[Pulp-dev] Fwd: Re: Changesets Challenges

Brian Bouterse bbouters at redhat.com
Thu Apr 12 21:00:34 UTC 2018 

On Thu, Apr 12, 2018 at 11:53 AM, Jeff Ortel <jortel at redhat.com> wrote:

>
>
> On 04/12/2018 10:01 AM, Brian Bouterse wrote:
>
>
>
> On Wed, Apr 11, 2018 at 6:07 PM, Jeff Ortel <jortel at redhat.com> wrote:
>
>>
>>
>> On 04/11/2018 03:29 PM, Brian Bouterse wrote:
>>
>> I think we should look into this in the near-term. Changing an interface
>> on an object used by all plugins will be significantly easier, earlier.
>>
>>
>> On Wed, Apr 11, 2018 at 12:25 PM, Jeff Ortel <jortel at redhat.com> wrote:
>>
>>>
>>>
>>> On 04/11/2018 10:59 AM, Brian Bouterse wrote:
>>>
>>>
>>>
>>> On Tue, Apr 10, 2018 at 10:43 AM, Jeff Ortel <jortel at redhat.com> wrote:
>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> On 04/06/2018 09:15 AM, Brian Bouterse wrote:
>>>>
>>>> Several plugins have started using the Changesets including
>>>> pulp_ansible, pulp_python, pulp_file, and perhaps others. The Changesets
>>>> provide several distinct points of value which are great, but there are two
>>>> challenges I want to bring up. I want to focus only on the problem
>>>> statements first.
>>>>
>>>> 1. There is redundant "differencing" code in all plugins. The Changeset
>>>> interface requires the plugin writer to determine what units need to be
>>>> added and those to be removed. This requires all plugin writers to write
>>>> the same non-trivial differencing code over and over. For example, you can
>>>> see the same non-trivial differencing code present in pulp_ansible
>>>> <https://github.com/pulp/pulp_ansible/blob/d0eb9d125f9a6cdc82e2807bcad38749967a1245/pulp_ansible/app/tasks/synchronizing.py#L217-L306>,
>>>> pulp_file
>>>> <https://github.com/pulp/pulp_file/blob/30afa7cce667b57d8fe66d5fc1fe87fd77029210/pulp_file/app/tasks/synchronizing.py#L114-L193>,
>>>> and pulp_python
>>>> <https://github.com/pulp/pulp_python/blob/066d33990e64b5781c8419b96acaf2acf1982324/pulp_python/app/tasks/sync.py#L172-L223>.
>>>> Line-wise, this "differencing" code makes up a large portion (maybe 50%) of
>>>> the sync code itself in each plugin.
>>>>
>>>>
>>>> Ten lines of trivial set logic hardly seems like a big deal but any
>>>> duplication is worth exploring.
>>>>
>>> It's more than ten lines. Take pulp_ansible for example. By my count
>>> (the linked to section) it's 89 lines, which out of 306 lines of plugin
>>> code for sync is 29% of extra redundant code. The other plugins have
>>> similar numbers. So with those numbers in mind, what do you think?
>>>
>>>
>>> I was counting the lines (w/o comments) in find_delta() based on the
>>> linked code.  Which functions are you counting?
>>>
>>
>> I was counting the find_delta, build_additions, and build_removals
>> methods. Regardless of how the lines are counted, that differencing code is
>> the duplication I'm talking about. There isn't a way to use the changesets
>> without duplicating that differencing code in a plugin.
>>
>>
>> The differencing code is limited to find_delta() and perhaps
>> build_removals().  Agreed, the line count is less useful than specifically
>> identifying duplicate code.  Outside of find_delta(), I see similar code
>> (in part because it got copied from file plugin) but not seeing actual
>> duplication.  Can you be more specific?
>>
>
> Very similar code or identical code, I think it begs the question why are
> we having plugin writer's do this at all? What value are they creating with
> it? I don't have a reasonable answer to that question, so the requirement
> for plugin writer's to write that code brings me back to the problem
> statement: "plugin writers have redundant differencing code when using
> Changesets". More info on why it is valuable for the plugin writer to do
> the differencing code versus the Changesets would be helpful.
>
>
> The ChangeSet abstraction (and API) is based on following division of
> responsibility:
>
> The plugin  (with an understanding of the remote and its content):
>   - Download metadata.
>   - Parse metadata
>   - Based on the metadata:
>     - determine content to be added to the repository.
>       - define how artifacts are downloaded.
>       - construct content
>     - determine content to be removed to the repository.
>
> Core (without understand of specific remote or its content):
>   - Provide low level API for plugin to affect the changes it has
> determined need to be made to the repository.  This is downloaders, models
> etc.
>   - Provide high(er) level API for plugin to affect the changes it has
> determined need to be made to the repository.  This is the ChangeSet.
>
> Are you proposing that this is not the correct division?
>

Yes I believe these problem statements suggest we should adjust the plugin
writer's responsibilities when interacting with the Changesets in two
specific ways. It's not exactly the language you used, but I believe the
following two responsibilities could be moved into the Changesets entirely:

- determining if any given Artifact or Content unit is already present in
Pulp (aka computing what needs tobe added)
- determining which content units need to be removed (aka computing the
removals)

^ goals are a restating of the problem statement that plugin writers are
asked to do differencing calculations when the Changesets could provide
that to the plugin writer instead.

