[Engine-devel] Using REST API in web UI - review call summary

Thu Nov 28 19:22:33 UTC 2013

----- Original Message -----
> From: "Michael Pasternak" <mpastern at redhat.com>
> To: "Vojtech Szocs" <vszocs at redhat.com>
> Cc: "engine-devel" <engine-devel at ovirt.org>
> Sent: Sunday, November 24, 2013 9:07:01 AM
> Subject: Re: [Engine-devel] Using REST API in web UI - review call summary
> 
> 
> 
> Hi Vojtech,
> 
> First of all it was a good "presentation" of requirements + suggested
> solutions - well done!,

Thank you :)

> few comments/questions inline.
> 
> On 11/21/2013 11:18 PM, Vojtech Szocs wrote:
> > Hi guys,
> > 
> > this is a summary of yesterday's review call, I'll try to highlight
> > important Q/A and things we agreed on.
> > Feel free to add anything in case I've missed something.
> > 
> > --
> > 
> > Q: Why don't we simply try to use existing Java SDK and adapt it for GWT
> > apps? (asked by Michael & Gilad)
> > 
> > A: This might be a viable option to consider if we wanted to skip
> > JavaScript-based SDK altogether and target Java/GWT code
> > directly; we could simply take Java SDK and customize its abstractions
> > where necessary, i.e. using HTTP transport layer
> > implementation that works with GWT. In any case, this would mean coupling
> > ourselves to Java SDK (which has its own release cycle)
> > and I think this would complicate things for us.
> 
> not sure i buy this one :), this is the purpose of any sdk, including the
> one you about to write, people that will use it, will be "coupling" to it ...

Of course, but by saying "coupling ourselves to Java SDK" I meant SDK perspective, not client perspective:

- someone else (you) maintains Java SDK and therefore controls generated sources (JAR or RPM isn't relevant here)
- another guy (me) maintains (fictional) Java/GWT SDK that relies on Java SDK + some (supported) customizations
- the only way I can impose changes in my SDK is through supported customizations as you control original (Java SDK) sources,
  i.e. the whole code generation process is driven by your SDK, so my SDK is coupled to your SDK's build/release cycle

For the sake of simplicity, I guess it's best to start with SDK that has no dependencies whatsoever. After all, there's no common dependency (aside from running Engine to provide XSD & RSDL) between Java & Python SDK too, if I understand correctly.

In other words, building on top of something existing (just because we can do that) isn't always appropriate/flexible/efficient, it always depends on given context and requirements.

> 
> > 
> > As proposed on the meeting, I think it's best to aim for JavaScript SDK as
> > the lowest
> > common denominator for *any* web application that wants to work with REST
> > API. oVirt GWT-based
> > UI can simply bind to JavaScript SDK, i.e. Java/GWT code that just overlays
> > objects and functions
> > provided by JavaScript SDK. Another reason is ease of maintenance - I'd
> > rather see JavaScript SDK's code
> > generation process to be independent of any other SDK (people responsible
> > for maintaining JavaScript SDK
> > should have full control over generated code).
> 
> what do you mean by "people should have full control over generated code"?

It's related to "coupling from SDK perspective" I mentioned above:
"the only way I can impose changes in my SDK is through supported customizations as you control original (Java SDK) sources"

(by "people" I meant "JavaScript SDK developers")

Full control means ability to change generated sources in whatever way desired, but assuming the idea of reusing/customizing existing SDK code, aspect of full control is lost in favor of reusing existing code. And of course, this assumes that existing code (Java SDK) provides everything we need, which might or might not be the case.

So I just vote for simplicity, generate JavaScript SDK the way like other SDKs (Java/Python) - not trying to reuse anything, just grab XSD & RSDL and generate sources.

> the purpose of
> code generation is to ease maintenance, i.e you/maintainer should not write
> the feature
> once it available in api, just run CodeGen and you'll get it for free, but
> this is zero control
> over code.

+1

I agree with you on this.

