[Pulp-dev] Fwd: Re: Changesets Challenges
Jeff Ortel
jortel at redhat.com
Thu Apr 12 15:53:40 UTC 2018
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
> <mailto: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
>> <mailto: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 <mailto: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?
>
>
>>
>> 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?
>
>
>>
>>>
>>>>
>>>> 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.
>
>>
>>
>>>
>>>>
>>>> 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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