>
>
>>
>>
>> So a shorter, simpler problem statement is: "to use the changesets plugin
>> writers have to do extra work to compute additions and removals parameters".
>>
>>
>> This statement ^ is better but still too vague to actually solve.  Can we
>> elaborate on specifically what "to do extra work" means?
>>
>
> Sure. Removing that vague language is one way to resolve its vagueness.
> Here's a revised problem statement: "to use the changesets plugin writers
> have to compute additions and removals parameters". This problem statement
> would be resolved by a solution that causes the plugin writer to never have
> to produce these parameters and be replaced by an interface that would
> require less effort from a plugin writer.
>
>
> I think it's the plugin's responsibility to determine the difference.
> Aside from that: without an understanding of the metadata and content type,
> how could the ChangeSet do this?  What might that looks like?
>

If I'm understanding this correctly, the Changesets already do this for
additions right? Help check my understanding. If a plugin writer delivers
PendingContent and PendingArtifacts to the Changesets as 'additions', the
Changesets will recognize them as already downloaded and not download them
right? If this is the case, what is the benefit of having plugin writers
also try to figure out if things should be downloaded?


>
>
>>
>>
>>
>>>
>>>
>>>>
>>>> 2. Plugins can't do end-to-end stream processing. The Changesets
>>>> themselves do stream processing, but when you call into
>>>> changeset.apply_and_drain() you have to have fully parsed the metadata
>>>> already. Currently when fetching all metadata from Galaxy, pulp_ansible
>>>> takes about 380 seconds (6+ min). This means that the actual Changeset
>>>> content downloading starts 380 seconds later than it could. At the heart of
>>>> the problem, the fetching+parsing of the metadata is not part of the stream
>>>> processing.
>>>>
>>>>
>>>> The additions/removals can be any interable (like generator) and by
>>>> using ChangeSet.apply() and iterating the returned object, the pluign can
>>>> "turn the crank" while downloading and processing the metadata.  The
>>>> ChangeSet.apply_and_drain() is just a convenience method.  I don't see how
>>>> this is a limitation of the ChangeSet.
>>>>
>>>
>>> That is new info for me (and maybe everyone). OK so Changesets have two
>>> interfaces. apply() and apply_and_drain(). Why do we have two interfaces
>>> when apply() can support all existing use cases (that I know of) and do
>>> end-to-end stream processing but apply_and_drain() cannot? I see all of our
>>> examples (and all of our new plugins) using apply_and_drain().
>>>
>>>
>>> The ChangeSet.apply() was how I designed (and documented) it.  Not sure
>>> when/who added the apply_and_drain().  +1 for removing it.
>>>
>>
>> I read through the changeset docs. I think this stream processing thing
>> is still a problem but perhaps in how we're presenting the Changeset with
>> it's arguments. I don't think apply() versus apply_and_drain() are at all
>> related. Regardless of if you are using apply() or apply_and_drain(), the
>> Changeset requires an 'additions' and 'removals' arguments. This sends a
>> clear message to the plugin writer that they need to compute additions and
>> removals. They will fetch the metadata to compute these which is mostly how
>> the changeset documentation reads. To know that they could present a
>> generator that would correctly allow the metdata from inside the Changeset
>> is I feel as non-obvious. I want the high-performing implementation to be
>> the obvious one.
>>
>> So what about a problem statement like this: "Changesets are presented
>> such that when you call into them you should already have fetched the
>> metadata"?
>>
>>
>> I'm not sure what is meant by "presented".  If this means that we should
>> provide an example of how the ChangeSet can be used by plugins (with large
>> metadata) in such a way that does not require downloading all the metadata
>> first - that sounds like a good idea.
>>
>
> Cool so this is transitioning to ideas for resolution. The solution to add
> documentation on how to do this with the existing interface is one option.
> My concern with adding additional docs on how to use the current interface
> better is that if users choose to follow the existing docs then they will
> have the stream processing problem once again. To me, this suggests that
> this new example should actually replace the existing documentation.
>
>
> Seems like both example would be useful.  I'm not convinced that all
> plugins would benefit from this.  For example: the File plugin manifest is
> small and would likely not benefit from the extra complexity.  For
> complicated plugins (like RPM), can differencing decision be made before
> analyzing the entire metadata (eg: primary.xml)?  Also, it's not clear to
> me how this would work using the Downloader.  Are you suggesting that the
> plugin would parse/process metadata files while they're being downloaded?
> Perhaps a better understanding of the flow to be supported would help me
> understand this.
>

Yes I am suggesting just that: that the Changesets could facilitate
parse/processing metadata files while actual content named in those files
is also being downloaded. I have a straightforward idea on how to achieve
this. It's short and easy enough to write up (no code), but I want to make
sure I'm not moving beyond the problem statement without others. Is there
more we want to do on these problem statements, or would answering a bit
about one way it could work be helpful?

Just to state my expectations: Moving beyond the problem statement I don't
consider to be a commitment to solve it; just an agreement on what we're
solving as we discuss various resolutions. Problem statements can also
always be revisited. Either way forward is fine w/ me, just let me know how
we should continue.


>
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>
>>
>>
>>
>>>
>>>
>>>
>>>>
>>>>
>>>> Do you see the same challenges I do? Are these the right problem
>>>> statements? I think with clear problem statements a solution will be easy
>>>> to see and agree on.
>>>>
>>>>
>>>> I'm not convinced that these are actual problems/challenges that need
>>>> to be addressed in the near term.
>>>>
>>>>
>>>> Thanks!
>>>> Brian
>>>>
>>>>
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>>>>
>>>>
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>>>>
>>>
>>>
>>
>>
>
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