> 
> > 
> > --
> > 
> > Q: What about functionality currently used by oVirt UI but not supported by
> > REST API? (asked by Einav)
> >    [For example, fetching VM entity over GWT RPC also returns related data
> >    such as Cluster name.]
> > 
> > A: Based on discussion I've had with other colleagues after yesterday's
> > review call, I don't think that
> > separate support-like backend layer is a good idea. Instead, this is the
> > kind of functionality that could be
> > placed in oVirt.js library. Logical operations like "get VMs and related
> > data" would be exposed through oVirt.js
> > (callback-based) API and ultimately realized as multiple physical requests
> > to REST API via JavaScript Binding.
> > 
> > oVirt.js client would be completely oblivious to the fact that multiple
> > physical requests are dispatched. In fact,
> > since HTTP communication is asynchronous in nature, oVirt.js client
> > wouldn't even notice any difference in terms of API
> > consumption. This assumes JavaScript SDK would use callback-based
> > (non-blocking) API instead of blocking one - after all,
> > blocking API on top of non-blocking implementation sounds pretty much like
> > leaky abstraction [1].
> > 
> > For example:
> > 
> >     sdk.getVmsWithExtraData(
> >         callbackToGetExtraDataForGivenVm, // might cause extra physical
> >         requests to REST API
> >         callbackFiredWhenAllDataIsReady   // update client only when all
> >         data is ready
> >     )
> 
> actually this the main bottleneck in moving UI to work on top of REST, and
> most interesting/complex part of this project,

Agreed, it's because UI "got used to" using internal backend interface concepts (actions, queries etc.) in the first place.. So we'll have to emulate what we used to use to prevent regressions, maybe improve/refactor in future.

> 
> you should think of very wise polling mechanism cause callbacks is a nice
> thing on paper, but behind the scene it all about polling:
> 
> - the entity/s till action got accomplished
> - add to this updating different grids
> - running multiple actions
> - showing events
> - and obviously much more

IMHO polling is just a workaround and indicates lack of proper notification solution.

Apparently, oVirt web UI isn't some CLI program for which HTTP request/response style is sufficient. oVirt web UI is dynamic, interactive web application that displays/updates data in real time. This is, in my opinion, quite a big difference.

I don't think callbacks are just a nice thing on paper. Callbacks are needed because the underlying communcation is async in nature:

- caller invokes API function and provides callback to execute when operation completes -> API is non-blocking
- polling attempts to detect change (i.e. operation completed) and notify the caller, so it's also some sort of callback -> this is more complicated compared to simple callback

> 
> and don't forget that every polled entity should be marshalled from xml to
> the javascript
> entity so at the end, "callbacks" mechanism will be extremely CPU consuming.

First of all, I don't understand how callback mechanism can be CPU consuming, can you please provide some explanation or use case?

Does Java SDK provide ability to poll Engine in order to get recent updates, and if yes, why?

Finally, polling makes things stateful, whereas SDK code should be stateless instead. If client wants to get recent updates, it should just use (stateless) SDK code to achieve this goal.

> 
> > 
> > [1] http://en.wikipedia.org/wiki/Leaky_abstraction
> > 
> > --
> > 
> > Last but not least, where to maintain JavaScript SDK projects: low-level
> > JavaScript Binding + high-level oVirt.js library.
> > 
> > I agree that conceptually both above mentioned projects should go into
> > dedicated "ovirt-engine-sdk-js" git repository and
> > have their own build/release process. However, for now, we're just making
> > baby steps so let's keep things simple and prototype
> > these projects as part of "ovirt-engine" git repository.
> > 
> > ... we can complicate things anytime, but we should know that any complex
> > system that works has inevitably evolved from simple
> > system that works ... (quote from
> > http://en.wikipedia.org/wiki/Gall%27s_law)
> > 
> > Regards,
> > Vojtech
> > _______________________________________________
> > Engine-devel mailing list
> > Engine-devel at ovirt.org
> > http://lists.ovirt.org/mailman/listinfo/engine-devel
> > 
> 
> 
> --
> 
> Michael Pasternak
> RedHat, ENG-Virtualization R&D
> 