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

[Michael Pasternak]

Hi Vojtech,

First of all it was a good "presentation" of requirements + suggested solutions - well done!,
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 ...

> 
> 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"? 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.

> 
> --
> 
> 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,

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

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.

> 
> [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
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-- 

Michael Pasternak
RedHat, ENG-Virtualization R&D