a lot more complicated, but I think modeling things the right
> way is worth carrying the extra code and complexity.
>
> Anyways, maybe I'm wrong and importer/publishers should exist without a repository, in which case I can
> definitely see the value in un-nesting the URLs.
>
>
> On Wed, Nov 29, 2017 at 2:21 PM, Jeff Ortel <jortel at redhat.com <mailto:jortel at redhat.com>> wrote:
>
> 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>
> <mailto: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>
> > <https://martinfowler.com/bliki/DDD_Aggregate.html
> <https://martinfowler.com/bliki/DDD_Aggregate.html>>
> > > > >
> > > > >
> > > > >
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